A couple more L78 set up questions.....

[Joel T.]

I've made some progress on get my NON-Stock L78 set up... The biggest difference from stock is that the car is running a non-stock cam and therefore is making only about 10" Hg at idle. So here is what I have done.. as well as a few more questions. Bottom line is that I am getting closer I believe. The car is running much better and doing what I expect it should..

1 - Replaced the VAC to one of those adjustable ones, set it with a Mightvac to start to pull in at 6" and be fully advanced at 8".. Gives me Duke's 2" rule
2 - Fixed the carb so as to have manifold vacuum versus ported....
3 - Have basic timing set at 8 degrees per specs

Here are the questions...
1 - How much advance should I get from the VAC when full engaged? Right now I am getting about 20 degrees.... I have no idea if that is good or bad
2 - Power Valve... Is there a rule as to which power valve to use? What's in there now kicks in when vacuum falls below 8.5" which is pretty close to idle vacuum... Someone suggested a 4.5" valve..
3 - Total advance - I'm seeing about 42 degrees (from TDC) when I am running the engine over 3000 rpm, at which point the advance appears to be all in... Is this OK? Again, I am being told that 38 degrees is the magic number...

Thanks!!

Joel

[Duke W.]

Re: A couple more L78 set up questions.....

That much overlap is going to want 38-34 degrees total idle timing for best quality idle. I think you'd be better off with a B28 VAC that provides 16 deg. at 8". Increase intial timing to 12-14 and limit centrifugal advance, if necessary.

Big blocks seem to like about 40-42 degrees total WOT advance assuming it doesn't detonate with that much.

Suggest you install a B28, set the total WOT timing to 40, then try some lighter springs.

Some guys think you rev the engine to 3000 and set the total, but if the total centrifugal doesn't come in until 5000 the engine is way underadvanced. It amazing the clueless, moronic advice in the Web. You have to keep increasing revs while observing with a timing light until it's clear that it is no longer advancing. This data is an absolute requirement before you start messing with the spark advance map. You have to know what your starting with.

My other suggestion - get rid of the high overlap cam and install an OE cam. That is a great high performance road cam that pulls about 14" @ 900 idle, has decent low end torque and reasonable fuel economy.

With head massaging, headers, and open exhaust it will pull close to 500 SAE gross HP on a lab dyno, which is only about 10 percent less than a similarly configured L-88, but it's actually pretty docile and easy to drive around.

Strokers with the OE cam retarded 4-6 degrees also make a nice combination - quite docile, but way more torque than the tires can put to the ground, however, some guys like it that way.

As far as power valves are concerned. I think it's a matter of finding the lowest vacuum rating that delivers good part throttle response and smooth acceleration. This can have a big impact on fuel economy, too. The lower the vacuum rating you can get away with, the better the around town fuel economy. But gas hasn't even his five bucks a gallon yet in most parts of the country, so who cares if their big block can get past 10 MPG.

I think most OE power valves are 6.5"

Duke