FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: bn69stang on November 27, 2013, 06:39:57 AM
-
when shopping for stroker kits for say a 428 block , the popular crank is the 4.25 stroke with the 6.7 chevy based rod and i was wondering what the rod ratio would be compared to 4.375 stroke and the 6.7 inch rod ? . Seems like everyone uses the 4.25 inch crank even with most of the 427 block builds . Cubic inchs and h beam rods with better bearing surface and forged pistons is why i want build my other block , all NEW parts and just want to get the most of this build .. Bud
-
It's easy to figure out, just divide the rod length by the stroke.
6.700/4.250 = 1.58
6.700/4.375 = 1.53
You're not going to notice any difference between the two. Rod/stroke ratio is a subject where you'll get 1000 opinions and none of them will be right....LOL
You will find a guy with dyno results showing that a certain rod works better with a certain head, then you'll find another guy that has tried the exact same combo and showed that another rod works better. Each engine is different.
If it were a race motor, I'd pick the shortest, lightest piston that was feasible, then find something to connect the piston to the crank.
For a street engine, I wouldn't really put any effort in worrying about rod/stroke ratios. With the 4.375" crank, you're not going to be able to play too much with rod lengths because the piston will get pretty short, and it will already require support rails with a 6.700" rod. With the 4.250" crank, you can play with a 6.800" rod, but it's hard to find pistons readily available with that compression height.
-
With that said you probably build more 4.25/ 6.7 combos on say a 4.16 or 4.17 inch bore ? , the rod ratio is not that far apart though . Have you built and dyno d any 428 blocks with the 4.375 stroke ? Bud
-
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.
Bottom line - everything you hear about long strokes and small rod ratios not revving or making top end power is BS. Go for the cubes - Jay
-
yeah i remember you saying that cubic inches rule the street , and i just wanted to get your opinions before buying any parts , this is again a street car with an occasional strip run . But i want a very healthy BIG BLOCK , and not sure at this point if it will have 1 carb or 2 or maybe a victor jr intake and a multiport EFI setup , and leaning towards a hyd roller so that this 463 or 477 c i motor will be a strong and reliable performer . And im looking forward to not having a motor with 40 year old rods in it .. Thanks Bud
-
The ratio is darn near meaningless in the context of other things.
But the 4.25 stroke does have a few distinct advantages over the 4.375 that make it worth considering.
First is cost on many builds - I can get the pistons for 390 based kits for a lot less money these days - pistons and rings are less than just the pistons for other combinations. Add in the extra cost for the crank and the 4.375 will cost about $800 more minimum
Second is packaging. The 4.250 is a drop in on most applications - assembles like a stock build. The 4.375 can tag the block in some builds, and the cam in others. Nothing that happens all the time - but another thing to closely check.
Third is the oil rail deal. The 4.250 does not pierce the oil groove - so zero issues there. But the 4.375 will require the spacer every time. That adds drag, and compromises oil consumption. I know that a million hot rod small blocks are built that way - so it will work OK, but absolutely zero OE builds will do this and I view it as something to avoid on street applications when possible. Just a risk with no reward.
-
Thanks for the feed back Barry , 800.00 for 14 cubic inches , i could spend that on the roller cam and lifters thru your shop , along with the other parts . I appreciate all the feedback you guys , and i want this to be a straight forward build with no surprises or difficulties .
-
And given this will be an high altitude build , my elevation is 5500 ft above sea level and there is not lot of air to fill the cylinders so the need to assemble the engine that will work , make great hp and torque .. Bud
-
n/m
-
Barry is referring to the pin height in the piston. When you go to a 4.375" crank and a 6.700" rod, the pin location moves up in the piston, and actually breaches the lower ring land for the oil ring. So, after you install the rod and pin on the piston, you have gaps between the lower oil ring lands on each side, where the pin had to slide through. The way that is addressed is for the piston manufacturer to supply a steel rail that fits on the lower oil ring land, making it a continuous support over the gap in the land where the pin went through. So, your oil rings go between the normal top oil ring land, and the steel rail that is sitting on the bottom oil ring land.
-
You'd be surprised on how many aftermarket pistons have that feature. Small block stuff, all the way up to 4.300" and 4.500" stroke BBF stuff. I've never had any issue with a piston being set up that way, knock on aluminum. Was talking to one of the writers of Modified Mustangs & Fords the other day, and he has a 347 SBF with way over 100k on the clock that doesn't use any oil.
It all depends on what other components you go with on how big the price differential is. If you use premium parts and compare apples to apples, the price on going bigger isn't all that bad. If you go with Probe/Icon/Race-Tec pistons, a cast 4.250" Scat crank, and I-beam rods, the price of a 4.375" rotating assembly is going to be a good bit higher. However, if you go with a premium 4.250" setup, with Diamond pistons, a forged Scat crank, etc, etc., then the price isn't going to be much different at all, and you may end up with a cheaper setup overall.
Figure out which route you're going to go, do some shopping, and then you can make a more educated decision.
-
This is one area where I have to go with the longest rod possible theory. I personally always try to use the longest rod available for a certain stroke, and I have always had good success with that process in building engines. There are a lot of variables in any engine equation, but torque production always increases with the longest rod for a given stroke, and hopefully everyone knows by now that more torque for a given displacement moves the vehicle easier. Over the years I have seen all the theories, heard all the arguments, and from my own personal experience from building 4 cylinder Volvos to Boss 429s to 635Pro Stock Chevies, I would rather have the longest rod possible in any performance build. Yes the short rods work, and require different camshafts, but for maximum durability, and torque AND horsepower production, I will take the longest rod possible to fit the combination EVERY time as the best power producer. Basically, in the long run every build is a compromise, but if I have a choice, I will use the longest rod available for a given stoke as my first choice. PS (I do have a degree in automotive/and a degree in Aircraft maintenance as well as 6 years instructor/professor experience. I have heard/read all the theories) Joe-JDC.
-
Long ago, a very wise engine builder of mine (Jerry Baker @ Pro Motor Engineering formerly of Des Plaines, IL, now doing stock car engines in NC state) said it this way. Only when one has optimized near 100% of a specific full-race-only engine package, come and see me about the miniscule gain a longer rod can bring. And yes, they more than dabbled in very high HP drag engines with long rods for sure.
His point was far more effort should go into other aspects of optimizing an engine's mix of parts, flow optimization, C/R, etc. He also agreed (sorry...it was a Chevy heavy shop, me with a BBC at the time, and his place with numerous NHRA award winning C thru E/MP damn near 10,0000 rpm SBC's!) that the old Smokey Yunick book claim of a near 1.9 -1 'ideal' rod ratio lead many SBC fans far astray. They all seemed to miss the point that for old, flow-limited SBC OEM iron castings in small CID engines, yes, a high-rod ratio did work the heads marginally better.....but all bets were off for larger, later and/or heavily ported heads in larger (i.e. >302CID) engines not limited by many rules. Certain cat lovers seem to always miss this critical point...LOL!
-
well again thanks for all the input , i think i have pretty much decided to use the 4.25 / 6.7 rod kit and go from on the rest of the build .. Bud
-
... and you may end up with a cheaper setup overall.
I rarely come out and disagree with you - but no way can that statement be true. The absolute best you can get is "equal" but the odds are generally against achieving even that.
The 4.250 stroke pistons are lower cost catalog parts even from Diamond unless you are doing a high compression deal or working on an odd bore size or ring pack. Best case is equal but never cheaper. The rail spacers will ad a couple bucks but that's pretty insignificant.
Scat does not offer a 4.375 steel crank (other than $$$ billet) so we are locked into RPM. They charge the same exact price for 4.250 as for 4.375 - both iterations require work before running them - so again the best possible case is equal. Of course - since Scat does sell superior 4.125 and 4.250 steel cranks for a bit more money I would go to those at any opportunity - but that gets away from the apples to apples comparison.
-
At the horsepower level i would get from a 463 inch stroker motor , non power adder with a target of 530-540 hp im sure the scat cast crank would be fine , its well below the rpm / hp levels its been pushed to in other builds . And thats 1.16 hp per cubic inch and is certainly a do able package ..
-
I was speaking in generality, didn't take time to add it up. It would come down to how much you charge for your stuff and how much I charge for my stuff. I get a decent break on rpm cranks so even with machine work that they need, I coukd still sell cheaper than what the forged Scats cost. As for diamond, MWD here, and a change to compression height, volume, etc is just around $40. Rods are the same price, rings & bearings the same.
You're not really disagreeing with me, nor am I with you, but I don't think the difference is all that big.
-
One of the points that hasn't come up here is longevity. A longer rod will load the sides of the cylinder bores less when the engine is running, because of a shallower angle between the rod and the cylinder bore. Basically this leads to less wear on the bore and a longer lasting engine. Not a big deal unless you are building 100K mile engines, but something to consider nonetheless.
I agree with Joe that the longest rod possible should be used in any performance engine, but I also agree with Bob that which rod length to use is pretty far down the list of important decisions. Given the choice between a smaller stroke and longer rod, or longer stroker and shorter rod (to get more cubic inches), I'd pick the longer stroke every time.
What we really need is a tall deck block with a raised cam, so that we can easily go to a 4.6" stroke and 7.500" rod, for a 559" engine with a 4.4" bore...
-
Jay you need to produce and sell that block lol , we need more options and new parts and cubic inches are on all our lists .. Bud
-
Here's what Darin Morgan and Buddy Reher had to say a few years ago on Speedtalk.com:
rod ratio, who cares??
Postby Darin Morgan ยป Thu Oct 28, 2004 7:17 pm
shawn wrote:I'm sure Darin Morgan will comment on this.They have a lot of experience wth testing different rod lengths. Here's a quote from Reher-Morrisons site from David Reher and his thoughts on the subject-
" We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I've seen dozens of magazine articles on supposedly "magic" connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued!
If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period."
This is located in the tech article "by the book" if anyone else wants to read the whole thing. Hope Darin does comment on this, he has some deeper thoughts on this with a GM backed test to back them up.
Shawn
DITO shawn.
You pretty much summed it up.
People put WAY to much importance on this preconceived " ideal rod ratio" idea. Rod ratio is not a primary consideration when designing an engine. You get the deck as short as possible so the piston does not come out of the bore. That gives you better manifolding which will make ten times the power any " ideal rod ratio" would net you. You shorten the pushrod and make the valve train stable above 9000rpm. You make the piston ring package as compact as possible to get the pin as high as possible and that will make for a light weight, balanced (not top heavy) piston design, THEN you decide what rod connects the piston to the crank. Its not magic, its simple mechanics. People look at what Smokey yunick said and they take it out of context in my opinion. He said you should put the longest rod YOU CAN not the longest rod YOU CAN CRAM JAM OR MANIPULATE into the engine. I see people all the time screw up the engine combination to facilitate some preconceived ideal rod ratio and they wonder why the thing wont turn up and make power. The difference in the GM 358 NASCAR test engine from 5.250 inch long rods to 6.1 inch long rods was maybe 2ft/lbs and 2 HP. Not much to worry about. That satisfied the GM engineers that there is nothing there. Does a short rod make more TQ? Does a long rod make more top end power? It probably does but its such an insignificant amount, its not even worth messing with! If there was a major advantage or power gain in this, it would have been proven a long time ago and we could all put this to rest but no one has. I wonder why???????????
Darin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
817-467-7171
FAX-468-3147
Visit our web site at
http://www.rehermorrison.com
-
That's an interesting post, Bob. Do you know what they mean by a shorter deck improving the alignment between the cylinder head ports and the intake manifold? I've got to believe that they are talking about runner length of the port and manifold runner, and how it relates to the plenum. Higher engine speeds will require shorter runner lengths, and a tall deck would mean that the plenum of a sheet metal intake would be pretty wide. So maybe the turn of the air/fuel charge would have to go horizontally across the plenum for a ways before it turned down the runner. Am I on the right track?
-
By shortening the deck you effectively change the angle of entry from intake to head. It is a small difference, but similiar to angle milling in effect. That is why the 8.900"-9.000" decked blocks make great power with the 9.200" intake manifolds that are machined to fit them. You straighten out the airflow---seems counterintuitive, but it works. Joe-JDC
-
Your analysis Jay of what they said is exactly right.
I think of it as an upside-down letter "Y" and we are talking custom tunnel ram intakes with carbs here. Giving a straighter shot from the plenum's C/L to the head's intake ports C/L by pulling the heads in closer to one another via a short deck block does increase HP due to maximization of flow. Other benefits include shorter, stiffer pushrods and apparently less drop-out of atomized gas from the now straighter air/fuel charge.
Today, all NHRA legal (500 CID max) Pro Stock engines have really short decks as Joe mentioned. In match racing such as the IHRA where folks like Jon Kaase's customers have crazy 800 CID+ Boss 429-based Ford engines, the extra tall deck heights do work to negate that straight shot.
This link provide a pic and some details of the GM block that, as supplied, is about 9.5" yet can be machined down to a mere 9.00", shorter than even a 351 Cleveland block at 9.2"!
http://hotrodenginetech.com/inside-warren-johnsons-pro-stock-engine/
-
I don't focus on rod length more than, as Joe put it, the longest (readily available, reasonably priced, and strong enough) possible, without making a crusade out of it. To put it very simply, a longer rod (which is closer to parallel to the cylinder all through the cycle) transfers energy to the crank, with a little energy pushing out against the cylinder walls due to the leverage. A shorter rod, with greater angularity, pushes against the walls just a bit more- very little, but a bit- so a (very) miniscule amount of power loss and additional wear and tear is just logical- but it would have to get to a pretty severe angle to be worth worrying about, and in the average range of rod ratios, probably not enough to notice or measure. So I go with the longest readily available and proper strength part that will fit, and don't spin about it any more. The 4.25/ 6.7 is a well proven combo that works pretty well (as is the 3.78 or 3.98/ 6.49), and the price is right- move on to @.050's and LSA's where you can REALLY fuss and spin LOL
I see examples of this every day at work, where we use large excavators, CAT 345's and 330's, and most everyone prefers a "long stick"- no pun intended- which is the vertical part that hangs from the boom- the connecting rod between the boom and bucket, as you can just reach farther and do more work with it. A "short stick" will "crowd" stronger horizontally, as in digging and loading the bucket, with more severe angularity (leverage)- like pushing against a cylinder wall. A long stick loses a small amount of horizontal strength, or crowd- which is good in an excavator, but wasted energy in an engine- as in pushing against the cylinder walls. Funny thing, the long stick and short stick lift the same amount of weight vertically- one can nit pick the slight difference in weight of the long stick (or connecting rod) but the total lifting work produced is still the same
-
Thanks Gene lol , @ 50 and L S A is will be down the line , and i agree that the 4.25/ 6.7 combo is proven and until i get my hands on a 427 block sometime down the line we will just build probably a 463 , and get on with the rest of the project .. Thanks again to all for the input , info and feedback .. Bud
-
As Kasse say's just give me a rod to connect the crank to the piston.
I love that one.
Mike Caruso
Do It Like You Mean It .....or Don't Bother!
www.AERA.org
Engine Professional Magazine Free To All