How much backpressure do i need on the engine

Posts: 205

i have an extra hole in my exhaust that i drilled out and put a bolt in just in case.

but when i remove the bolt it lets out twice the amount of exhaust and is alot louder which i hate for just anyday riding but it give it alot of noticeable torque.

what will happen if i do not have enough backpressure on the engine.

Posts: 616

good question...3 weekends ago i was riding at the mud hole and my muffler seperated from the pipe. straight crack. so i was riding for about 4 hours with about 10 inches of just pipe right off the head. more torque was noticeable. i also would like to know what could be damaged with no back pressure. Question

could it mess up the valve spacing? or warp the head?

Posts: 205

hey stang i meant to tell you something.
we are constantly in the mud and water!

notice when you are in the mud your engine bogs when the muffler is underwater?

install a snorkle! i know everything is about to protest because it kills the power but let me explain.

i welded about a 3/4 pipe onto my muffler and fastened it with some ubolts to the engine carage.

okay yes it killed the power but i drilled a hole about as big as my pinky finger in the end of the muffler just below my snorkle so now when i bury the engine in water the it forces the exhaust out the snorkle,

used to if i let the engine idle underwater it would kill the engine and once it let water into the cylinder but now it just sprays a mist of water out the snorkle and leaves me with plenty of power because it redirects the exhaust from the hole i drilled in the muffle to the snorkle.

i have had it pretty deep several times and it never hurt the engine but im not saying that it wont hurt it. try it at your own risk cause im a noob

Posts: 616

how the heck did you get your buggy like that? do you have a wench on a branch up there or something?

Posts: 205

lol yeah i used my 2k winch and just sat down and pressed the button. it is attatched to branch

Posts: 12

Hey Guys
Some info on back pressure. The short simple answer is as follows. Back pressure is good for low end torque. This is why you will see "stepped" head pipes on performance engines. At low end grunt this restriction builds torque, as you start to accelerate, the increase in exhaust gases build and then as it hits the second larger part of the pipe, it creates a drop in differential or vacuum and simulates a venturi like the carburetor. The faster the rpm the better the flow at this point. It’s kinda like the reverse of a carb. A carb uses a large opening and then decreases the size as you go toward the engine to make more velocity because air is drawn in and speeds up. The exhaust is pushed or forced out so the same principle applies. That being said, do not run with short head pipes because during the deceleration process, the engine now will draw in "cold" air that can damage valves especially after a full RPM hammer run. How short is too short........That will take some calculations, but know if you bust a pipe out in the track, run as slow as possible for as little distance as possible to get back home. Hope this helps.
Keep the petal down and the buggy upright!

Motofreak

Posts: 616

thanks for that info. very helpful on my end. i think ill just make another exaust at work this week when i get bored and keep it around for a spare when im out riding.

Posts: 205

so what happens if i have the stock length but cut a huge hole in the tip of the muffler to let more exhaust out. i wouldnt think that would cause it to suck air

Posts: 648

A good old school technique for tuning that pipe for back pressure and optimum flow is to run a length of primary pipe the same interior diameter as the exhaust port, paint a line down the length of the tube, run the motor at full operating temp for a while. the exhaust heat will burn the paint to a point down the tube.That is the point where the exhaust gasses start to cool and contract. That is the point of reduction in back pressure.add a couple of inches and cut the pipe at this point then add one pipe size larger. this creates a vacuum/void that causes each exhaust pulse to suck the preceding pulse along behind it like a train. no set length on the second pipe but the longer the better. run it for convenience then step up again to match at least a 1 1/4 inch or better inlet size muffler of choice. The bugle style pipes used on briggs motors were first designed using this technique.We used to find our optimum V8 header collector lengths this way. If your using the stock pipe, do the paint thing and substitute a larger pipe from the burn point to connection point of the stock muffler then open up the muffler outlet to match. getting the step up in at the right spot is the key.

Posts: 205

so the tip on my muffler needs to be the same diameter as the pipe going into the engine?

what about these power pipes,they have a huge exit hole for the exhaust.

im just worried i cut too big of a hole at the end of my muffler