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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 would like to chime in... in a nutshell... If you want to be fastest at the dragstrip, go with ...

  1. #41
    Member Rich L's Avatar
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    Thumbs up Maximum boost...

    I would like to chime in...

    in a nutshell...

    If you want to be fastest at the dragstrip, go with low compression high boost setup.

    You'll have an inefficient, max hp motor thats good for racing.

    If you want to maintain some streetability...

    go with higher compression and lower boost setup. You'll have a more efficient stoplight street racer.

    It is all a compromise, and there are a ton of variables to factor in when trying to consider the best setup for you.

    I would recommend you guys that are really interested in this topic to check out a book called "Maximum boost" by Corky Bell.

    It covers the matter at hand... Designing, testing and installing turbocharger systems. Its a good read and will give you alot of insight on this topic.
    2002 A4 Z28: SLP LM exhaust and HSW 100whp dry shot.

    NA: 323.4WHP@5360 RPMs, 344.2ft-lbs@4610 RPMs,
    N2O: 417.0WHP@5200 RPMs, 467.6ft-lbs@4210RPMs

    NA: 13.351@107.48mph, N2O: 12.324@117.88


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  2. #42
    Junior Member jakesz28's Avatar
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    Great book it is sitting beside me.

    He even states that "A turbo engine must never be reduced to a low-compression slug."
    The first nitrous powered LT1 in the 8's.
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  3. #43
    Member Rich L's Avatar
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    Quote Originally Posted by jakesz28 View Post
    Great book it is sitting beside me.
    He even states that "A turbo engine must never be reduced to a low-compression slug."
    Its like the turbocharging bible!

  4. #44
    Junior Member jakesz28's Avatar
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    Well I'm going to bring this to the top. I have been spraying my car for two seasons and I'm planning on building a motor over the winter. This has me considering a turbo set up again.

    The ? is where to put it front, back, buy a kit, or make my own?


    ___________________________________
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    Junior Member CPViolation's Avatar
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    Quote Originally Posted by BlackLT1Z28 View Post
    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.
    You're right,
    for high boost applications, it's good to have a low static compression ratio.
    I ran my motor without the blower pulley. It was.... slowbra for sure.
    I have a stock KB pulley with a mild tune for open track events. It makes plenty of HP/Torque for a 52 year old man. Besides, I'd be doing burn-outs around the track.


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  6. #46
    Member Rich L's Avatar
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    Quote Originally Posted by jakesz28 View Post
    Well I'm going to bring this to the top. I have been spraying my car for two seasons and I'm planning on building a motor over the winter. This has me considering a turbo set up again.

    The ? is where to put it front, back, buy a kit, or make my own?

    ___________________________________
    11.002 at 112 coasting thru the traps.
    Like I said: It is all a compromise, and there are a ton of variables to factor in when trying to consider the best setup for you.

    As far as turbos here are my 3 humble opinions:

    1) I would say that it would be easiest to do a STS kit, but you will have to settle for the odd single outlet exhaust look. (which most people hate... unless you like that "sleeper" look) Bonus: no IC needed for low boost apps.

    2) Traditional single turbo setup. Probably better for bigger turbos... higher HP output, probably better spool up time and less worries about water splashing in the intake. But more time and expense to install... and you will have to shell out extra cash for a nice big cool looking FMIC.

    3) Custom Twin turbo setup. I never seen it on a LS1 Camaro... but have seen some killer setups for corvettes. Very expensive and exotic. Im sure someone has done it before, that would be a PITA to try to make fit... but boy it would be cool.

    Lastly...

    Dont rule out a supercharger. Do your homework. I hear the Procharger kit is one of the best. It will cost a bit more than the sts turbo kit.... but then again... with a SC you can have true duels or much more flexibility with headers and exhaust!

    (for the record.... I have NEVER heard anyone ever say anything good about the Powedyne SC kit, so be weary of it.)

  7. #47
    Junior Member CPViolation's Avatar
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    I've seen the tt set-up for vettes.
    It seems a little scary with the turbos in the rear. Although there is little lag, too bad if you bottom out.
    Jeff

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    what about 10.75:1 compression.Is 5lbs boost doable?

  9. #49
    Laugh while you can... mistermike's Avatar
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    Here's the deal with compression and boost, to clear up some blatant misinformation in earlier posts. Statements have been made that too much compression causes detonation. This is utter nonsense. Boost and compression are exactly the same thing, with one exception that I'll cite later. Let's say you have a 10:1 compression motor with 5 Lbs of manifold pressure (boost). Your effective compression is 13.4:1 Now let's drop the static CR to 9:1 You can now run 7.2 PSI for the exact same effective compression of 13.4:1. The exact same number of air / fuel molecules are being elevated to the same pressure in the combustion chamber.

    There are still two elements in the detonation equation which are lacking. Those are temperature and spark advance. Here's where the intercooler makes the big difference. Any compression of air raises the temperature. It doesn't matter if its a turbo, roots, twin screw, or simply the normal compression of a piston during the compression stroke. Compress X volume of air to Y times atmospheric pressure and you will raise the temperature by the same amount. The difference with compressing the air outside of the engine is that it can be cooled before being induced into the combustion chamber. This cannot be done in the normal process of squeezing the air with a piston in the motor. A small amount of heat may be lost into the surrounding metal, but it's not significant compared to passing it through an intercooler. That's it in a nutshell. You can get an intake charge that is overall cooler using external compression with an intercooler.
    We still haven't addressed timing, where myth still abounds. The purpose of spark advance is to cause peak cylinder pressure to occur at around 14-16 degrees ATDC. This is geometric, determined by the mechanical lever advantage of piston, pushrod, and crank. It never changes, but spark varies with RPM, naturally. Unfortunately, decades of hot rodding inefficient piston / valve / head designs led to the belief that more timing produces more power always. More timing produces more power only until peak pressure occurs at the optimum spot. Further advance lowers power. Modern cylinder head design produces much faster combustion than in previous years, so far less spark advance is needed. Add elevated temperatures due to a blower or turbo, and the combustion occurs even faster, requiring less timing. Frequently, forced induction setups are detonation limited, that is detonation occurs before timing brings the peak cylinder pressure to the 14-16 degree range. The only cure for this is to drop the temperature, or add octane. It's informative what you can accomplish with certain combinations. I have an Australian friend who used to develop "tunes" for General Motors in Australia before branching out on his own. He runs a 383 LS1 stroker with 10.8:1 static compression with 15 PSI of boost in a daily driver that runs 10.22 in the 1/4 and gets better fuel economy that a stock 346 going for groceries. Naturally, he could get more power lowering the compression, and the car would be a pig of a DD, but once you've got your 3900 Lb grocery getter into the low 10's, how much faster does it need to be?
    Last edited by mistermike; 07-30-2007 at 02:20 PM.
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  10. #50
    Senior Member predator's Avatar
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    Quote Originally Posted by mistermike View Post
    Let's say you have a 10:1 compression motor with 5 Lbs of manifold pressure (boost). Your effective compression is 13.4:1 Now let's drop the static CR to 9:1 You can now run 7.2 PSI for the exact same effective compression of 13.4:1.
    He runs a 383 LS1 stroker with 10.8:1 static compression with 15 PSI of boost
    What?!?!?!?!

    Ok I can understand that you can cool boost, but WHAT?!?!?!?!?!?!?!?!

    I was gonna try to use simple physics to disprove that statement, but lets just put it this way... been too long since i had thermo and i dont want to figure out what im doing wrong... so in a nut shelll WHAT?!?!?! thats 21.8:1 compression man... completely denying all things mechanical (such as head gaskets, head bolts, ring) what kind of fuel are you running? 130 octane? does the fact that the fuel exits the pump in a counterclockwise manner down under affects its detonation?

    Im gonna say NO F'n WAY... term cobras run 8.5:1 compression and start detonating pretty bad around 16-18 psi... your gonna tell me your throwing 3 and a half mor atmospheres at a combustion chamber with no issues....

    -me

    p.s. heres the link to me totally bum fuzzling myself http://www.ls1.com/forums/showthread...340#post908340
    Last edited by predator; 08-01-2007 at 03:21 PM.

  11. #51
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    Blatant information or whatever you wanna call it, in simple terms: if you run boost with too high of compression, your waiting for disaster to happen...and soon enough it will. In '03 I ran a Vortech V-1 with 8lbs on a stock bottom end, lt1 edit professional tuning, and the result...kaboom. That's why I said I would never do a blower setup without building the bottom. Yes it's costly, but if you want a reliable motor that will last, it's the right thing to do. I don't believe in cutting corners.

  12. #52
    Senior Member predator's Avatar
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    Wait a second... Lawdog? Is it you old budy? did you pull the KB off the marauder and throw it on a GTO?

    This the engine bay pic?



    -me

  13. #53
    Laugh while you can... mistermike's Avatar
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    The gentleman in Australia runs only a few degrees of spark advance and uses a 50/50 alcohol-water spray pre blower to reduce charge temperatures and suppress detonation. It's a 2 bar speed density tune, so spark and mixture remain under control for all operating conditions. http://www.chipmaster.com.au/monaro.html

    A stock compression LS1 @ 10.2 static can tolerate about 13 PSI before the heads begin to lift due to bolt stretch. ARP head studs and a stock GM MLS gasket can take a lot more than that. It's useful to remember that the stress on parts due to compression, blower or otherwise, is insignificant compared to the pressure due to combustion. As long as there's no detonation, things are pretty predictable, but naturally at those charge densities, mistakes are costly.
    Last edited by mistermike; 08-01-2007 at 03:54 PM.

  14. #54
    Senior Member predator's Avatar
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    dude that is like 606 hp... not saying its anything to sneeze at, but a guy i know from local is pushing like 13ish psi to a camaro and he won the most hp for a long hauler on the power tour... think he laid down like 720 to the rears...

    im still curious about this 21.8:1 compression ratio...

    -me

  15. #55
    Laugh while you can... mistermike's Avatar
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    His latest dyno is 670 on that car. This is on a "heartbreaker" Dyno Dynamics, which makes a Mustang Dyno look generous. Dynojet would be about 20% higher or around 800. His build is a few years old, using regular LS1 heads ported. AFR's and such weren't available yet. Here's some of his other work. http://www.ls1.com.au/forum/showthread.php?t=56108

  16. #56
    scot w.
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    Quote Originally Posted by Rich L View Post
    I would like to chime in...

    in a nutshell...

    If you want to be fastest at the dragstrip, go with low compression high boost setup.

    You'll have an inefficient, max hp motor thats good for racing.

    If you want to maintain some streetability...

    go with higher compression and lower boost setup. You'll have a more efficient stoplight street racer.

    It is all a compromise, and there are a ton of variables to factor in when trying to consider the best setup for you.

    I would recommend you guys that are really interested in this topic to check out a book called "Maximum boost" by Corky Bell.

    It covers the matter at hand... Designing, testing and installing turbocharger systems. Its a good read and will give you alot of insight on this topic.
    Well this is my .02 worth. I don't think it's all a Compromise! I have a 10:1 or 10:5-1 compression V6 engine running as much as 25lbs boost with 93 pump gas and Methanol injection. NO DETONATION!!! I recomend to anyone that is running a turbocharger that runs on the street and takes it to the track should really think about Methanol injection very seriously. No high priced race gas, No burnt out o2 sensors, Lower inj duty cycle. A NO BRAINER! Once you install the Alky kit it alows you to run on 93 pump gas and always in KILL MODE! No more detuning for the street without racegas! Iv'e installed several kits and every customer said: "This was the best mod iv'e done, I should have done it years ago!" I would only install the progressive alky kit seeing it rise's the alky amount while the boost raise's. the kit also comes with numerous safe failures.

  17. #57
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    Gentlemen, the information in the posts before mine is largely inaccurate. If you want to learn about boost related topics, I highly suggest going to turbomustangs.com and opening the same discussion. There, 90% of the people offering advice have real experience building boosted engines, and many have a better working knowledge of the physics involved. This thread is out of control and has so much misinformation in it that I lost count of the number of posts I wanted to quote and correct. Doing so would largely be a waste of time anyway, and I don't want to get into complicated arguments that noone but the posters will read. So, here is a physicists attempt at summing up the difference between boost and compression in an elementary manner. If you want to discuss the subject in more detail email me @ Chris.Arnold@us.army.mil or come over to the turbo forums.

    First, get rid of these ideas of compression and boost being the same thing. Let's discuss and define the basics: preignition, detonation, pressure, and compression. "Effective compression" is only an idea that gives an amateur an inclination as to whether or not he's toe-ing the line of detonation. In reality, a custom engine's characteristics are so different from another's that it's effective compression is meaningless. I don't want to get overly complicated about effective compression, so suffice it to say that given a certain octane fuel, preignition and detonation/knock, does not happen at any particular effective compression ratio because air/fuel temps may not be the same at that effective compression in one motor vs. another. Theoretically, we could continue compressing our air-fuel mixture ad-nauseum and as long as we kept lowering the temperture and staying below the ignition temp of our fuel, it would never ignite.

    In reality, preignition is caused by a temperature that is high enough to ignite a fuel. This typically comes from an overly hot spark plug tip. As you know, temperature rises proportionately with pressure. If during the compression stroke the unignited air & fuel reaches its ignition temperature before the spark, it will burn and it is said that your engine has preignited. Preignition is a very bad thing because your engine is trying to continue to compress gasses that are burning and are trying to expand. The pressure in the cylinder spikes astronomically, and no gasoline-powered combustion engine built today can withstand the forces of this severity. Detonation happens similarly, but after the spark. As the flame front travels through the cylinder, pressure escalates very quickly and the unburned fuel reaches ignition temperature before the flame front ignites it.

    Now that we understand that it is temperature that causes fuel to ignite, lets talk about compression and boost

    Compression: The more you compress air and fuel before you ignite it, the more efficient its burn will be, which will produce among other things, more power, more pressure, and more heat. This happens because you have a smaller combustion chamber volume filled with the same amount burning air & fuel, which causes higher cylinder pressure throughout the entire power stroke of the engine. Therefore, an efficient engine whether on boost or off will always make more power given more static compression.

    As you can see compression in and of itself is independant from boost.

    Boost: Obviously, pressurizing the air in the intake tract adds more air-mass, and all important oxygen, into the cylinders with each stroke. Unlike upping compression, the reason boost makes more power has nothing to do with efficiency, it has to do with quantity - burning more fuel and oxygen will result in the production of more power in a given engine. The problem as already addressed is that pressurizing the air will raise temps before it's even in the cylinder. Fortunately, there are a lot of ways to drop those temps before they reach the cylinder: intercoolers, aftercoolers, methanol/water/ethanol/alcohol/nitrous injection etc...

    Now let's talk about how compression and boost affect each other. Once the fuel and air is in the cylinder we're back to trying to keep the air/fuel below its ignition temp. That's a little harder to do without detonation, though, for two reasons: 1. the air/fuel temp is already higher 2. a higher mass of air and fuel in the same volume, which will proportionately increase cylinder pressures both before and after compression. That's it, in a nutshell! That's really all you have to understand to figure out the rest.

    How does all of this affect power output of our motor? Well, if you can run high compression AND max out your power adder on the fuel you run, then go for it, that's how you'll make the most power. All of these injections on the street are designed to either increase octane, decrease intake charge temperatures or both. If you're just running plain old pump gas, you can't feasibly max out any correctly sized turbo or supercharger AND run high compession. It is most-likely always the case that you'll make more power by dropping compression and increasing the amount of air/fuel you are burning (boost) than by decreasing the air & fuel (boost) and increasing compression. The efficiency of compressing the air/fuel more does not compensate for the added amount you can burn.

    That said, do not take this information, run to your nearest engine builder and tell him you want to run a 6:1 CR 30 psi street car. Such a car would not likely even run, and dropping your CR will have adverse affects on your production of torque/power at low RPM which is a necessary component of building boost with turbos in the first place. Compression is absolutely essential to the operation of any internal combustion engine.

    I hope this helps somebody.

    Chris
    Last edited by Chris Arnold; 08-03-2007 at 06:39 PM. Reason: To correct the difference between preignition and detonation

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    Senior Member predator's Avatar
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    So is it of your opinion that setups as mentioned are streetable without "bleeding" alot of the presure off with a cam?

    I mean great right up, but you sound like a senator in a court investigation...

    -me

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    Preignition vs. Detonation

    Attached is an article by Allen Cline, who helped develop the Northstar motors and works with aircraft engines. It's the best clarification of the subject I've seen.

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    Quote Originally Posted by predator View Post
    So is it of your opinion that setups as mentioned are streetable without "bleeding" alot of the presure off with a cam?

    I mean great right up, but you sound like a senator in a court investigation...

    -me
    That is a great question! I'm not sure which setups you're specifically referring to, but I think that topic is as yet beyond my experience. I think anything can be "streetable" though. With all of the things we're able to do to bring down intake charge temps and raise octane without even running race fuel, I think almost any wild boost combination can be streetable, but it depends on your definition of streetable.

    However, the subject you want to get into is absolutely fascinating to me. I'm still trying to wrap my little brain around the physical concepts at work in the "high compression- cam bleeding" engines. I certainly believe there is something to raising compression ratio and bleeding off pressure with the cam, which is why the top dollar race engines do just that. The benefits of increasing compression ratio must outweigh the cost of bleeding off the pressure.

    Interestingly, I resurrected a thread last night on that very subject after I wrote up my post above. I think you should refer to it at turbomustangs.com. It's title is "compression vs cylinder presure, advance-more, retard-less help me understand" and it's in the "other tech questions" forum. Nobody has answered the topic to my satisfaction, yet. When someone does, I'm going to contact them in the hopes of better understanding of all the physics involved there. Who knew engines could be so interesting from a physics perspective.

    Chris

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