Langadorf

I've been trying to adjust the timing to get rid of all my engine knock. What I am doing to test this is I get going about 30 in 3rd gear (TH-350) and then just floor it. If I'm too advanced I hear it knock and I just retard the timing until the knock stops. Currently I have 29* of mechanical advance (vacuum disconnected) all in by 3200 RPM (stiffest springs) and no knock. If I advance the total or go w/ a quicker advance I get knock.

What I'm wondering is if the carb (Holley 750 DP) is just going too lean when the secondaries just dump open causing the engine to knock. It seems when I ease into the pedal, or downshift and then floor it, I don't hear this knock even with the timing advanced more.

My engine specs are as follows:

383 LT1 (modern) running 11.7:1 compression
Holley 750 DP (stock jets)
GM Carbed Dual Plane Intake
Comp Cams CC306 w/ 1.6 RR (duration @50 230i/244e w/ lift .544i/.576e)
Aluminum Heads "Stage II" ported (flows 265i/190e @.50 w/ 2.02/1.6 valves Hooker LT Headers
Dual Flowmaster Exhaust
3.55 rear end
2500 stall
TH-350 trans
160 deg thermostat
MSD 6AL
Meziere electric water pump

It seems I can run around 34* timing w/ a quicker advance if I'm not flooring it while cruising in 3rd gear. Is it possible that I'm getting knock still but just can't hear it over the engine?

Does this sound like too much timing or too lean a fuel mixture?

Thanks!

Vetterodder

You said that you have 29º of mechanical advance but you didn't say how much initial you have. The combination is the total and that is the figure that matters the most. Typically, the "heavy" springs delay max mechanical advance until 4,000 rpm or higher, while the lightest springs allow max at 2,500-3,000 rpm. I suspect that you might not be revving it high enough to get an accurate reading with the heavy springs. For a baseline, rev it to 4.5k (vac advance disconnected) and set your total to 36º. Pinging is load sensitive and is more likely to occur when the engine is straining the most, like at full throttle/mid rpms. It may also occur under light throttle which allows the vacuum advance to be fully deployed with the mechanical advance at or near total.

HrdAtWrk

I'd try going to the airport and filling up your gastank with some 130 octane avgas. Either that or a couple bottles of 104+ octane booster. :D

Langadorf

When I said 29* mechanical advance, I was refering to TOTAL mechanical advance (initial+centrifugal). I have run the engine up to 4000 RPM checking the timing and it's at 29* during 3500RPM and up so I'm pretty sure it's pegged out.

If I set it to 36* total I get engine ping/knock.

Matt Gruber

with 11.7:1 you are pushing the limits with a 383.
29 sounds fine.
an A/F gauge would likely show a lean condition in one or 2 cylinders.
if you floor a th350 or any auto. at 30 it should go into 1st gear, this could be part of the problem

Langadorf

I have the kick-down cable disconnected. I prefer to manually shift down when needed.

70PurpleLS7

Throughout this whole topic, unless I missed it, you don't mention anything about what type of gas your running - premium or regular? 29 degrees total I would guess would have the car running like a dog once the rev's come up.

With 11.7 compression anything but premium fuel is gonna ping when you get aggressive. BTW - check the plugs to indicate whether your too lean.


[Modified by 70PurpleLS7, 12:03 PM 3/17/2003]

Matt Gruber

i've got 29 with 9.7:1 iron heads and 87 octane.
runs strong thru 5700.

my trans downshifts anything over 1/2 throttle, at low to medium speeds.
for a while it was downshifting at 1/4 throttle! it was "anticipating" that i'd like WOT soon.
was a lot of fun, then i think the modulator went bad, and i never felt the need to change it.

The Money Pit

What does Comp Cams say about the cam you have in relation to the CR you're running?Might be a bit too small for 11.7 CR. You could pull a few plugs to determine if you're running too lean.
For what it's worth I've been messing with my distributor curve too,and have settled on 15 initial,36 total in by 3200.I haven't hooked up the vacuum advance yet,but no pinging here.
I run just over 10.1 to 1 CR and with AFR aluminum heads and I noticed a difference between the XE-274/XE-284 cam.The XE-274 (w/1.60 rockers)was close to what you're running and seemed to have more low end punch,as you would expect.