Highest compression I can run at 8 psi boost?

[douglas vincent]

So I am about 3 weeks away from getting another engine to play with. I want to bump the compression up as high as I can AND run 8psi boost w/out blowing it up. Race gas is not out of the question.

I would like to stick in some 10.5-11 to 1 pistons and start the supercharger off at about 4 psi and then work it up to 8 psi. With that high of of compression, I figure I could hit up to 250 plus chp at 8 psi.

Don't tell me to turbo. I could have done that a long time ago.

I just want to have that huge amount of torque on demand at the rallycross. Right now there are few cars out there that can beat me power to ground wise but they are out there. If I could at another 60 hp, I would be in the winners circle (providing I dont eat 11 cones like last time).

And as for reliability, I dont really care. If the engine lasts a season thats fine. If it lasts half a season thats fine. I just want it to last for a few hard runs. And then I can learn from it. There is mathamatics, reasoning, prediction and reality. I go for reality since I don't have the money or patience for the previous, although I still arn't two stuupid.

The current supercharged setup is running great at 4-5 psi and while "quick" still needs some serious working over to get it up to 8 psi and that extra 30ish hp that it is seriously lacking.

[vrg3]

I think it depends a lot on two factors: engine management and intercooling. A Roots blower, high boost, and high compression add up to a great risk of detonation.

Higher compression is likely to require less spark advance under most conditions for good combustion. You really should have some way to tune ignition if you're going to modify compression. The alternative is to just dump in enough fuel to stave off detonation, which might kind of work suboptimally, or might not work if you're too far off.

Unless you have a really good intercooling setup, I don't know if it'll be possible to prevent detonation with 10.5:1 compression and 8 psi from a Roots-type supercharger, even on race gas.

Some people might suggest you accept a certain amount of detonation as the price of performance. But I don't know if you could reasonably expect the motor to last even one hard rallycross run that way.

This is all just me guessing though. I'm eager to see your results if/when you try this.

[douglas vincent]

I run the stock Subaru AWIC for the intercooler. I will have the 430 cc injectors (or whatever the turbo/australian ones are) over the stock NA ones. I have, just to start off, the rrfpr, and the walbro 255 lph fuel pump, and the J&S safeguard, just to start off.

Hopefulley this will get me running at least at high compression and low boost. And then if it runs, and it will by god!, I can spend more money and dyno tune it with an actual program.

[vrg3]

Oh, that's great that you have a J&S Safeguard. That'll give you a good extra bit of protection against excessive knock.

I think the Australian RS injectors might actually flow the same as EJ22T injectors. Not sure about that, though. Do you care at all about drivability? Because I don't think the ECU will be able to smoothly deal with 30%-50% extra fuel at lower loads.

Can the Safeguard be set up with some kind of base retard, so it always retards timing a little?

[THAWA]

wooooooohoooooooooo! some form of timing control!

anyway, if I may suggest once again having the drive belt set to the correct tension, and having some form of boost control, a $15 MBC would work perfectly, and you could make it yourself . I don't know if you've got one yet but a boost gauge should definately be in the car. What ratio is the rrfpr at? If you're wanting to tune it on a dyno you should really look into getting a stand alone, there's only so much the stock ecu can do.

[vrg3]

How would you make a $15 MBC for a belt-driven supercharger?

[Brat4by4]

Have you looked into water injection? This would stabilize your flame front similar to race gas. I don't know if I would up the compression, though. Seems like you would have to start backing your timing to atdc

There is boost - compression - timing. More of one means less of the other. For the most torque I would look at dropping boost to keep as much compression and timing as possible. Upping the compression would mean you need to drop both of the others. I'm not trying to explain it to you, I'm just trying to look at it a different way.

I vote for stock compression and more timing with water injection keeping the engine from detonating to death.

[Matt Monson]

brat4by4 read my mind. Most of the guys who have boosted the 1.8l blocks over the years went with water injection prior to intercooling. Seriously consider water injection for your rig. But I still think 10.5:1 is about the max you can run on your ghetto EM...

[THAWA]

How would you make a $15 MBC for a belt-driven supercharger?

I would assume the same way for an exhaust driven turbo. Eatons have wastegates right?

[vrg3]

I don't think so... They don't have any exhaust gas to waste.

They have bypass valves, but those aren't used to control boost, I don't think -- they're just to bypass the restriction of a slow-moving supercharger when you're idling and stuff.

I think boost is pretty much a function of the speed of the supercharger, so the only way to adjust boost is to change the pulley ratio.

[douglas vincent]

The SC is clutch controlled but you still need a blowoff valve, or an air bypass valve like I do now. When your manifold pressure is negative, the SC is off. When you give it enough gas to pop the manifold pressure up towards zero, the SC clutch kicks on.

If and when i do this, I will not be running stock EM or timing, or at least after I start boosting it over the stock level the SC puts out with the stock crank pulley, a big 2 psi with the intercooler in place.

And if I blow it up, well, I'll just rebuild it. Getting better now. I am going to hit 12 hours now on a rebuild now. Thats 12 hours to jack the car up, yank the engine, pull the heads and clutch, pull pistons and rering it, reinstall rebuilt heads, install clutch, all the various parts and reinstall and drive away. In about 12 hours. I suspect I could get it down to 10 hours and just one day but I would have to have EVERYTHING I needed sitting right there.

[evolutionmovement]

What do you use to pull the piston pins?

Steve

[douglas vincent]

I took a piece of 1/4 steel bar and bent and ground a hook in the end.

If you need one, and don't want to try to make your own, I would make you one for shipping costs plus $10.

[evolutionmovement]

Cool, thanks, but I was just wondering if you had the special tool or whatever it is they sell. That's a fast rebuild. You've got me wondering how quickly I could do it.

I remember reading about a (supposedly) reliable Porsche 911 turbo that a guy was running with high compression. I remember he had a lot of money in engine mgt, but I don't recall there being any water injection. I'd try to find the article, but I know it's buried. For what it's worth I'd go with 10.5 and work boost up from there, but I tend to be cautious - especially with new custom pistons. Then again, if you get some tougher ones they may be able to survive the required playing around with boost.

Steve

[dzx]

s-afc

[Matt Monson]

What do you use to pull the piston pins?

Steve

I use a bent piece of wire clothes hanger...

[evolutionmovement]

Seriously? Like a viagra user with an expired prescription, I wouldn't think it would be stiff enough.

Steve

[-K-]

I know there are a good amount of 2.5RS owners with turbo kits. I think most of the kits run 8psi or a little less with just a rrfpr at 9.5-10:1 comp. They last at least a while... I would start at 10:1 and see how it goes. Are you going to get forged pistons?