Horsepower, compression ratio, airflow?

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IQ52

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So, how much airflow increase through the cylinder head, will give the same horsepower increase as raising the compression ratio one point? When does airflow win? When does compression ratio? Where's your proof, huh, who has tested this?
 
I might be totally off base with this, but it would seem to me that if compression ratio trumps airflow, then the airflow (cylinder heads) would be a restriction. I would think I would want my engine to have capabilities to outflow the bottom end. Just my thinkin....however distorted, convoluted and WRONG it might be. LOL
 
comp ratio is just a "get as much as possible thing" ... for street you want 9.5 and maybe 10.5 if you have perfect quench/alum heads/perfect cooling/perfect curved distributor/cam overlap to prevent detonation. but more than likely 9.5
on race gas you want minimum 11.0

for flow stuff it should all be matched as a setup: head flow, cam , carb flow, exhaust tube size, torque converter, rear end ratio all need to mesh. tweaking your heads for an amount of flow would be a waste of time if you have 7.5:1 compression ratio.

but for all out race builds every bit counts so yes the flow numbers become important.
 
IQ52 said:
So, how much airflow increase through the cylinder head, will give the same horsepower increase as raising the compression ratio one point? When does airflow win? When does compression ratio? Where's your proof, huh, who has tested this?
Click to expand...

The compression ratio doesn't "give" power like an increase in airflow does. The engine is an air pump, nothing more. More air in=more power. Since increasing compression ratio does not alter airflow, it does not increase horsepower one bit. Remember, however, that "horsepower" is a derived figure, and it's derived from torque.

That said, increasing compression does increase efficiency (which is an increase in torque). You can have awesome heads that flow a lot of air, but you're going to need more fuel too. you're getting more power, but it's because you're burning more stuff. Increasing compression ultimately ( and in much more complex ways than I will get into here) creates power by getting more energy from what you already had. That's why bumping compression is so much fun...it works on an any motor and it produces gains just about everywhere. Until you can't afford the octane required to run the high compression. Enter: Rudolph Diesel.

So, where is peak torque? Where an engine is most efficient. This is also where the motor is "fun" to drive.

You can slap badass heads on a stocker. Low-end will probably suffer, and you gotta compromise somewhere in order to get that air inside the engine at the RPM that you want it in there.

There's no direct correlation between compression and airflow regarding which is worth the other, because you're comparing apples and Chevies. They're dependent on each other in operation, but they're independent in what they do.

Make sense?

Edit: Besides this, increasing compression "one point" is not a very valid way to look at how compression is measured. We look at a point of compression as one integer-digit (a whole number) increase in the ratio, right? 9.0:1 has 9 "points" of compression, 10:1 has 10 "points".

Well, if our compression was was 1:1, and we go up one point to 2:1, we just doubled our compression ratio "points", a tremendous increase in compression ration, no?

If our compression was 100:1 and we go up one point to 101:1, we just increased our compression one point, for 1% increase in compression ratio. Not so great... A point is always a point but a point is not always the same.

No testing required.

Clear as mud, right?
 
+agree

for me, i also build in the "fun factor"

i'm building mine for street use... so i'm going 9.5:1. reason being i dont want to have to worry about detonation. but i want good power. since i'm not going for ET's i went for a decent build with edelbrock performer rpm package, kb240 pistons (it's a 400 bb) and i decided not to go with a stroker kit since that would push my cost up $2k for the kit and because they all have higher comp ratios i would be needing to run E85 if i want street gas. the E85 would mean i need an EFI kit add $2k more.

since i dont want to add $4k and would like to use that money elsewhere i'll settle for a good performing 400 with nice top end / holley 4bbl / headers / good flow exhaust and compression similar to the old bangin' 383's.

i will have a good fun factor ratio :D

something i can go hop in go to the gas station fill up with 92 octane head to the beach just have fun with. a good ole boy car
 
You dont need a 2k efi to run e85 on the street. there are several carb companies that either make e-85 convertion kits or make complete e-85 carbs.
 
Sounds like nobody knows, doesn't it? Alright, an increase from 8.5:1 compression to 9.5:1 compression or 10 cfm all across the board. This means 10 cfm increase at .100" lift all the way to .500" lift, or an increase from 8.5:1 to 9.5:1. Which will give a greater average horsepower increase?

Did anyone really think I was talking about 1:1 or 100:1?
 
IQ52 said:
Sounds like nobody knows, doesn't it? Alright, an increase from 8.5:1 compression to 9.5:1 compression or 10 cfm all across the board. This means 10 cfm increase at .100" lift all the way to .500" lift, or an increase from 8.5:1 to 9.5:1. Which will give a greater average horsepower increase?

Did anyone really think I was talking about 1:1 or 100:1?
Click to expand...

Well, not trying to be rude here, you're still not getting it. What cam? What valves? What pressure drop? Any change in spark timing? Blah blah blah.

I know the answer, as do many people on this board. There is no straight-up answer. You might get some good guesses from the guys that have built many many motors.

Are you wanting to know if you'd be better off to spend money bumping compression or doing heads, or is this a bench-racing thing?

You're asking questions that are so vague, we're not sure what it is you want to know, and we don't know what you are grasping or not. From your first post, you could have meant a change from 8:1 to 9:1, or you could have meant from 12:1 to 13:1. HUGE DIFFERENCE.

And to be specific, neither increases horsepower. They can only affect torque. ;)
 
IQ52 said:
Sounds like nobody knows, doesn't it? Alright, an increase from 8.5:1 compression to 9.5:1 compression or 10 cfm all across the board. This means 10 cfm increase at .100" lift all the way to .500" lift, or an increase from 8.5:1 to 9.5:1. Which will give a greater average horsepower increase?

Did anyone really think I was talking about 1:1 or 100:1?
Click to expand...

um... if your under carbed and under cammed and you increase your head flow a bunch there wont be much difference at all...

or if you have dual 1000 cfm carbs on a dual quad setup huge cam and a tiny little stock exhaust manifold, guess what? same thing

the variations on hp due to head porting will vary depending on the rest of the setup....

the variations on upping compression ratio always improve HP, but at the cost of getting too high and having problems with gas octane levels.

sorry but what you are asking is for hard numbers when said numbers dont exist... it all depends on the rest of the setup.

now, if you have a great setup on all of your build, but your heads are the bottleneck, stock 318 heads or something, then obviously it will gain you more than raising your comp ratio.

but paying to have heads CNC ported can be quite expensive... the only reason to do that on steel heads nowadays is if you need matching numbers for a resto. aluminum heads are the way to go. having aluminum heads ported is good to do if you have the money and if the rest of your setup warrants it and if you need every last bit of HP for racing. CNC porting is not money well spent for street use
 
for instance adding 10cfm to indy heads on a big block is not the same as adding 10cfm on 302 heads. you wont get the same results on HP/TQ curves
 
Who has tested this, in any configuration with the flow and compression being the only variables? Anyone? Do you really think I don't get it? Maybe my point is that the numbers don't exist. Or do they?
 
I think it's all really just tossing out ideas, simply because even if we all had a flowbench and dyno, every single engien package would be unique. Too many variables.

I think it's a lot like the "2hp per cfm" guideline. There's a lot of people that can exceed that. But, "in general" you can expect that.

As far as my own determinations... I start with fuel type, intended hp level, and displacement and go from there. Now hypothetically, given the choice between as much compression as I want, or as much head flow as I want, I'd go with head flow.
 
Im my eyes and going back thru some of my dyno sheets and what I have learned over the years Id say they will come out equal. 10cfm gain or 1 point compression, the same power. If you have an engine that can breath better, its effective compression will increase, as it is getting more in the cylinder, more to squeeze. If you run a smaller space, more compression, you get the same effect. Ill take more compression as the engine will generally run more efficient with higher compression. Less fuel, more power.

This is why Circle Track 2bbl engines(or NASCAR restricted engines) can run 12 to 1 static compression, or MORE, on a lower octane than what you might think. They can not fill the cylinder as much, so they have to squeeze it harder to get the power. The effective compression may only be 8 to 1.
 
I take for granted when you use "effective compression" you mean dynamic compression? Am I off base there? Cause a dynamic compression "only" 8:1 like you describe is outta the range of pump gas, if I am not mistaken.
 
Effective or running compression=dynamic. Just different wording. 8 to 1 effective is not out of pump gas range. I shoot for 8.5 to 1 and run them all the time like this, iron or aluminum, on 93 octane pump gas. If the engine is set up right, you wont have any issues.
 
MRL Performance said:
Effective or running compression=dynamic. Just different wording. 8 to 1 effective is not out of pump gas range. I shoot for 8.5 to 1 and run them all the time like this, iron or aluminum, on 93 octane pump gas. If the engine is set up right, you wont have any issues.
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Ok, thanks for the clarification. I am not too familiar with using dynamnic compression in making comparisons, so I was not sure.
 
MRL Performance said:
Im my eyes and going back thru some of my dyno sheets and what I have learned over the years Id say they will come out equal. 10cfm gain or 1 point compression, the same power. If you have an engine that can breath better, its effective compression will increase, as it is getting more in the cylinder, more to squeeze. If you run a smaller space, more compression, you get the same effect. Ill take more compression as the engine will generally run more efficient with higher compression. Less fuel, more power.

This is why Circle Track 2bbl engines(or NASCAR restricted engines) can run 12 to 1 static compression, or MORE, on a lower octane than what you might think. They can not fill the cylinder as much, so they have to squeeze it harder to get the power. The effective compression may only be 8 to 1.
Click to expand...

Great response and kind of along the lines of what I was thinking in terms of it comes down to personal experience and individual builds.
Next question if I might...
Your answer sounds like it's based on a race oriented restricted engine package... i.e. limited heads, carb, cam, whatever. With no restrictions other than pump gas, hypothetically, which would you go with: compression or head flow and why?

I build primarilly street engines and my limits for open chamber iron heads is 8.25:1 dynamic, and with closed chamber aluminum and quench up to 8.75:1 dynamic in order to run on pump 89 w/10% ethanol.
 
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