Does anyone else's SC pop this much on decel?

[Hotwatermusic]

I have an '83 SC engine (w/ P's and C's bumped out to 3.2L) in a '74 chassis. Flowmaster muffler. Normal CIS. No cat. Normal heat exchangers.
Every time I lift off the throttle at anything over 3k, I get a backrap that almost sounds like a machine gun. Just a rapid series of pops and crackles. If I take it out of gear, it doesn't do it. Does anyone else's car pop this much on throttle lift-off? It might be hard to hear, but listen as it bounces off the wall of the exit ramp.


http://www.youtube.com/watch?v=VS9Y4mj39Bs

[timmy2]

Yes... Sounds great!

[James Brown]

combination of several factors, negative torque (car driving engine instead of engine driving car), CIS, and the flowmaster. You paid for it, enjoy!!!!

[Joe Bob]

"Usually" backfire and popping on decel is an an exhaust leak.

[porsche930dude]

if your deceleration valve is unplugged it will do that .

[Hotwatermusic]

I don't know if my decel valve is unplugged, but if I'm not doing any damage then I'm going to leave it as is. I love the sound but a little concerned is was TOO much and something was wrong. Thanks guys.

[scarceller]

If you have any sort of open exhaust you will hear this on decel. Reason for this is that on decel the CIS cars still fuel the engine slightly and under this very high vacuum scenario you have very small amount of air fuel mixture entering the cyl. Here the flame front speed is significantly reduced and requires tons of extra ignition timing to compensate for the slow flame front speed but the CIS cars don't have programmable ignition timing so the spark plug fires a bit to late for the given condition, the end result is that air/fuel is still burning when the exhaust valve opens.

I know this issue well as I have removed this exact problem area from my 84 Carrera by adding tons of ignition timing at very low (decel) loads. I run SSIs into a 2in2out MK pipe and my car did this terribly till I altered the ignition in the DME.

With a CIS engine you just live with it or put back a quieter exhaust.

[schumicat]

even with my decel valve plugged (while doing some troubleshooting) mine didn't pop nearly that much.

[kjchristopher]

Originally Posted by scarceller

I know this issue well as I have removed this exact problem area from my 84 Carrera by adding tons of ignition timing at very low (decel) loads. I run SSIs into a 2in2out MK pipe and my car did this terribly till I altered the ignition in the DME.

With a CIS engine you just live with it or put back a quieter exhaust.

In this spirit, the popping lessens if you don't have the vacuum retard connected on the distributor.

[911SauCy]

Mine does the same thing, condition lessens quite a bit when the car is "hot".

My situation also includes no cat, a "muffler-delete" and a plugged decel valve...

GOD I LOVE THAT SOUND!

[scarceller]

Worth noting is that many modern day EFI systems allow for shutting down fuel and the better ones even provide shutting down both fuel and ignition during decel.

Some engines can become very annoying with excessive popping but I tend to agree some popping can sound real nice.

[jasonlee]

my does that, and I really like it, running a cat bypass, and flow master muffler. Enjoy it.

[tazzieman]

Since installing SSIs and a Monty muffler I've noticed the popping , quite nice really and in keeping with backdating ! As I drive down 2 long steep hills each morning , it is part of the dawn chorus now.

[spuggy]

LOL. That's not really a problem, sounds nice... CIS 930's pop, bang, burble. cackle and backfire more than that factory stock - including flameballs if the turbo is hot enough to ignite the fuel that's causing it.

If you don't like it (or want to avoid the attention), try holding the throttle plate a fraction off the stop on the overrun (e.g. don't just take your foot completely out of throttle)

[Gordo2]

My 83 SC is more pronounced (worse, better?). Two known causes for my exhaust rumble and pop on loaded decelleration:
#1. I disconnected & plugged the vacuum line for the deceleration valve years ago - it wasn't working and was causing the idle to intermittently hang at ~ 1300 RPM. this alone caused some burbling with the stock exhaust.

#2. I installed a cat bypass pipe and modified 1 in - 2 out muffler. The flange that mates the cat pipe to the muffler is warped and leaks. Last fall I replaced the gasket and was able to get a leak free connection. The popping and burbling nearly disappeared (back to levels associated with decel valve). Since then the gasket has blown out again and I'm back to the full on rumble.

Gordo