spark knock

[matt rupert]

just installed a 350-30over with dart 202 heads, edelbrock air gap intake, edelbrock 650 carb, large cam-specs unknown, stock hei distributor,msd 6al, 10:1 compression, approx 400hp.running 91 octane gas. my problem is spark knock at medium acceleration. total timing is at 30 deg at 3500 rpm. should i back off the timing--should i try installing a curve kit --should i be using vac advance on a motor like this with low vacuum. any advice would be greatly appreciated

[MagicRat]

Originally Posted by matt rupert

--should i be using vac advance on a motor like this with low vacuum.

Uh oh.... what vacuum source are you using with this?

You must use the ported vacuum source for the distributor. This has no vacuum at idle, but develops progressively more vacuum as you increase throttle and RPM.

Ported vacuum is not affected by a radical camshaft, like manifold vacuum is.

If you are using a manifold vacuum source, you are getting the inverse of what you want. Manifold vacuum will give you full vacuum advance at idle, with progressively less as the throttle is opened, which cuts power. This also may be producing the spark knock.

[MrPbody]

Another question: WHICH Dart heads? Pro-1s? Iron Eagles? Chamber volume?

Jim

[matt rupert]

vacuum advance is hooked up to ported vacuum on carb, the heads are dart steel heads with some work done to them. not sure of exact specs on cam or heads, bought someone elses project that ran out of cash

[MagicRat]

Being a aficionado of Canadian gasoline, I think that the ordinary premium fuels (91 octane) may simply not be up to the task, regardless of your tuning.

In my experience, anything approx 10:1 or over often produces knock and ping on 91 octane. Also, theoretically, wedge-shaped combustion chambers like you are more susceptible to this than the hemi-heads and modern DOHC heads.

I would suggest drain as much of your 91 octane as you reasonably can (use it in another vehicle) and go to a Sunoco station and fill up with Ultra 93, and see if the situation improves. FWIW, Ultra 93 is the only gasoline I can get that makes my Ford 429 (10.5:1) truly happy.

My search shows 3 Sunoco stations in Windsor.