Compression ratio vs Porting

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hbbuddha

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Hopefully a simple question. I'm only planning on very light porting of my 714 heads (late model 302's or early model magnums depending on who you ask) really just more clean up then porting.

As what point would a higher Compression Ratio be canceled out by the heads ability to breath?

As an example (using arbitrary CR numbers): if I upped my CR to 9.5 I'd feel hide power gains but would 10.5 feel any different than 9.5 based on the heads ability to breath?
 
High flowing heads are mostly helpful in the upper RPM range.

In general a 1 point bump in compression is 3-4% gain.

I believe cam and dynamic compression ratio plays a vital role here as well, generally 7.5-8.5 is the number to shoot for:

http://www.wallaceracing.com/dynamic-cr.php

It's all a system and one part works with the other to give you the end result.

PS: Get a good converter if your going automatic. :D
 
Well, I'm a little confused then. Although I haven't opened the link yet. But I thought the stock CR for a LA318 was around 8.5 stock. And even higher before 1968? Maybe I'm wrong.

http://www.allpar.com/mopar/318.html

That link should be to stock 318 CR.

And I figured, adding the swirl heads with small combustion chambers would help raise that?
 
We just did some dyno testing on air flow increases versus horsepower increases. If you increase the compression ratio one point from 9.5 to 10.5 you will get approximately 3-4% increase. So a 300 HP engine would increase from 9-12 HP.

Theoretically if you increase the cylinder head flow on a race engine you could gain 2-2.25HP/CFM. But we found on the dyno, on a engine in the 400 HP range, if you change nothing but the cylinder head flow you will gain 1.25 HP/CFM. So, a 10 cfm increase would give you 12.5 horsepower increase. A novice, just cleaning up a port, could possibly get 10-20 cfm gain if you follow these instructions.

http://www.forabodiesonly.com/mopar/showthread.php?t=167865

Horsepower wise, roughly 10 cfm would equal 1 point of compression. If you could gain 20 cfm in your cam lift range you would get twice as much horsepower gain as you would with 1 point of compression.
 
IQ52. That's great info. Did you find in your dyno testing a point where the flow hindered any added compression?
 
At what rpm do you begin to see the hp gains from increased flow? Is it significant below 4000?
 
hbbuddha said:
IQ52. That's great info. Did you find in your dyno testing a point where the flow hindered any added compression?
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Never made any tests like that. I would expect if you maintain the required octane for the cylinder pressures, then flow isn't going to negate the advantages of increased compression. What we have done with turbo charged diesel engines shows that the better the cylinder head flows the more power it makes even beyond 80# boost.

65DartGTConv said:
At what rpm do you begin to see the hp gains from increased flow? Is it significant below 4000?
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Gains are seen throughout the entire rpm range.
 
IQ52, I suspect that your IQ is missing a digit. Should probably add a 1 in front of that 52. Thanks for your work and sharing your findings.

Logically, I'm having trouble wrapping my head around it though. I imagine a buff dude running a mile vs a buff dude with asthma running a mile. But that's probably a bad example.
 
Compression happens with the valves CLOSED. Flow happens with the valves OPEN. Two different things, not a lot of interplay.

hbbuddha said:
IQ52, I suspect that your IQ is missing a digit. Should probably add a 1 in front of that 52. Thanks for your work and sharing your findings.

Logically, I'm having trouble wrapping my head around it though. I imagine a buff dude running a mile vs a buff dude with asthma running a mile. But that's probably a bad example.
Click to expand...
 
I'd like to add that airflow gains at mid lift are much more important than high lift, as the valve spends most of it's time there. It's quite easy for a novice to attempt porting and actually lose hp and ruin the casting. The best things that can be done for a beginner is a good valve job (pay to have one done if you don't have the tools/experience), some light blending of the seats into the first couple inches of the port, a gasket match, and knocking down any obvious casting imperfections like flash and parting line mismatches.
 
A basic clean up, blending and port matching is all I'll be attempting. If I feel good about it, I'll go to the junk yard and pull another set of heads to play with. But being my first time, I'm going to keep it simple.
 
451Cuda said:
I'd like to add that airflow gains at mid lift are much more important than high lift, as the valve spends most of it's time there. It's quite easy for a novice to attempt porting and actually lose hp and ruin the casting. The best things that can be done for a beginner is a good valve job (pay to have one done if you don't have the tools/experience), some light blending of the seats into the first couple inches of the port, a gasket match, and knocking down any obvious casting imperfections like flash and parting line mismatches.
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X2!
A good valve job and then MINOR Bowl blend will give you more cfm then a mild port job.

A basic valve job don't flow much air. and Thats what you get, unless you ask for something better.;)
 
805moparkid said:
then explain dynamic compression...
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If I had to guess it would be that the cylinder volume is greater than the port volume. So as the piston pushes with the valve open, there is still pressure being built, just not as much as if it were closed. Pressure is flow restricted so piston creates pressure, port creates restriction. Somehow I think you knew this.....
 
dano said:
If I had to guess it would be that the cylinder volume is greater than the port volume. So as the piston pushes with the valve open, there is still pressure being built, just not as much as if it were closed. Pressure is flow restricted so piston creates pressure, port creates restriction. Somehow I think you knew this.....
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you are correct that valve operation (timing, lift and duration, etc) have no effect on STATIC compression, this is because it does not take into account above, its simply the compression given by stroke/bore/head gasket/etc.

When you get into DYNAMIC compression the game changes. You are now looking at what the actual effective/dynamic compression ratio of the engine is. If you have 10:1 and put a cam with no overlap it is going to have a ton of cylinder pressure because you were able to fill the hole with little to no loss thru overlap or a valve being open. Now with a big cam and a ton of overlap your cylinder pressure will be less due to the valves being open and overlap, your effective compression just dropped. With this your static compression hasn't changes, bore/stroke/comp height) havn't changed BUT because of a bunch of factors based on cam, valvetrain and heads your effective has changed, and in the end its the one that matters...
 
A different twist: pocket ported heads,good valve job,needs less camshaft to hit the same numbers.(with lower compression).
 
To expand on the same twist. More flow at lower lift increases "area under the curve" making more power for a given cam profile. A good valve job and blended bowls will help low lift flow, so will bigger valves. Bigger valves with a good valve job will help even more. Rebuilding those "302" heads with 360 valves (1.88/1.60) and a good bowl blend will really wake up a 318. Don't go too nuts on the cam, one of the modern 268 profiles would make for a really nice street combo.

But, to answer your original question getting 10% more air (and FUEL!!) into the cylinder will make more power than squeezing the same amount 10% harder before you light it.
 
I think in terms of the basic question - head flow will always trump compression. A combination of cam desigin and port flow can be done to equal minor static compression ratio increases. That's pretty much what a modern lobe does. In terms of your own heads - 3 angle is standard now, but a modern performance valve job is a 5 angle and is done using cutters - not stones and tapered pilots. At least that's my definition. If you do that, all you have to do is casting flash and blend the throat cut and you're done. That type of valve job, along with a back cut intake valve, can increase low and midlift flow considerably. Then just don't over cam it because high low lift flow will flow both ways and reversion an mixture dilution become a problem.
 
Difference between compression and more cfms is you can only go so high with compression especially in a street engine. Most seem to try to squeeze too much CR out of their engine. 9:1 CR can run most street cams and run fine on today's gas, squeezing it up to 10:1 CR is only gonna get ya 10 -15 hp on most street engines but everything must be right to run on pump gas. The worst with more air flow depends on how you go about it could lose you some bottom end power.
 
273 said:
The worst with more air flow depends on how you go about it could lose you some bottom end power.
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NO!

If having more airflow hurts power it's because more fuel wasn't added with it. Too much airflow is like too much oral sex... A great problem to have that none of us are likely to experience.

Remember, an engine is an air pump. Fuel + air go in, torque comes out. Nothing more, nothing less.
 
thesiren74 said:
put some 360 heads on a 318 that's whats its like lame! until about 3500
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Yes, because installing 340/360 heads on a 318 will lower compression at least 1.2.

If you want to put 340/360 heads on a 318, use 10.5 pistons and you will end up with 9.2 compression. -> I done it. Ran great on standard pump gas.
 
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