UrS4

Hey guys hoping you can help educate me on exhausts (namely the SLP LM),

I went through the exhaust stickies and did some searching but I was hoping for a straight answer.

I thought that when the SLP LMs first came out everyone said that they lost hp (3-8hp, I hope that is not that much at the wheels?), because they were too free flowing and there was no backpressure. The back pressure aided in fuel scavenging in the cylinder providing power and torque. And yet with more flow, it is better because it removes all the spent gases and fills the cylinder with more fuel and air (instead of burnt fumes) to make more power. So a little back pressue is good but this benefit can be overcome by better flow??? However, back to my questions about axle backs. . .

This is confusing to me because countless people say that the stock mufflers flow so well that changing axel backs is purely for sound. So which is it???? If the stock mufflers flow so well and you can't gain hp from changing them out, how can you lose hp when changing to another free flowing pipe? If you need back pressure, gained by restricting flow, how do the stock mufflers flow so good?

Or is it that the stock mufflers flow pretty well, about as good as any other muffler, but provide enough restricted flow to maintain back pressure?

Lastly, numerous people say that there is a significant amount of hp is to be gained from changing out the mid pipe (catless or catted) and headers. Isn't the hp gain due to better, less restricted flow? If that's the case how can reducing the flow by changing out the mufflers to LM hurt power?

thanks guys

bl1nk

Ok, well as far as the LMs, until someone provides dyno results that LMs lose power, its just a rumor. Anything that improves exhaust flow will be beneficial to the engine, even more so in FI situations. So my answer is no they wont lose power, and also the same for gaining. While NA, the improved flow is not enough to gain you actual HP so everyone says its just for sound, which it pretty much is, but anyone who is blown will tell you switching out the stock exhaust for something better can net you quite a bit of HP (and thats just axle backs). Headers - longtubes are the best, some claiming up to 30 hp but more realistically 20 at the wheels, shortys go for about 3-5 at the wheels as well as sounding more obnoxious (IMO, I heard them and didnt like the sound). Midpipes are good, O/R setups can claim up to 10 HP or so with an X pipe with the H following behind by about 5, high flow cats are slightly under those numbers. The midpipes also change the sound alot from what Ive heard and been told. Havent heard anything about how longtubes change the sound, youd have to ask someone who has them.

rmays06

I haven't heard any that lose power. The ones I looked into (which is most of them) make like 5-10 HP. Lots of info on the internet and dyno sheet out there.

mlawdawg

You have to look at where they make/lose power. Usually, with some cat backs you may lose a little low end torque or HP. If yu have other changes (s/c, intake, lt header, mid pipes) to increase flow before it gets there, it likely won't matter.

billyjo

I changed from my headers back with high flow cats and the gain in power was incredible. It made her breath a ton easier and all the work i added to the front of the engine like cold air intake and such now has the same going out. I noticed increases power on on levels. I hope this helps

CrazyAl

I think this explanation might help a little bit, even if I do get flamed for it:

There is a long-standing notion amongst hot rodders that "too little backpressure is a bad thing" or "some motors need a little backpressure to run right" This is a rumor, and it is NOT TRUE. Backpressure in the true sense of the word is ALWAYS a bad thing in exhaust systems. Backpressure reduces scavenging ability because it prevents the exhaust gases from leaving the cylinder as easily as they otherwise could.

After making this statement I'm sure a lot of people are now thinking: "That's crap. I took the mufflers off my old whatever and I lost power...." or something to that effect. The problem is that this is not a valid comparison. When you change an exhaust system in that manner you are changing TWO things. Backpressure is one of them. The other is the resonant frequency of the pipe. The resonant characteristics of the exhaust is often confused with backpressure, and that is where the real power-loss culprit is.

Think of it this way. Your exhaust system is basically a pair of long tubes that start at the engine and go back to the tips behind your bumper. Any tube will resonate at a particular frequency. This is the principle by which a pipe organ or a brass instrument (like a trumpet) works. Changing the length or the diameter of the pipe alters what it's resonant characteristics are. Resonant properties are important because they are what determine the optimum performance range of the exhaust.

For example, many-hot rodders will relate to this example:

Headers with short, large-diameter, primaries are good for high RPM power.

Headers with longer, narrow, primaries are good for low-rpm power.

Both sets of headers have negligible backpressure. If you put them on an engine and added a pressure gage, you probably couldn't measure the difference between them. However, their powerband characteristics would be different because their resonant frequencies are different. (A long pipe resonates at a lower frequency than a short pipe).

If this is still not clear, go read The Design and Simulation of Four-Stroke Engines by Blair, published by SAE.

Anyway, suppose someone put your mufflers of choice on a car and Dynoed it (Before and after). Let's suppose that the car did loose a few HP.

Backpressure would NOT be the reason why. The reason is that the resonance properties of the exhaust changed. And if you look at where (as in where in the rpm range) they changed, you could fix the problem by adding or removing tubing to bring everything back closer to OEM spec.


I have heard that the LM's loose power, having read that here on the forums. But like Blinkage said too, I have yet to see any kind of dnyo charts to back that up! I have also heard some brands of axle-backs claim to add HP--and usually those claims come from people trying to sell you them. Again, I have never seen a dyno chart that supports that claim either.

Based on the dyno runs that I have witnessed, I am going to conclude that swapping out your axle-backs for LMs or any other brand has a negligible effect on HP. In other words, no effect, or the effect is so small it's not measurable or significant.

UrS4

Thanks CrazyAl,

That really helped!

Time to install my SLP LMs