Race Car Exhaust

[markmac]

So as to not completely highjack pjoyaux's thread on his race car, as I start to get back after mine (with the hopes of finishing it and driving it at some point before I die.....).  I have a pretty good list going, exhaust is one of them that has impact on quite a few other things (DS oil line routing, diff/trans oil cooler mounting etc.,) so I really do need to get it figured out.

 

Doing some 'track' research, most events be it race, drivers' school etc., have sound limits.  What I have found so far is that most range between 90db and 105db. Very few have zero (unlimited) sound allowance.  The SVRA event at Thunderhill last weekend had a limit of 108db, and that track is about as rural as it gets.  The way my exhaust is now, 115db++  no question.

 

When the car was being raced it had large diameter tube headers (4--->1) that ran into a collector and then a straight pipe out the side (part way up the passenger door).  They raised the floor to help keep things up and away from the pavement below.  The ID of the exit pipe is 60mm (2.36+/-).

 

Talking with Abarth now on some resonator/exhaust options that might work, all of those though mean going out the back end, not the side.

 

 

[Preyupy]

After years of racing everything from 1920's Bentleys to late 90's F1 cars meeting sound limits is a tricky problem.  

The biggest thing is to know WHERE they are measuring the sound?  Which side of the track are they on? You can lower your "measured dB" by making sure the microphone can't SEE the end of the exhaust pipe.  If the Mic is on the right side of the track run the pipe out the left side etc.  Side exit exhausts are the hardest to get within limits.  If you run it out the back you can put a tip on it that you can aim to the left or right depending on the track and where the sound control station is located.  This alone can cut 2-4db off the top.   There are a lot of good mufflers out there now that do a good job of sound control without making a big dent in your HP.   There is also the change in sound pressure levels depending on weather so the fact you were at 102 dB  one day does not mean you won't be over the 103 dB limit the next day. 

 

Also know that it isn't just EXHAUST noise you need to pay attention to. If you are running an air box on the intake and have it ducted to the front of the car the intake noise can sometimes be 98+ dB the sound control people don't care where the sound is coming from just that you are too loud.  A few years ago, at Laguna Seca the B Sedan group was running and there were a number of Alfas that were getting busted for noise (I think they needed to be under 100db that day) and they kept working on the exhaust systems and could not get under the limit.  One of them finally took the hose off the airbox that was running to the headlight bucket and suddenly they were legal. 

 

Sometimes you just need to DRIVE the car differently as well.  Once you know where sound control is located you may just have to "pedal it" as you go by.  I raced a big block Corvette with side pipes at Laguna Seca once when there was a 103db limit.  The 427 Cobra that I was racing was also running side pipes.  We both had as much muffler on the cars as we could fit, with shields, tips turned down etc.  The sound control shed is on the hill between Turns 5 and 6 before the bridge.  It is on the Right side of the track about 70' feet from the edge of the track.   Mac and I would come out of turn 5, short shift to 3rd gear, dive to the left side of the track and go to 1/2 throttle until we went under the bridge and out of range.  They kept telling us we were "getting close" at 101.5-102 db all weekend.  On the last lap of the last race of the weekend we both lit up the tires coming out of 5 and laid 4 black tire marks all the way up the right side of the track to the bridge.  If we had not gotten the checkered flag that lap we would have been BLACK FLAGGED for noise!  

 

My recommendation is run the exhaust all the way to the back of the car, Fit in as much muffler as you can fit.  Design it with an adjustable tip that you can AIM away from the Sound Nazi.  Be prepared to modify your cold air box if needed. Also be prepared to "pedal it" if you have to.  

[visionaut]

What's your thoughts on the Supertrapp pancake-style muffler and it’s effectiveness and ability to adjust volume?

 

That could fit size-wise at the tip of Mark’s side-exit. Maybe with a center resonator. But I’ve no idea if they work or hold up at 115db 😁

 

Tom

[tzei]

Absolutely no no!

Check David Vizard’s vids att youtube

[markmac]

Great info Byron.  It does have an air box system, same as it was raced in 1975 ( I have all the original pieces just like in this BW picture), the intent was to use it in some format.  The snorkel on the box is connected to a fitting on the bulkhead.

 

This is what we are playing with.  Need to get some more clarification from Abarth.  These resonator/mufflers would go to a Merc 250/280 SE.  Same header but would change the collector for two pipes going out.  Those are roughly 1.5" each.  The problem is that the resonator (B) and muffler (A) together are 36" (+/-) and I just don't have that much real estate in the back end.  To make it work we would have to run the pipes on the underside to the resonator, then out of the resonator over the axle and then to the muffler.  It seems like having the two 1.5" pipes going in/out would be sufficient to move the exhaust.  Not the same as the header and straight pipe but.....

[Preyupy]

I am not a big fan of the supertrap plate mufflers. By the time you put enough plates on the end of the pipe so they don't eat up 30% of your HP you haven't lowered the noise level enough. 

 

The big problem with the original exhaust on this car is the pipe is sticking straight out the right side of the car.  If the noise nazi is on the right side of the track you are going to have a huge problem. 

 

I would not use those Abarth mufflers, they are exceedingly heavy and huge.  Try one of the Magnaflow mufflers.  Figure out what space you have and go looking for a muffler that fits.  Maybe even round resonator type muffler up under the car along with a muffler out back.  It really depends on how quiet you need to be. 

 

 You might consider an OVAL tube to run over (or under) the rear crossmember.  I have built a number of 2.5" systems for 2002's over the years and it can be done fairly easily.    

[2002iii]

Maybe something like this:

Found Here.

[TobyB]

So what I found was the round Magnaflow- which was a 'glass pack' arrangement-

was that it was darn near 103 at the tailpipe.  At the rear.  If I got the turndown to aim away from sound,

I was usually at 101.  Then I stuck a Supertrapp on the end of THAT to run at Mission with a (98?) limit,

and had to take enough plates out that it hurt lap times.

 

Went to the FlowBastard and now don't make it to 95.  And it doesn't seem to slow me down enough to notice.

 

t

 

 

 

 

edit, from the other thread we're un- jacking here:

Quote

They use noise- canceling technology like you see in headphones. They are tuned for a specific engine and they pit the specific wavelength and amplitude of the undesirable sound waves against each other, canceling them out.

 

That sounds so good in theory.  In practice, it MIGHT make a 6dBSPL cancellation-

IF you held the engine at the exact right rpm to generate an exact standing wave of just the 

right frequency....

Noise cancelling headphones use active processing, and even that way the consumer version's not

all that hot (15dB, maybe).  The Military version are far better, but cost more than a nice 72tii with

a matching enclosed car trailer...

 

t

 

Edited March 6 by TobyB
trying to un- jack the other thread without dropping the car on someone.

[steve k.]

I highly recommend Burns Stainless

Burns Stainless: Performance Exhaust, Collectors, Race Mufflers

BURNSSTAINLESS.COM

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High Performance Stainless Steel Exhaust Parts. Burns Stainless offers exhaust parts, tubing...

I ran their muffler on my race car and passed all sound limits including San Diego where they were a bit rediculous. 

You get to pick the internal diameter, external diameter, andlength. They are also serviceable and can be repacked. They are super light: 3 to 4 pounds. They are not cheap, but in this situation you get what you paid for. 

 

Our rules were that the exhaust has to exit behind the driver seat, so I ran the header to a short pipe to muffler, another short pipe and turn down right after the driver seat. I did not take it to the side. 

 

Steve K. 

 

 

[markmac]

Thank you all, all really good info and gives me a place to start.

[visionaut]

I’ve been eyeballing Marks existing exhaust system design, specifically the header and the ‘collector extension’ side exit pipe, trying to understand the engineering that’s potentially behind that design to see if that might factor into making changes to it.

 

My takeaway is it’s not optimized for flow volume and inertial scavenging but for velocity and wave scavenging. And the length of the collector extension might have been calculated to optimize that wave scavenging.

 

The header is obviously massive. Huge primaries feeding a very short 4:1 collector. The primaries look to be equal-length, though there looks to be some unintended diameter variations due to the bending.

 

But the primaries diameter are so much larger than the exhaust valve that it would hurt inertial scavenging. And the primaries aren’t arranged in firing order around the collector for a spin contribution to inertial scavenging. And the collector has large merge angles which hurt inertial scavenging. So it’s likely not optimized for that..

 

That leaves wave scavenging - which isn’t about gas moving up/down the header tubes like inertial scavenging but about sonic waves moving down/up the exhaust. Harnessing sound wave reflections at volume transitions in the exhaust — the most important being the inversion at the exhaust exit where the volume change is infinite (in this case at the end of the side-exit pipe).  So its distance from the exhaust valve would matter.. ( You want the total length to be a harmonic multiple of the exhaust pulse timing. The length determines the arrival time of this negative pressure wave, which you want to occur just before the exhaust valve closes and while the intake valve is opening.)

 

What I don’t know is if the original exhaust system, and particularly the side-exit extension on the header was optimized for length or if it was just a practical ‘routing away’ and cut off where it happened to reach the side of the car. If it’s an engineered length for power optimization you’ll want to know that in determining an alternative exhaust.

 

This is where a program like Pipemax could help…

 

[TobyB]

Yeah, my understanding is that by the time you pass 5500, you get far more assistance by maintaining the wave front,

and that inertial sizes become a flow restriction.  

The V8 guys will tune each runner on the headers a little differently to broaden the peak of the effect-

they sacrifice ultimate HP boost in favor of power over a wider band (for them, they're talking about a 500 rpm band, ususally)

 

I never had the time, science, brainspace, brainpower or instrumentation to go very far down that route!

 

t

 

[steve k.]

A couple of cool articles

 

Exhaust Header Theory and Proper Exhaust Tuning

BURNSSTAINLESS.COM

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In order to explain the effect of exhaust tuning on performance, let’s take a quick look at the 4-stroke engine cycle.  The first step in the 4-stroke process is the intake stroke.  With the intake valve open, the piston travels down the cylinder pulling a fresh air and fuel mixture into the...

 

Muffler Technology - Sound Measurement

BURNSSTAINLESS.COM

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Muffler Technology – Part 1 Sound Measurement Over the past few years, as residential homes have been built closer and closer to race tracks, there has been a strong movement towards reducing the  sound from race cars. At some tracks, such as Laguna Seca in California, the...

 

 

There are a lot more interesting things to read there. I need to find an article explaining the distance from the collector to the expansion. That should allow some thoughts on the distance of the muffler/resonator from the collector. 

[Preyupy]

Just be careful about dumping the exhaust under the car (even behind the driver)  it helps with sound levels but you stand a chance of Carbon Monoxide poisoning (happened to me at an endurance race in Portland years ago) and the other big problem is trying to keep the Differential cool, at least dump it behind the Diff.   

[visionaut]

7 minutes ago, TobyB said:

Yeah, my understanding is that by the time you pass 5500, you get far more assistance by maintaining the wave front,

and that inertial sizes become a flow restriction.

Yep.

 

Inertial designs are typified by the features you find on tri-y  and step headers - smaller diameter longer tubes with more volume/speed transitions, each segment sized for its gas flow effects. They’re a big aid in the midrange. And why in general for high rpm power-focused application folks use a 4:1 to maximize exhaust speed.

Inertia mainly contributes below the torque peak whereas wave scavenging mostly aids above it.

 

 

Steve - there’s lots of info and test data from the v8 boys on collector extensions and lengths..

 

Most always they’re an improvement over bare collectors, though some of that is coming from folks getting headers with too-large primaries and the extensions are dialing it back some.  The oft-cited ‘cut it where the paint mark wears off’ trick doesn’t get the best length ( tho it’s not a bad start to test from).