Seeking CR and Ignition Suggestions
 

I've been building a 3.0 fitted with domed Ross pistons, Elgin Cam grind 280/270, Webers and Pre-74 exhaust. I'm at that point of down selecting the CR and threads I've looked at here give me pause. So I'm asking for suggestions...

Intended use is street with desired low to mid torque in the 3000 - 6000 rpm range. Since I'm socal, it's 91 gas unless I swing by John Wayne for some AVGAS to mix (Nyet.)

Shown below are my calcs for CR. They indicate that significant shimming is required to get the value below 10.

I did let George at Ollies talk me into added the second plug port even though I didn't plan on twin-plugging during the build. Given gas and intended use, should I consider it?


http://forums.pelicanparts.com/uploa...1112243461.jpg:cool:

What lobe centers are the cams on?

Hi John,

110. You did these last July from SC CIS cams.

Hi Gary,
Seems to me like 10.5 and the twin plugs. IF you shim the barrels out to drop it to 9.8-1 you will lose the quench, which might create more problems then it cures.
BTW AVgas is designed to atomize at high elavations... not good for an engine operating at sea level. You should be able to find a Unocal station with 100 octane. You can spike the 91 with a little 100 and see what happens.

I ran some numbers and I get different figures than yours. (I could be wrong.) Judging from my figures it looks like you would need to increase the combustion chamber volume by 10cc to get the compression down to 9.5:1.
That's quite a bit. Looks like to do it you could:
(A) Increase deck height to 2.2mm (which is crazy IMO)
(B) Reduce piston dome volume 10cc (is there that much meat?)
(C) Increase combustion chamber volume 10cc

"B" looks like the only one with a chance of working.

http://forums.pelicanparts.com/uploa...1112275135.jpg

-Chris

Gary,

There is a Unocal that sells 100 unleaded at the pump off of the 5 freeway and La Paz in Mission Viejo. There is also a Texaco that offers high octane leaded and unleaded in the City of Orange.

I am sure there are quite a few others, but these I am familiar with.

Ralph

9.5 with single plug,
10.5 with dual plug for us octane starved Californians...

-Wayne

The pocket for the second plug should add a little volume to the chamber but whittling down the top of the piston would certainly be the best solution.

Chopping a little off the top of the dome in addition to sinking in some deep valve pockets might be able to give you the volume you need.

Alrighty then. Thank you all for your input. My course has been slightly adjusted to aim towards 10.5 CR supported by dual plug ignition. I will do some higher fidelity measurements, so today I'm off to buy some hollow-core solder. Thanks again - stay tuned.

I am a pigg.....keep the c/r up and solve the problems