3.6 Muffler change...loss of HP and torque

[rdane]

I went from a 2 in and one out muffler to a two in and 2 out muffler.

The one out seems pretty constricted and basic. The 2 in and 2 out seems like it would flow better.

Problem is muffler change alone dropped the hP a full 5HP but worse yet 15# of torque.

Engine is a '95 3.6 with factory heat exchangers.

Would adapting a 2/2 out Monty work better? Any one done any testing on the dyno with mufflers for 3.6 transplants?

[JGalt]

Can you identify the brand and model of before and after mufflers?

[rdane]

First one was built with a small square Flowmaster, the 2/2 was built with small dual glass pacs.

[Steve@Rennsport]

Originally posted by rdane
First one was built with a small square Flowmaster, the 2/2 was built with small dual glass pacs.

I've never seen any decent results with any glass packs, even going back to 1978 on that.

These engines need a crossover, either incorporated into the plumbing or the muffler, to make good low & mid-range torque. Something like the Series 80 Flowmaster (loud) or a factory-style muffler works FAR better in every respect.

The Monty mufflers are well done but would need different flanges to mate with the 993 HE's.

[DW SD]

I would think of a 2 in 2 out system as similar to the above, except with two outlets, which would offer the cross-over advantage which Steve mentions.

Does your new one have a combined plenum (like above photo) OR are they two separate straight out pipes made with 2 separate glass packs?

Cross overs, amazingly make a big difference. I have little experience compared to Steve, but I am aware of their ability to boost low and mid range, too.

Could you post of a piture of your new setup? So we understand the system?

Doug

[mb911]

As what doug says the cross over helps the low end and midrange but you will gain some drone from that set but that depends on the internals to as how much. I may have a solution shortly for this. Good luck

[Bill Verburg]

On an even fire engine a cross over gains nothing on a properly designed header collector muffler setup


B boom
S silent

911 6 is even fire
L- B S B S B
R- S B S B S

generic V8 w/ 90deg crank is odd fire
L- B B S S B B S S
R- S S B B S S B B

generic V8 w/ 180deg crank(Ferrari) is even fire
L-B S B S B S B
R-S B S B S B S

Chevrolet sb V8 is odd fire
L-B S S B S B B S
R-A B B S B SS B

the odd fires benefit from the cross over because they have 2 consecutive cylinders firing on the same bank, so the collecter either needs to be too big most of the time or to use the cross over to take up some of the excess load.

the other option is a 180 deg exhaust where 1 left cylinder is piped to the right collector and 1 right cylinder is piped to the left collector.

the 180 deg exhaust effectively makes a 90 deg V8 into a 180 deg v8

Champ cars and Ferrari use the 180 deg flat cranks, that's why the sound so different, a Chevy or Ford v8 will sound the same w/ the 180 header setup.

180 deg headers aren't common because of cost and packaging constraints. Early Ford GT 40 w/ the bundle if snakes exhaust was the last one I ever saw. Cross over is a fair substitute that uses much less space and is much easier to implement



The Flowmaster is a particularly good design becuse if the inlets are properly done the collector sees it as atmosphere, all the glass pack designs are seen as extensions of the collector and will therefore affect the acoustic tuning of the pipes sometimes for the better sometimes for the worse.

For the Flowmaster to be seen as atmosphere the pipe needs to penetrate the inlet stub

[jluetjen]

One conclusion that I've drawn from exhaust design is that there are two distinct zones that need to be considered for exhaust design. Specifically they are what I would call "low speed" performance which is from idle up to a maybe 1000 RPM above the torque peak, and "high speed" performance which goes from there up to the red-line. My understanding is that the "low speed" exhaust performance is generally dominated by a combination of inertia and accoustical tuning (think of a slide trombone), while the "high speed" performance is dominated by flow rate (think of a fire hose).

Then you need to layer on top the engine configuration (as Bill Verburg described) and the cam design and intake configuration. The neat thing about 911's is that they all have the a solid engine configuration (120 degree boxer flat 6 with even exhaust pulses) which simplifies that particular variable. But that still leaves you with 2 problems and at least 4 variables to manage (including the collector and muffler designs).

Unfortunately most of us have to resort to swapping parts to see what seems to work out best, but this approach will never tell you why or provide any predictive information. Never the less, I'll wade in with my $0.02.

1) We can most likely start to build a database of examples to understand what's happening.
A) Cam design. Does the cam have much (if any) overlap?
B) Primary lengths. How long are the primaries in the heat exchangers?
C) Primary ID. What's the internal diameter of the primaries? Is there a step?
D) Secondary length? How long is the pipe after the collectors. If it continues into the muffler, you need to include that length too as Bill shows.
E) Secondary ID. What's the secondary internal diameter. Does it change?
F) Muffler configuration. Baffles? Glass? Steel-wool type stuff?
G) Cross-over pipe. Is there a crossover? Is it in the muffler?

2) The next thing is to describe the results fully so that we can understand the change in the torque curve.

A) Just looking at the change in HP isn't meaningful since that just describes the limit of the exhaust's or the engine's "high-speed" performance.

B) Just knowing the peak torque number describes how well the system is in tune at that one RPM.

C) In this case I really think that at torque curve comparison tells the most

Some hypothetical cases.
1) The early 911 race system (a la a 906) was tightly tuned for maximum performance from the torque peak ( with a comparitively high peak torque number at around 5000 RPM) up to the red-line (around 8000 RPM). By virtue of being so "in tune" at 5000 RPM (with a strong low pressure pulse at the exhaust valve and a pressure pulse at the intake valve during overlap), the system was almost completely "not intune" at 4500 RPM when the reverse was true of the pulses. (For those of you with band experience, think of tuning a trumpet by tiny changes of the small tuning slide). The result was a big hole in the torque curve at 4500 RPM.

2) A more "out of tune" system would have slightly different primary and secondary lengths resulting in the low pressure exhaust pulse arriving at the exhaust valves at a different time from when the high pressure intake pulse arrives at the intake. The result would be less torque at 5000 RPM, but no hole at 4500 RPM either. So this design might have a wider torque curve and a more driveable motor.

3) Playing around with the cross-over designs can augment or reduce the affects described in cases 1 and 2. In many cases the muffler designs on most street 911's tend to be in this realm.

4) Changing the pipe diameters can (in combination with the valve size, port diameter and cam lift and duration) affect the amount of flow at peak RPMS -- which means HP. But they also affect inertia which impacts the magnitude of the pulses described in cases 1 and 2.

So, would a Monty work "better"? Well, it depends...
- What changes are there in the secondaries?
- What changes are there in the cross-over design?
- What changes are there in the muffling material or design?

While the number of outlets is the most visible change, it also in many cases may be the least influential.

[Bill Verburg]

One conclusion that I've drawn from exhaust design is that there are two distinct zones that need to be considered for exhaust design. Specifically they are what I would call "low speed" performance which is from idle up to a maybe 1000 RPM above the torque peak, and "high speed" performance which goes from there up to the red-line

thats true, when I raced single cylinder motorcycles we tuned for the track. On the tight low speed and hilly circuits we used smaller longer pipes for low speed torque optmization, the faster flatter tracks we used bigger shorter pipes to enhance top end.

The trick is always to keep the gas velocity high. This is done by keeping heat in the pipe, and having the pipe no bigger than absolutely necessary for the highest rpm that is commonly used.

W/ a single cylinder m/c it's easy and cheap to experiment and swap, w/ a 911 it's not.

But the basic principles are always the same[list=1][*]keep heat in to keep gas velocity up, these days Jet Hot coating is the way to go[*]longer pipes enhance torque across the rev range[*]shorter pipes enhance hp at the top end[*]use the smallest pipes that cary the full unrestricted flow at the highest rev point, again to keep gas velocity as high as possible for the longest period of time[*]tune is relatively insensitive to fairly large differences in primary pipe length[*]tune is relatively sensitive to collector configuration, size and termination[*]acoustic effects are only significant when there is a relatively large amount of valve overlap available[*]even fire engines/exhaust configurations do not benefit from x-overs[*]odd fire engines do benefit from x-overs because they prevent the overload condition where 2 consecutive gas slugs try to exit through the same collector(slowing gas velocity)[/list=1]

[Lukesportsman]

Well written by both John and Bill.

"The Flowmaster is a particularly good design becuse if the inlets are properly done the collector sees it as atmosphere, all the glass pack designs are seen as extensions of the collector and will therefore affect the acoustic tuning of the pipes sometimes for the better sometimes for the worse.

For the Flowmaster to be seen as atmosphere the pipe needs to penetrate the inlet stub". BILL

The difference between a baffle race muffler and a glass pack muffler is one of those areas that MANY racers of MANY brands never consider or appreciate. To each their own and sometimes due to collector length there certainly are circumstances where packed mufflers might work, but they are NOT interchangeable.