FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: Joey120373 on July 14, 2020, 09:35:05 PM
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Given the following criteria, what would you say is a good “max safe on premium” CR
6500 rpm MAX, more likely 6000rpm
Aluminum heads with good quench
5000lb truck with 3.73 gears
4500 feet in elevation and likely never going to go below that.
93 octane gas
I know most would say 9.5-1 to 10-1 , but that is usually accounting for sea level, and I am no where near that so it seems to me I would be leaving some on the table not accounting for that in the CR.
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Need more info about your cam.
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Bore and stroke?
I think in some circumstances a 352 would be under more load at low rpm for longer and more likely to detonate than a 505. On the other hand, a smaller bore makes for less distance and time for detonation to occur.
paulie
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Gonna need to know the cylinder head you plan to use and what cam you will use.
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You can see a trend here, need details, I think you were talking 505 inches, but lost track of the other discussion. I'll add, as much as anything, need intended use, but something else to think about.
When you talk about an RPM goal it is looking at the HP peak. If I read between the lines, seems like you are talking about a max shift point. The reason I bring it up is that a 5000 lb truck with 3.73 gears, likely has some tire on it to, and the cam required to go 6000-6500 (6400-6900 shift point) likely doesn't match the gearing and weight very well.
My recommendation would be to look close at maybe a 5800 RPM HP peak at the most, and get there with a good head. That way you can keep cam short and early for pulling that truck around. That way, you won't need a lot of cam, and therefore, you won't need a lot of compression.
FWIW, you can look in the dyno section. We have all done some truck motors, they range in both budget and HP, with my 461 EFI motor being the baby of the bunch. I can tell you, my low budget iron head 461 peaked at only 490 hp and 5000 rpm and VERY quick in my 4500 lb 4x4 short box.
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IIRC I could run a full point of compression higher at 5000-7000 ft altitude vs sea level.
Also, the same cam will feel smoother at altitude - the air is more compressible, so the cam will feel 5-10 degrees smoother than at sea level.
Downside as you know is, the car/truck is slower too. The 1st time we took our truck down to sea level in CA it was like I'd added a blower almost.
Beyond that, as stated above, need more detail. Everything matters - is the heat crossover blocked? Underhood air? Doggy-dog IMCO air cleaner or open? Cold air scoop or tubes? How cool can you keep it? 180F is a lot more octane tolerant than 210F.
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Wow, big thanks for all the lengthy responses.
IIRC I could run a full point of compression higher at 5000-7000 ft altitude vs sea level.
this was more or less what I was hoping to confirm. Mainly just is it safe to bump the CR up on any given engine to help account for the lower air pressure at altitude. I muddled things up by giving particular engine/application criteria.
Details! Yes, need to provide more details, but I wasn’t really looking for an exact answer, more of a rule of thumb. I am at least 6 months to a year or more out from starting to build the motor, thus far, it will be 504-520 cubes, jays heads, if not jays probably pro ports.
The rpm range given as 6000-6500 MAX was truly max, or shift point, in all reality i agree that somewhere in the 5500-5800 peak HP is where I would want to be, and as said, rely on good heads to carry some of that rpm. Truck might see some light towing duty but for the most part it will be a fun weekend toy.
I know jays heads will most likely be overkill for this application, but the motto of get the best head you can afford is one I happen to agree with. The cam will definitely be a custom grind and I will enlist an expert for sure.
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Wow!! jays heads may not be what you want for a truck on the street, unless you are running a very large cubic inch engine. Yes, he has designed a great head but it leans more to racing. With the flow #s His runners have, I believe i saw 345 cfm from the get go? That's where others end after porting pretty much. 500+ci and 5000 rpm and his heads are now awake and ready for action. anything around 2500rpm (street ish) wont be very responsive. I would love to have his set up but the 482 street with lots of strip time that i'm planning wont be enough for his set up i'm afraid. Would definitely need to run the 8 stack unit on the street as his other intakes would really hurt low end performance.
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Wow, big thanks for all the lengthy responses.
IIRC I could run a full point of compression higher at 5000-7000 ft altitude vs sea level.
this was more or less what I was hoping to confirm. Mainly just is it safe to bump the CR up on any given engine to help account for the lower air pressure at altitude. I muddled things up by giving particular engine/application criteria.
Details! Yes, need to provide more details, but I wasn’t really looking for an exact answer, more of a rule of thumb. I am at least 6 months to a year or more out from starting to build the motor, thus far, it will be 504-520 cubes, jays heads, if not jays probably pro ports.
The rpm range given as 6000-6500 MAX was truly max, or shift point, in all reality i agree that somewhere in the 5500-5800 peak HP is where I would want to be, and as said, rely on good heads to carry some of that rpm. Truck might see some light towing duty but for the most part it will be a fun weekend toy.
I know jays heads will most likely be overkill for this application, but the motto of get the best head you can afford is one I happen to agree with. The cam will definitely be a custom grind and I will enlist an expert for sure.
Bench racing is fun and certainly one of the cool parts of building a high end engine like you are doing. I just wouldn't look for those types of rules of thumb. An example, my own 489 is 10.7:1, before I built the 461 for my truck (which is 9.5-ish, I forget) I had a 10:1 445 and it was fussy as all get out on fuel, but the 489 will run on almost anything. The difference? cam and head design.
I would recommend you pick a head, I personally think Jay's may not be right for your application, and would go a CnC BBM or a Trick Flow. I would then cam for the RPM range, and based on cam, pick the compression ratio. You end up getting to the thought process you want, but rather than a rule of thumb, you can tie it to some real parts.
I will say though, don't only think about peak flow numbers, you need clean, fast flow, and if it's going to be a wide RPM range of power, a 335 cfm Trick Flow with a fast port may be the right head.
That being said, I am in NO WAY saying Jay's heads aren't awesome, and couldn't be used, but in a lower RPM motor, you may not need to go there
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Wow!! jays heads may not be what you want for a truck on the street, unless you are running a very large cubic inch engine. Yes, he has designed a great head but it leans more to racing. With the flow #s His runners have, I believe i saw 345 cfm from the get go? That's where others end after porting pretty much. 500+ci and 5000 rpm and his heads are now awake and ready for action. anything around 2500rpm (street ish) wont be very responsive. I would love to have his set up but the 482 street with lots of strip time that i'm planning wont be enough for his set up i'm afraid. Would definitely need to run the 8 stack unit on the street as his other intakes would really hurt low end performance.
I certainly don't agree with most of these comments. I will try to explain why.
First, it is important to realize that flow numbers are not everything! Flow velocity through the port is at least as important, and perhaps more important, because it directly impacts how quickly the cylinder fills after the valve opens, and how long it keeps filling as the valve is closing, while the piston is coming up from bottom dead center on the compression stroke.
On other FE cylinder heads, the only way to get big flow numbers is to increase the size of the port. This increase in cross sectional area of the port reduces port velocity at a given airflow rate, making for a low velocity, sluggish port with a 425-440 cubic inch engine. Especially at lower engine speeds, a low velocity port is going to make for lower torque numbers and a less responsive engine. In that case you need a lot of cubes to increase the airflow and get up to a decent port velocity, so that you get good cylinder filling as the valve is opening and closing, and good low end torque.
My cylinder heads, on the other hand, do not have a significant increase in the port's cross sectional area. The cross sectional area at the port opening in my heads is about 4% larger than a factory medium riser port, and quite a bit smaller than a factory 428CJ port. So, for the same given airflow rate, the velocity through the ports in my heads will be about the same as for a good stock or aftermarket head.
The reason my heads flow so much better than other heads is that the port is raised nearly an inch and a half over the stock medium riser port location. This makes the short turn dramatically better than any head that relies on the stock port configuration. Also, since the ports in my heads are straight in like a tunnel port or a cammer, rather than hooking towards the center of the engine and aiming the flow at the cylinder wall, flow into the cylinder is much improved.
At the higher airflow rates, velocity through the ports in my heads will be much higher than can be achieved with a stock type or aftermarket head. Again this is achievable because of the very generous short term radius, and the straight shot into the cylinder. Without going into all the details, an increase in port velocity without separation of the flow from the port wall is going to result in better cylinder filling and more power.
I think it is probably true that the 4V and 8V intake manifolds would cost some low end torque when compared to something like a dual plane manifold. However, it's possible that the heads may crutch that to an extent, making the combination of my heads and intake as good or better than a stock type head with a dual plane intake. I won't know on that until I do some dyno testing on a smaller engine, like a 390 stroker.
For what it's worth, I do plan to make a version of my heads available with a smaller intake valve, and a slightly reduced port size, so that they can be used on a 4.08" bore. I'll reduce the size of the runners in the intake to match. I'll build a 390 stroker to test these heads, and probably make it a 600-650 HP engine, so that I can compare it with some other aftermarket heads. It will be a while before I can get to that, though ::)
Also for what it's worth, I plan to run the existing heads on all my street cars. I expect that they will perform better than any aftermarket head that is currently available.
Once again, flow numbers are not everything. Drawing conclusions based only on flow numbers will only lead you astray in your quest for performance.
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Thank you for explaining that much better than I could.
We’re your heads hogged out with huge cross section on the ports I would not have considered them. The small port to huge flow ratio is precisely why I want to use them.
That, and this motor may not stay in the truck with a relatively small cam, it may well get pulled out and put into something else, so the heads will allow me to go pretty much anywhere I want to with the motor.
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First, it is important to realize that flow numbers are not everything! Flow velocity through the port is at least as important, and perhaps more important, because it directly impacts how quickly the cylinder fills after the valve opens, and how long it keeps filling as the valve is closing, while the piston is coming up from bottom dead center on the compression stroke.
This is what a lot of people miss, getting caught up in flow numbers. Even Kaase basically says flow numbers are a distraction.
Stock 351C 4V head port is huge, about 260~270 cc. And in stock form, they barely get to 280 CFM. Kinda sad sounding for a port big enough for a comfy gerbil den. The bottom of the port doesn't do much either.
(http://raceabilene.com/kelly/hotrod/images/falcon/2007351c/4vHeadIntake600.jpg)
So we crutch it using port tongues to fill the lower part of the port. This reduces volume into the 220cc range. It also raises the floor and give a better shot at the pocket. With a few days of updating the pocket and blending the tongues into the port, now we're running 340+CFM at only .500 or so. Not bad for old tech.
Or, you can just buy an updated part - shown is an AFD "4V". Same, about 220cc intake and the flow is never less and mostly way better than the worked, stuffed old iron part. Velocity is better and the chamber is completely re-designed also. The 4V closed I run handles 37~38 degrees timing. The AFD - 26 degrees. And - no labor. Just bolt it on. With the updated port runner and pocket, it works great on even a 351 cid engine and even better when you add CID.
(http://raceabilene.com/misc/351cNotes/2015/AFD4V/AFD_4Vf.jpg)
For extra credit - a 351C 2V port - these run in the 180~190cc range.
(http://raceabilene.com/misc/351cNotes/4Vgasket2Vhead.jpg)
Wow, looks like the AFD, must be pretty close with a little work. Well, not so much. Even when carefully ported, at about .450 or so lift and 28 in/hg depression you will likely hear a loud BANG as the air flow through the port fails to make the short turn. The floor is very flat and while velocity is good, the air can't handle that sharp curve. On a set I have, we hit about 250 CFM then ka-pow, lost about 30 CFM and did not recover until nearly .600 with nothing after that - 254 CFM max. Not much good for velocity right in the middle of typical lift range, would be pretty disruptive IMHO.
Bottom line - learn a lot when head shopping. Way more to a good head than a set of flow bench numbers.
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I have to disagree with some of this on flow bench numbers. Flow benches are used to show gains in flow, or loss, or turbulence, and at what lift of the valve or velocity. A flow bench, if used properly, will show what works and what does not when removing metal. The flow bench only shows what has been done to the head, not how good it CAN be or what needs to be done to correct the problem. It takes years of trial and error to show how to get a head to keep flowing up to any lift desired, and folks today take someone's opinion as stated on the internet as gospel. If a port is shaped properly, the flow bench will show that, and reflect exactly what the engine is capable of producing in horsepower. I have used three different flow benches in three different shops over the last 40 years, and still depend on my flow bench to show me where I can make improvements or just let something go out the door. Velocity and shape are very important, but valve jobs can make or break a good port, and the flow bench will show that if you take the time to verify those different valve jobs. Lots of time, and work, but I am still convinced that a flow bench is worth the effort so that when I send something back to a customer, I am confident it will perform at the level expected. Joe-JDC
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To the OP , "most" guys that run above 4,000 ft "actual " altitude ( not relative) run a full point over seal level compression ratios. Why? the lack of air density leaves you compressing LESS air so adding more calculated static compression compensates some to restore the energy had at lower altitude where there is more "air" per square inch so to speak.
Re cylinder heads,
I fully agree that a flow bench is a "tool" which "can" lead to HP gains , NO QUESTION there and total respect to those who use them every day. "I" feel it is flawed because it tests a "static" valve position at a constant vacuum number. This is NOT reality , obviously. Look at how porting ides changed when "wet flow" was explored by high end cylinder head guys. Unless the valve is in motion and the vacuum source is from a reciprocating piston the "measurements" aren't realistic. Well IMHO anyway. Neither Goodyear , M/T or Hoosier make slicks for flow benches , I checked.
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The reason my heads flow so much better than other heads is that the port is raised nearly an inch and a half over the stock medium riser port location. This makes the short turn dramatically better than any head that relies on the stock port configuration. Also, since the ports in my heads are straight in like a tunnel port or a cammer, rather than hooking towards the center of the engine and aiming the flow at the cylinder wall, flow into the cylinder is much improved.
If that's the case, I stand corrected, assuming intake options support the behavior. In fact, that's awesome, much like the CHI heads took off on the Clevelands, these would do the same. No need to explain airflow to most of us, I mistakenly assumed that this was a high port with big honking intake options for big inch RPM engines and volume was very high. I certainly wasn't basing it on flow numbers because 345 isn't much more than a TFS head if that is the number. What is your port volume, assuming of course it will change with throat diameter...? Second question would of course be, with that given volume, is the port significantly longer or shorter for comparison?
I will say, in truck packaging, if a long, properly sized intake runner with a reasonably sized plenum could be used with available hood clearance, it'd likely do very well. To your point though, the head is only one part of the port and same caution needs to be taken there if unable to run stock design intakes
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On a very large displacement engine, with very large intake ports, would something like the old 351C Weiand dual port make sense? Especially at lower rpms, in a heavy vehicle?
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Offenhauser made them. The problem is too much surface area. In the day , we "cut back" the separator from the flange almost three inches and it helped allot.
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Offenhauser made them. The problem is too much surface area. In the day , we "cut back" the separator from the flange almost three inches and it helped allot.
Helped how? Higher rpms?
They cut back the shared runners on the old Chrysler 300 long Rams, to help high rpm performance.
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The cross section area and low RPM port velocity was my initial concern. Jay addressed that (in theory) way back for me. Good to see this topic come up again with more discussion. A 390, 428 or 427 based project is still up in the air for me. Depends on what the actual power is vs. available engine blocks/strength.
Another point is to recall that the cross ram intake is under development as well. Generally these have the butterflies lower which reduces the relative air volume on the vacuum side i.e. valve to butterfly. Another bonus for throttle response and and low RPM manners. Should complement the heads well. Again in theory.
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I fully intend to use the cross ram intake, it’s what pushed me over the edge on using the heads.
Jay can chime in with more detailed specs, but to answer some questions I think I know the answer to,
The port cross section is only slightly larger than a stock MR, the port volume is larger but that is because it is a longer port, not because it is a bigger port. The heads flow something like 417cfm on the intake, so it’s not just a little better than current offerings, it’s a LOT better.
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The port volume is bigger not just because of the added length, but also because of the valve size. When you go to a larger diameter valve, you need a larger diameter bowl under the valve. Plus, the more tulip shaped the valve is, the taller the bowl needs to be. When I do the smaller valve version, the port volume will shrink up some. All those dimensions are kind of designed to work together. Part of the theory is to ramp the port velocity up along the intake tract to the choke point in the port, and then slow it down so it can negotiate it's way around the short turn and the valve while keeping the fuel in suspension. I don't claim to know everything about this, and people like Blair or Joe who spend lots of time on the flow bench probably know more, but I know enough to get the port design into the right ballpark. I think that has been born out with the flow numbers, and hopefully will be confirmed with the dyno testing. But again the really big advantage of my heads is to throw off the basic FE architecture that all the other heads out there use, and improve the design with a raised, straightened intake port.