Back pressure

Hi guys, just a question.

I took the Pride N Joy in to my mechanic last week to see about why it was backfiring on decel.  I thought the timing was wrong and so asked him to check it.

He said it was set at 28 degrees advanced and that he adjusted it back to where it should be.  I asked if it still backfired.  He said the popping, BLUB, BLUB, BLUB, BLUB I was hearing when I would pull my foot out of the throttle was due to running true duals with no cats and Flo-pro mufflers, thereby producing little to no back pressure on the system.

Does anyone else here have the same problem, and what would be a simple fix to this annoying little symptom?

Thanks in advance

15" woofers?


I don't know if the backpressure senario is on target or not, but I'm leaning towards "not".
The "blub blub" on decelleration is the engine sucking cooler outside air into the exhuast and colliding with the really hot exhaust gasses, creating "thunder". (Ask yer weatherman )
Actually, Ken has explained this phenomenon before...I just don't remember how he does it!
But...my explaination sounded good, no?

I have true duals on my '78 with low restriction mufflers and have no problem like that. I know there needs to be some back pressure for proper exhaust scavenging, but unless you go to really large pipes (like 3" for that small block), I can't imagine you having a problem.

What most performance exhaust system changes accomplish besides lowering back pressure is they change the rpm range the engine makes its best power. You may notice a difference in performance but it's as likely from that than a real increase in horsepower and torque...the hp and torque peak has moved to a different rpm range, not really increased. That's a trick manufacturers used to play to advertise hp and torque ratings...read the rpm it's rated at. It may be a completely wrong rpm range for the planned use of the vehicle. It's also why trucks and the like are rated at far different rpm readings than cars. Trucks need the low rpm pulling torque more than cars in most cases.

Your problem sounds more like a carburetion/timing type problem than exhaust related.

Take it to a dyno & have them check your air fuel mixture , I'm guessing you might be lean. Pull your plugs & check the color. I'm with gunslinger on this as sounds like carb & or timing.

Alan

It's not the back pressure, it's the flow that makes the difference.

When the exhaust valve closes there is no supply for additional gas and the flow from the valve is stopped.  An object in motion tends to stay in motion.  This includes gas.  Due to the movement and lack of supply, there is a vacuum created in the exhaust.  This happens every time any exhaust valve shuts.  This vacuum is less with more back pressure due to the fact that the flow is restricted.  Pressure rises and drops but does not go all the way to vacuum.  The problem with the pressure is you don't get all the exhaust out of the cylinder, thus restricting air/fuel in, and performance suffers.

These pulses ALWAYS happen, but with increased throttle there is more total volume and the effect varies.  At decell there is little exhaust supply.  So the system can pull air/ or more air/ into the exhaust system.  The pulsations can cause some of the sound changes.  But when there is left over fuel, which is always the case but varies depending on carburation, the fuel mixed with the fresh air and oxygen.  Put fuel oxygen and heat in one place and you get a secondary explosion in the exhaust system.  POP!

Different carb setting, or a smaller size pipes change the effect.

Yeah, what Ken said.

I told ya Ken knew!!

Axchewally, it's the same thang I said , onlee Ken hab a mooch better way wid werds!! All that teknickel stuff, ya know! It reely makes weigh more sins when Ken says it!



Adams' Apple38948.846724537

I once had a plug gap close up on me when the plug was dropped during installation. Not noticing, the plug went in, every time I took my foot off of the gas she would pop out of the exhaust. Eventually found the plug, regappd it, popping gone.

May seem obvious, but if you haven't done so already, make sure your plugs are good and gapped correctly.

Good luck.

Paul

If you have an exhaust leak in the system, it can cause a backfire when you get out of it.

Steve

[QUOTE=LukesVette]

I once had a plug gap close up on me when the plug was dropped during installation. Not noticing, the plug went in, every time I took my foot off of the gas she would pop out of the exhaust. Eventually found the plug, regappd it, popping gone.

May seem obvious, but if you haven't done so already, make sure your plugs are good and gapped correctly.

Good luck.

Paul

[/QUOTE]

This would leave unburnt fuel in the cylinder which causes detonation in the exhaust..