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High Compression

This is a discussion on High Compression within the Internal Engine forums, part of the LSx Technical Help Section category; Ok gang. I'm wondering who around here is running higher than stock compression. I'd prefer not to hear about what ...

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    High Compression

    Ok gang. I'm wondering who around here is running higher than stock compression. I'd prefer not to hear about what you've heard is the theoretical maximum compression we can run on these engines - trust me, if I change my cam around I could run 25:1 like a diesel if I wanted to. A real world example, my sportbike runs 12.4:1 and can handle being pounded on with engine temps @ 220F on 93 without issue.

    I'm looking for what people are doing, and what the rest of your top end is set up like. What fuel do you use?

    Does anyone have coated pistons - it has been written that the new ceramic coatings that can be put on piston/heads when done you can run a full 1.0 higher compression than otherwise on most engines?

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    🤔 11s. blackbirdbrain89's Avatar
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    with a alum block im saying 17-18-1 would be pushing the limmits of block flex

    on a ls1 with the right cam u can run 12.0-1 on pump gas with out any issues

    its all about the DCr the cr only has so much to do with it

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    Senior Member bills98ta's Avatar
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    I'm 14.0-1 with +12cc domed Lunati ( forged ) pistons, 5.3 milled heads with 2.02 / 1.6 valves. 25* timing with a 150 shot and Sunoco unleaded 100 octane.

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    Member JRENIGAR's Avatar
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    Quote Originally Posted by bills98ta View Post
    I'm 14.0-1 with +12cc domed Lunati ( forged ) pistons, 5.3 milled heads with 2.02 / 1.6 valves. 25* timing with a 150 shot and Sunoco unleaded 100 octane.
    Damn, that outta run.

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    Yeah, that isn't a street car by any means.

    I still want to drive my car on the street, so 100 octane fuel won't cut it. I need to still be able to put pump gas in the tank. I am not, however, at all opposed to running a water/meth injection system to act as an octane booster and lower my IAT to get around any problems with pre-det.

    I know we can safe and easily do 11:1 on our engines and 93 pump gas without issue.

    Is there some general rule out there that says that for each 1.0 more compression you run you'll need an octane of X more to keep timing advanced?

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    🤔 11s. blackbirdbrain89's Avatar
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    what cam shaft are u going to run with 11.0-1?


    i have a hr281 in my car and ive done a lotta R&D and im going to mill my heads to 59.5cc and use .051 head gaskets from gm and

    that will put me @11.2-11.5 -1

    and i plan on running 93 oct with out issues

    with cam im running i can go to 11.8 with it but i dont wanna push my luck....

    but to mill heads that small im using a after market head thats all ready at 62 cc so not to mill to much off

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    Gonna depend on the cam for sure - I ran my 408 at 12.2:1 on 93 octane spraying a 250 shot on it... able to do it because of how much overlap my cam had... all depends on the dynamic compression ratio...

    I know you can easily run 13:1 on E85, so if you are gonna spray a healthy dose of meth on it, you could get get into the mid 12's I would think... gonna take some experimenting to find out though... don't think there are many out there doing it.

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    Right now my cam is 232/234, 113, @.592 and was spec'd for running 11:1 with the trickflow narrow angle heads milled slightly. This cam is still streetable without issue. I mean, if I get caught out in the rain, and I'm pulling out up a hill, traction control usually give me a hand. Its definitely lost some off idle tq over the stock cam - no surprise there - I knew it would.

    What kind of overlap did your cam have?

    If there were more stations around me that sold E85, I would definitely be running it. Unfortunatley, there are only 2 nearby, and they're both 15+ miles away. Should be able to do 14:1 on E85 with no issue, its 104 octane. Won't get good gas mileage, but then again, who builds a 600HP engine with a mind on gas mileage.

    Methanol, without looking it up, I believe is ~120 octane rating, so yeah, tuning in the meth injection is just a matter of stoichiometry. What I don't neceessarily want, is to have to be spraying meth to drive it gently - windshield washer fluid is cheap, but not if I need to go through 5 gallons of it per tank of gas.

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    My cam was a 248/254, .615"/.622" on 113 LSA. Keep in mind it was a 408.

    How much power are you trying to make?

    They have a meth kit for naturally aspirated engines that works based on vacuum readings and sprays progressively. Would just take some dyno time to get the meth, fuel, and timing all straight to make the most of the setup... Personally, I would buy methanol from a place that sells race gas and mix it 50/50 with distilled water - cheaper than boost juice and better than windshield washer fluid (most washer fluids aren't enough meth to make them work properly - I've tested it on the dyno because cheapo people insisted on trying it...)
    Last edited by LSXZ28; 09-27-2010 at 04:17 PM.

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    I am thinking that 500/450 hp/tq at the wheels is within reach if I can roll the compression up in the >12:1 range.

    I've seen the setup that I'm aiming for do 470/425 on 11:1...

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    Quote Originally Posted by gashkerwin View Post
    I am thinking that 500/450 hp/tq at the wheels is within reach if I can roll the compression up in the >12:1 range.

    I've seen the setup that I'm aiming for do 470/425 on 11:1...

    You might be able to get close to that... everything else about the combination will have to be right! The biggest problem I see is that you are in kinda uncharted territory - not many have tried it... If you overshoot on the compression and can't get it covered with the meth, you are screwed into race gas... But, I would think that if you use one of the bigger nozzles for the meth, and then lean it out as needed on gas to compensate, the concentration would be high enough to make it work... getting the meth coming in at the right load might be tricky too. Interesting project idea - wish I could try it...

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    What did your 408 do in NA mode? Thats a big ole cam you were running...

    See, the whole uncharted territory thing is the main thing that is appealing about it.

    I dunno. Its a ways off I suppose, since theres no point in doing the heads I'd suppose until I get off the stock rear anyway.

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    Moderator Firebirdjones's Avatar
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    I've done similar stuff on several other engines. Two I have currently and drive primarily on the street.

    What I've found, 11:1 is a piece of cake with aluminum heads and a decent camshaft. 11.5:1 is doable.
    I generally don't push it much past that on a street motor, simply because you never know the quality of gas you get from 1 tank to the next. And gas is getting worse every year, with talk of raising ethanol content to 15% now.
    More than anything the tuneup has to be spot on for these scenerios to work. I tend to lean towards a safer setup that has a little fudge factor, but that's just me. I have tried to push the envelope however with a few antique builds.........

    I have run as much as 10.5:1 on a cast iron head with enough overlap on the camshaft, and pump gas was fine with 38 degrees total timing and dinosaur heads, drove it daily for years. To take that further, I've had a few of the DZ 302's and recently a 70 LT1. Both of these motors are 11:1 from the factory with ancient cast iron heads,,,and the factory solid lifter cams, which have a fair amount of overlap. These engines are fairly easy to run on pump gas with 38 degrees of ignition lead if you have the fueling right.

    I built a vintage 427 L88 with 12.5:1 compression and factory aluminum heads, with the factory L88 cam (which is no slouch for a factory piece) Duration numbers at .050 in the 260 range. It would only run pump with timing conservative, but made best power with 40 degrees and a 50/50 mix of pump and 110 octane.

    On the same token, I built a pontiac 455 with modern aluminum heads and 11:1 comp,,,260+ duration at .050 and that thing would run on cat pea on the dyno So there is a fine line somewhere in between. I just recently freshened a 406 with cast iron heads, 10.2:1 compression with a muther thumper camshaft and it runs effortlessly on 91 octane without a hint of complaining and 36 degrees of total timing. Since I've run more compression with other cast iron head builds I felt very comfortable with this one. I could probably get away with less octane if I wanted. And that was the idea for those long road trips. Like I said, I like a little fudge factor anymore.
    My own personal experience with some dinosaur motors. With todays high tech coatings, you can get away with more, but it's costly for such a small percentage gain in HP. But, if money is no object........
    There are plenty of high end engine builders that know how to get these combos to work.

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    Quote Originally Posted by gashkerwin View Post
    What did your 408 do in NA mode? Thats a big ole cam you were running...

    See, the whole uncharted territory thing is the main thing that is appealing about it.

    I dunno. Its a ways off I suppose, since theres no point in doing the heads I'd suppose until I get off the stock rear anyway.
    It ran 10.60's @ 127mph - but that was with a semi-tight nitrous converter, 28" tires, and 3.73's... if I woulda been trying to optimize NA, I would have had a 4000 converter and 4.56 gears. But, it was a street car built to run on nitrous...

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