FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: c-reed on October 31, 2019, 04:56:15 PM
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I remember back in the 90's when the small block stroker scene hit, i was really into the foxbody mustangs at the time. Every kit that you would see advertised for sale would list its rod/stroke ratio. But nowadays you don't even hear about it anymore. Why is that. Is it not as big a deal as it was once thought to be? Or is it because that most kits sold are marketed more toward high performance, assuming a higher cfm flowing top end.
I'm no expert but it is my understanding that a longer rod tends to make better power when used with "bigger" heads, intake, cam...etc. and the shorter rods perform better with more velocity at a lower rpm. Is this correct?
If so, what about other applications like towing were you want more power down low or performance builds using factory small port heads and/or intakes. Wouldn't they benefit from a shorter rod?
For instance I've been thinking about a 428 street build, standard bore and a 4.125 stroke using a 6.535 or 6.385 BBC rod, a relatively square engine. Using smaller port heads and intake for good velocity (TFS, or maybe stock iron) , not too radical of a HR cam and a good intake to match (streetmaster/ dominator or ?). How would the shorter rod perform vs the 6.7 or 6.8
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I don't use "IMO" much because I usually try to speak based on experience. However, this is one of those topics where there's a lot of correct answers.
IMO, "rod/stroke" ratio doesn't carry as much weight as choosing the correct rod length for the displacement and induction.
Also, IMO, a large, slow moving intake port will not like a long rod because the piston spends more time at TDC, when it needs to be moving to "draw on" the intake port. Some camshafts will help with this, but I don't like long rods on a large, lower velocity, intake port.
In most cases, I use a long rod and a short piston. That's all relative, of course, depending on the combination and the application. In some instances you can't get away from it, but in most I don't like a heavy piston. I did a road race 289 and used a 2.875" crankshaft with a 5.700" rod. It used a 200 cfm ported factory cylinder head and made almost 400 lb-ft of torque with it, with 445 hp @ 7000. I had to do a lot of machine work to make that combination work because of the rod journal dimensions, but I feel that it was worth it.
What would I use on yours? I like 6.800" rods with that combination. Gets the piston down to a 1.288-1.300" compression height, gives stability, but makes the piston lighter. Also, on some crankshafts, the counterweight will get into the wrist pin bosses with a short rod. The last two 4.125" stroke engines that I've built, that's what they got. I also use a Molnar rod, which is a ton lighter than most of the I/H beam rods out there. Makes for a very lightweight rotating assembly.
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It makes almost no difference. Smokey Yunick's book is the most misunderstood and misquoted book after the bible.
I have read his deal on rod length and it only applies to a very specific situation. And even then it scratching for the last available hp, as in a high dollar racing scene.
I would worry more about piston heights and weights, due to rod length.
JMO,
paulie
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There are like 10 people on the face of the earth who can maximize the tune up based upon rod ratio. I'm going out on a limb here and assume you are not one of those 10 people. I'll admit, I am not.
It's really one of those bench racing topics. Everyone knows about rod ratio but, again, they can talk about it only in a theoretical sense...you know...how a piston accelerates or decelerates at different positions in the cylinder and the dwell periods and all that super critical stuff 'n' junk. Practical application belongs to those 10 folks. About the only thing we can address in our knowledge base is that a longer rod reduces load on the thrust side of the cylinder. It can also make for a lighter piston with a shorter compression height. Just don't run into the rings for the sake of rod ratio. That's not a good trade off.
So, with that in mind, the prevailing opinion for the past 40 years has been to put the longest rod you can into the engine. Of course, you have to consider the engine's architecture. There are cams, cylinder walls, and piston compression height limitations to consider.
And if you can't run a longer rod, or just don't want to go to the trouble for such a small gain, then put yourself in the company of the Pro Mod guys running those 1,000ci mountain motors with impossibly short rods.
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There are like 10 people on the face of the earth who can maximize the tune up based upon rod ratio. I'm going out on a limb here and assume you are not one of those 10 people.
HA! You are correct sir, I am not.
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Six year old posts to review:
http://fepower.net/simplemachinesforum/index.php?topic=1461.msg12803#msg12803
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I built myself a spreadsheet and graph for this a while back, because I'm curious like that. The sheet compared rod length and what it did to piston position/velocity/acceleration vs crank angle. When I compared between a 6.7" stroker rod and 6.8" stroker rod, I had to make the graph full-screen and zoom in ridiculously far to see any semblance of difference between either one of the graphs. Bumping the rod length to 7" I could finally see some separation in characteristics, still not a large amount.
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I think rod ratio is a lot more important when it comes to long term engine endurance. If you have a low rod ratio, the force against the cylinder wall from the piston is greater than it would be with a high rod ratio, and so an engine with a low rod ratio will wear the cylinder wall and rings more quickly. This is one reason why you see high rod ratios in production engines; they are designed to last over 200K miles. If you only need to go 30K or 50K miles between rebuilds, you can certainly tolerate a lower rod ratio.
For a performance engine like most of us are trying to build, you are usually trading off rod ratio with cubic inches. For example, you can build a 427 with stock stroke and stock rod length, and get a rod ratio of 1.71. On the other hand, you can build a 482" 427 stroker, with a 4.25" stroke and a 6.700" rod, and get a rod ratio of 1.57. Which will make more power, given equivalent performance parts? Of course the larger engine will; cubic inches always wins.
I'm currently putting together what has to be the most god awful engine for rod ratio ever conceived. 4.75" stroke, 6.625" rod length, rod ratio of 1.39. It has the longest rods that will fit, but still a very low rod ratio; nevertheless it will certainly make well over 1000 HP. It is replacing the 4.60" stroke, 6.700" reciprocating assembly, that had a rod ratio of 1.45 and made between 930 and 1005 horsepower, depending on cams and induction. I spun it regularly to 7800 RPM at the track.
My advice - put the longest rods you can reasonably use in the engine, but don't get overly concerned about rod ratio; in terms of performance it won't have a big effect.
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When 347 small block ford stroker cranks came into prominence we saw quite a lot of talk about stroke to rod length issues because this combo had a very short rod and engine life was a concern. Might be why not so much being printed these days. IMO.
Mike.
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a quote from Jay in the 6 year old link machoneman posted:
"For what its worth, I am a firm believer that rod ratio is not a particularly important parameter for most performance engines. If you are running in a class where cubic inches are limited and/or stroke is limited, there may be some benefit to certain rod ratio and head combinations. For all other applications, cubic inches will trump the effect of rod ratios. As an extreme example, my 585" SOHC has a 4.6" stroke and a 6.625" rod, making for a rod ratio of 1.44, which is extremely low in anybody's book. That engine made 960 HP and revved to the limiter (7600) faster and more aggressively than any other engine I've ever had. On the track I had to set the shift light at 6800 RPM in order to make the 1-2 shift before the engine hit the limiter. "
My question was not so much about changing cubic inches with stroke but only the two variables of rod length and compression height. I understand about changing the rod angle and piston weight. I was really referring to how it would "perform" not about the most power between the two.
Jay mentioned how fast his 585 SOHC revved. If you could make a street engine rev without changing gear ratios and target your induction in that given RPM range is more of what i was getting at, but it seems the overall consensus is to not worry about it and use the longest rod readily available. Just dreaming at work really, it's how I get through the day 8)
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Use the longest rod you can until the piston gets too short. I like taller pistons than is currently in fashion, maybe in the 1.35-1.55" range. I wouldn't want to go to either extreme. A Mopar 440 has excessively tall and heavy pistons. 347 Fords have pistons that are pretty short. I know people run both extremes successfully, but I think the best is in the middle. A fairly short and light piston with the ring not in the pin is what I like to see.
If I had to pick a ideal number it would be around 1.35" or 1.40". That is short enough to make the piston light and long enough to have the rings out of the pin, and give good piston stability. The pistons in my next engine will likely be around 1.58". That is just how the parts that I could find and afford went together. I am okay with that.
JMO,
paulie
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And just for more clarification i wasn't talking about targeting a specific ratio #, it was more of a longer rod vs a shorter rod question. I probably could have worded it better.
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I do a lot of engines with oil rail supports, which means the pin intersects the oil rings. The only time I have suffered any consequence from that is when the support has spun for whatever reason. Other than that, I see no difference from the engines that have the pin/ring intersection. With as many 347's that are rolling around, the weaknesses should have been found with that setup by now. One of the previous editors of Modified Mustangs (Marc Christ) told me that he had over 100k miles on his Fox Mustang with a 347, with no oil consumption or issues.
I do believe a lot of the voodoo about intersected rings has stemmed from internet bench racing. The sheer number of rotating assembly combinations out there between Ford/Chevy/LS/etc. would be staggering.
A stable piston does make me feel better though, even if it's just a warm and snuggly feeling inside. I've had pistons in here from .990" pin height to 1.900" pin height and even though I've torn down lots of engines with 1.040-1.090" compression heights with no perceived wear, it just seems to me that the piston has to be rocking more than in an engine with a taller piston. On FE's, we really don't have to worry about that sorta thing.
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I do a lot of engines with oil rail supports, which means the pin intersects the oil rings. The only time I have suffered any consequence from that is when the support has spun for whatever reason. Other than that, I see no difference from the engines that have the pin/ring intersection. With as many 347's that are rolling around, the weaknesses should have been found with that setup by now. One of the previous editors of Modified Mustangs (Marc Christ) told me that he had over 100k miles on his Fox Mustang with a 347, with no oil consumption or issues.
I do believe a lot of the voodoo about intersected rings has stemmed from internet bench racing. The sheer number of rotating assembly combinations out there between Ford/Chevy/LS/etc. would be staggering.
A stable piston does make me feel better though, even if it's just a warm and snuggly feeling inside. I've had pistons in here from .990" pin height to 1.900" pin height and even though I've torn down lots of engines with 1.040-1.090" compression heights with no perceived wear, it just seems to me that the piston has to be rocking more than in an engine with a taller piston. On FE's, we really don't have to worry about that sorta thing.
That is all true, Brent. I've never built or even used a 347. I just don't "like" super tall or super short pistons. No doubt they can be made to run good and be reliable. I just have a height range that I think is best overall. You work with the parts you have, but if I could wave a magic wand, I would always be around 1.35". Good combination of light weight and stability. And no support rails. :)
JMO,
paulie
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8:1 is the only rod ratio I really care about, eight rods to one crankshaft LOL. I do try to use the longest rod I can to minimize piston weight while balancing it withthe most stability I can with skirt length. But in the end I don’t put a lot of time in it on my street builds
ON EDIT: Corrected the gibberish....LOL I originally did this response via phone....never again
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It makes almost no difference. Smokey Yunick's book is the most misunderstood and misquoted book after the bible.
I have read his deal on rod length and it only applies to a very specific situation. And even then it scratching for the last available hp, as in a high dollar racing scene.
I would worry more about piston heights and weights, due to rod length.
JMO,
paulie
+1 I cornered Smokey ( rip) at the PRI show ( in an elevator) and asked him how he could make a statement like "use the longest possible rod'' and he replied that was taken out of context and pertained to a specific non ported SBC head /bore/stroke/ class combination. HE said rod length can make or break a combination , and about that time we were at his floor.
Randy
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I'd add to plovett's comment that for 99.9% of us, it makes no difference. Bench racing, racing rod rations, etc. Light weight piston makes RPM easier on the rod. Other than that, use what fits IMHO.
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I like and respect Smokey, a lot (may he RIP), he even worked with Fords a little :) and was a true innovator but, he did say, in one of his books, that he thought that the reason you could get more HP from a higher rod/stroke ratio, was because there was a longer dwell time, at TDC, building more pressure, for the down stroke.
Personally, I think, if there is more HP for the longer rod, it's because of less pressure on the piston to cylinder wall contact and less friction.
I know the rod length to stroke ratio has been around since, at least the '20's (probably before the turn of the 19th century) but, I look at it as a maximum angle.
A 317 Linc (3.5/7.062) has a 2.018/1 ratio, the angle is 14.35 degree, Jays 4.75/ 6.625 combo 1.395/1 and that angle is 21 deg.
What I look at for longevity is what NASCAR runs and that's 16.5 - 17 deg (9,300 rpm/500 mi) and for what will reliably work, I look a sprint cars 19.5 deg. and try to keep in those areas, striving for the lower angle and longer rod. Also, you can't deny that the sprint cars basic 1.015 C/H works on bores over 4".
Because what we talk about, is very subjective and opinion based mostly, I would like to see some dyno tests based on those perimeters, the FE would be a good test bed for it but, that old 317/368 Y block Lincoln with it's 10.94 deck height could yield more info but, at a much higher price.
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Power from dwell time is ridiculous . A piston that is not in motion is doing anything . If an increased burn time of a couple of nano seconds increases HP I've never seen any proof of it. Rod length in some engines can make significant changes. Being in the aftermarket performance piston industry for 20+ years has let me see thousands of different stroke / rod / head combinations and the cams that go with them. I have made rod length suggestions that "woke up" some combinations like alcohol sprint cars with SBCs and All Pro 13* heads. Rod length changes will move the torque curve around .
The FE is one where rod length doesn't make much difference except maybe in a tunnel port or cammer.
Randy
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The long rod does change timing requirements on engines that don't have a very modern combustion chamber. The better the combustion chamber, the less difference it makes. I have dyno tested different engines with same crankshafts, heads, but different rods and pistons, and found that timing can change as much as 4* for optimum power and torque. I have also tested the same shortblock with as many as four different sets of heads, and found the timing needed was the same with the same rods and pistons with different compression. Personally, I feel the rod length has more to do with timing than folks can imagine because they never really tested back to back changes. When you go from a 6.300" rod to 6.750" rod in the same engine, you will find things that you never thought of and some real head scratching for answers. The more I think I know, the more I get surprised in real back to back testing. Joe-JDC
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Joe, do you have the data from those dyno tests? I'd love to see how it changes things.
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The long rod does change timing requirements on engines that don't have a very modern combustion chamber. The better the combustion chamber, the less difference it makes. I have dyno tested different engines with same crankshafts, heads, but different rods and pistons, and found that timing can change as much as 4* for optimum power and torque. I have also tested the same shortblock with as many as four different sets of heads, and found the timing needed was the same with the same rods and pistons with different compression. Personally, I feel the rod length has more to do with timing than folks can imagine because they never really tested back to back changes. When you go from a 6.300" rod to 6.750" rod in the same engine, you will find things that you never thought of and some real head scratching for answers. The more I think I know, the more I get surprised in real back to back testing. Joe-JDC
In my opinion, that is one of the most pertinent posts regarding rod/stroke ratios that I have ever seen. Ignition timing seems like it would be affected by minute changes in piston velocity/position much more than other variables. I have never been into the rod/ratio argument, but ignition timing could measurably be affected by it.
JMO,
paulie
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The long rod does change timing requirements on engines that don't have a very modern combustion chamber. The better the combustion chamber, the less difference it makes. I have dyno tested different engines with same crankshafts, heads, but different rods and pistons, and found that timing can change as much as 4* for optimum power and torque. I have also tested the same shortblock with as many as four different sets of heads, and found the timing needed was the same with the same rods and pistons with different compression. Personally, I feel the rod length has more to do with timing than folks can imagine because they never really tested back to back changes. When you go from a 6.300" rod to 6.750" rod in the same engine, you will find things that you never thought of and some real head scratching for answers. The more I think I know, the more I get surprised in real back to back testing. Joe-JDC
Joe, can we assume longer rod needed less total in your tests?
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Yes, less lead time. The events happen quicker with the shorter rod, requiring more lead with older combustion chambers. Joe-JDC
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Yes, less lead time. The events happen quicker with the shorter rod, requiring more lead with older combustion chambers. Joe-JDC
Makes perfect sense, increased dwell time at TDC with the longer rod means the flame needs to travel across a smaller 3 dimensional chamber compared to a shorter rod that is dropping slightly quicker. Thanks
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Funny, I was just about to ask:
“With better fast burn chambers, wouldn’t dwell time matter less?”
Thanks for answering that before I asked it Joe.
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I have dyno tested different engines with same crankshafts, heads, but different rods and pistons, and found that timing can change as much as 4* for optimum power and torque. I have also tested the same shortblock with as many as four different sets of heads, and found the timing needed was the same with the same rods and pistons with different compression. When you go from a 6.300" rod to 6.750" rod in the same engine, you will find things that you never thought of and some real head scratching for answers. The more I think I know, the more I get surprised in real back to back testing. Joe-JDC
With the timing change did the torque change in those applications and what I think everyone wants to know (at least me), did the longer rod, increase torque?
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I will have to see if I can find the dyno sheets/graphs, but if memory serves me correctly, there was a shift in torque peak. A manifold change, or spacer change, or camshaft degree change, header collector change, will also affect the torque peak or torque averages. It then becomes a tuning quest to find the best of those changes for the final combination that repeats pull after pull. Joe-JDC
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Thank you, Joe.
Regarding the "shift" was it in ft/lb or rpm?
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Ok, a couple of points to consider. I went back and asked my friend Ted Eaton to confirm my observations. Ted has his own dyno facility, and balances engines, builds FEs, and has raced FEs almost as long as I have. He has a 64 Thunderbolt Clone High Riser FE, 66Fairlane with FEs, and has years of dyno experience. He said that the longer rod does in fact require less timing, which helps with lessening detonation issues, and the shorter rods actually improve torque slightly since the rod moves the crankshaft off TDC quicker with the initial combustion explosion. It is difficult to quantify since very few back to back dyno tests have been done for this actual comparison. The long rod does help with reducing side loading of the piston skirts, and helps with ring longevity due to detonation reduction issues. He will try to check his dyno sheets to see what it did with the torque curves when he gets some free time.
What most folks forget is that torque is a measure of work performed, and it can be manipulated with camshaft degreeing, intake manifold changes, carburetor changes, carb spacers, timing changes, valve lash changes, and header tubing, length, primary tube size, tri-Y, four into one, collector size, merge size, and collector extensions, and probably more that I can't think of right now. It is not purely a function of cubic inches, or stroke, or rod length, or compression. If it were, these other modifications would not affect torque, but they do. Joe-JDC
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I'm familiar with Ted Eaton's dyno work on Y blocks and have read some of his articles on dyno testing. He knows what he's doing, for sure! I look forward to anything he can offer, in this area.
My racing has always involved, engine size limitations so, it's one more thing that can help.
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Ross,
I would think the later ignition timing requirement would be a result of the slower approach to TDC since the spark is in advance of TDC. I fully agree that the reduced area by the piston being near TDC longer would benefit from less advance.
As Joe mentioned it is key to stay focused on "our" FE engine and not generalize. For example a modern Nascar engine usually has 28* total timing with 12-1 compression on "E85" type fuel. An FE could easily be 6-8 degrees more with the same compression and fuel.
Short rods "tend" to have torque at a lower RPM than a longer rod. Some builders tailor the RPM of the torque band by changing the rod length for different tracks ( circle mainly).
Side loading does become a factor when rod ratios dip below 1.5-1. Fortunately for us the 4.375-6.7 rod is still 1.53 or very close to a GM 454.
I am open to criticism / correction / different opinions.
Randy
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What most folks forget is that torque is a measure of work performed, and it can be manipulated with camshaft degreeing, intake manifold changes, carburetor changes, carb spacers, timing changes, valve lash changes, and header tubing, length, primary tube size, tri-Y, four into one, collector size, merge size, and collector extensions, and probably more that I can't think of right now. It is not purely a function of cubic inches, or stroke, or rod length, or compression. If it were, these other modifications would not affect torque, but they do. Joe-JDC
I think all those variables affect volumetric efficiency, differently at different rpms. So I simplify and say torque is a result of displacement, compression ratio, and volumetric efficiency, at any specific rpm.
JMO,
paulie
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to further support JDC my local speed shop Performance Clinic was just talking to me about SBC old heads verses the late 90's Vortec heads , example was a 1980 Chevy L82 big chamber 72cc aka big chamber LT1 heads 2.02/1.60 verses late 90's Vortec 62 cc heads similar prep 2.02/1.60 , both engines have the same 9.5 to 1 compression , same cam , same Performer intake and carb on two seperate engines 355 cubes only difference was the heads , Greg says they can match in horsepower but the Vortec Combustion Chamber produces 18 to 22 more ft lbs of torque through out the pull with both engines having the same HP , his thoughts was Combustion Chamber difference for the improved torque with less timing , not quite FE related but still apples to apples with different/improved torque curve on similar builds
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As an owner of one of those long rod, tall deck Lincolns, I can give an observation.. The chamber in the Lincoln head is not efficient. Odd shape, deep, and large volume.. Yet my my 12:1 roller cam motors make best power at 32 or 33 degrees..Less than any stock head FE I have worked with.. Another observation is when I tear down an old Lincoln there is never much bore wear and the piston skirts do not show wear. As far as torque, it is no better than any other engine of similar cubic inch and compression.
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Yes, less lead time. The events happen quicker with the shorter rod, requiring more lead with older combustion chambers. Joe-JDC
My 452ci tunnel port with 6.800 rods needed the same timing as the 427ci version with stock length rods. (40*) I think the inadequacy of the old chamber design was such there wasn't going to be any noticeable difference changing the rod length.
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Those two combinations have almost the same rod/stroke ratio.
452 - assuming 3.98 crank. 6.8/3.98" = 1.709
427 - 6.49/3.78 = 1.717
Is my assumption on the 452 correct?
thanks,
paulie
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You know I didn't do the math , the 452 was 3.99 stroke.
I guess that's why it wanted the exact same timing.
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In the engines that most of us are likely concerned about, I don't believe this as a variable worth chasing. In the case of a 4.25" stroke FE, the only rod lengths I have seen are 6.7" and 6.8". At 6000rpm there is a max piston velocity difference of .26 feet/sec. Through the whole stroke, the maximum that the 6.7" setup piston gets ahead of the 6.8" setup piston is .005". The info could prove worthwhile when comparing sizeable internal dimension differences, but when comparing our options available for a common stroker FE, I just can't see any added torque/timing benefit expectations.
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Would wrist pin offset have any significant effect on rod angle and or piston side thrust in an engine with a marginally too short rod?
Mike.
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Offsetting the pin changes the "pivot point" so the piston acts like it has a longer rod because of the change in angularity. Obviously this also reduces side load and "piston slap" because the offset is toward the thrust side of the piston. Many of us recall swapping piston offset from thrust to non thrust to see a small power gain. Cast and some replacement forged pistons still have pin offset but "most" aftermarket forged pistons are "on center". Piston designs have come along way in the last twenty years as we learn more about how to manipulate sizing and skirt shapes to obtain maximum possible stability with minimal drag. Very thin rings are common now and a challenge to keep sealed against the cylinder walls.
Randy
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Ross,
I would think the later ignition timing requirement would be a result of the slower approach to TDC since the spark is in advance of TDC. I fully agree that the reduced area by the piston being near TDC longer would benefit from less advance.
As Joe mentioned it is key to stay focused on "our" FE engine and not generalize. For example a modern Nascar engine usually has 28* total timing with 12-1 compression on "E85" type fuel. An FE could easily be 6-8 degrees more with the same compression and fuel.
Short rods "tend" to have torque at a lower RPM than a longer rod. Some builders tailor the RPM of the torque band by changing the rod length for different tracks ( circle mainly).
Side loading does become a factor when rod ratios dip below 1.5-1. Fortunately for us the 4.375-6.7 rod is still 1.53 or very close to a GM 454.
I am open to criticism / correction / different opinions.
Randy
No argument here on different designs having very different requirements, heck you know my position on 3-dimensional chamber shape and flame path. I don't even consider an "FE" to be a single thing. Quench pads, chamber shape, crevice area all vary and can result in a huge difference
However, in my opinion, pre-TDC piston behavior is less of a factor (although still a component) than flame travel during dwell and immediately post-TDC, especially at RPM. I'd have to do some math to back it up and it's certainly RPM, chamber size/shape and fuel dependent, but the spark shouldn't drive a significant growth in flame and heat expansion on the upstroke, that is merely "when" the spark needs to happen to allow expansion during dwell and through the power stroke
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Ross ,
There is a video ( youtube maybe) of an engine study with a quartz , semi transparent "cylinder head" so flame travel could be observed . It is interesting to see how the flame "propogates'' across the chamber.
Yes there are "far" more efficient chamber shapes than the common FE we deal with. The larger the chamber , the more space there is to be "filled" with exploding fuel and air before they react against the piston and transfer the energy. Early ignition timing wastes energy (potential anyway) as the piston is further away from TDC when spark is initiated. "Ideal" timing would be zero so the piston isn't fighting against the explosion , but the lack of enough dwell time makes it impossible to do power wise.
Randy
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My point exactly Randy, we must be talking past each other. That's Chamber 101, and there are many ways to skin the cat, but in the end, there is no such thing as an "FE chamber". Heads alone vary in design, but more than that, quench distance, piston design, builds are all over the map, and it's not just an FE thing. I would also tell you that with an FE chamber, the common 38 degrees is likely due more to the fuel than anything else...reason being is "it doesn't get much worse than that" in terms of needing that much lead at WOT, even in other engine designs.
When someone thinks chamber, they need to think in 3 dimensions, shape, as well as how those moving shapes interact, and not only with flame expansion but also with churn and airflow. Although that is not the discussion here.
One very important word police point, there is no explosion in an engine, its a controlled burn with heat expansion that pushes the piston down. I am usually not a definition guy but 40 years ago, when I was about 12 years old, I used to argue with my dad about that during his builds, and guess who was wrong...not him LOL There were many times that I have admitted that over the years...
Back to the real story...if anything in my post made you think that I advocate more advance over less for some reason, I do not. I advocate for the right advance,across the whole RPM and load range, which makes a significant difference drivability and power. The "right" advance for a compression, chamber design, fuel, rpm, not to mention other inputs like cam design, even elevation, etc, will never fight the piston. That was my point exactly, the burn will not expand fully until the piston is at the top of it's stroke, not when the plug fires, (unless you have too much advance, which you shouldn't, or pre-ignition) so my position is that pre-TDC rate of chamber volume change is less a factor with a long rod than dwell and the immediate start of expansion.
However, even THAT position I think is minimal, I think there is MUCH more to be gained with a tight quench and proper upper and lower quench pad cooperation and 3D chamber shape than with the slight change in dwell and acceleration with a rod change. However this post isn't going to change two old dogs that have slightly different opinions on how to skin the same cat with chamber design :)
FYI - It's going to be very hard to directly compare the two engines coming up on the dyno, but one is your 24 cc D-cup with a crappy D2 iron chamber and .041 quench, the other is your 24 cc dish with a TFS chamber and .041 quench. It will be hard to draw a conclusion based on piston alone, because all of the different variables in the engine, compression and cam alone are worlds apart, despite the cylinder volume (less the head, being identical) but it still may give us a little info on two very different chamber designs. It would be neat if total timing on both was lower than 36-38, no doubt the TFS head wsill be, but we will see what the iron head will do as a comparison. The issue of course is we know the TFS chamber works better, so we really won't know how much the piston contributes...however, we will know that a standard FE likes 36-38, and if it comes in on the low side, the quench and D-cup will have helped. We will have that info for both on 15 Nov.
If you want to BS about that, I'd rather put it in one of the "guess the HP" posts, or start a new post and not hijack here
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Ross we are on the same page! It is this medium of communication that skews things. The semantics for a true explosion where the "vessel" or "host" is sacrificed is a point taken. In the video I mentioned , there was a noticeable "flash" after ignition followed by a rapid "burn" away from the ignition source. I am assuming you call it a controlled burn because the "host" is not destroyed after the "medium" is ignited. When I speak of the FE chamber I am relating to the most common 72-75 cc chamber used in cast iron FE heads designed in the late '50s fpr '60s engines ( two year lead time at minimum back then) not the early smaller chamber versions or the MR/Tunnel Port machined camber versions.
NO , I know you are not an advocate of large amounts of ignition timing , we discussed that over the phone when I was explaining my Nascar findings to you. We both agree that "proper" timing across the rpm band is the goal , no question there. Back in the "stone age", I thought that "curving" a distributor was a waste of time and all you needed to do was pull the point plate and replace the heavy spring with another light one and set the total timing when the centrifugal advance stopped advancing. This of course resulted in unknown ( possibly detrimental) amounts of timing at lower rpms. Now with modern electronics timing curves can be manipulated to "ideal" situations. I fully AGREE long rod to stroke ratios require less timing . NO argument there.
Back to chambers . With all of the attention paid to flow bench information , chambers have been "flared" around the valves and some now sport revised "kidney shapes" that dramatically aid airflow into and out of the chamber. This is not new news to you at all. In the past when some of these kidney shaped , shallow depth chambers were designed , we made "mirror image dishes to follow the thinking of reducing quench areas and combat quench related detonation. What we found as an offshoot was that as the chamber became more efficient , chamber matching quench pads weren't as critical because the chamber shape itself curbed detonation.We then looked at air intake and piston shapes that let air enter without "hitting a wall" in the mirror image dish. We also looked at what was happening on the compression stroke with regard to cross flow from the quench pads. These developments are not "guaranteed" improvements without the ability to test them in various applications because as you noted chambers vary all over the place and some just don't respond well. I COMPLETELY agree with you that some chambers MUST have mirror image dishes and the tightest possible quench to gain the most power. NEVER an argument there.
Your upcoming testing will be very interesting , and I look forward to seeing the results. Dale's dyno is very "honest" unlike some of the "happy" dynos out there. Using the same facility time after time keeps the results relative.
Best of luck.
Randy