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Maritimes Guy
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97 Posts
Discussion Starter #1
Just wandering, my buddy's got a indy 500 1990 or so with a 400 liquid in it, Hes got hes muffler gutted and its sounds kinda good actually.

I wanna know if it's dangerous to do that for the engine? any of you people who runs their sled like that?

any power gain or loss?
 

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SD Rider
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4,337 Posts
Pretty bad idea. Without proper backpressure, you're gonna be backfiring a lot and hurting the engine more than anything else. It probably won't run very good and you will definitely lose power that way. If you want to melt down a cylinder in fairly short order just to get your sled to sound cool, then hey, who am I to judge.
 

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Hank Scorpio
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112 Posts
Just wandering, my buddy's got a indy 500 1990 or so with a 400 liquid in it, Hes got hes muffler gutted and its sounds kinda good actually.

I wanna know if it's dangerous to do that for the engine? any of you people who runs their sled like that?

any power gain or loss?
Like stated in previous posts, it's probably not a good idea. Not just back pressure loss, but compliance with DNR officers and the like. Gotta protect the sport!:thumbsup:
 

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Registered
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323 Posts
Well gusy here is what I can tell you, I have done some playnig around with stock exhaust, some on purpose, some because it was broken and needed repair. Not sure how your buddy "gutted" the stock muffler on his 400/500, I had a 500 and the pipe/muffler is all 1 piece. most of the new sleds you see these days have a 2 piece setup, the actual "pipe" that come off the engine, and the "can" or muffler at the end.

The one piece setup is the pipe and can all in 1, the "pipe" is hiding inside the one piece design.

As far as back pressure, Yes a 2 stroke relies heavily on back pressure to perform correctly, but most of the back pressure is in the "pipe" that come off the engine.
Picture a foot long piece of pipe, now slide a funnel over the pipe to about half way, exhaust leaving the engine expands and is rushing out and trying to exit on the large side of that funnel and find its way to the smaller pipe in the center to actually get out, this causes the back pressure the engine needs, how long that center pipe protudes inside the pipe determines the degree of back pressure.

\/ < << @ @\
\/ [email protected]__\___________
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____________________________ exhaust out
exh turbulance>> /
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I have a trick I have done with the single piped 400's and 500's that worked well, I will be glad to share if anyone wants to know
 

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Registered
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323 Posts
my diagram didnt work so well, sorry
 

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Maritimes Guy
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97 Posts
Discussion Starter #7
Thanks everybody, I think i'll just stick with my stock exhaust,i knew 4 stroke engines needed back pressure but just was'nt sure about 2 bangers.
 

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1997 Polaris Indy 500
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9 Posts
Thanks everybody, I think i'll just stick with my stock exhaust,i knew 4 stroke engines needed back pressure but just was'nt sure about 2 bangers.
Actually, the opposite is true. 2-stroke engines need a certain amount of backpressure to keep the air/fuel mix from getting sucked out the exhaust when the piston uncovers the transfer port. A 4-stroke engine relies on valves to open and close the ports, and also has the advantage of having the piston forcing the exhaust gasses out as it travels upward with the exhaust valve open. A 4-stroke engine will perform better the less backpressure and more velocity the exhaust has.
 

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Certified Mechanic
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56 Posts
2 strokes are designed to run on specific exhaust configurations. This is why 2 strokes have such goofy looking pipes. Sound may be cool for some people, but it will most likely make you lose a little performance/hp. 2 stroke exhaust "back-pressure" is made from the shape of the pipe; it causes almost a rolling effect through the pipe and this is where the back pressure is formed. The silencer/muffler/can is another pressure point for the exhaust. Without the propper pressure the exhaust will blow out faster causing improper burn-off for the fuel after the power stroke. If you want good sound and better performance, look into aftermarket silencers. I run an SLP silencer on my F7 and it sounds glorious! Its fast, fun, sounds great, and no performance was robbed from the swap. They are fairly inexpensive; $300.00
 

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Sconnie
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1,745 Posts
Actually, the opposite is true. 2-stroke engines need a certain amount of backpressure to keep the air/fuel mix from getting sucked out the exhaust when the piston uncovers the transfer port. A 4-stroke engine relies on valves to open and close the ports, and also has the advantage of having the piston forcing the exhaust gasses out as it travels upward with the exhaust valve open. A 4-stroke engine will perform better the less backpressure and more velocity the exhaust has.
Too true!! gotta let the 4-stroke breath!
 

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Maritimes Guy
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97 Posts
Discussion Starter #12
This might be true to a certain extand for 4 stroke engines but,(example): when you start putting like 3 inch exhaust on a stock 1.6 liter honda civic,you loose back pressure and hp/tq. No need to increase exhaust airflow without increasing engine effiecency(port and polish,upgrading camshaft/valvetrain,higher flow heads,higher rpm,compression ratio ...and on and on). right?
 

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Sconnie
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1,745 Posts
Well it has to do with the velocity of the exhaust gases too. You want an ideal size pipe that in effect 'pulls' the exhaust gas out of the cylinder when the exhaust valves are open. headers are a good example of how this works, the lengths are matched to the pulse of each exhaust stroke, so that the 'pulse' when it rebounds from further down the pipes meets at the end of each header pipe. so im not sure exactly how to put it into words but it in effect pulls the exhaust stroke out as it maximizes the velocity. yes smaller pipes will give more tq and larger will give more peak hp. ideally you want them matched
 

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i vote no
 

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MarcusSport440
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55 Posts
isn't the pipe gonna give it some back pressure? i got a 91 sport 440. if your familiar with it. its got a single pipe and can. doesn't seperate. its stupid. i want it to be a little bit more louder. Any way i can gut it out a little bit?
 

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Maritimes Guy
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97 Posts
Discussion Starter #16
Tryed the gutted pipe, did not like the side effects...had a so so sound, but after the motor warmed up, I had hardly any blue smoke coming out of exhaust(normally i have plenty). I take it as it was a bad lean condition. donno what caused the lean condition??

anyways, took that off and stock pipe will stay. reajusted air mixture screws:1 turn and 1/4 from seat. 1 turn out is factory setting right? starts up now at 2500rpm,for 3-4 min. then drops at 1500..starts first pull warm and holds idle too. Plugs are light greyish-brown color.

the only thing i dont like is my 100 psi and 95 pto side...ahh well, i'll stay like this for now and im gonna send block for boring next summer. .040 over.
 
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