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  1. #1
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    Compression confusion

    Can anyone tell me why I need to build a low compression motor 8.5-1or so to run a supercharger on a small block Chevy motor. And yet I can buy a supercharger for a 2010 camaro with compression around 10.4-1 and apparently have no problems.

    ''Courage is being scared to death
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    Anthony


  2. #2
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    Modern Ls engine use aluminum heads (which don't trap as much heat) and super efficient combustion chambers that let you run less ignition advance to light the flame. The more efficient chambers allow you to increase compression ratios without detonation, and that includes forced induction as well.
    You could try to run 9:1 on an old 350 chevy with aluminum heads. If you want to limit boost to 6-7 psi then you can probably make 9.5:1 work. BUT you'll make more power with less compression and more boost.
    Greg
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  3. #3
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    Quote Originally Posted by anthonys 69 View Post
    Can anyone tell me why I need to build a low compression motor 8.5-1or so to run a supercharger on a small block Chevy motor. And yet I can buy a supercharger for a 2010 camaro with compression around 10.4-1 and apparently have no problems.
    I'm with you..I'm still trying to figure out the details. I get that aluminum heads help reduce cylinder heat allowing higher compression as well as combustion chamber design, but when I'm running AFR's 185 aluminum heads on the "old school windsor SBF", 9:1 is about all that you can go with 91oct and 12 psig boost intercooled at 24deg timing for peak HP. Now that I'm running an LQ9 with AFR 215 LSx aluminum heads and 8.6:1CR, people are saying that is way too low? The factory ZR1 LS9 is 9:1 with 10.5 psi boost intercooled. I think I'm good running up to 15 psig intercooled boost reliably on 8.6:1.

    LS cars running 10.5:1 can only run 5-6psi boost on 91 oct and must run an intercooler to prevent detonation where as a 9:1 at 5-6psig need not run an intercooler. Bumping back up to 10psig will require an intercooler again. Of course if you have to pull back timing to prevent detonation, this reduces horsepower.

    The way I see it, people are keeping the higher compression for the crisp off boost response and mildly boosting (5 psi with intercooler) for the top end HP. This does come at a cost..timing is being pulled to prevent detonation. This is where I'm not sure by how much as I'm new to LS engines. Down to 18 deg under boost?
    Ron DeRaad
    68 Camaro RSx
    Darton Sleeved LS9 - 434ci (4.155x4.00)
    AFR LSX245 Heads (12:1cr)
    660hp/588tq

  4. #4
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    OK big thing in tuning, air/fuel ratio and exhaust gas temps per cylinder. while cruising you can live in the 13 to 13.5 range under boost if you keep the AFR in the 11 to 11.5 to one you will have little issue, intercooler is good for boosted no matter the compression, also if you go looking at www.turbomirage.com/water you can source parts for an inexpensive water/methanol injection system.
    Honestly if you use a little common sense and have a knock sensor in place and watch that AND if you have water/meth(ie washer fluid for cheap skates ) system in place.
    tuning is the key and which system you are using for fuel control. fuel injection can be easier to tune on the go, and if you use a Holley carb go to the turboforums.com and look up "the Crutch " carbs, which are basically externally activated power valved carbs.
    As for me I have driven 8.5 to one, properly tunes LS1s and they seemed to be pretty crisp on take off but then they used the proper gearing and converter to make car a pleasure to drive. Boost would just make tons of torque.
    I am working on my 302 Chevy to run good off boost till I get my turbo setup built.
    Because I am building a short stroke engine I am also looking to do it cheap and so I am trying to get a set of decent used STD 350 pistons, Ill trim them to fit, and use 6 " rods and bore my 283 over 0.125" THEN use a set of double hump heads too keep the compression up. Im figuring 10 to 15 psi with two small turbos. intercooler and Megasquirt and my own DIS setup.
    Lee Abel
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  5. #5
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    In this months issue of camaro performers there’s a 2010 camaro with 10.4-1 compression running a whipple supercharger at 9.2 psi of boost and a 100hp shot of nitrous. so I check this web site http://www.google.com/url?sa=t&rct=j...Rgz_wEgnqQhgto to see what kind of static compression it would have, this web site recommends not going over 12.4 static compression, and this car would be closer to 16.5-17 to 1. And the # this site is giving is about the same as others. What gives my sbc has 8.5-1 so can I give it 16 psi
    ''Courage is being scared to death
    - But saddling up anyway''.......John Wayne...

    Anthony

  6. #6
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    Quote Originally Posted by anthonys 69 View Post
    In this months issue of camaro performers there’s a 2010 camaro with 10.4-1 compression running a whipple supercharger at 9.2 psi of boost and a 100hp shot of nitrous. so I check this web site http://www.google.com/url?sa=t&rct=j...Rgz_wEgnqQhgto to see what kind of static compression it would have, this web site recommends not going over 12.4 static compression, and this car would be closer to 16.5-17 to 1. And the # this site is giving is about the same as others. What gives my sbc has 8.5-1 so can I give it 16 psi
    The nitrous will help cool the intake charge and as MonzaRacer stated, tune is key..

    It's all about controlling detonation. Many contributing factors come into play. Compression, Timing, Boost level, Fuel used, Heads Used (Alum, Chamber Design), Coatings, A/F ratio, Intake Temperature,..yada yada.. You need to look at all the details of the engine before thinking the engine can run max power on 91 oct. You can easily detonate any boosted engine at 8.5:1 if not set-up properly.

    GM designed the LS9 to be very reliable at 9:1 at 10psi on 91oct. The higher you push the envelope without looking at the contributing factors of detonation, the likelihood of engine meltdown increases.

    My Ford suffered melt down with only 2 deg timing added. That coupled with a hot day equaled disaster. Before my mistake of the timing, the little 302 at 800hp was kicking but. I told myself that my next engine will have a little more safety margin built into it.
    Ron DeRaad
    68 Camaro RSx
    Darton Sleeved LS9 - 434ci (4.155x4.00)
    AFR LSX245 Heads (12:1cr)
    660hp/588tq

  7. #7
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    I’ve been studding this for sometime and I understand that to much timing , lean mixture, to high a spark plug range or intake air temp even carbon build up can cause detonation and piston melt down. That engine is running hypereutectic piston and making a lot of hp the combustion pressure at ignition must be enormous to make that kind of hp. The only thing I can think of. if you mean by tune is the computers capability to control timing and mixture with the aid of knock sensors and o2 sensors so precisely and fast it saves the motor. Which could never be duplicated with a carburetor. A few year back You were the man if you could make 1hp per ci of motor ‘350 ci making 350 hp’ this engine is making almost 2hp per ci. I know the ls platform is a great motor but I still cant get over the enormous pressures that engine must be generating. So I’ve got a sbc 406 ci motor, magnachager tvs2300 supercharger, edelbrock e Tec cylinder heads [ aluminum ] bell intercooler, fuel injected, forged pistons, running 8.5-1 compression. once I’ve got my fuel management system [recommendation would be appreciated]. What kind of boost do you think I could safely put to the motor. That is the question??????? thanks
    ''Courage is being scared to death
    - But saddling up anyway''.......John Wayne...

    Anthony

  8. #8
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    Quote Originally Posted by anthonys 69 View Post
    I’ve been studding this for sometime and I understand that to much timing , lean mixture, to high a spark plug range or intake air temp even carbon build up can cause detonation and piston melt down. That engine is running hypereutectic piston and making a lot of hp the combustion pressure at ignition must be enormous to make that kind of hp. The only thing I can think of. if you mean by tune is the computers capability to control timing and mixture with the aid of knock sensors and o2 sensors so precisely and fast it saves the motor. Which could never be duplicated with a carburetor. A few year back You were the man if you could make 1hp per ci of motor ‘350 ci making 350 hp’ this engine is making almost 2hp per ci. I know the ls platform is a great motor but I still cant get over the enormous pressures that engine must be generating. So I’ve got a sbc 406 ci motor, magnachager tvs2300 supercharger, edelbrock e Tec cylinder heads [ aluminum ] bell intercooler, fuel injected, forged pistons, running 8.5-1 compression. once I’ve got my fuel management system [recommendation would be appreciated]. What kind of boost do you think I could safely put to the motor. That is the question??????? thanks
    People will throw a little richer A/F ratio to help cool the cylinder to prevent detonation but at the same token horsepower will suffer. Knock sensors are good to have when you are on the edge of detonation to back out timing. Tuning with wideband O2/Lamda is a must. I was running 2.6hp/cu and I didn't have knock sensors on mine and I went from 24 to 26 deg timing. Bad move..I didn't hear any pinging what so ever and horsepower was very strong at 11.5 A/F. I didn't increase the timing for horsepower purposes but for egt reduction. I was running 1700deg and was afraid I was going to melt the turbos not to mention cooking my ceramic coating. Increasing timing reduced egt to 1500 but the heat goes right back in the cylinder. DOH!

    There are lots of good FI systems out there (Accel DFI, Fast XFI are the ones I'm familiar with) Find one you can get the most local support on. Helps when you have someone local you can talk to.

    As for your engine, i'm thinking 8-10psi with intercooler. Not saying you can't, but 15psi might be pushing it on 91oct if you are looking for safe. (You are 93oct back east?)

    Although it's twin turbo, it's good boost related info for SBC. pg23 & 26
    http://static.bankspower.com/Twin-Tu...PowerGuide.pdf
    Ron DeRaad
    68 Camaro RSx
    Darton Sleeved LS9 - 434ci (4.155x4.00)
    AFR LSX245 Heads (12:1cr)
    660hp/588tq

  9. #9
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    How many people actually run a knock sensor on an aftermarket engine management system? All the factory supercharged (or turbo) engines that I know of use them, and that's how they get away with higher static compression. They will pull timing and/or add fuel automatically at the first hint of detonation.

    Ray Kaufman - Wyotech Chassis Fab and High Performance Instructor. Words of Wisdom from an old master... at Asylum Custom Interiors website

  10. #10
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    Tune is key. It doesn't matter if you have hypereutic pistons, forged or whatever it may be, if the tune is out of whack then then engine will suffer. I am running the "crutch" style carb on my setup. I am running a stock engine at 10 psi. I still have to finish tuning just have to find time. With 8:5:1 you should be able to run 16 psi easy as long as the tune is correct.
    David Butler
    "If you're not first, you're last"
    67 Camaro Twin Turbo SBC (changing from Blow-thru to Holley HP EFI)

  11. #11
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    I'm no expert, but from what I've read...

    The old adage, "there's no replacement for displacement" is mostly true. This is because most NA engines are capable of 1.1- 1.3 ft/lbs of torque per cubic inch of displacement. Some newer designs and race engines are getting above that number. Most of our SBC engines make 1.0 -1.2 ft/lbs torque per cubic inch. High horsepower engines shift the torque band higher in the power band since HP is simply a function of torque*RPM/5252.

    The above torque figures are with an engine running at normal atmosphereic pressure of 14.7 lbs/sq in. The easiest way to get an engine to make more torque per cubic inch is to increase the air pressure the engine operates in. To increase the air pressure, we used forced induction. Forced induction makes the engine think it is an bigger air pump than it really is. On a typical 350 engine, 7.5 pounds of boost will simulate a 525 cubic engine. Give a 350 14.7 - 15 lbs of boost and it is capable of consuming as much air as a 700 CID NA engine. Given normal atmosphereic pressure is 14.7 lbs, add 14.7 lbs of boost doubles the amount of air an engine is taking in. 29.4 lbs of boost is tripling the amount of air an engine takes in. You can do the math.

    In the real world, the engine can handle XX amount of cylinder pressure. Force too much air in and the engines goes south pretty quickly. So, the trick is to give it as much boost as possible and keep the combustion pressure below critical mass. This is why lower CR allow more boost.

    Back to your question, should you use 8.5 or 10:1 compression. It depends on how much boost you want to give the engine or in power terms, how much torque per cubic inch you want to make. The lower the compression ratio, the more boost you can give it before combustion pressures get too high.

  12. #12
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    Quote Originally Posted by exwestracer View Post
    How many people actually run a knock sensor on an aftermarket engine management system? All the factory supercharged (or turbo) engines that I know of use them, and that's how they get away with higher static compression. They will pull timing and/or add fuel automatically at the first hint of detonation.

    I was looking at adding one to my Accel DFI Gen 7 system. And should have to the Ford. Most likely would haved saved it from meltdown. Now that I have the LQ9 at 8.6:1 and 10psig-intercooled, I'm expecting to have more than enough safety margin built-in and not needing one. If I was running 10:1, I would definetly add it. But then I would need larger turbos cus that would go beyond the 800-840hp the turbos are good for. I think they will be max'd at the 8.6:1. Or at least I'm hoping.
    Ron DeRaad
    68 Camaro RSx
    Darton Sleeved LS9 - 434ci (4.155x4.00)
    AFR LSX245 Heads (12:1cr)
    660hp/588tq

  13. #13
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    If atmospheric air pressure 14.7 psi and a pressure gauge is zeroed to show 0. At 9.2 psi of boost on the camaro in the camaro performer magazine is putting over 50% more air into the engine than the engine is designed for. That’s a lot of pressure, if 12.4 to 1 static pressure used to be the safe limit. [That’s the compression ratio plus boost pressure] and he’s running 16.5 -17 to 1 what is the max safe limit now. Without tune being an issue
    ''Courage is being scared to death
    - But saddling up anyway''.......John Wayne...

    Anthony

  14. #14
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    This is making for an interesting read thanks for the information and learning experience
    ''Courage is being scared to death
    - But saddling up anyway''.......John Wayne...

    Anthony

  15. #15
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    Quote Originally Posted by anthonys 69 View Post
    If atmospheric air pressure 14.7 psi and a pressure gauge is zeroed to show 0. At 9.2 psi of boost on the camaro in the camaro performer magazine is putting over 50% more air into the engine than the engine is designed for. That’s a lot of pressure, if 12.4 to 1 static pressure used to be the safe limit. [That’s the compression ratio plus boost pressure] and he’s running 16.5 -17 to 1 what is the max safe limit now. Without tune being an issue
    “Safe limit” is relative to a given engine. What is safe to one may not be to another. GM’s safe (reliable) on the LS9 is 10.5psig on 9:1CR (~13-14:1 under boost). If you use knock control and intake temp timing retard functions, it helps squeeze more power when the conditions are favorable. If you get a bad tank of gas or run in Vegas on 115deg day, power will be decreased as the computer backs off gobs of timing. Reminds me of the twin turbo Mosler in the desert doing the mile pull on The Car Show. Morning was strong, by afternoon was no power due to the heat.

    In favorable condition 16-17:1 will be safe to an engine that is designed for it. During unfavorable conditions, the computer will back out tons of timing via knock and high intake temp, etc.

    I was running 15:1 (under boost) on the Ford 302 and 91oct and it was just fine. This didn’t leave much room for error as I found out the hard way.

    I for one am done with pushing the envelope and will stay around the GM safe range. If I get to my 800hp turbo limit, I'm good.
    Ron DeRaad
    68 Camaro RSx
    Darton Sleeved LS9 - 434ci (4.155x4.00)
    AFR LSX245 Heads (12:1cr)
    660hp/588tq

  16. #16
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    Thank Ron are there aftermarket fuel management system designed with boost in mind that can control timing like that. What fuel system are you running. thanks
    ''Courage is being scared to death
    - But saddling up anyway''.......John Wayne...

    Anthony

  17. #17
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    Absolutely, many MAP sensor options with stand alone FI systems(1,2,3bar..etc)

    I run the Accel DFI Gen 7 and it has the ability to run a knock sensor.
    Ron DeRaad
    68 Camaro RSx
    Darton Sleeved LS9 - 434ci (4.155x4.00)
    AFR LSX245 Heads (12:1cr)
    660hp/588tq

  18. #18
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    cool thanks ill look int accel
    ''Courage is being scared to death
    - But saddling up anyway''.......John Wayne...

    Anthony

  19. #19
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    http://www.theturboforums.com/smf/th...art-2-to-come/

    This guy built crap and ran like stink, but it was tuned. and too many people get greedy. boost is addictive. Figure this, the guy made big numbers due to tuning, and a scrap engine, while extra compression is one thing, your getting greedy, turbos or superchargers are not included WITH compression, they ARE compression, but adjustable. Lets say all you have is 86-87 octane how do you take away from 10 to 1 or 11 to 1 compression? So you can rattle your engine till it blows? Heck no. On a turbo car you dial in more water/meth injection and less timing and turn turbo wastegates down nd still live and drive with little or no issues. And again being able to tune for boosted wideband numbers in the 11 t o1 to 11.5 to 1 AFR or even richer for safety.
    Marty , on his grenade had lots of chassis dyno time to dial in his numbers and it didnt take him long after he had the base to keep it in tune.
    He found it had a narrow window of timing wants, and it still made ludicrous power for the limited amount of parts he used. Read the first 20 ish pages, you will be blown away on his car.
    Lee Abel
    AFTERMARKET PERFORMANCE

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    1978 C10 Long bed , On air and trailer puller
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  20. #20
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    thanks ill take a look
    ''Courage is being scared to death
    - But saddling up anyway''.......John Wayne...

    Anthony