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high compression vs lower compression?

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If you're shooting for 600whp then you'd be running a lot turbo lb/min flow. I'd go slight lower compression like 8.3:1. I was restricted to 27 psi on 9:1 block with Holset HX52 because the head started lifting after though. Albeit it made 598whp but I was planning to run way more boost than that.

Keep in mind, making the clean and head is surface cleanly, go to the Felpro Permatorque head gasket and use the A1 or ARP L19 head studs or else you'll regret having to spend more time and money digging in there again.
 
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A higher compression motor will make more usable power on a street car. The motor will make power lower in the RPM range and will spool a turbo faster. Your max boost level will be less on a high comp motor all other things being equal, but its power that makes a car fast, not psi.
I would rather have a motor that makes 600 hp at 10lbs over the one that makes it at 30 :cool:
 
A higher compression motor will make more usable power on a street car. The motor will make power lower in the RPM range and will spool a turbo faster. Your max boost level will be less on a high comp motor all other things being equal, but its power that makes a car fast, not psi.
I would rather have a motor that makes 600 hp at 10lbs over the one that makes it at 30 :cool:

Yeah. But when you run high compression with a turbo you are more likley to blow up your motor...:ohdamn:
 
Yeah. But when you run high compression with a turbo you are more likley to blow up your motor...:ohdamn:

not true at all. its just that different rules apply to high compression motors compared to low compression motors.
most people just take it as higher compression means its easier to blow up.

personally i think its best to keep a balance of the two.
medium boost and medium compression.

it keeps the turbo at a better efficiency and it spools up faster compared to a 8.3:1cr motor and crazy boost
 
Yeah. But when you run high compression with a turbo you are more likley to blow up your motor...:ohdamn:

Yeah, oh course if you were stupid enough to match the amount of boost you're running to the same as a lower compression engine. Even though running higher compression you're set back by the amount of boost you can run it doesn't make the car less powerful.

With my setup all clear and with FelPro PermaTorque head gasket, either A1 or ARP L19 head studs and a clean surface block that won't stop me from boosting more than 27psi with it already making 599whp. My intentions are running 40 psi+ and who knows how much power I'll be making by then.

Also, running alcohol base fuel like E85 uses less energy than the equal amount of gasoline thus you can running a higher compression block.
 
It depends on what size engine you plan on running, the turbo you use, its efficiency range and boost level.

The bottom line is the volume of air and fuel crammed into the engine makes the power. You don't need higher compression if you run high boost. The boost creates whats called dynamic pressure that adds to the static compression of the engine along with carrying more cubic feet of air than the engine can naturally pull. Higher static compression also reduces the amount of timing you can run under boost to prevent detonation.

I would shoot for 7.5-8:1 ratio, 2 liters, BW258 turbo and 35psi of boost. I am running a similar set up on my 91 GSX producing 490whp limited by the fuel system. This is using no meth/water injection or E85 blending, straight pump 93 with the 10% ethanol required in my area.

I have ran as high as 9.5:1 on a boosted motor and the small sacrifice in spool time with lower compression is well worth the top end gain and lack of tuning issues. The camshafts, cam degree setting and tune can vary the spool to an extent, so there is no solid figures you can give someone for the lack of compression due to the impact of variables. For example: advancing the intake cam will raise the cranking pressure and give a better low end spool.
 
i don't understand what you mean by this?
Also, running alcohol base fuel like E85 uses less energy than the equal amount of gasoline thus you can running a higher compression block.



ever wonder why bmws are running around 10:1CR and turbod..

and just to clear things up when i say high compression i mean 11:1

the setup im going for is a hx35, somewhere between 9.5:1 to 10:1 CR, e85 and possibly a 2.1 motor.
i won't be able to crank the boost way up but as 4sfed said lower static compression ratios i will be able to run alot more timing and fully take advantage of e85
 
Mt current build is an e-85 engine, so i went 9:1. I believe that some of the big power e-85 cars are running 11:1. If i were doing a pump gas only car 8.3:1 is a good setup. It just takes more time to tune a high cr boosted engine. By the way, the stroker guys are running way less timing than you would on a 2.0 9:1 build.
 
donniekak- are you talking about the 2.3?? i just like the idea of the better rod ratio with the 2.1, wasn't really doing it for displacement but ill take that as a bonus. i have yet to hear any flaws by design on paper with the 2.1. if anyone has any input on the subject i would like to hear it.
 
With E85 you can run more timing with higher compression. Alcohol burns cooler and the lack of heat allows higher cylinder pressures without pre-detonation. I'd feel comfortable running it with 9-9.5:1 static compression.

In my area E85 is very hard to come by at the pump. The only place I know of that has it available within 20 miles of me is military only. If it was more convenient for me to get E85, I would run it. However some precaution needs to be taken that you use alcohol friendly components and replace those that are not. I don't have any experience running a high alcohol based fuel with a factory fuel tank. Although I have heard stories of people having problems with factory tanks and fuel lines in other cars. Maybe someone here has some input on that?
 
I know a couple of local guys running 9:1CR and 35-40 psi. One turbo is a 4094 and the other is a T67. They both run 100% E-85 and 9s. They are building 10:1 CR or higher on their next motor because of the great results with E-85.

I am running 9:1CR and 30psi on an HX-35. 50/50 blend of pump gas & E-85. 21-22* timing and 12-12.5:1 AFR. No tuning issues at all. I ran that time <---- at 26 psi on the first run of the night then my clutch started slipping and I never got the tune dialed in.

Pay attention to critical details. IAT, coolant temps, plug heat range, cams used. I have a CAI in the bumper, 170* coolant temp, BR8ES (non-projected) 272s (some overlap helps reduce the cyl pressure) I have 185-190 psi on a compression test at sea level. There are other things when building a motor like quench pad, polishing the CC & piston tops, forged pistons run cooler, perched valves reduce the hot edge of the valve, blah blah. All theses things (and more) contribute to the overall reaction of a particular motor to boost, timing & CR. I am not an expert at all, but I am really able to push my setup to the ragged edge without any knock to speak of and I have done all of that to my car. I am hoping for an 11.2 @ 125-127 on my next trip to the track :D
 
I'm a big fan of splitting the difference between CR and PSI myself. Too much of either can have devistating side effects, but too low and you're sacrificing potential. My current build is a 6bolt w/ 8.3:1 pistons and a Hyundai head with 5cc smaller CC's. Should end-up being about 8.7:1CR, well above stock but still lower than a lot of newer turbo cars are running. It's gonna be a street car so the CR boost will help with driveability and off the line and it should still be able to handle higher boost (30-ish) for the top end.
 
I doesn't matter if you have a low comp engine or high comp engine. The main thing your aiming for is cylinder pressure. A 9:1 engine making say 1500psi of cylinder pressure at 20psi is the same as an 8.3:1 engine making 1500psi at 30 psi. The disadvantage of the 8.3:1 is that IAT is going to be higher because of the higher boost levels. The advantage of the higher comp engine is the static cylinder pressure. The cylinder pressure is higher at any rpm level out of boost. That is why the turbo spools faster on a higher cr engine.
 
Doesnt madder what people have done. Overall 600whp is going to need way more than 60lb/min on pump gas to make that power.

If running e85 or race fuel go high compression. If your running pump gas go low compression to save yourself a tuning disaster. Some cars just seem to never knock where others knock like no tomorrow. I would stay in the low 8's like suggested. My next motor will have very high compression, with custom pistons. E85 loves it.
 
stay away from high compression unless you're looking to always be fidgeting with tuning and trying to find some exotic fuel. if you have e85 in the area, then i think high compression is worth it, needing less boost and usually giving better mpg than low compression. you don't need as big of a turbo to make the same power, and the engine already has the potential to spool a turbo quicker, so it's win/win.
 
I'm not a member of any automotive engineering boards or anything, but I'm surprised no one has brought up the whole thermal efficiency thing with high compression builds yet...
 
Maybe, just maybe because tuners on this site want power not mileage.
Just saying.

Edit: Opps dead thread revival. No mileage in that.
 
so if your planning on building a street car with about 600 whp on gas (just useing an example) would it be better to use higher compression pistons or lower?
Hey
I have a 1998 Ls1 iron block Twin Turbomaking 650 RWHP.
Lots of rip offs in my drive to get it all to work.
My Engine is 9.5:1
All high grade components
**Due to trying NOS
Felpro Gaskets
Larger ARP Bolts
** Rocker arm train: tied all rockers together
I pulled a lifter
I used the STS rear mount Turbo
It took 3 shops & $6000 to get them to work properly
Corkey Bell should be on speed dial
5 psi now, I have beat:
Lamborgina
Ferrari
Turbo Porsche
From rolling starts, otherwise it would have been mire ca lengths
 
Hey
I have a 1998 Ls1 iron block Twin Turbomaking 650 RWHP.
Lots of rip offs in my drive to get it all to work.
My Engine is 9.5:1
All high grade components
**Due to trying NOS
Felpro Gaskets
Larger ARP Bolts
** Rocker arm train: tied all rockers together
I pulled a lifter
I used the STS rear mount Turbo
It took 3 shops & $6000 to get them to work properly
Corkey Bell should be on speed dial
5 psi now, I have beat:
Lamborgina
Ferrari
Turbo Porsche
From rolling starts, otherwise it would have been mire ca lengths
Lots of people in here making that and more with 2.0 liter 4 cylinders. I suggest you try some of the Ls based forums, or yellowbullet to learn how to maximize the power from your engine.
 
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