Exhaust back pressure or lack there of...

[Jon Sailer]

I did a quick search and could not find a definitive answer to my latest 'worry'. Yeah, I know, I need therapy.

I have recently pulled off my single-in/dual-out Fabspeed exhaust. I replaced the muffler with the Fabspeed 3.2 Carrera Muffler Bypass Pipe. This is basically a pipe that bolts up to the converter. When I purchased the car the Cat was replaced with a steel bypass pipe as well.

So hopefully someone can tell me if there is any danger in running with the stock heat exhangers, plain steel cat by-pass along with the Fabspeed muffle by-pass. I am not sure if this would give me any more or less power than keeping the muffler on and is there any possible risk to my stock 3.2 from running with a lack of back pressure.

[911nut]

The PhD engine specialist that lives at the end of my street says that you get all the backpressure that you need from the stock system and bypass pipe.
Backpressure improves efficiency by holding the fuel/air charge in the cylinder while both intake and exhaust valves are open. It doesn't take much backpressure to do that.
Putting a premuffler on is in the catagory of putting SSI's on. Some people swear by it, others wondered why they went to the trouble and expense of doing it.

[Wil Ferch]

I'm not quite sure what you did, but the overall net difference isn't all that much to start worrying about going lean ( or somesuch worry) as you "open" up the exhaust with the mods you indicate.

however, I'm still confused. The hot-dog running left-to-right at the tail end of the car is a muffler, and the cat runs alongside the engine on the driver's side. Replacing the cat with a "pre-muffler" is understood, but you seem to be talking about 2 premuffs??

..."bolts up to the conveter" ...and later you also say..."plain steel cat by-pass along with the Fabspeed muffle by-pass. ..."???

This description can't be right.....or ???


--Wil

[IanW505]

Wil, i think if you check the fabspeed website it'll explain it. The car already had a cat-bypass pipe, then he installed this fabspeed pipe which is basically a straight pipe running from the cat-bypass out...aka no muffler. Is that right, or am i just totally on crack? This is a track car i assume?

-Ian

[Wil Ferch]

It was the "bolted up to the converter" ...along with the other stuff...that has me confused.

If this is a track car...and uses a pre muff ( *instead* of cat)...followed by a straight pipe out...then this might be a whole different ball game.

--Wil

[BK911]

"Backpressure improves efficiency by holding the fuel/air charge in the cylinder while both intake and exhaust valves are open."

Not to argue with a PhD, but that doesn't make any sense to me. The whole point of overlap is to use the exhaust gas inertia to help start 'pulling' the intake gas mixture into the combustion chamber. If you pressurize the combustion chamber, you are hurting volumetric effeciency by impeding the incoming charge.

[BK911]

I'm still confused on the whole back pressure theory. My understanding is...

A pressure wave starts when the exhaust valve opens, travels at the speed of sound until it hits a drastic area change (collector), and then reflects back toward the exhaust valve. Tuned exhaust time this wave to bounce back from the exhaust valve right as the valve is opening to help pull the charge out of the combustion chamber. Then there is also tuning the equal length runners so the pressure wave passes the collector as another exhaust valve is opening to help create a vacuum in the other runner, helping the exhaust gas escape. Or something like that.

[RaceProEngineer]

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[bigrubberjeep]

Well I am no exhaust expert, but if a motor is infact an "AIR PUMP" the easier the air comes out the better right?

Why else would drag racers use a straight pipe out of the headers?

[spinner]

I would recommend you put the Fabspeed 1-in/2-out muffler back on. The straight pipe affects your low-mid acceleration; more so with a cat-bypass. I had a straight pipe with a cat and the low end was affected although the top end is improved. The sound was pretty serious..

[Bill Verburg]

The only danger is to your(and innocent bystanders) ears. Assuming the l sensor is hooked up, The Motronic will adapt to the increased volume of air(if any) moving through the engine

If you have equal length headers on an engine w/ significant valve overlap(not a stock 3.2) then the exhaust can be designed so that a negative pressure wave will be at the exhaust port at a certain point of the rev range.

None of that applies to stock 3.2 exhaust where you are only looking for sufficient volume of flow w/o significant obstruction

[BK911]

"Well I am no exhaust expert, but if a motor is infact an "AIR PUMP" the easier the air comes out the better right?"

Sort of, but there is a little more to it. If it was that simple, then why run any pipes at all? Just run right out of the exhaust ports.

Once the pressure wave is taken into account, then yes, the easier the air comes out the better.

[RaceProEngineer]

The following is neither NEW, nor UNIQUE, nor INNOVATIVE, but here goes anyway:

On our 3.0L racer, we use equal-length headers from the exhaust ports, and straight megaphones out the back. Since we are required to retain the Motronic FI, we have an O2 sensor connected in line.

Because of the "direct path" from the outside into the exhaust heads, we take precautions, like covering the open ends of the megaphones while the car is outside (not running).

In the PCA National stock classes, there are very few performance enhancements allowed on the motor, but getting rid of back pressure is allowed, and it is HUGE! Differences in throttle response, both on and OFF, are like night and day!! It actually begins to feel (and sound) like a performance car.

Bill: Ear plugs are standard equipment in the helmet bag, but the spectators are on their own. I am glad to hear (*pun intended*) that there will be no collateral damage to the motor from lack of back pressure. We had our fingers crossed about that . . . . .

Ed LoPresti
RacePro Engineering
New York

[Randy Webb]

There are numerous books on this principle (sometimes called scavenging). And you don't have to go into the mathematics of wave superposition to read (some) of them either. Use the search engine to find at least one cite by me to the guy who I think first discovered or at least first measured it.

Or just take BV's description above. If it "Just doesn't make sense" then it is time to look at the diagrams in the books.

[sammyg2]

Drag racers who have superchargers use straight pipes.
Drag racers with naturally aspirated engines have headers with carefully tuned pipes and collectors, and they are only after power in a very narrow rpm range.

Less backpressure generally increases power in the upper rpm range but reduces the torque in the lower to mid range.
The reason for this is the scavenging effect of a tuned exhaust system helps improve the VE by increasing the filling of the combustion chamber. Not only does it help to pull the spent gasses out of the chamber, if it is set up correctly it will also pull some unburned air and fuel mixture, then at the last possible moment before the exhaust valve closes, the shock wave pushes most of the good stuff back into the chamber. Kind of like a mini supercharger.
reducing the back pressure reduces the density and also temperature of the exhaust gasses a little effecting the speed of the shock wave.
That is what usually moves the power higher up in the rpm.

[Bill Verburg]

On our 3.0L racer, we use equal-length headers from the exhaust ports, and straight megaphones out the back. Since we are required to retain the Motronic FI, we have an O2 sensor connected in line

The Motronic will work fine w/o an O₂ hooked up as long as the chip was burned to optimize open loop operation.

As it is when the system goes to open loop at full throttle it just reverts to l = .85 - .90 any way.

In order to maximize performance l should be known and set for max. power. This is where dyno time and wideband O₂ are important tools used to optimize your chip.