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compression questions

This is a discussion on compression questions within the Internal Engine forums, part of the LSx Technical Help Section category; I heard that whether you can run pump gas with high compression is all in the tune....is that true? I ...

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    Moderator 98TransAmWs-6's Avatar
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    I heard that whether you can run pump gas with high compression is all in the tune....is that true? I really want to get as high as possible on the compression on pump gas if possible although I am not opposed to running race gas. I just don't know where you get race gas or an equivalent.
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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by 98TransAmWs-6 View Post
    I heard that whether you can run pump gas with high compression is all in the tune....is that true? I really want to get as high as possible on the compression on pump gas if possible although I am not opposed to running race gas. I just don't know where you get race gas or an equivalent.
    Well, I don't think you would want to run race gas. I used to do that when it was $4 a gallon and back then that was alot. Now pump gas is $4 a gallon
    Race gas now is about $7-$9 a gallon depending on the brand you buy and which octane (there are many). Really good stuff used in Pro Mod, Pro stock, is $15 a gallon. It's available at the track. Don't know of any local gas stations selling it around here anymore. Back East in Ohio it was relatively easy to find around town. Out west,,,,not so much...

    With all that said, I'd stick to a pump gas motor. I've completely revamped my engines within the last few years to make power on pump gas and weed off the race gas. Just getting too expensive for a street car that may see the track only 10% of the time.

    How much compression on pump gas is going to depend on tons of variables. You have dynamic and static compression. You can change things around with cam timing, early intake valve closing, late intake valve closing, Lobe seperation, intake centerline and overlap will all play a roll. The shape of the combustion chamber, quench, the shape of the piston. You can play with coatings on the piston and combustion chamber to keep things cooler and allow more compression.
    How cool the engine runs plays a role, and as you mentioned, the tune plays a roll. You can bandaid a light ping issue with more fuel (but may give up some ponies). Not enough fuel and pinging can be a problem. A heavy car with no gear makes it more prone to detonate, and lighter car with more rear gear is less prone to detonate. There are general guidlines and always exceptions to what you can get away with.

    Generally speaking, aluminum heads allow more compression (they dissipate heat). How much depends on the total package. 11:1 is pretty doable with an aluminum head and a sharp tune. I've had slightly higher than that with pump gas and it's been fine, but the camshaft bleeds off alot of cylinder pressure too.

    Cast Iron heads seems to be hit or miss. Most stick to 9-9.5:1 but you can easily do more if the tune is spot on. My 69Z is 11:1 factory with cast iron heads and it's fine on pump gas. That has alot to do with the short stroke, large bore, and a camshaft with alot of overlap, a car that only weighs 3200 lbs and has a 4 speed with alot of rear gear.
    Another example is my 56 nomad with a 10.5:1 327 and cast iron heads. Runs fine on pump gas using an L79 camshaft, heavy car, but alot of rear gear. Again, tune is spot on.
    My chevelle used to be a race gas type engine until I switched to aluminum heads and a big roller cam. It's only 10.6:1, heavy car, but alot of rear gear. That thing will run on cat pee

    There is one thing to keep in mind despite all this though. Pump gas quality is only going to get worse, so build something that would run on cheap stuff, then run the best pump gas you can find for a little extra cusion.
    Last edited by Firebirdjones; 06-22-2012 at 04:37 PM.

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    Spaz is My Mentor SMWS6TA's Avatar
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    FBJ, I just score on a set of LS2 pistons & rods for $100 for my LQ4 block. With LS3 Heads what should the compression be vs. a set of 243 heads vs a set of 5.3 heads? Just looking for the best combo.

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by SMWS6TA View Post
    FBJ, I just score on a set of LS2 pistons & rods for $100 for my LQ4 block. With LS3 Heads what should the compression be vs. a set of 243 heads vs a set of 5.3 heads? Just looking for the best combo.
    This is just a guess because there are too many variables that can change.

    The 5.3 heads will give you the most compression. Without having specs it's just a guess but 5.3 heads with about a 58cc chamber and a flat top piston would give you over 11:1. Somewhere between 11:1 or 11.5:1 more likely. That's doable on pump gas with the right camshaft and a good tune, but it's probably pushing the limits.

    243 (LS6) heads have a chamber volume of 64cc and should yield at least 10.5:1 but again that's a ball park depending on variables.

    I'm not certain what the chamber volume is on those LS3 heads but I'd assume they are bigger than 64cc LS6 heads, so compression would be even less yet.

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    Moderator 98TransAmWs-6's Avatar
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    Thanks for the info FBJ. I know I am going to run TFS 215 heads milled to 58cc if possible, with the Vindicator Cam with a 112 LSA. I would need to figure out how thick of a gasket to run and talk to a tuner about what else they can do for me.. I will definitely keep it on pump gas, I wasn't sure how expensive race gas was compared to pump. I believe LS3 heads are 68cc, I may be wrong but I think that is correct.

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    Spaz is My Mentor SMWS6TA's Avatar
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    Quote Originally Posted by Firebirdjones View Post
    This is just a guess because there are too many variables that can change.

    The 5.3 heads will give you the most compression. Without having specs it's just a guess but 5.3 heads with about a 58cc chamber and a flat top piston would give you over 11:1. Somewhere between 11:1 or 11.5:1 more likely. That's doable on pump gas with the right camshaft and a good tune, but it's probably pushing the limits.

    243 (LS6) heads have a chamber volume of 64cc and should yield at least 10.5:1 but again that's a ball park depending on variables.

    I'm not certain what the chamber volume is on those LS3 heads but I'd assume they are bigger than 64cc LS6 heads, so compression would be even less yet.

    I have to find my notes but 68-70cc for the chamber is what's off the top of my head.


    Here is what I'm a little confuse on...what is better to have a high flow rate or high compression rate? I think both would be best but it is not always easy to achieve.

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by SMWS6TA View Post
    I have to find my notes but 68-70cc for the chamber is what's off the top of my head.


    Here is what I'm a little confuse on...what is better to have a high flow rate or high compression rate? I think both would be best but it is not always easy to achieve.
    Best to have both. A good flowing head with the smallest port volume is ideal. That is easiest done with expensive aftermarket heads. Compression is always good to have.

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