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high compression and boost

This is a discussion on high compression and boost within the Forced Induction forums, part of the LSx Technical Help Section category; I have had a turbocharged car running approx. 20 pounds of boost. But my question here is why does everyone ...

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    Junior Member jakesz28's Avatar
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    high compression and boost

    I have had a turbocharged car running approx. 20 pounds of boost. But my question here is why does everyone think they have to lower their compression to run a turbo or supercharger? At what point is it more efficent to lower your compression? Does a 6 to 1 comp. motor running 30 pounds of boost make more power then a 10.5 to 1 motor runnig 7 pounds? Intercooler effiency to me would make more since then lowering my compression to 9to 1.

    I have considered turbos and superchargers before but I currently spray. I just don't like having to fill a bottle.
    So with the right turbocharger and intercooler how much boost could you run on a 10.5 to one compression motor with forged internals?
    The first nitrous powered LT1 in the 8's.
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    Member cailey37's Avatar
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    The reason you lower your compression ratio is because of the octane required to prevent detonation. If you tried to run a 10.5 to 1 motor on 20lbs of boost you'd have to run racing gas to prevent detonation. What happens is the inlet air temps will rise when you compress the air more. Thus intercoolers and methanol injection systems. They lower the temps so you can run more boost.....if you plan on driving your car daily then you have to be able to have the right fuel for it.......it would get pretty expensive putting race gas in it all the time......

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    Junior Member jakesz28's Avatar
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    Yes I realize the how octane effects detonation. My Question is at what point does compresion have to be lowered? If a stock LT1 can handle 5 pounds of boost and I built a 9.5 to 1 compression motor with every thing else the same(cam heads exhaust etc) and only ran 5 pounds of boost the horsepower would be lower. So I would have to raise the boost to compensate.
    Max horspower will be on the edge of detonation. By increasing the amount of boost you raise air temps.
    What is going to be more effient, 10.5 to 1 with 5 LBS boost or 9 to 1 with 10 LBS of boost?

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    Member cailey37's Avatar
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    Well the answer is just that.....when does the motor start having a detonation problem???? You would have to have the car hooked up and monitoring everything to have an answer to your question of when does it need to be lowered......but previous builds suggest that a stock motor pushing more than 7-8lbs of boost would be on the edge of the limit without any means of cooling the charge. As for which one would be more efficient...well.....I would lean towards the 10.5 to 1 but it would just depend on the setup.......motors react differently....

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    LS1 FTW! 85mcss's Avatar
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    Good topic! i have also put a lot of thought into this as i am in the middle of building a SC 355... if we can get plenty of info, im sure this could be a stickied topic!

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    Senior Member 440 rwhp trans am's Avatar
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    good question

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    Junior Member jakesz28's Avatar
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    That is what I thought too. But not many replies. Personallly I wouldn't go below 10 to 1. I want as much power that I can get without heating the air any farther then required. The more boost more heat. Lower compression moters mean less torque.

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    Pathelogical Liar BlackLT1Z28's Avatar
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    My GUESS would be that a little higher CR with a little less boost would be more efficient, as far as fuel consumption. Think about it...let's go to extremes here for a moment. Say you were running a 7:1CR with 50PSI, then you would need to fill all that extra space up with air and fuel, lots of air and fuel (Because ATDC, there is a lot of area left in the combustion chamber). But if you were running a 10:1CR at 9PSI, then you would have less space to fill with more static compression.
    But that's just a guess, I'm no expert on physics.
    Another thought makes me think that there is an equilibrium between CR and boost. Wouldn't it kind of make sense that if you drop one and raise the other, that the output would be the same?
    I don't know, this could be a good thread with the right information.

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    Junior Member CPViolation's Avatar
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    Quote Originally Posted by jakesz28 View Post
    Yes I realize the how octane effects detonation. My Question is at what point does compresion have to be lowered? If a stock LT1 can handle 5 pounds of boost and I built a 9.5 to 1 compression motor with every thing else the same(cam heads exhaust etc) and only ran 5 pounds of boost the horsepower would be lower. So I would have to raise the boost to compensate.
    Max horspower will be on the edge of detonation. By increasing the amount of boost you raise air temps.
    What is going to be more effient, 10.5 to 1 with 5 LBS boost or 9 to 1 with 10 LBS of boost?
    My stock motor is running 15 lbs. of boost with a mild tune. With the right pulley and tune I'd trust 20+lbs.
    Try running 20lbs./10.5:1 Comp ratio and pump 91 octane. Especially with a stock bottom end.
    '03 '04 Cobras came with a pretty stout bottom end.
    Jeff
    Last edited by CPViolation; 12-08-2006 at 09:10 PM.

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    Quote Originally Posted by CPViolation View Post
    My stock motor is running 15 lbs. of boost with a mild tune. With the right pulley and tune I'd trust 20+lbs.
    Try running 20lbs./10.5:1 Comp ratio and pump 91 octane. Especially with a stock bottom end.
    '03 '04 Cobras came with a pretty stout bottom end.
    Jeff
    I don't know if I would trust 20psi at 10.5CR on pump gas. That's a lot of heat that would be generated. I think you would experience detonation with that combo. But I thought the cobras came from the factory with an 8.5:1 comp ratio. That engine was built around the blower, hence the low comp ratio...could be wrong though.

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    Senior Member 440 rwhp trans am's Avatar
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    sarge!!!! i want to here your opinon!

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    Junior Member crazycat's Avatar
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    Good question, I would like to know before I rebuild my motor.theres got to be some experts on this, some dynos would be good...

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    Hello , I felt I just had to join in with this. I think it more about heat management and and torque production. Obviously a lot of fuel and air in a tight space will detonate because the particles will burn to fast and explode. If you lower c/r and have a larger volume you can still pack in plenty of fuel and control the burn without detonating. It's not so crowed in there All that extra fuel takes time to burn so it's still burning for longer during the downstroke. You gain more torque which is a factor used to get rpm's. This is how I see it. Hope it helps.

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    Quote Originally Posted by jakesz28 View Post
    Yes I realize the how octane effects detonation. My Question is at what point does compresion have to be lowered? If a stock LT1 can handle 5 pounds of boost and I built a 9.5 to 1 compression motor with every thing else the same(cam heads exhaust etc) and only ran 5 pounds of boost the horsepower would be lower. So I would have to raise the boost to compensate.
    Max horspower will be on the edge of detonation. By increasing the amount of boost you raise air temps.
    What is going to be more effient, 10.5 to 1 with 5 LBS boost or 9 to 1 with 10 LBS of boost?
    Maybe your question should be 'which will be the most efficient on boost or off boost to suit my driving style' ?

    If you rarely drive on boost then you want a high compression atmo engine with a fast burn and low boost. If you drive on boost more of the time then you might want a low comp engine. The torque rush is addictive. For the road it's always a compromise and anywhere over 9:1 with low/high boost would be nice and sharp giving a bit of both worlds. Either configuration will burn fuel quickly if you drive in the efficiency zone. Once out of the zone you'll get losses in the same was as when on or off the cam. If you want power then it's low boost for me but for flexibility it's high c/r. A low boost combo will be inefficient because you'll plant the pedal every chance you get , imo.

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    The Bandit Wesman's Avatar
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    From what I understand, its better to pull in more air and compress it less, as opposed to pulling in less air and compressing it more.

    Compressing the air too much results in detonation, so the less compression and the more boost, the better

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    Quote Originally Posted by Wesman View Post
    From what I understand, its better to pull in more air and compress it less, as opposed to pulling in less air and compressing it more.

    Compressing the air too much results in detonation, so the less compression and the more boost, the better

    Well, you sure said it better then me!

    It's like people at a party, being close is cool. Shoulder to shoulder and something's bound to kick off!

    Boosted.

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    Member Runn_WS7's Avatar
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    Really good question...
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    BOSSONNOS
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    You can run 15lbs 20lbs of boost quite safely on a forged bottom end with a blower or turbo, we currently run a Large single turbo on 12 psi on a stock LS2 with 10.9 CR with no detonation and it makes over 600 rwhp and is daily driven BUT we have a higher Octane rating on our fuel here which is 98.
    Heat soak is a big killer with FI engines, control the heat and you control detonation.

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    Junior Member BLWN 231's Avatar
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    i could be all wet here, but i have always heard that more power is to be had from lower c/r and higher boost. if you really think about it isn't it really 6 in one and half a dozen in the other? all boost really is is more air in the same amount of space, hence more compression, so, theoretically, at some point they are always going to equal out. i think the point made earlier about driving style is very valid. ie: (the following numbers are strictly ficticious) say john's motor has 10.5:1 compression pistons, and with 7 psi, john has 0 knock and 450 hp, now why is that any different than ralph's motor with 8.5:1 compression pistons with 17psi, 0 knock, and 450 hp? i'm nearly positive these numbers are far from accurate, but i think i made my point. i guess the only argueable issue is that john has more power at lower rpm's where as ralph needs boost more. like i said earlier, i could be all wet, but i think its nearly the same no matter how you look at it, or at least it could be intelligently argued from either side of the fence.

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    Senior Member predator's Avatar
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    isnt this kinda like a mid size vs. large cam argument?

    will a low comp high boost motor will show more peak hp, but a high comp low boost motor show more power under the curve?

    using only detonation on 91 octane fuel as a factor (no blown internals) what is the best street motor? would it be smarter to run 10:1 comp with like 7 psi or 8:1 compression with 20 psi?

    maybe one of you stang guys can help us out with this... can yall find a dyno sheet of a s/ced mach 1 (nothing crazy just basic mods) and a term cobra with basically the same hp ratings?

    would this not show us what we want to know? 2 reasonably close motors compression and boost are only differing factor (in genearal)

    -me

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