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Discussion Starter · #1 ·
It seems if you have a head milled down to raise compression ratio this will change the heads air flow spects.
So,what is more important air flow or compression when trying to make max HP and torque?
 

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BOTH!!

but i would say that airflow and the airflow carracteristics of a good set of heads is the most important, you can only go so far with milling a set of heads before you start running into valve clearence issues, angle milling so the ports match up, getting the quench to an acceptable height, bla-bla-bla, the more air you can flow typicaly they more power will be made, but dont just look at max airflow, you need to pay attention to the flow at .400" up and also the rate of acceloration that the airflow takes
 

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zz71s said:
It seems if you have a head milled down to raise compression ratio this will change the heads air flow spects.
So,what is more important air flow or compression when trying to make max HP and torque?
How much of a trade-off?? I generally view airflow as much more important than compression, but you can't totally ignore one for the other.
 

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Discussion Starter · #4 ·
The best flow on one of the heads I was looking at went from 260 to 230ish.
Was going to have them milled to a 58 cc chamber from a 65 cc chamber
so I could keep the 10 to 1 compression ratio,
 

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with a 65cc chamber you would be at around 9.5 ish c/r this is still fine and much easier to tune for pump gas, this is all going off the assumption that yor calculations are correct, im not sure i understand the 260-230ish comment, as flow numbers are suppose to go up as the lift on the cam/valve inceases untill you reach the max lift the head should really be used for in which time the flow will decrease but that is not the number you should pay attention to, you want to look from .300" on to about .600" lift depending on the max lift on your cam, i dont know the application or what motor this is but a max flow of 260 isn't too bad for a good street/strip motor, but not all out strip. i was once told that what ever a heads max flow is multiply that by 2 and you will have a round about number of power those heads can handle, but that is under the absolute best combo with the most ample tune
 

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260 cfm down to 230-ish would not be acceptable to me, not for so little gain in compression. 10 cfm in this case would be all I would be willing to give up, don't be too hung up on the compression ratio.
 

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You didn't mention what cam you would be using. This has to be factored into the mix as well as the intake and carb. Too much or too little lift and duration and the lift / duration combination will affect everything else from idle speed to possibly needing a stall converter if you run an automatic and even your rearend gear ratio may need to change.
 

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Good thread
i was thinking about it because i have a 355 with a zz4 cam
Duration @.050 208/221 Lift: .474/.510 112 LSA
1.5 roller rockers, 1 5/8 headers and "128" 58cc aluminum heads.-
Im getting a set of brand new 64cc heads,and maybe a Superram.-
Should i worry to leave some compression but the upgrade will be the better intake/head combo ? or should i angle mill the heads ? i guess if i mill the heads i will have to mill the intake base as well ? i was told to use a thinner head gasket .28 to get some of my original compression back,what you think i should do guys ?
Thanks !
 

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Discussion Starter · #10 ·
The 260 cfm was at 050or 060 cant remember.
3300 pound car zz4 short block lt4 hot cam auto gear m22w ,rpm performer and 355s.
It will save me some money if I dont have the heads milled.
So a 65 cc chamber and the zz4 would be around 9.5 to 1ish do you all think.
 

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Calderone said:
Good thread
i was thinking about it because i have a 355 with a zz4 cam
Duration @.050 208/221 Lift: .474/.510 112 LSA
1.5 roller rockers, 1 5/8 headers and "128" 58cc aluminum heads.-
Im getting a set of brand new 64cc heads,and maybe a Superram.-
Should i worry to leave some compression but the upgrade will be the better intake/head combo ? or should i angle mill the heads ? i guess if i mill the heads i will have to mill the intake base as well ? i was told to use a thinner head gasket .28 to get some of my original compression back,what you think i should do guys ?
Thanks !
Any opinions ?
 

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start a new thread on your last question, you'll get a better response


thats a mess of questions, when angle milling the heads you will need to mill the intake as well, gasket will never seal, im not a big fan of cams with a LSA above 110 unless its going to be a daily driver or fuel injected, im not real sure exactly what your asking in the rest of the questions
 

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zz71s said:
It seems if you have a head milled down to raise compression ratio this will change the heads air flow spects.
So,what is more important air flow or compression when trying to make max HP and torque?
Both but there are associated circumstances that mange their selection.

Compression has a large effect on engine efficiency which then controls both fuel economy and power output. An engine needs a relationship between the fuel it burns and the power ot produces. An engine that burns a lot of fuel for the power output simply throws unused energy out the tail pipe. This costs money and power.

The compression has two aspects one is Static Compression Ratio (SCR) which is the mathematical ratio of all the cylinder volumes divided by the volumes above Top Dead Center (TDC) of the piston. This needs to be in agreement with the cam timing as well. Long duration, late intake closing cams need more compression to recover power lost to low RPM reverse pumping. This latter thought will lead into air flow and the Dynamic Compression Ratio (DCR). The higher than ordinary compression used with long duration cams is not usable with short cams because it results in excessive low and mid range DCR that isn't compatible with available fuel octane ratings.

The DCR gets into the air flow capabilities and cam timing of the engine. This is a variable compression ratio that changes in proportion to the instant volumetric efficiency of the engine at any particular moment of operation. Roughly speaking, when engine RPM is low to moderate and the throttle mostly closed, the density of the mixture is low (another way of looking at it is the manifold vacuum is high). In this situation the air flow in the ports is also slow and the cam's intake opening time After Bottom Dead Center (ABDC) results in reverse pumping of the intake charge by the piston back into the intake. At this point dynamic compression is low and close to the statically calculated ratio. At the other extreme of high RPM, wide open throttle; the mixture density is high (manifold vacuum is low) and mixture speed in the ports is high. This gives the mixture a lot of inertia and it will continue to flow into the cylinder against a rising piston until the intake valve is closed. At this point the Dynamic Compression Ratio is higher than the Static Compression Ratio. These are the effects of the fact that gases are compressible and will change the number of molecules for a given volume (density) based upon the forces being applied.

An engine's torque peak is a good indicator of the point where breathing (volumetric efficiency) is as good as it will ever get. Above the torque peak the ability of the cylinders to get a full breath starts to drop off becoming shorter and shorter of breath as the revs go up. What's happening is horsepower keeps going up for a while, this is the cylinder producing less power on every revolution, but being able to make more revolutions in a time period, so torque is falling but horsepower is raising for a while at least. Assuming nothing mechanical limits the RPMs by a failure, the horsepower will continue to a peak after which it falls off. This peak is the point where the engines ability to get even a short breath is no longer enough to keep going on and the horsepower curve flops over. So one can construe that Volumetric Efficiency falls off above the torque peak and for practical purposes of getting power ends with the horsepower peak.

So the answer to your question is both compression and air flow are important, and work together. You can trade them around, but this has to be done in relationship to how the engine is to be used. Obviously having and engine with variable compression and variably sized ports that can change with speed and power demand would let you build and engine that was flexible and maxed for efficiency and power in all conditions expected of it. But the engineering of such a situation is simply beyond our technical capabilities.

Getting to your basic question of head milling affecting air flow, it doesn't have any effect. Milling doesn't change the dimensions of the ports at all, other than lowering the head and manifold a few thousandths of an inch. It doesn't change the location nor angles of the valves or ports. To a great extent the same can be said of angle milling the head. This will change the angle of the valves to the cylinder bore but not by much. But more importantly it doesn't change the angle of the valve in relation to the port, where heads built with the valve angle changed to that lesser than Chevy's usual 23 degrees like the 18 and 15 degree heads, also, move the valve relative to the port which improves breathing.

Bogie
 

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Discussion Starter · #15 ·
The tech at the head manufactors tech line told me about the change in air flow when milling there heads its also on there web sight.
The tech also gave me the 260 to 230ish numbers.
 
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