Can I use 5/8" square pipe for an exhaust?

[tonyslowrider]

I have a 4 stroke motor, the exhaust is 7/8" like a 2 stroke exhaust. But I want to use a 5/8" square pipe for a custom exhaust. I cut off the end of the 7/8" pipe that attaches to the motor, and welded it to the square pipe then bent the square pipe. Will it damage the motor or make it run bad? Or will it be ok?
Basically what I need to know is, will using a pipe that is slightly smaller than the usual exhaust pipe cause a problem with the motor? It is only a 2/8" difference.

 

[wheelbender6]

It shouldn't hurt your engine. You may lose a few hundred RPM at the top end.

 

[crazyman]

Does your engine have a round exhuast port or a rectangle? If its rectangle, it may be benefical.

 

[tonyslowrider]

IT has a round port,But I made the square pipe round at the front,Then I welded the end of the 7/8" pipe that attaches to the motor to it, So it's like the original exhaust.

 

[loquin]

You don't say how big the engine is, which could make a difference. (A 25-35cc motor may have no problem with a narrow exhaust. A 50cc or larger motor could well have 'breathing' difficulties.)

The cross-sectional area of a 7/8 (inside) round tube is .6 square inches, and the area of a 5/8 (inside) square tube is .4 inch, so you're looking at 2/3 the area when using the square tube.

BUT square tube measurements are normally outside (they are normally used in structural builds.) What is the inside measurement of the 5/8 square tube? If it has a 1/16 wall thickness, then the inside measurement is 1/2 x 1/2, and this means that the cross-sectional area is only .25 square inch. If it has a 1/32 inch wall thickness, you have a .32 square inch area.

If you only have half the exhaust area (or less!) you'll be throttling the exhaust gas flow considerably...

 

[HeadSmess]

nice use of maths '.'

square pipe in itself wont do any harm, but using the wrong size will.

do a funky 3 pipe job

 

[tonyslowrider]

I measured the square pipe top to bottom inside, it was 5/8, corner to corner it was 7/8, then I made the end of the square pipe round so now the pipe measures between 3/4 an 13/16.

 

[Quenton Guenther]

If this is a 49 CC 4-stroke [HS 142 or Honda GXH50], the exhaust is major, and can alter the motors performance greatly.

We ran over 20 different exhaust setups on a dyno over a 4 day period, and the results were surprizing.

Not only I.D. but length, and interior design were major factors. We tested 1", 7/8", 5/8" and 1/2" I.D. pipe. We tested lengths from 24" to 48".

We watched the motors run very lean to very rich, and in one test a piston was destroyed [hole in piston from high RPM in a lean condition on a Honda motor].

Results indicated a 7/8" I.D [round] flex pipe was by far the best, and the length controlled the torque curve.

Have fun,

 

[loquin]

The interesting thing about a square pipe is that it may provide a little more muffling than a round pipe, Quenton, even if the cross-sectional area is the same.

The pressure pulse propagation speed through a tube is proportional to the diameter. The larger the diameter, the faster the pulse propagation speed. A 3/4 square tube has almost the same area as a 7/8 round tube.

But, the 'diameter' of the 3/4 square tube varies from 3/4 inch out to 1-1/16 inch, corner to corner. The pressure at the corners of the tube would get to the end of the tube a little faster than the pressure at the flats of the tube. This would cause the pulse in a square tube would get 'stretched' a bit as it passes through the tube. The average pressure would stay the same, but the peak pressure would be less; the pulse would be spread out over more time.

And, back to the OP's question... Based on your testing Quenton, it sounds as if while that 5/8 square pipe would be just too small for a 50cc class engine, it might be OK for a 35cc motor.

 

[Quenton Guenther]

Hi loquin,

Exhaust is one of the most important considerations on a small 4-stroke motor. During my 15 years racing Motorcycles, I won most races because of tuned exhaust systems. On my record setting Whizzer [over 70 MPH], the exhaust system is the reason my flat head motor can turn over 8,000 RPMs.

As an example....... During tests on the 49 CC HS motor we found the interior of the pipe made a large difference in overall performance. We tested LP gas pipe, the same I.D. and length, however it ran almost 4 MPH slower. Next we tested electrial condutit with a smooth interior and it managed to re-claim the 4 MPH difference, but is impossible to bend without "kinks". We also tested the length of each pipe, and watched the torque on the DYNO shift around. We found shorter pipes added top end, and long pipes add low end torque [exact opposite of test pipes on a Ducati Desmo racer & HD XR750].

Don't have a clue about using square pipe, as I have never tried it.

Have fun,