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Do noisy exhausts sacrifice significant horsepower?

Joined
11 January 2021
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611
Location
Ontario, Canada
I just got back from NSXPO in Atlanta. I was initially worried that I wouldn't be able to keep up to the cars that had been upgraded since my car is basically stock. A lot of the cars there seemed heavily modified. However, I noticed a curious thing on 3 of the 5 rides I took. In these 3 cases, the car in front of me had an exceptionally loud muffler. I assumed this would be associated with a significant power increase and I would struggle to keep up, but the opposite appeared to be the case. These guys stomped on it and made a lot of noise but accelerated very slowly. In several cases, their slow acceleration during passing actually seemed unsafe to me. I was able to keep up without even downshifting and practically ran into the back of them when I downshifted to maximise acceleration, because I had anticipated they would step on it. It seems folks are slapping on loud exhausts for some sort of safety benefit of everyone hearing them without considering the lost horsepower of these ill-tuned units? After the 1st occurance I had assumed that the driver was still learning to shift a manual transmission, but after the 3rd different car like this I am perplexed.
 
On a stock engine, changing the after cat exhaust system will likely have no detectable effect on horsepower, either up or down. It is primarily noise level / sound quality and perhaps weight reduction effect. On an NA1 NSX replacing the cast iron exhaust manifold with the later NA2 tubular style manifold or an aftermarket version may offer some small power improvement. However, exhaust manifolds do not alter the noise level significantly although thin wall headers can be ringy compared to a cast iron or heavy walled manifold which you will notice inside the car and when standing next to it at idle.

I don't know exactly what is causing the phenomena you experience. However, I offer some possibilities
- all that exhaust noise provides a psychological input that causes the driver to think they are going faster than they really are and they don't put the pedal right to the medal (max power only occurs when the throttle is 100% open)
- not all modifications are improvements. Who knows what other changes have been made to the car which may have had a negative effect.
- if they make changes that significantly alter the volumetric efficiency this requires a fuel map re tune. No revised fuel map means the tune is incorrect which may have a negative effect on horse power. The ECU goes into open loop operation at high engine load so you cannot use close loop O2 fuel trim to 'tune' the engine at high engine output.
- "All catback - no motor" - they spent more money on the exhaust system and less money on life sustaining maintenance (the engine is 30 years old and getting tired).
 
Your question is about exhaust flow. In a race engine, you want the highest exhaust flow possible because it creates the least resistance to the pistons inside the combustion chamber as they move up on the exhaust stroke. This results in more horsepower from the engine because the pistons have to do less work to move the exhaust gasses out of the engine. Placing any restriction inside the exhaust tract (like a muffler) slows down the flow of the gasses and reduces horsepower. Keep in mind however that all of this really only applies at high RPM. Thus, loud (e.g., free flowing) exhausts tend to produce more horsepower in the upper RPM band of the engine. On that NSX, that's above 6,000 RPM, or VTEC). The practical application is to use larger primaries and larger collector pipes to reduce any possible volume restriction on the gas flow. This is why race cars have massive headers and exhaust pipes- they want the best possible flow at 8,000 RPM. They don't care about torque at 3,000.

However, initial acceleration is a function of torque, not horsepower. A downside of free flowing exhausts is that they tend to reduce torque at lower RPM because they are not optimized for low RPM scavenging. At lower RPM, the exhaust velocity is slower. The way to speed it up is to use a smaller diameter pipe, but this will hurt high RPM flow because it isn't enough volume to support the flow at those high engine speeds.

Honda tried to strike a balance between large enough pipes for VTEC power, but small enough to preserve mid-range torque. The aftermarket exhaust producers by and large design to max out the hp on the dyno to sell their product. They use larger pipes to accomplish this, but they lose the torque. It's very common for NSX owners to be surprised by the throttle responsiveness and initial pull when they put the OEM muffler back on the car. It happened to me going from a RM Racing exhaust, which uses huge pipes. You can gain as much as 10 lb/ft or more under the curve by going back to OEM, but you will lose power up top.

So, what was happening is that your torque advantage was kicking in during acceleration and you were keeping up. :)
 
I’ve noticed that loud exhausts are usually paired with big wheels and tires which really slows down low horsepower cars like the NA1/NA2 - maybe that is what’s happening.
 
I’ve noticed that loud exhausts are usually paired with big wheels and tires which really slows down low horsepower cars like the NA1/NA2 - maybe that is what’s happening.
True- larger rear wheels effectively gear the transmission longer. I never understood owners who spent $$$$ to get the NSX-R final drive and then slap big rims on their cars. It totally undoes it!
 
Then there is rotational inertia - a biggie when it comes to acceleration and deceleration. Along with lower horsepower the NSX brakes are small to reduce unsprung weight.
 
Nsxpo drives are the best...squeaky brakes, blatting exhausts, folks with little to no tire tread left, and jockeying for positions at the starting hotel..
 
To this day I still hear people bark about "gassing" brake pads in reference to Asbestos...

I also hear the word "backpressure" used a lot from the old guys, and some kids, as if every new car is still carburated. Some people don't know the facts and don't wanna do the research, understandable, some are misinformed. Its easiest to tell people that the correct way to find out what exhaust is best for X or Y application is by actually checking, like with a dyno. In theory we all know that noise is a byproduct, and thus a more effecient engine may be quieter, emit less heat out the tailpipe, all that stuff. Few people go through the money to get the absolute most optimal exhaust, but in the end its basic marketing. Racecars are loud, so people buy louder exhausts. On some cars you may net as little as 1 horsepower, on cars like the STi you can loose power. Same with the big wheels and big tailpipes. If the tailpipe rattles like crazy, odds are its not a muffler but a resonator, theyre lighter and louder, but paired with a big pair of wheels and wide tires, your 0-60 time might be a tad slower due to the added rotational mass, more than compensating for the weight saved on a lighter and louder exhaust.

On a similar note, they are still selling the "turbulator" which is a shroud you stick into your intake pipe that helps "atomize fuel" by spinning the air. Works great on direct injection cars :p
 
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Best would be non silenced straight pipes, maybe with a cross over to help extraction, but no restriction is always better. Does that mean all noisy exhausts are better ? Probably not, but the point is while there little correlation between increase in noise and power, the best theoretical setup is the noisiest (and banned everywhere so of little relevance..).
 
Best would be non silenced straight pipes, maybe with a cross over to help extraction, but no restriction is always better. Does that mean all noisy exhausts are better ? Probably not, but the point is while there little correlation between increase in noise and power, the best theoretical setup is the noisiest (and banned everywhere so of little relevance..).
I have an old Hennessey exhaust on my '91 and it has to be completely empty inside. It's super light and super loud! I'm tempted to cut it open and see if there's suppose to be some fiberglass packing that's blown out. I drove it that way for four years and does sound pretty damn cool, but it would be nice to tone it down a hair.

Edit: Forgot to mention it still has OEM cats on it.
 
I'm trying to remember the actual numbers from the folks dynod locally (columbus ohio) from the 2014 NSXPO folks which i co-ordinated but was not able to attend. The comptech/header/intake/exhaust was a quite significant high rpm HP increase and a solid 26 plus increase in torque. I'm thinking around another 6 HP from cat removal. If you don't have emissions tests, pull those cats off IMO! That said of course the HP/TQ "under the total curve" was likely not quite as much, in other words the HP around the 3-4k range went down 5-10. But my take is you are still are looking a solid 20 hp/tq from a header. The take at that time was that most exhausts only (except comptech according to them-but backed up dynos') was that many aftermarket exhausts resulted in a loss of 6-12 peak HP. Bottom line the headers are a must and DO work, the exhaust not so much, and some exhaust systems can certainly cost you throttle response. One thing that Honda engineers excel at is absolute outstanding throttle reponse. Some exhaust systems can f that up. As a very past motorcyle mechanic i purchased a 1984 750 honda interceptor. The throttle response of the motor was turbine like-right now! As a past Harley guy i had to put a K&N exhaust which supposedly netted about a 6 hp increase, but it DID lessen my throttle response from razor sharp to still very good. PS - Removal of cats for most exhausts WILL cause resonances that can be VERY irritating. My very old HKS does not have an issue with no cats, but from what know almost all others do. Just my thoughts mostly from way back when.
 
I can't say whether a noisy exhaust would make less horsepower, however I recently replaced my Pride LTW exhaust with an original NSX-R exhaust and definitely noticed better mid-range power with little, if any, power sacrifice at the top end. This leads me to believe that the C30 engine benefits from a little backpressure that lower restriction exhausts don't provide.
 
I have an old Hennessey exhaust on my '91 and it has to be completely empty inside.
My NSX came mod'd from Hennessey (Stage IV) and I thought that I understood that Hennessey used modified Borla SS exhaust. Mine sounds great BUT I never looked inside.
 
I can't say whether a noisy exhaust would make less horsepower, however I recently replaced my Pride LTW exhaust with an original NSX-R exhaust and definitely noticed better mid-range power with little, if any, power sacrifice at the top end. This leads me to believe that the C30 engine benefits from a little backpressure that lower restriction exhausts don't provide.
The NSX-R exhaust is optimized for mid-range scavenging MUCH better than the Pride. :)
 
It's my understanding that the old HKS units are exactly as stock except 1/4" larger thru-out. What do we know about the R exhaust? I'm guessing that HKS perhaps copied the R unit (?)
 
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