Which mufflers?

[Stallion]

i was told it was running rough when i bought it , because it had old gas and carbon build up......so i took it out on the freeway and was rolling along at 60 mph when i decided to dump it....well when the 4 bbl kicked in things got ugly performance wise.....so i let off on the throttle , that is when i had a MASSIVE backfire.....all of a sudden my car is REAL loud.....i didnt see parts out the rearview mirror so i went to the house and looked underneath.
Wow....that was some explosion......there is a seam in these mufflers that were on it, and one was blown out down the seam....top to bottom.the reason the timing was at fault is that i had unburned fuel in the exhaust system that wasnt burning when it should and when it dumped it the 4 bbls really added to it...when that fuel hit the red hot exhaust ....well the rest is history.
timing is set now and my vette flys!

Technically, what does it mean and what happens with a "backfire"? Could you explain this concept to me?

Thanks!

TR

 

[Chas]

Backfire = Explosion!

if you put a small amout of gas under pressure and ignight it it will explode.its the basic function of a combustion engine. if the timing if the gas....compression...spark.....explosion....exhaust....dont happen at the right time you will have excess gas sometimes someplace....not always....and it will cause the backfire ...or explosion when not needed causing the driver to have a small heart attack.

 

[Stallion]

Okay, I see. But you said that your Vette was running "rough". Why exactly was it running rough? What was causing this timing problem that resulted in backfire?

Thanks!

TR

 

[Chas]

gas....compression...spark.....explosion....exhaust....

it has to happen in the properly timed sequence......in my car the timing is 12' above top dead center.....it makes it all happen in the proper timing. if it is off ... say 2' below top dead center...as mine was....it is starving for gas or air or spark.....so it runs bad.....when timing is set to factory specs. it makes it all happen and the driver is rewarded with a smooth running automobile.

 

[Stallion]

Okay, I think I understand. Thanks, dude!

TR

 

[MaineShark]

Just a little more on timing: the spark actually occurs before the piston reaches top-dead-center. This is done because the gas takes a certain time to burn. If the timing is right, the pressure increase will not travel down to the piston, until after it has crossed TDC, and is coming back down. If it happens before the piston crosses TDC, the combustion actually pushes backward on the engine, reducing power and stressing the crank. This is called pre-ignition. If it happens too late, the gas might still be burning when the exhaust valve opens.

Now, on the other hand, if the mixture is too rich, not all the gas will burn. There won't be enough oxygen to burn the rest, so the gas vapor will get pushed out into the exhaust system. Exhaust, as we all know, is very hot. So, when the vapor collects in the muffler, it wants to ignite. If some oxygen seeps in... boom!

Back to the muffler topic: I have some Dynomax Ultra-Flo's sitting here, but I need to have a new exhaust bent before I can install them. I am eagerly waiting to see what they will do, and I'll be sure to mention it here when I finally get them installed.

Joe

 

[Stallion]

Wow, that sounds like it's not too good for your Vette. How exactly do you set/re-calibrate your timing in case you have a problem such as backfiring?

Thanks!

TR

 

[Hotred94]

I have Flowmaster 40s under my vette. Idle sound is great, then there is a period of heavy resonation at about 1500, then they quite out above that. They also sound great when you let off the gas at speed. I'm trying to quite out the resonation somewhat because they make my ears ring after a long drive. I thought I would try a crossover pipe in the rear. If that doesn't work I'll probably change mufflers. No good going deaf if you want to hear the rumble.

 

[MaineShark]

Wow, that sounds like it's not too good for your Vette. How exactly do you set/re-calibrate your timing in case you have a problem such as backfiring?

Thanks!

TR

The adjustment is done at the distributor. To read the timing, you use a timing light. Think of a strobe. Every flash "freezes" motion. Well, the timing light's flashes are triggered by the spark signal (a sensor clips onto one of the plug wires). There is a mark on the balancer (at the front of the engine, bolted to and rotating with the crankshaft). There are also marks on the block. By watching how the balancer mark "moves" related to the other marks, you can adjust the distributor to put it at the correct place.

Basically, the timing light freezes the mark every time the plug fires. The mark tells you where the crankshaft is at that moment. You can masure its position by comparing it to the marks on the block.

That's base timing, anyway. You also have "timing advance" - as the RPM's go up, the timing advances further, since the crankshaft's speed has increased, but the speed of combustion is still essentially constant (being a product of chemical processes, not engine speed). So, you can light the mixture sooner, and the faster-moving crank will still have time to get the piston out of the way before the pressure wave hits. This is where you will hear things about "vacuum advance" (since vacuum is related to RPM, you can use it to control the advance mechanism). Other systems use weights and springs, so the centrifugal force generated by the faster motion causes the weights to move, which can control advance. Modern systems use electronic controls that read the RPM directly, compare it to curves stored in a chip, and advance or retard the ignition, accordingly. A significant amount of the power difference between older engines (ours, for example) and their modern, electronically-controlled descendants is due to the more accurate ignition advance curves.

Joe

 

[Red76Vette]

I got 2 40s and it is to quiet I wish I would have got something a little louder. I love setting alarms off in the parking lot. I work in Newport beach and a lot of nice cars in the lot when I drive in and park I will set off at least one alarm fun!!!

Late, Chuck

 

[Stallion]

The adjustment is done at the distributor. To read the timing, you use a timing light. Think of a strobe. Every flash "freezes" motion. Well, the timing light's flashes are triggered by the spark signal (a sensor clips onto one of the plug wires). There is a mark on the balancer (at the front of the engine, bolted to and rotating with the crankshaft). There are also marks on the block. By watching how the balancer mark "moves" related to the other marks, you can adjust the distributor to put it at the correct place.

Basically, the timing light freezes the mark every time the plug fires. The mark tells you where the crankshaft is at that moment. You can masure its position by comparing it to the marks on the block.

That's base timing, anyway. You also have "timing advance" - as the RPM's go up, the timing advances further, since the crankshaft's speed has increased, but the speed of combustion is still essentially constant (being a product of chemical processes, not engine speed). So, you can light the mixture sooner, and the faster-moving crank will still have time to get the piston out of the way before the pressure wave hits. This is where you will hear things about "vacuum advance" (since vacuum is related to RPM, you can use it to control the advance mechanism). Other systems use weights and springs, so the centrifugal force generated by the faster motion causes the weights to move, which can control advance. Modern systems use electronic controls that read the RPM directly, compare it to curves stored in a chip, and advance or retard the ignition, accordingly. A significant amount of the power difference between older engines (ours, for example) and their modern, electronically-controlled descendants is due to the more accurate ignition advance curves.

Joe

Wow! Sounds like quite a process you must go through. Do you usually do timing modifications yourself, or do you need a lot of special tools and/or pay somebody else?

Thansk!

TR

 

[MaineShark]

It's really up to the individual. I enjoy working on the Shark, so I don't mind it. For someone who isn't careful with it, messing up the timing can cause a lot of damage, so that person might be better off to have a mechanic do the work.

Joe

 

[Don&Kelly]

We like "Flowmasters on our '81 -