Art_H

You could put a tad more SCR and then use a water injection system. Water injection on stock engines has little advantage. However, jump the SCR and it helps quite a bit. No more detonation... Though WI really shines on a Forced induction setup.

As stated in earlier posts, compression is your friend. It has been said that a 1 point jump in compression (9-10:1) will result in about a 2-4% increase in power @WOT This does diminish as the ratio increases. The even better thing is that with that 1 point jump, during throttled conditions, ( where the engine is 95% of the time) gets up to a 25% jump in power. Just think what the DCR is @ idle. I have not run numbers, but I'd say around 1:1 or so, maybe less.

A good way to look at the issue is the cranking pressure.( A LA compression test ) The relationship between the cam and SCR becomes very obvious. I have an original '70 LT1 'vette engine in my boat. That came stock with 11:1. Though the cam @ .050" was 246 deg.(solid) I have never heard the thing ping once, and in a boat, it's like towing uphill all the time. With iron heads and a carb, no less.
Cranking pressure: D. Vizard wrote that a cranking pressure between 200-210 psi is possible on pump gas. What that means is that first you find the cam you want for the RPM etc. then find the SCR that will put your Cranking pressure arond 200 psi.

The thing I would do is thermally coat the chambers, and valves. Your Mahle pistons should already be coated. There is a write up by Popular Hot Rodding on a small block Chev that runs on 87 and makes 540 or so Hp. Uses therm coatings.

Do look into the water injection though, even just to find out about it. If you have the ability to alter the ECM then you can run lean with WI and it has no prob. The NOx even drops, that is normally a by product of running lean due to the extra heat.

MY 0.04

MadBill

Good points, Art!
Re idle conditions, the first time we fired up an engine with cylinder pressure sensors fitted, we thought we'd screwed up the installation, because the peak pressures at idle were barely 100 psi. When you think about it though, it doesn't take much peak pressure to make zero B.M.E.P...
When I Googled "Dynamic Compression Ratio", several of the calculators I found took a crack at cranking compression, e.g.: http://www.rbracing-rsr.com/comprAdvHD.htm but I haven't had a chance yet to do a reality check (and of course there are a lot of other variables like engine temperature, engine sealing, etc. that could affect results)

Steve Bryant

iTrader: (1)

Thanks a lot for this reference to the Engine Masters article. I don't know how I missed it, as I almost always buy the magazine, and I should just subscribe.

After reading and re reading the article it's interesting to note that they used pistons with a 29 cc dish to yield a 8.65:1 static CR. The cam they used (244/248 @ .050") would be huge for my purposes. However, a cam with this much duration will produce a lot of power and torque at and above 4,000 RPM's on a dyno.

The bottom line is that in order to run 87 Octane for a street driven vehicle, they significantly lowered the static CR. The dynamic CR would be lower I'm sure.

This makes me think that more dish would be a good thing. Any comments?

Thanks,

Steve

DAPSUPRSLO

iTrader: (14)

Sorry, have nothing to add, but this is a terrific thread which addresses many of the same questions I have too. Good work guys