[QUOTE=wadzii,Oct 12 2010, 04:54 PM] wow.. you dont understand

On a turbo motor the static compression is directly related to the amount of power you can make on a specific amount of octane.

On pump gas 8.9:1 compression on a 2 liter will be safe to 500-600hp ish.

stock compression is lucky it works as well as it does, 350-400 is about all you'll get on stock compression with pump gas, even then every motor I have torn down that was making that much power, regardless of who it was tuned by showed signs of detonation or pre-ignition on the wrist pins and rod bearings... big cams work well in alot of situations including 8.9:1 in an s2000.

 

Here, ill help with a tad bit of info.


dynamic CR.

 

Trust me I fully understand dynamic compression.

and where do you get that a dished piston destroys the quench etc whatever else you said?

the dish is designed to work with the chamber, its not just arbitrary.

low compression pistons arent for race motors, they are for street motors... BIG difference there.

You can do all that on a race motor where the fuel will be carefully monitored etc.

you are correct in that you design the setup around the dynamic compression. where do you think the limits of the compression come from? thats right, the FUEL you are using.

If you want a street car to make more than 400hp and have it live even when the gas station puts 1/2 water in your tank then you run with low compression.

Street motors are about SAFE, first and foremost. These pistons in an f20 will make 500hp on pump all day every day. 10:1 will last longer than 11:1 but what happens when you get a little bad gas, or the air filter gets a little too dirty? What happens when a vac line melts and you get some good quality boost spikes?

If i was building a race motor you're damn right it wouldnt be using 9:1 pistons.

 

Easy vegas. Wadzii is on our side lol

 

ok ok.
Thing is, ANYONE who sets up an F20C with 8.9 to 1 and stock cams
Has like 4 or 5 to 1 dynamic when under vtec.
I know your adding a ton of volume once on boost, BUT you forgot every day driving. The burn is TERRIBLE.
It ends up hard as hell to tune, gets shitty mileage, the VE is terrible and the car is a dog to drive.
Thats what im trying to say. I have nothing against wadzii, im just trying to get the point out there that.... you simply cant just do this and walk away. Its wrong no matter how you calculate it.
Thats why the 3mm and 2mm gaskets simply shouldnt be produced.
And this piston, IF it is setup properly which im sure it is You still should not drop the static below 10.
Because the dynamic will be near spot on, or a tad low with it at 10 to 1 static. (for pump gas madness)
I have yet to get anyone to send me the REAL or exact cam specs for the F20C.
But the ones i did get, i calcd the dyn and it was spot on where i build all my motors for pump gas. I mean, you know honda would have that shit spot on.
Either way, you have to realize the only time you want static that low, is when you have a fixed cam profile, a shitty chamber, and likely a shitty tuner.
Mo betta now? Not attacking wadslow, just what he was sayin. I disagree, and passionately.
Engine building and tuning is what i love most.

 

You seem to consider yourself an oracle on FI engines.

Any chance you could post your ignition and target AF tables you use to get 700hp in stock configuration on pump gas to help the collective knowledge base.

 

Im no oracle. I just build a ton of engines. Nobody i know of has gotten 700 on pump gas, it has only happened on race gas. But so far most every s2k build i have done with boost has made 500ish at the flywheel on pump gas. And they are all daily driven.
The point of the discussion is why NOT to use the spongecake gaskets. And i was elaborating on why NOT to drop the static compression with the stock cams.
So, nice try. LOL.

 

I tend to agree with what your saying.

For a race motor only, 9:1 is stupid imo. Most guys in that scene from what I've seen/heard are starting to run higher compressions. I'm talking 10.1/etc on 1000whp+ 4 cylinders.

Seems the old lower comp better is starting to fall to the wayside. However for a street driven motor, i.e. like mine, I dropped it to 9.1

I've kicked around in my head 100 different times if I should have went 10.1, and I think I could have...but I am using methanol, and I feel I can get a little more "life" out of the methanol w/the lower compression. Meaning I won't have to spray it as soon b/c the lower comp will take more boost out of the box. So effectively I broaden the range on the pump/nozzle I have.

 

will you agree with me that above a certain compression ratio (dynamic or static) gas will not be able to burn in a controlled manner right?

basic logic says that if you double the pressure pushing stuff into the motor you double the compression ratio... ISH

lets say that at 20:1 pump gas just spontaneously explodes.

if we take the static compression numbers, a 10:1 motor will work at a pressure ratio of 2.. so 14.7psi of boost.. any more and you blow shit up.

now, a 8.9:1 motor by the same logic will work up to about 19psi (doing math in my head so i may be a psi or so off)

dropping a point in compression doesnt really effect power on a turbo motor so long as the turbo is big enough... from my experience in anyway.

so figure 20hp per lb of boost.. you'll make 80 more hp on the 8.9:1 motor and still be safe.

I will not build a street car motor with anything more than 9.5:1 because most customers are not tuners, they are not builders. for the most part normal people dont recognize the signs when something is going wrong. That usually ends up with a melted motor. A little lower compression buys everyone a little more margin for error, especially when using pump gas.

the reason for not using the "sponge" headgaskets is they kill the quench. You cant have quench when there is 3mm between the top of the piston and the head. These pistons maintain piston to head clearance in the quench areas.

 

Also.. IMHO, cam selection on a turbo car is much more Dependant on exhaust back pressure than dynamic compression ratio. Dynamic compression ratio is easily fixed by more boost.