Actual gains from exhaust modifications

Just curious, on what some of the gains of exhaust mods are, I mean on anything, car, truck, diesel, bike, weedwacker. I couldnt sleep last night and apparently this is what I think about. dyno graphs and % total gain for both HP/TQ get you internet cookies

Well I dont have dyno numbers, but when I had my civic I ran 17 in the 1/4 stock. After full exhaust and "colad air intake" I ran ~16.7. And my buddys camaro gained about 20 HP with full bolt ons as well according to the dyno he was on.

when I bought my Integra LS (new in '01) it didn't take long before I bought a header for it (DC Sport 4-2-1) … I put the car on a chassis tyno and it came up as 105 whp

went back a week later with the header installed (no mods to the rest of the exhaust system) and it measured 121 whp … (~ the same temp / humidity)

adding a CAI (not exhaust, but certainly part of the air-in-air-out cycle) and the result was 131 whp

and torque increased a corresponding amount … from (to the best of my memory) about 95 ft/lbs to ~115 ft/lbs

after replacing the cat back with 2 1/2" piping and a low restriction muffler I didn't get it rechecked … doubt that that would have made much more than 1 or 2 hp difference … statical noise

This is the info I was looking for keep them coming guys! I would like to see gains from a larger downpipe on something with a hairdryer

Well no experience, but from what I have read you can gain quite a bit opening the exhaust up a few inches. More than I can say for a N/A car, since N/A needs x-amount of back pressure. Snails dont need near as much. But thats jsut what I have read in an article or two about it. So I could be wrong and please, anyone feel free to correct me.

Long and short depends on the car, I know most turbo cars do much better with an efficient downpipe (wrx, MS3, GTI and so on) but not so much with the cat back exhaust.

With N/A cars it is really all in the headers from my understanding, I think Keith would definitely be the man to chime in here.

Well, the wagon gained a ton of power with an exhaust. The factory was a 2" crush bent exhaust for a 307...then I made it worse by installing a 350. Long tube headers, high flow cats, high performance (whatever that means) Y pipe and 2.5" mandrel bent tubing all the way back made an incredible difference. No numbers to back it up, but it was the difference between towing in 4th and 5th with 3 bikes in the trailer.

My Formula that I had seemed like it gained at least 25 HP with intake, LT headers, ORY, and Hooker cat-back. So to say the least, opening air flow you should see some noticable gains.

Depends on the car.

Most newer cars don't get anything out of a CBE except a little sound. Ditching the primary cat and doing with headers is where most of the gains are on newer vehicles.

Rotaries make a lot more power when you open the exhaust up. Most of it is likely due to removing the cats, but there's quite a bit that goes into the tuning of a rotary exhaust system for max gains.

My E36 S52 race car picked up a nice fat 18 ponies between 5100 and redline by going to a 3" mandrel bent single 2:1 (from 1.75 dual w/ xover). I gave up some torques down low but... a race car never sees 3-4k unless there is a yellow flag so it's all win. Unless you forget the ear plugs.

EDIT: Also note the intake was also opened to the extent the rules allow... more air in, more air out. The tune was matched to the changes so - it's not just an exhaust mod.

In reply to DaveEstey:

Not so much on a renesis, granted I have a test pipe on mine but that is for safety (and it was free lol) the renesis engine as I'm sure you know is stretched almost all the way from factory.

z31maniac wrote: Depends on the car.

The gains on my 2nd Gen RX-7 were significant because I had more than one cat and a collector type exhaust manifold (square).

I think there's some improvements to be had on legacy small blocks and the inline-6 on my Impala (even though there are no catalytic converters) because of the crappy manifold design.

I think there's less to be had on my Alfa GTV6 because it comes with sort of an "improved manifold" design (its still cast iron, but it's in the shape of headers) and the exhaust is reasonably well tuned (but I bet I'd pick up a few points).

I think there's even less to be had on a more modern engine (say my buddy's new Civic) because most engines come with a tube-type manifold, and catalytic converters aren't the corks they once were.

If I were to that point I would do what I could to squeeze the last few ponies out, but wouldn't get my hopes up too high.

My rule of thumb has always been that exhaust mods are something you need to do to accommodate other mods. In other words, on a stock engine, the gains will be minimal. But if you start modifying the engine, the exhaust becomes a choke point that must be addressed to get the full potential of the other mods. There are always exceptions, of course, and IMO older cars had more restrictive stock systems then newer ones do. The OEMs have gotten pretty good at making exhaust systems efficient and quiet.

In reply to Tom_Spangler:Well said. If you start modifying, pretty much everything involving the engine is a choke point somewhere in the process. If you improve A, then B is an issue, and so on.

Once you start getting past those choke points, the next issue you'll probably encounter is the break points. Your ride will usually show you what the weakest links in your drive-train are, sooner more often than later.

It all depends on what the stock setup is like. Some cars respond very well, some respond barely at all. The OE engineers are good at their jobs, but constrained by things like cost, packaging and sound levels.

As noted, you address the choke point. On the Mazdaspeed Miata, for example, changing the muffler alone does little. But the downpipe will gain you more power and response. With a more free-flowing downpipe, the engine will now respond to a different exhaust. The combination of the two is good for 23 hp on an otherwise-stock car, which is about a 15% gain.

Dyno!
Exhaust alone: http://www.flyinmiata.com/tech/dyno_runs/MSM_exhaust.pdf

Exhaust with downpipe: http://www.flyinmiata.com/tech/dyno_runs/FMspeed_exhaust.pdf

Headers are different and very cool. If designed properly, you can use the timing of the exhaust pulses to create standing waves in the pipes. This gets you regions of high and low pressure - and if you put one of those low pressure regions at the exhaust port, you suck the exhaust gases right out of the cylinder. This makes them more efficient than no restriction at all in that area of the rev range. The coolest part? If you're of a musical bent, you can hear it. I built a mad scientist header for one of my cars and the audible harmonics were wild.

Adding an open air filter and header to my 6.5 HP kart engines nets about a 3+ HP gain, fwiw.

89-95 Taurus SHO's gain 12-20whp on average by deleting the catalysts on the y-pipe. The rear bank goes straight into the top of a catalyst and goes 90* to go through it. Worst choke point I've ever seen on a vehicle.

Conversely, a catback or header were useless on my 318ti as it had a nicely designed 4-2-1 tubular manifold stock. It lost a little on the bottom end in exchange for negligible gains up top....the 32 lbs reduced from the flywheel made all the difference on that car.

Did this..

Gained 60 decibels.

It varies tremendously, depending on how compromised the stock system is.

GM fabricated the Fiero headers for the V6 out of high alloy tubing, but they did it using an assembly machine that pinched the stubs of the primary pipes to fit into the main primary that they all (3 per manifold) joined to. Needless to say, the pinch reduced flow. I can spend a half hour with a high speed grinder (or 10 seconds with a milling machine, not including set up time) and simply bore the holes to gasket size removing the part that intrudes on the gas flow. You can pick up 10-11 bhp from that mod alone.

Guess they figured it would have been one added machine operation, and they didn't bother despite the relatively huge pay off and small cost.

BTW, if you own a V6 Fiero, they also hid another significant bottleneck where the collectors join in a 'Y'.

GM also made a bad choke point in the 3800 series engine on w-bodies(grand-prix, mont carlo etc.) on the downpipe. Whose idea it was to put a dip in the exhaust and put the O2 sensor on top rather than on the side with a straight pipe leading to the cat? Just replacing that alone I think adds 5-10 HP.

tjbell wrote: Just curious, on what some of the gains of exhaust mods are, I mean on anything, car, truck, diesel, bike, weedwacker. I couldnt sleep last night and apparently this is what I think about. dyno graphs and % total gain for both HP/TQ get you internet cookies

While it obviously has a lot to do with how restrictive the stock setup is, here is the basic take on a gasoline NA exhaust. Most gasoline engines have valve overlap. There is a period while the exhaust is closing that the intake valve begins to open. During the exhaust stroke the cylinder pressures rise as the upward movement of the piston pushes exhaust out. As the moving exhuast mass reaches peak velocity, the piston is beginning to slow down and actually causes a small vacuum. The cam events are timed so that the intake valve opens early enough to take advantage of this scavenging and begins to pull intake charge in even before the piston starts to drop. You want an exhaust size/flow to match that tuning. Going too big causes the exhaust flow in the pipe to be slower and have less inertia, therefore less scavenging and less intake charge gets pulled in. Going too small means that you start running out of flow at higher RPMs and it restricts breathing. In this way, smaller exhuast flows favor lower RPM torque production and higher flows favor high RPM torque production. Generally speaking, 300 mph is a kinda magic number. Exhaust speeds higher than that begin to get exponentially more restrictive. Choosing a "proper" exhaust flow will mean you take advantage of as much scavenging as you can while not reaching restrictive flow velocities before redline.

If you add a turbo, the general rule is the bigger the better. This is for two reasons. First, cams ground for turbos tend to have little or no overlap, so the scavenging you would get is gone; having been replaced with positive intake pressures. Second, turbos only slow down when faced with backpressure. Just make it big and get it out of there.

Diesels are a similar story regardless of turbo or not. Most diesels don't have any valve overlap so scavenging is non existent. Turbo diesels respond well to larger exhaust for the same reasons a gas turbo does.

In two stroke applications, scavenging is a big deal. Since the crankcase provides all the pressure for moving intake air and getting rid of exhaust is more passive, maintaining proper exhaust size and tuning is pretty critical. Much research is put into the shape of 2-stroke exhaust and it shows with some of the aftermarket mufflers and pipes for things like dirt bikes and snowmobiles.

Here is a google image search with some 2-stroke exhaust designs. Strange how they go from tiny to huge to tiny again. Linky

Every car seems to have its weak link. I had a 215 hp 4.6L Ford Truck and the hot mod was to cut out a cone-shaped restriction in the air cleaner box. It necked down to just over 2" to make it quiet at full throttle. Removing that cone was often times responsible for 15 hp. Until you removed that weak link, no amount of exhaust mods would net you anything but noise. Contrast that with something like a Cummins 5.9L in a dodge pickup that has an adequate intake but you can pick up 25 hp by removing the muffler.

BeardedJag wrote: GM also made a bad choke point in the 3800 series engine on w-bodies(grand-prix, mont carlo etc.) on the downpipe. Whose idea it was to put a dip in the exhaust and put the O2 sensor on top rather than on the side with a straight pipe leading to the cat? Just replacing that alone I think adds 5-10 HP.

Because its GM. That's not an insult, its just truth. Even Land Rovers and Ferraris have shortcomings that result from trying to engineer something that functions before the deadline, its just that lower-priced marques have more. Then the aftermarket has years to come up with a solution and everyone says "why didn't GM do that?"

Turbo cars gain enormous power from exhaust typically.

Similar gains of 10whp at peak and near 20whp at redline going from stoc ~2.25" to 3" single exhaust on small (2.8-3.2l) v6's.

Imho I really have not seen the "na engine needs backpressure" play out. For me, less has always been better.