How does SLP, and other companies claim a HP increase on GTP'* exhaust? From some reading, there seems to be a general consensus of 10-15 hp increase for exhausts on a GP. Would we see the same gains? What can i do to avoid paying $500 for an exhaust?

it will be differnt for the GP'* they have a VERY restrictive u bend in the factory exhaust. That is where the 10-15 comes from. also headers are thrown in the mix along with the high flow cat and mufflers

HP from exhaust depends on where the restriction is. For the GP'* its the ubend for us its the manifolds (at least on the 00+). You can put the nicest exhaust you want on a Bonneville but if your still running stock manifolds it wont help much.

Now, if your changing it just for the sound, thats a different story.

With your Series II manifolds you're definetly having those as your bottleneck.

With your current mods your next move should be headers or ported manifolds. Both are a lot of work but well worth the upgrade from stock.

GET SOME HEADERS.

*&* are the cheapest to get.

Then comes SLP and the big dog, TOGS.

Actually its TOGS then the BIG DOG TOGS.

Too add confusion to this place, here'* my opinions.


Go to the local strip. Are they out there bolting pipes up or taking them off?

Terms are being missused here on BC a lot.

Backpressure is the amount of restriction in an exhaust. Backpressure doesn't make more torque. Backpressure can only make the gasses exiting the combustion chamber stay there more then not.

Scavenging, or the Scavenging effect, is how a good exhaust system or header design gives the engine more power. Performance exhaust is all about getting the exhaust gasses out from the combustion chamber as quick as possible. The exhaust pipe has VERY little effect on this. All engines benefit from this, highly tuned ones get A LOT.

Header design is the most important part of power in an exaust system. The key to the most power is getting it to exit fast. That usually means keeping it as fast as possible while giving ample room for al the gasses (diamter). The biggest constraint is the collector design, and length of the collector. As long as possible, while having very narow angles between primaries, and flow benched joints.

The exhaust diameter is the same, as long as you can get the air out as quick as possible, it doesn't matter what size it is, but if you make the diameter 3x the size of your collector, what'* the point?

Oh yeah, get some damn headers.

The stock cat-backs on both the GP and Bonnie flow pretty well. A nice 2.5" exhaust system with high flow mufflers will get you 10 to 15 HP depending on the application.

It really isn't necessary though. The power won't be that noticeable.

PEM'* or headers will give you probably over a 15 HP gain. Some headers can give more.

My downpipe/cat/ubend combo probably yielded me 10 to 15 HP.