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If I'm going from 9.8:1 to 11.1 how much HP gains am I looking at?

Also is there a special cam needed for higher compression? Would I be able to run my current cam and set-up on 93 octane?

Pontiac 455 bored .040 over
Edelbrock Performer intake
Comp XE Cam 240/256 507/510 LSA10
2600 stall,
750 holley, vac snd
stock 6x-8 heads, 3 angle valve job (going to 6x-4 which is going to increase the compression)
stock bottom end
373 gears.
 

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From what I have heard, you can assume around 20 hp per point of compression. This is just an estimate, and will vary. As far as you combonation running on pump gas, the only way that you will have any chance is if you are going to be running aluminum heads, and your quench is right (distace from top of piston to deck of head, should be around .045).

Adam
 

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it depends on the rest of the combination....lower RPM engines will have less impact than higher RPM engines as far as total increase in power.
 

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how much hp?

How much gain for each increase in ratio depends on the engine, how its built, head design, etc. There is not hard fast rule on it.
Higher compression shows its greatest effects at higher rpm, where the breathing time is much shorter, and cylinder filling isnt quite so good.

With your engine setup, cam wise, you will probably see some good gains from going to the higher compression. The biggest concern is whether you can run that high on 93 octane, due to other factors like combustion chamber size, etc.
Around these parts, with a steel head engine, about the maximum anyone is running with any big blocks (427 and up) is about 10-10.5:1. Thats with a heavy cam and steel heads.
 

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The lean mean donut machine
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compression increases also help with large cams in the lower RPM ranges. It helps when the overlap is hurting performance... More compression from a diminished charge will feel like the cylinder is filled more completely than at a lower compression level. It will make large cams seem less radical or enable engines to operate more smoothly at lower RPM. It will also help torque and vacuum slightly as well.
 

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I can't remember the exact specs. I have it written down somewhere. The way it works is you get 4% more power each point from 7:1 to 10:1. From there each additional point nets around 2%. I know that is close enough for what you are asking. If you want I can go digging through the archives and get the article, but I am in the ballpark.

You can't say how much hp. Every engine will gain more or less. That is why it is more accurate with a % of hp.
 

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Thats gonna be very border line for sure you may have to knock back the timing a bit on pump gas which will take away any gains you made from the increased compression.
 

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Robinson Robin, I think you hit the nail on the head. With iron heads and 11:1 it is not going to run on pump gas (and if it does it won't for long). You honestly need to be in the area of 9.5-10:1 to run iron heads on pump gas (I think that's about the practical max). The slight gain you "might" get from 10:1 to 11:1 will be negated with reduced timing and broken parts.

Royce
 

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Bah humbug!!! lol. ANyway I run about 10.5 or 11:1 on the street. I admit I have had someproblems with spark knock, but I think I've got them taken care of, and we noticed the plugs to look a bit lean, so it needs a bit more fuel anyway. That coupled with a new set of 3.73 gears in leu of the 2.73, and I think I'll be fine. It depends a lot on the car and the conditions. I wouldn't want an 11:1 truck motor that will be towing a boat, but in a car, you're much better off. I run 93 and some marvel mystery oil, but I also admit that I've had to come down to that from mixtures of 110. I also haven't CCed my heads, so... who knows. But I think it can be done...

K
 

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I read somwhere that 1 point increase in comp usually nets an average of 15 hp. I have no proof. But I did read it somewhere in a pile of my old mags.
 

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camaroman7d said:
Robinson Robin, I think you hit the nail on the head. With iron heads and 11:1 it is not going to run on pump gas (and if it does it won't for long). You honestly need to be in the area of 9.5-10:1 to run iron heads on pump gas (I think that's about the practical max). The slight gain you "might" get from 10:1 to 11:1 will be negated with reduced timing and broken parts.

Royce
I have said it a million times now. My 400 is 11:1 with iron heads and 91 octane gas. My timing is set at 18o initial and 16o mechanical.

What you gain from 10: to 11:1 is more vacuum, better throttle response, better gas mileage, and more power.

You have to do your homework before attempting to go over 10.5:1.
 

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lluciano77,
How did you calculate your compression ratio? Were the heads cc'd, pistons cc'd, how far is the piston in the hole, how thick is the head gasket? What is your quench? Without actually measuring, which most people don't do or have the tools to do (home builders). There is no way for you to know your TRUE compression. If you buy a set of heads that are supposed to be lets say 64cc, when you actually measure them they can/will be from 63 - 66cc in some instances. The same goes for pistons.

If you are estimating based on what the advertised compression ratio of the pistons are, then you can easily be 1.5 points off. there are a lot of "11:1" engines running around and in reality they are closer to 9:1.

I am not trying to start an argument you, may have very well measured everything. I just have a hard time believing you are running 11:1 with iron heads on 91 octane.

While the engine is new you might get away with it for a while but, once you start building up carbon and the compression goes up slightly this will end the party. Nevermind a bad load of gas.

If your engine is actually 11:1 with iron heads more power to you. I can tell you I have seen first hand what happens to "true" iron headed 11:1 engines on the street (broken ring lands, hammered bearings, poor performance).

Just cause you "have said it a million times" doesn't make it a fact.

Royce
 

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Discussion Starter #15 (Edited)
lluciano77


Sent you a PM :)

If not pump gas how about aviation fuel? I have a gas station that sells it for air boats. Aviation fuel is what octane? 110?
 

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The lean mean donut machine
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I currently run 11.68:1 compression with IRON heads, no detonation with 14* initial, 36* total timing. I had my 305 heads cc'd at 59 instead of the stock 58 after I ported them, I have a 4.1" bore gasket with a thickness of 0.038 inches, and flat top pistons set in the bore 0.005". Do the CR math and 11.68 is what you get. I attribute my combo working from the high quench area of the 305 heads, polished chambers, special waterless coolant, and a canister of octane booster. Throttle response is awesome and it idles pretty smoothly for the cam that is in it.
 

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Okay here goes....11.6:1 verified down to cc'ing the eye brow's even...BUT I have aluminum heads, a 160F thermostat huge radiator (you HAVE to keep it cool) gentle advance curve, a ton of cam which will bleed off some cylinder pressure (who cares about compression ratio, its your cylinder pressure that counts) and I live at 5000 feet of elevation. That last part makes all the dif.
Humidity, ambient temp and elevation will play an important part in where you can go with this exorcise.
And yes I run pump gas most of the time....but there are hot summer nights when I will blend some race fuel and advance the ignition just to get the low end punch back.
 

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Elevinpointsixtoone,
You hit the nail on the head with the elevation. You actually need to run slightly higher compression to make power up there. You are also correct that cylinder pressure is what counts, the problem is (even with a big cam) at higher RPMs the cylinder pressure still builds, if it didn't you wouldn't make any power. You also have aluminum heads, that makes a 1 point difference right there. Not exactly the same as running 11:1 with iron heads at a lower elevation on 91 octane gas.


Mad Maggot,
It must be that octane booster, LOL :rolleyes:
How do you know you have no detonation? Just curious

Royce
 

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camaroman7d said:
lluciano77,
How did you calculate your compression ratio? Were the heads cc'd, pistons cc'd, how far is the piston in the hole, how thick is the head gasket? What is your quench? Without actually measuring, which most people don't do or have the tools to do (home builders). There is no way for you to know your TRUE compression. If you buy a set of heads that are supposed to be lets say 64cc, when you actually measure them they can/will be from 63 - 66cc in some instances. The same goes for pistons.

If you are estimating based on what the advertised compression ratio of the pistons are, then you can easily be 1.5 points off. there are a lot of "11:1" engines running around and in reality they are closer to 9:1.

I am not trying to start an argument you, may have very well measured everything. I just have a hard time believing you are running 11:1 with iron heads on 91 octane.

While the engine is new you might get away with it for a while but, once you start building up carbon and the compression goes up slightly this will end the party. Nevermind a bad load of gas.

If your engine is actually 11:1 with iron heads more power to you. I can tell you I have seen first hand what happens to "true" iron headed 11:1 engines on the street (broken ring lands, hammered bearings, poor performance).

Just cause you "have said it a million times" doesn't make it a fact.

Royce
Yes. It is all calculated and accounted for. I can dig up all the specs if you want.

I have been driving this car everyday in traffic to and from work for over a year. No pinging.

My compression read with a compression tester is at 220 psi. on all cylinders.

camaroman7d said:
Elevinpointsixtoone,
You hit the nail on the head with the elevation. You actually need to run slightly higher compression to make power up there. You are also correct that cylinder pressure is what counts, the problem is (even with a big cam) at higher RPMs the cylinder pressure still builds, if it didn't you wouldn't make any power. You also have aluminum heads, that makes a 1 point difference right there. Not exactly the same as running 11:1 with iron heads at a lower elevation on 91 octane gas.


Mad Maggot,
It must be that octane booster, LOL :rolleyes:
How do you know you have no detonation? Just curious

Royce
Aluminum heads only allow .5:1 more compression. Not one point. Trust me Camaro man, I have done my homework.

11.68 wouldn't be slightly higher compression. The altitude isn't going to make up for that much difference.
 

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I notice that you guys have brought up cranking compression and cams.... what are you alls lobe seps on the 11:1+ engines? and total duration?
 
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