Exhaust Design & Maintaining Scavenging Performance

[Muzez]

Hi Guys, 

Need a little help understanding scavenging and the impact of changing pipe size through an exhaust setup. Is there a negative or positive impact to reducing exhaust pipe diameter after a header collector? 

Context:

I have a stock 1977 280z with L28 and no modifications. I will be installing a new exhaust later this year. I have an MSA 6-1 which features a 2.5" collector and am planning on buying a Zstory muffer which also features a 2.5" inlet.

I read on the forums that for an unmodified setup, the 2" or 2.25" setup is ideal for torque due to scavenging/velocity and that 2.5" may have a positive impact on high RPM HP, but at the expense of my wonderful low end torque. If I dropped down to a 2" or 2.25" pipe immediately after the header collector and ran that back to the muffler, would that provide the scavenging required to maintain torque for an unmodified engine? Or would that negatively impact exhaust velocity and therefore is a bad idea? 

[AK260]

Basically, at any point that the exhaust gasses encounter a change of diameter larger than where they came from, they expand (lose energy / velocity) and send a shock wave back up the exhaust tract.

Now, in your situation you are not expanding but taking an existing travelling pressure wave and compressing it - in effect, this is creating a bottle neck / back pressure.

For proper scavenging, you ideally want the pipework to be the same diameter from the collector point all the way back to a resonator / muffler to avoid suffocating (smaller pipe) or introducing new expansion harmonics (larger pipe).

I had an MSA 6-3-2 header with 2.5” pipe all the way back to a turbo muffler with no resonator in between. The torque curve was utterly delicious.

[grannyknot]

Keep your exhaust pipe the same dia. as the collector.

[Muzez]

@AK260 brilliant! As long as it is not going to impact that delicious delicious torque curve, that is what I am interested in. Thanks much for the explanation on scavenging. Watched a few videos on YouTube which had me spinning!

[AK260]

[mention=32052]AK260[/mention] brilliant! As long as it is not going to impact that delicious delicious torque curve, that is what I am interested in. Thanks much for the explanation on scavenging. Watched a few videos on YouTube which had me spinning!

Most welcome. With a straight through muffler / improved fuelling I would have been able to pull another 500rpm out of the top end.

I used the exact same set up on my modified L28 and found it running out of puff at the same sort of point. The Z story classic muffler I used on that setup after blowing up the turbo muffler (long story!) freed up about 1k-1.5k rpm more. In fact it now sprints into the rev limiter at 6.7krpm (but it is a different engine so I wouldn’t have gotten that much out of the L26).

Without a centre resonator, it was however, unbearably loud for my soft ears (especially between 2k-3k RPMs where our L6s tend to “resonate” / drone). It would likely have been because of the much more aggressive cam and cam timing. It will probably be OK on yours with a stock cam. But be on notice for a centre resonator in case you have the same effect / your ears are as soft as mine. I’m “that guy” at music concerts with tissue in my ears! [emoji1787]

I don’t know if Zstory does pipe work for the MSA header back - you have to ask Sean - but the fit under the car is pretty spot on.

[texasz]

2 hours ago, AK260 said:

I don’t know if Zstory does pipe work for the MSA header back - you have to ask Sean - but the fit under the car is pretty spot on.
 

Not exactly.  I've talked with him about this and he does not do anything that directly fits the MSA headers out of the box, it will require a small bit of pipe and fabrication to mate the two systems.  It did not sound like very much, and knows of others who have mated the two systems so it can certainly be done.

[Sean Dezart]

I have a prototype to fit between my line and the MSA 6-1......but I'm not keen on the 6-1 design for anything other than its' conceived role - that of track and drag-runs.

It's not ideal for street cars- it makes noise but noise isn't performance. It creates exhaust drone at highway speeds whereas MSAs 6-2 doesn't.

On both your drawings, you're missing a resonator which'll not only eliminate the exhaust drone from your 6-1 but will increase the scavenging effect. Fit one of my complete lines without the resonator and not only will it be noisier (no better sound, just more decibels to p*ss off your neighbours and the cops) but should be down as much as 30-40hp on a mildly-tuned performance engine.

 

I'm not advocating buying one of my systems so this is what I'd reccommed :

MSA (no-smog) 6-2 (Code: 15-6001 Price:$252.87) 2.5" pipe then convert that 2-1 pipe to become 1x pipe at the rear of the transmission then fitting a resonator before continuing out to your muffler. The header isn't expensive, sell off your 6-1 to recover something, get MSAs Performance exhaust (Code: 15-6021 Price:$174.95)

With selling off the 6-1, it should cost you about $350 then spend some on extending that secondary pipe and fitting a resonator. You won't get better performance for less !

[Muzez]

On 11/27/2021 at 5:01 PM, Sean Dezart said:

On both your drawings, you're missing a resonator which'll not only eliminate the exhaust drone from your 6-1 but will increase the scavenging effect. Fit one of my complete lines without the resonator and not only will it be noisier (no better sound, just more decibels to p*ss off your neighbours and the cops) but should be down as much as 30-40hp on a mildly-tuned performance engine

@Sean Dezart Thanks for the feedback! Yeah, from what I heard the resonator is a must. Had the opportunity to see one of your mufflers in action a couple of months ago. Love your stainless setup so that is where I am headed. Might try to put the headers up for sale this winter and see if I get any takers. 

[Sean Dezart]

Keep in touch - my next batch due in just before Easter in April.