exhaust backpressure question

[ride_or_die86]

hey guys, this might seem like an amateur question, but my buddy recently gave me a set of pipes that have almost non existant baffles in them. just universal slip ones that he had on his bike but were "too loud" (like thats possible) anyways, i'm thinking of taking my mufflers off and welding these shorties on. is there a big issue with back pressure? i don't want to detonate the engine, obviously, but for the look i'm going for these pipes are mint. ive seen other sprtbikes run with almost open exhaust or a flow-through system. will it harm the motor at high rpm's with nothing holding the exhaust back? the bike is an 86 750F.

[masonv45]

About 90% of your backpressure comes from the common collector under your bike. So if you only replace the mufflers, you probably won't have to rejet.

Now if you are replacing an entire system, I have a 4-1 Kerker on my bike, and it is LOUD. There is no collector and the muffler itself provides the backpressure.

You shouldn't have to weld the shorties on. If it is a slip on, you probably just need a reducing collar and a clamp.

[derstuka]

Well, you need to look no further than racing engines to see that less backpressure can mean more power, however you would need to re-jet your bike as it now can flow exhaust much more freely, but you may lose out low-end torque in the process.

Well, more power, that's why people put larger diameter, or shorter exhaust pipes on their car/bike in the first place, but in the process you can lose low end torque, because the same volume of air moving through a larger pipe flows at a lower velocity, or in a very short pipe creates less suction, which leads to poorer scavenging, so less fresh air into the cylinder with each stroke, and so forth. Some mistakenly think that the loss in torque is because of "less backpressure" as though backpressure was a good thing. It's not. The best exhaust systems simply take all the flow restrictions out of the smaller diameter piping thus decreasing backpressure for higher horsepower, but maintaining the same high flow rates for good low end torque.

I know that some engines seem to sometimes need a certain amount of back pressure to run best, however, as long as you re-jetted your engine corrected so you don't burn a hole in your piston, or burn your valves, seems like it should be ok. I can remember running my old Fizzer with straight headers to see what it was like and all the low end torque was gone but the midrange was killer....without rejetting of course.

[ride_or_die86]

thanks guys, so i'm guessing ill have to fatten the carbs up with bigger jets to compensate for the increased exhaust flow? something like a dynojet complete kit or similar? ive got a friend who is an absolute pro at anything carb related, so it shouldn't be a big issue to get that right. i rode the bike once when i put it back together, with just the collectors. the straight pipes sounded sick at idle, but id didn't go above 3 grand for fear of engine damage. how much is a basic carb kit? i may as well rebuild them while im at it.

[masonv45]

Go up one size on the slow and main jets. See if the bike will run decent. If it does, then take it to a dyno with an exhaust analyzer to determine where you are rich/lean.

If it doesn't run decent, keep going up in size. Typically cheaper to order the jets individually than to buy the kit...

[RedDukeRider]

In addition to my VFR, I own a KTM Duke. At a ZRXOA rally back in the spring, a shop showed up at the rally with a portable dyno.

I have a set of Staintunes (with baffles) on my Duke and did my first run on the dyno. 53 hp. I took the end baffles out and the numbers dropped down to 50. There has to some back pressure - the amount depends on the exhaust cam duration more than anything. The only way to know for sure is to put it on a dyno and try it both ways.

Louder doesn't not necessarily equate to more HP.

[masonv45]

Originally Posted by RedDukeRider

Louder doesn't not necessarily equate to more HP.

Try telling that to the HD crowd...

[RedDukeRider]

Originally Posted by masonv45

Try telling that to the HD crowd...

Anytime I get that "loud pipes save lives" BS, I ask them "Why don't you just put a piece of duct tape on your horn button so it blows continuously when you ride?" I hate loud HD's with a passion!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Speaking of which: One of my winter projects is to mount dual Fiamm Freeway Blasters (139 db's) on my VFR. $19.95 each from Pep Boys. Got a high tone and a low tone. Don't know if you've ever heard them both go off at once, but they will wake the dead. Using an Easterbeaver.com heavy duty dual horn harness I got from Jim Davis. Lots of juice for those babies!

[squirrelman]

You alone will enjoy the loud pipes.
don't you care about anyone else??
don't care about excessive, needless noise??
Grow up and learn to blend in, not blast your precious right to make a racket.