PDA

View Full Version : exhaust popping


dbarnes
08-21-2003, 08:32 AM
I have a 350 chevy with a 6+/blaster coil and Pertronics point replacement. I'm getting some popping out the exhaust. The heads are new Edelbrocks I put on about 6 months ago. The compression test I did then showed 170-180 psi in every cylinder.
The original base timing id 4 degrees BTC. I'm wondering if I should maybe do 6-8 degrees BTC? Or any other thoughts on popping through the exhaust?

On another note. I tried putting the car on a Sun type scope and I could see the multiple dischages on only the ODD cylinders (1,3,5,7). Any idea why this would be??

msdtech1
08-21-2003, 09:38 AM
Before you advance your initial timing you should determine what your centrifugal timing is because if you bump your initial up 2-4 degrees it is also going to affect your total timing as well. 4 degrees does sound very low for a small block chevy, and a few more degrees would probally help it but make sure you check your total timing. What is the max timing of your engine (initial timing + centrifugal timing = total timing) that you are running right know? As for the multi-spark indication on your sun scope, the MSD Ignition does not know which cylinder it is actually firing to. The distributor sends a signal to the ignition, which in turn causes the coil to fire. Each time the coil fires it is exactly like the previous one because it is just repeating the process.

MSD Tech1

dbarnes
08-21-2003, 10:01 AM
Well, according to your support FAQ, I can't use my adjustable timing light. Last time I check it (before the electronics) I think I had mid 30 degree total advance. SO, can I use the timing light I have or?? What is the base timing you guys recommend for a non-smog small block chevy?

msdtech1
08-21-2003, 10:10 AM
A non-adjustable timing light in combination with a degree tape for your harmonic balancer is the most accurate method for checking your timing. We have experienced problems with some of the dial-back/digital timing lights not being able to cope with our multi-spark feature which in turn causes fluctuations in the timing reading. If your timing light seems to be consistent throughout the rpm you should be ok. Depending on the centrifugal advance of your distributor, most SB Chevy engines run anywhere from 8 to 12 degrees initial advance.

MSD Tech1

dbarnes
08-21-2003, 10:17 AM
Well, I think I'll do another compression test (possibly a leakdown to rule out a funky exhaust valve) and check the timing again. If that checks out, i'll go from 4 BTC to 8 BTC and see what happens. Stay tuned.

msdtech1
08-21-2003, 10:28 AM
That is a good idea. If you have a leakdown tester that will point you in the right direction if it is a mechanical problem. If it is electrical let us know and we will try to help you sort it out.

MSD Tech1

dbarnes
08-25-2003, 02:22 PM
Well, compression test shows 180 PSI in each cylinder and the leakdown test is virtually identical for each cylinder (all in the GOOD range with ~10% loss in each). I hear NO noise from the exhaust during the leakdown test. SO, my popping has to be card and/or ignition. Is there a good "general" rule of thumb for mechanical and vacuum advance numbers for a mild street motor?

msdtech1
08-25-2003, 11:56 PM
For a performance street engine we normall set our distributors up with 21-25 degrees mechanical advance and set the initial timing between 8-12 degrees. Our vacuum advance canister provides 10 degrees of advance at 15lbs of vacuum.

MSD Tech1

dbarnes
08-26-2003, 08:02 AM
SO 39-47 degrees of TOTAL advance?? (Base+mechanical+vacuum)

msdtech1
08-26-2003, 12:27 PM
With 8 deg. initial + 21 mechanical = 29 degrees total
With 8 deg. initial + 25 mechanical = 33 degrees total
With 12 deg. initial + 21 mechanical = 33 degrees total
With 12 deg. initial + 25 mechanical = 37 degrees total

Please note that the vacuum advance is only added when vacuum is present. At wide open throttle position you will only have your initial plus your mechanical timing added.

MSD Tech1