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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 need input in a few things. First I am trying to raise the compression to the max I can ...

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    Moderator 98TransAmWs-6's Avatar
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    compression questions

    I need input in a few things. First I am trying to raise the compression to the max I can run on 93 octane. And this leads to my other questions. I was thinking of getting 5.3 heads and a thicker head gasket but I am not sure how thick. Another thing is stock for stock are the 5.3 heads better than ls1 heads? Oh and this is with the vindicator cam which it's specs are: 240/244 .605/.596 112 LSA.
    Last edited by 98TransAmWs-6; 05-25-2012 at 10:50 AM.
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    Moderator Firebirdjones's Avatar
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    The general consensus from the engine builders I hung around was that thicker head gaskets are a bad idea because it increases the quench area, cuts down on turbulance and promotes bad fuel mixture,,,,which when all mixed together will actually create detonation or make it more suseptable.

    It's best to run a thinner head gasket to make quench as tight as possible, which promotes turbulance and better fuel distribution throughout the combustion chamber,,,,this helps to fight detonation.

    The amount of quench you can get away with depends on the engine build, alot of things are taken into account. One issue that I remember discussing is the thickness of the actual rod bearing being taken into account in the event of a spun rod bearing,,,so you don't contact the cylinder head with the piston and do more damage.

    Generally speaking, from building gen 1 sbc's/bbc's, a .041 compressed thickness head gasket worked well if you zero deck the block, as this would give you exactly .041 clearance from the flat portion of the piston to the flat portion of the cylinder head/combustion chamber. I've even seen that envelope pushed to the extreme on a particular 355 built where the block was decked below zero so the piston would protrude out of the cylinder slightly. This allowed 11:1 compression with a flat top piston and a 67cc head. Just a sneaky way to make a little more power, since flat tops promote better flame travel and that was something they didn't want to sacrafice with a pop up piston.

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    Quote Originally Posted by Firebirdjones View Post
    The general consensus from the engine builders I hung around was that thicker head gaskets are a bad idea because it increases the quench area, cuts down on turbulance and promotes bad fuel mixture,,,,which when all mixed together will actually create detonation or make it more suseptable.

    It's best to run a thinner head gasket to make quench as tight as possible, which promotes turbulance and better fuel distribution throughout the combustion chamber,,,,this helps to fight detonation.

    The amount of quench you can get away with depends on the engine build, alot of things are taken into account. One issue that I remember discussing is the thickness of the actual rod bearing being taken into account in the event of a spun rod bearing,,,so you don't contact the cylinder head with the piston and do more damage.

    Generally speaking, from building gen 1 sbc's/bbc's, a .041 compressed thickness head gasket worked well if you zero deck the block, as this would give you exactly .041 clearance from the flat portion of the piston to the flat portion of the cylinder head/combustion chamber. I've even seen that envelope pushed to the extreme on a particular 355 built where the block was decked below zero so the piston would protrude out of the cylinder slightly. This allowed 11:1 compression with a flat top piston and a 67cc head. Just a sneaky way to make a little more power, since flat tops promote better flame travel and that was something they didn't want to sacrafice with a pop up piston.
    I think I mixed it up saying thicker and not thinner. So how would I figure out how thin to go on a stock ls1 minus bolt-ons and my future cam? What other ways can I raise compression besides heads?

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    Moderator Firebirdjones's Avatar
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    On a stock LS1 I'd use the stock GM MLS gaskets. Only thing I don't like is they advertise a .054 thickness, so with a stock LS1 short block that hasn't been decked (so I'm sure the pistons are down in the hole) along with this gasket the quench is going to stink.

    I'd be shocked if the advertised 10:1 compression in the LS1 were actually 10:1 (most stock engines never are near the advertised ratios)

    The easy way to bump compression on a stock LS1 is with the cylinder head swap you mentioned, with heads milled down to 60cc or even less. But without knowing how far down the pistons are in the hole it's impossible to figure exactly what you'll have for a final ratio. It is an easy way to bump power though. You can easily find a full point in compression with just a cylinder head swap,,,but what the final might be is just a guess without all the variables.

    Some of the heads on the market claim 11:1 compression when bolted to a stock LS1, but I don't know if that's a guess just from the CC change of the cylinder head, or if someone actually took the time to figure bore/stroke/piston depth/piston cc/gasket compressed thickness and bore diameter of gasket used/ etc.....Even the same head will cc different after a valve job depending on the valves used and how far they are sunk into the seats.

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    Alright cool I will do that. I am looking at getting heads with 58cc. Another question not about compression though. I have to take my cam out due to metal fragments in there but I don't think I can afford to buy a stall too and I want to swap the cam out for the vindicator and don't feel like doing this twice. I should probably wait until I can get the stall too huh? I just am dying to get the car up and running, maybe not driving but running.

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    Moderator Firebirdjones's Avatar
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    Hmmm,,,,don't know the specs on the vindicator but judging by the name I'd guess it's healthy.

    I personally would not do a camshaft without a converter to go with it. Don't think you would be too happy with the outcome.

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    Quote Originally Posted by Firebirdjones View Post
    Hmmm,,,,don't know the specs on the vindicator but judging by the name I'd guess it's healthy.

    I personally would not do a camshaft without a converter to go with it. Don't think you would be too happy with the outcome.
    It's 240/244 .605/.596 112 LSA I figured that it isn't a good idea but since it won't be driving anywhere until it gets a 4000 stall I wasn't sure if that would make a difference.

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    Quote Originally Posted by Firebirdjones View Post
    Hmmm,,,,don't know the specs on the vindicator but judging by the name I'd guess it's healthy.

    I personally would not do a camshaft without a converter to go with it. Don't think you would be too happy with the outcome.
    Car trying to push you through stoplights=BAD

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    Quote Originally Posted by az gt eater View Post
    Quote Originally Posted by Firebirdjones View Post
    Hmmm,,,,don't know the specs on the vindicator but judging by the name I'd guess it's healthy.

    I personally would not do a camshaft without a converter to go with it. Don't think you would be too happy with the outcome.
    Car trying to push you through stoplights=BAD
    There aren't any stoplights in my garage....

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by 98TransAmWs-6 View Post
    It's 240/244 .605/.596 112 LSA I figured that it isn't a good idea but since it won't be driving anywhere until it gets a 4000 stall I wasn't sure if that would make a difference.
    That's starting to get healthy, I'd want a stall in that no question. I guess if you don't plan to drive it until you do a stall I see no harm in it. You'll have to get it tuned anyway regardless.

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    Quote Originally Posted by Firebirdjones View Post
    Quote Originally Posted by 98TransAmWs-6 View Post
    It's 240/244 .605/.596 112 LSA I figured that it isn't a good idea but since it won't be driving anywhere until it gets a 4000 stall I wasn't sure if that would make a difference.
    That's starting to get healthy, I'd want a stall in that no question. I guess if you don't plan to drive it until you do a stall I see no harm in it. You'll have to get it tuned anyway regardless.
    Okay thank you so much for your help!

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    IMHO That cam would be to big for either head stock, due to port size; valve size; and also spring size, to me that is cutting it close on your springs also. It purely depends on what your planning on doing with the car. I had in an '04 goat ported ls6 243's with a comp 232/234 @.050 .580"lift and 112 lsa and it is plenty for the 5.7 with 3500 stall, in reality it locks up before 3k every time, also that car is not the friendliest daily driver, it's not bad, but it does lope pretty hard.


    Depending on what your doing with the car I would go with the 5.3 heads and a lower to mid range cam. I.E. 226/228 @.050 .580" lift. That would match those heads gread plus, the 5.3 heads WILL increase your compression. That would make for a torque monster of a motor that would be great in the street.

    Most engines that see a cam that big I.E. 240 plus duration. are 400 plus inch strokers putting down around 500 ft/lbs, the larger the displacement the better it will respond to a larger duration cam, so that cam in a 408 would act like a much smaller cam than if it were in a 350. (if it were in a 350 power band would be approx. 5,500 to 7,500 so your talking forged everything and BIG heads.
    Last edited by Jay37; 06-18-2012 at 02:31 PM.

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by Jay37 View Post
    Most engines that see a cam that big I.E. 240 plus duration. are 400 plus inch strokers putting down around 500 ft/lbs, the larger the displacement the better it will respond to a larger duration cam, so that cam in a 408 would act like a much smaller cam than if it were in a 350. (if it were in a 350 power band would be approx. 5,500 to 7,500 so your talking forged everything and BIG heads.
    Speaking of the LS engine sure, with 15 degree heads they don't need so much camshaft to make power. Gen 1 sbc's and bbc's however ran cams with this kind of .050 duration from the factory but with LSA spread out a bit to keep them streetable.

    My 69 DZ 302 factory camshaft is 254* at .050 duration but lsa is 114 which keeps it managable on the street.

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    Quote Originally Posted by Jay37 View Post
    IMHO That cam would be to big for either head stock, due to port size; valve size; and also spring size, to me that is cutting it close on your springs also. It purely depends on what your planning on doing with the car. I had in an '04 goat ported ls6 243's with a comp 232/234 @.050 .580"lift and 112 lsa and it is plenty for the 5.7 with 3500 stall, in reality it locks up before 3k every time, also that car is not the friendliest daily driver, it's not bad, but it does lope pretty hard. Depending on what your doing with the car I would go with the 5.3 heads and a lower to mid range cam. I.E. 226/228 @.050 .580" lift. That would match those heads gread plus, the 5.3 heads WILL increase your compression. That would make for a torque monster of a motor that would be great in the street.

    Most engines that see a cam that big I.E. 240 plus duration. are 400 plus inch strokers putting down around 500 ft/lbs, the larger the displacement the better it will respond to a larger duration cam, so that cam in a 408 would act like a much smaller cam than if it were in a 350. (if it were in a 350 power band would be approx. 5,500 to 7,500 so your talking forged everything and BIG heads.
    lol who the hell would run that cam with stock springs? I have done the research on what I want and how to achieve it. I talked to Kevin at Vengeance racing and told him my goals and he came up with a cam/head package to reach them. I just can't afford the heads atm. I plan on running TFS 215s in the future which have a 13.5 degree valve. The cam would be going in with a 4000 circle d stall. But atm the cam is going on the back burner for now since I am low on car mod funs atm. I think Vengeance racing knows what they are doing and wouldn't steer me in the wrong direction as far as my goals. My short term goals is to get as close to 500 rwhp on the ls1 as possible, Vengeance suggested the Vindicator with TFS heads. My long term goal is a 511 inch stroker using the tall deck version of the lsx bowtie block.

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by 98TransAmWs-6 View Post
    lol who the hell would run that cam with stock springs? I have done the research on what I want and how to achieve it. I talked to Kevin at Vengeance racing and told him my goals and he came up with a cam/head package to reach them. I just can't afford the heads atm. I plan on running TFS 215s in the future which have a 13.5 degree valve. The cam would be going in with a 4000 circle d stall. But atm the cam is going on the back burner for now since I am low on car mod funs atm. I think Vengeance racing knows what they are doing and wouldn't steer me in the wrong direction as far as my goals. My short term goals is to get as close to 500 rwhp on the ls1 as possible, Vengeance suggested the Vindicator with TFS heads. My long term goal is a 511 inch stroker using the tall deck version of the lsx bowtie block.
    If that is your goal, it's doable. There used to be a few cam/head LS1's out there that made 500rwhp through a manual, but it takes a healthy camshaft and a really good set of aftermarket cast heads.
    The stall needed in an auto for a combo like this won't really show the true potential of the car on a dyno, so don't expect it to be a dyno queen. Where you'll have the big advantage is at the track

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    That is exactly what vengeance told me. Which is fine as long as it performs. It would be nice if it dynoed pretty high but wouldn't be the end of the world if it didn't.

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    Well looks like I'm confused. I thought you were referring to the stock heads, not aftermarket heads.
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    Quote Originally Posted by Jay37 View Post
    Well looks like I'm confused. I thought you were referring to the stock heads, not aftermarket heads.
    The initial question was about compression and stock heads. I was thinking of getting the cam putting .650 lift springs and stuff in the stock heads plus having them milled to 58cc to raise compression until I could afford TFS 215 heads. Sorry for the confusion.

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    Member Jay37's Avatar
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    Well if your still interested... a 58-60cc (5.3) head would put you at 11.0/11.2 to 1. every 5.3 head i've measured comes out at about 58.5 cc so that would be just over 11., that is why some people go with a little thicker gasket, because that is a little high for pump gas... IMHO.



    heres the math.-


    cyl vol+piston/clearance volume+gasket volume+chamber volume divided by- piston/clearance volume+gasket volume+chamber volume
    (43.2+.230+.48+4.068)/(.230+.48+4.068)=10.041 to 1 -stock ls1
    so if you switch the 4.068(66.68cc) head volume to 3.569(58.5cc)
    (43.2+.230+.48+3.569)/(.230+.48+3.569)=11.09 to 1 stock ls1 with stock 5.3 heads
    switching to a .048thick (.573vol with 3.9 bore)vs the recommended .041thick (.48vol with 3.9 bore.)
    (43.2+.230+.573+3.569)/(.230+.573+3.569)=10.88 to 1 stock ls1 with thicker head gasket and stock 5.3 heads

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    Member Jay37's Avatar
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    also ls1's are a zero deck engine so the piston/clearance vol. .230 (3.7cc's) is the extra room around the head gasket+deck to piston+piston dome. So in our case ls1's have no dome and no deck to piston volume. they only have play around head of piston to ring and head gasket. I.E. head gasket bore not being exactly 3.9.
    some ls1 pistons come OUT .008" none go in, therefore taking AWAY from the piston/clearance vol number
    (-.095 vol with 3.9 bore) that would bring the piston/clearance vol DOWN to .135... so all the compressions are capable of being .2 higher.

    You would still benifit from flat decking because of the fact that the deck height from GM can change up to .008" which seems small but if you plug the numbers it changes compression notably and also affects the quench notably. so going consistant and running a .41 gasket with -.008 deck height will give a quench of .033 which is tight, but GM does it so why wouldn't I? the tighter the better, as long as it's safe...
    Last edited by Jay37; 06-20-2012 at 01:39 PM.

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