How important is compression?

[wyrmrider]

I've built lots of those motorhome motors
we developed the KB quench pistons just for that application with cam to match
with the stock pistons and with stock gears and converter stick with a short cam with lots of lift even with aftermarket heads
You CAN make more top end power with a big cam but it will get suck gas and get hot- you are not doing super stock are you?
quench pistons and the right cam reduces EGT by around 800 degrees in a motorhome motor
the aftermarket combustion chamber helps as the stock 452 or any BBM head after the 67 915 sucks big time what with the spark plug location etc
Mike Jones has the best cam in this class bar none and he will give you an exhaust lobe to match your build
next would be a Lunati Voodoo in the 256 range or if you are a perfectionist in setting up your heads with light valves and springs the HOWARD with a 15 in the comments column in the 256 range works COMP, CRANE etc have NOTHING in this duration range Comp's smallest is 275 and that's way to big for a smog build
even if you do raise the compression you still have to determine where you want the torque curve
and forget roller tip rockers unless you are willing to spring for the B3 kit-- go to B3racingengines.com and read all 4 tech articles
1.6 rockers help but there are better places to spend your money (that's IMHO)

 

[hoover]

Would there be a power difference between aluminum vs iron heads if the ports and quench/compression ratio?
Does heat dissipation have much affect?

 

[steve340]

A few guys I have talked to who know what they on about recommend 0.5 of a point extra static compression ratio to compensate for the loss of combustion chamber heat when you go to an alloy head.
I have gone a bit higher CR on an engine with alloy heads following their advice but I am not sure if there really would be a difference if all other things were equal.

 

[IQ52]

I'm tired of the aluminum vs iron crap, and does aluminum suck heat out of the combustion chamber, change the power or detonation resistance.

But I did do some iron head tests on a low compression 440. It was what we had laying around and I wasn't going to spring for new pistons to raise the compression because this test was to be on the cheap. Every part we used was already on the shelf except for the new cast iron piston rings and the cheap Summit Racing 6400 camshaft.

So, these dyno tests were just for fun.......and not how I would have built the engine if I was looking for a more balanced engine design. AND I will no longer speculate what it would have done if we had changed this or that on the engine.

https://www.forbbodiesonly.com/moparforum/threads/iron-head-test-starting-on-78-440.58687/page-4

What did we do with the Summit Racing 6400 cam after the 440 dyno test? Well it went into a low compression 400.

https://www.forabodiesonly.com/mopar/threads/dyno-a-7-5-1-400-sure-why-not.293381/

 

[JoePole]

I'm tired of the aluminum vs iron crap, and does aluminum suck heat out of the combustion chamber, change the power or detonation resistance.

But I did do some iron head tests on a low compression 440. It was what we had laying around and I wasn't going to spring for new pistons to raise the compression because this test was to be on the cheap. Every part we used was already on the shelf except for the new cast iron piston rings and the cheap Summit Racing 6400 camshaft.

So, these dyno tests were just for fun.......and not how I would have built the engine if I was looking for a more balanced engine design. AND I will no longer speculate what it would have done if we had changed this or that on the engine.

https://www.forbbodiesonly.com/moparforum/threads/iron-head-test-starting-on-78-440.58687/page-4

What did we do with the Summit Racing 6400 cam after the 440 dyno test? Well it went into a low compression 400.

https://www.forabodiesonly.com/mopar/threads/dyno-a-7-5-1-400-sure-why-not.293381/

Great real world info IQ. Got any similar low comp on LA360 of 5.9 Magnum by chance?

 

[Ceedawg]

My 64 Savoy all steel race car had the stock 73 low comp 440. Good intake-carb.509 cam 452 heads 4.56 gears and 14-32 tires ran 7.60 in the eight mile, on Regular gas! Changing compression could pick up 5-6 tenths imo

 

[Challenger340]

Forget hp.... start thinking Torque because imo, it's more relevant on the street ?

 

[Charger Dana]

To the OP, to go along with Doug a change in CR is not linear- if you have an 8:1 motor and drop by 8, 3-5% drop per 1CR implies only a 40% decrease. However a zero CR wouldn’t run at all.

 

[mopar 3 B]

Isn't this what turbos and super chargers are for? Enough air and fuel, compression is meaningless.

 

[Cranky]

Have you done a compression test on it yet? I'm curious to know what it is. I've dealt with a few low compression engines and yes, too much cam will only bleed off more cylinder pressure and that's not what you want but the wrong 'RV' type cam might raise cylinder pressure too much causing ping and low compression no quench engines are already prone to pinging especially if it gets on the warm side. I also know that too much cam will cause the low compression engine to like a lot more timing than you probably thought.....

 

[Cranky]

Isn't this what turbos and super chargers are for? Enough air and fuel, compression is meaningless.

Forced induction engines don't like a lot of compression. Usually, the higher the compression, the less boost you can/should run unless you like holy pistons.....

 

[mopar 3 B]

Forced induction engines don't like a lot of compression. Usually, the higher the compression, the less boost you can/should run unless you like holy pistons.....

To much boost will make even low compression pistons holy.

 

[IQ52]

Great real world info IQ. Got any similar low comp on LA360 of 5.9 Magnum by chance?

Maybe this...............

https://www.forabodiesonly.com/mopar/threads/lets-put-a-mild-8-2-1-360-on-the-dyno.307012/

Forget hp.... start thinking Torque because imo, it's more relevant on the street ?

Good advice.

 

[1968rt]

Good read on making horsepower with a low compression 440. They made 449HP with a 7.5 compression shortblock https://www.forabodiesonly.com/mopar/threads/dyno-testing-a-stock-1972-440.249866/page-2

 

[biomedtechguy]

So, these dyno tests were just for fun....
https://www.forbbodiesonly.com/moparforum/threads/iron-head-test-starting-on-78-440.58687/page-4 the Summit Racing 6400 cam after the 440 dyno test? Well it went into a low compression 400.
https://www.forabodiesonly.com/mopar/threads/dyno-a-7-5-1-400-sure-why-not.293381/

THIS!! CLICK THE LINKS AND LEARN!.

@IQ52 certainly has commented quite a few times about CR and power output, and with his experience and accolades, if Jim LaRoy says it-believe it!
He has built many low compression engines that make HUGE power.

 

[451Mopar]

Forced induction engines don't like a lot of compression. Usually, the higher the compression, the less boost you can/should run unless you like holy pistons.....

Forced induction is compression of the air (boost). The boost vs. compression is more of a thermal management issue and fuel octane. Every time the air is compressed (induction or compression) will increase the temperature, and if the air is too hot for the fuel octane, you have self ignition like a Diesel engine.

 

[Cranky]

Forced induction is compression of the air (boost). The boost vs. compression is more of a thermal management issue and fuel octane. Every time the air is compressed (induction or compression) will increase the temperature, and if the air is too hot for the fuel octane, you have self ignition like a Diesel engine.

Yup.....and that is where an intercooler comes in handy. And doesn't the Hellcat use the AC system to help cool the boost charge?

 

[biomedtechguy]

Yup.....and that is where an intercooler comes in handy. And doesn't the Hellcat use the AC system to help cool the boost charge?

BRILLIANT!

 

[IsaacP]

I'm tired of the aluminum vs iron crap, and does aluminum suck heat out of the combustion chamber, change the power or detonation resistance.

But I did do some iron head tests on a low compression 440. It was what we had laying around and I wasn't going to spring for new pistons to raise the compression because this test was to be on the cheap. Every part we used was already on the shelf except for the new cast iron piston rings and the cheap Summit Racing 6400 camshaft.

So, these dyno tests were just for fun.......and not how I would have built the engine if I was looking for a more balanced engine design. AND I will no longer speculate what it would have done if we had changed this or that on the engine.

https://www.forbbodiesonly.com/moparforum/threads/iron-head-test-starting-on-78-440.58687/page-4

What did we do with the Summit Racing 6400 cam after the 440 dyno test? Well it went into a low compression 400.

https://www.forabodiesonly.com/mopar/threads/dyno-a-7-5-1-400-sure-why-not.293381/

Wow that's great info and really encouraging. Thanks!

 

[andyf]

I have read a lot of forums lately and want some opinions. I hear a lot of guys talking about compression and some say you can't make any "real" power with a low compression engine, but I have also read that for any given engine, an increase in compression by one point only adds roughly 3-5% more power which isn't much. And then you hear guys say that air flow is more important than compression and they say if you have good flowing heads and cam you can make good power. I guess my question is, if increasing compression only adds 3-5% more power, why do guys talk like compression is so important? Which side is right?

It just all depends on what you're trying to do. The new Trick Flow heads are so good that you can now build a very streetable, pump gas engine that makes 600 hp. So if you want a 600 hp big block for cruising around town and going to the track a few times a year you don't need more than 10:1 compression.

If you want a 800 hp NA big block then you'll probably need some compression. Although, I've gotten close to 800 hp with a pump gas 470. The best pulls I made with my dry sump 470 were at the 780 hp range. That was a very trick engine but compression was only 10:1 so it didn't have a bunch of compression.