Well I'm still having a ton of problems with pinging. I can't run anything over 93 octane here, and even when my initial timing is at 6 deg, 20 mechanical advance, and 20 vacuum, I'm stilling pinging bad under load. This much retard is already making my headers glow and I don't want to go any lower.

When I got my car the engine was already built pretty hopped up. Is there a way I can determine the compression ratio? Maybe that will help me figure out my problem.

It's the 350 L48 block bored and stroked to a 383, 30 overplate
3.750" stroke, Keith black flat top pistons 10.5 compression
Trick flow 23deg heads

I have various other specs from the build sheet on the engine, but nothing that clearly lists the compression ratio. Maybe it's 10.5 like the pistons say but I'm not sure?

Anyway, any help that you guys can give would be appreciated. If you can tell me my compression ratio from that info or how to determine it I'd be eternally grateful. I'm really tired of this thing pinging, and mostly out of ideas.


Josh
defsegx2008-03-23 14:04:46

Use this link and fill in the blanks:

http://www.csgnetwork.com/compcalc.html

Scott
ranger32008-03-23 20:20:35

well, i can't claim to know it all, but, i do know roughly that an engine with 8.5-9.0 compression ratio should register about 180 psi on a compression gauge... there abouts....  as for the headers glowing, that is usually a timing issue as you've tried to adjust... i might suggest trying another carb maybe... glowing headers is usually a result of raw fuel in the exhaust burning(seen many times on our race cars using motorcycle engines) no doubt its not a chevy small block, but works the same in theory.. :) 

Determining actual comp ratio is difficult without knowing ALL of the particulars, such as bore size, stroke, piston pin height, piston shape, ring placement on the piston, chamber volume(heads), and even head gasket thickness. All of these things effect comp ratio.

Sounds like you may have a lean fuel condition, or even a vacuum leak.
How is the vacuum connected to your distributor? As in...where does it come from? Is it "ported" or direct? If you are running ported vac, try moving it to a direct source
You may also need to fatten up the fuel mixture a bit...it really sounds like it is running lean...
hth

I agree.  With flat top pistons and 64 cc heads like TrickFlows usually are, you should be at about 10.5, but that can vary greatly depending whether the heads have been milled, block decked, gasket thickness, etc.  But assuminf 10.5, you should be able to run about 35 degrees total advance, which is usually about 10 or so initial.  Glowing headers and pinging almost has to mean a very lean condition.  When and engine is modified to increase airflow like yours is, the mixture usually becomes too lean.  Try going up about four sizes on the jets and see if there is any improvement.  You don't mention carburetor, but with a modified 383, you probably need about a 750.

Good luck
Larry

Hi guys. Thanks a ton for the replies. I have a 795CFM Edelbrock 1903 Q-Jet on right now. I don't have any idea what the jetting is as I didn't set up the carb, but I've got lots of suggestions. The vacuum is also set up as ported. When I switched to direct, pinging got even worse.

So far I think I'm going to try disconnecting the vacuum and seeing if it still pings. If not, I'm going to do a few things. I'm going to richen the idle mixture screws by about 1/2 a turn since the RPMs are so low when it pings the most, I'm going to limit the vacuum to about 6 deg of advance, advance the initial to 10-15, and try to get mechanical to 14-18. This should give me around 40 total and since my cam is a decent size I'm hoping this solves my problem.

If that doesn't fix it I suppose I can try the jetting, but I really don't know anything about carbs at all.

Josh;

Have you checked for vacuum leaks?  I had a mid rise dual plane that the guy that build mine up had put on and it didn't have a flat surface on it.  It was warped at the intake runners, not level at the carb and the water neck was also off so bad no matter how much RTV you put on it, it wouldn't seal. 

The old carb cleaner trick spraying in short bursts at the gasket seams found the probs initially and I would guess you probably have done that.  Just thought I would mention it.

I have not actually. Didn't even think about this. I didn't know vacuum leaks could cause pinging to be honest. defsegx2008-03-24 11:55:21

I have said before here, get the Edelbrock book for your carb, they are online in PDF format, and go from page one until the end. They give you step by step procedeures to set this carb up, AND ITS NOT HARD. The only time we get pinging from our stroker, is when it starts getting hot, about 200 degrees or so. I run with a 165 fail open stat, and dual electric fans, and the world is fine now!

To determine the compression ratio, why can't you just do this: