What is the upper limit with compression?

[neilca]

I am building a 2.4 liter engine for my race car. It will be twin plugged. DC70 cams. I thought I had some good pistons and biral cylinders to use, but now I question if I want to use them. The compression ratio I have measured is 10.7:1 and the bore is 84mm. To get the most out of this engine I want to run 86mm pistons above 12:1 compression. I know Nickies are the right answer but my wallet doesn't agree. What can you run on birals? Can I bore the factory ones to 86mm?

[tadd]

Can you jump to 90 mm? Birals go out of round real bad. The four cylinder guys don't even bother and use(d) cast iron. Maybe Supertec has some selected QSC cylinders for reasonable?

On the other note, 13:1 on 100 octane and twin plugs is quite doable.

Someone needs to build a propane race car with liquid injection (temperature sensitive rating).

I've heard of 12:1 on pump gas in a street motor. It's all about cam selection, and your going to want a lot.

t

[neilca]

90mm would knock me out of the class, I am limited to 2.499 liters. AA Performance has 86 mm birals new for $500 get a set of 12.5:1 J&E's for just over $1000.

Now the decision point. Can I run pump gas at 10.7:1 with the DC70 cams? I had planned on running race gas (106 oct) @ $9.50 a gallon. Will jumping to 12.5:1 be worth the extra expense?

[HawgRyder]

If you look up Bill Elliot (NASCAR) he once ran a V8 up to 17:1 on race gas.
There were special mods done to to the pistons in the area of the land in between the first and second ring.
This may apply to our engines, or not, but it does show that with ingenuity it can be done.
I think he had almost a 25% advantage on the competition for a short time....and then they banned it.
Bob

[AlfonsoR]

^^ the Bill Elliot stuff sounds interesting. Thanks.

[tadd]

Neilca:
Didnt porsche do a short stroke 2.5, like 89x66? My math says 90x66 is 2519 cc. Thats 3.3 cc per cylinder... something couldn't be fugded to make that up? Im told the nickel silicon carbide can be put on quite thick and be stable. A bore of 89.5 mm on a 66 crank is 2,491 cc. Thats 0.25 mm extra thickness.

Or perhaps a 65 mm stroke since you want to use GT3 rods?

It would let you use all aluminum cylinders for reasonable.

t

[Flieger]

While the shorter 65mm stroke would be nice and allow a better rod ratio, Porsche did, in fact, make an 89.5mm bore for use with the 66mm stroke, IIRC. They raced the ST in the "under 2.5 liter" class at one point.

[neilca]

Tadd,

I looked at a short stroke motor when I started but decided I wanted the long stroke to get me off the corners. I tend to look at the area under the torque curve rather than peak HP.

I did some checking and with the DC70 cams and my 10.7:1 pistons I have a piston to valve clearance problem of about 0.200". I also measured the piston to wall clearance and it was 0.007". Looks like I am going to have to do something different.

I looked at AA Products birals new 86mm are $499, QSC Nikisils are $980, I am getting a quote on boring my birals to 86mm.

I also have been in contact with Ross Pistons in El Segundo, they are willing to work with me on developing a piston that will work. Apparently, they have some software that you send them a mold of your combustion chamber and cam specs and they can develop a piston. I was thinking I would like to then take their model and have a stereolythe part made mock up th engine and varify everything clears and I get the compression I want.

All it takes is time and money.

[tadd]

Neilca:
You'd have to prove that the long stroke has more torque than the short stroke. Yes, it is a 4 mm stroke difference, but it is a only 4 mm difference.

Birals have a long history of going out of round from thermal stress. It only gets worse once they are cut thinner. If you can swing the QSCs that would be the way to go IMHO.

t