i just dont understand why you need to rejet a 4banger if you put an exhaust on

the exhaust isnt used to create power in the same way a 2 stroke does, the exhaust stroke is totaly seperate as well also unlike a 2 stroke,

so whats it all about???

 

ooh thats a good one but i think i have the answer. k even though it is a separate stroke entirely a better flowing exhaust will rid the chamber of more exhaust gasses and let more fresh air come in. this will increase in more fresh air wich needs more fuel. but since this is a totally separate stroke and does not help as much as a two stroke that is probally why two stroke respond better to pipes.

 

thanks for the reply, i just started to write another question but managed to make sence of it by the time i had writen it, LOL

good info there

thanks

 

I disagree.
The only reason exhaust makes ANY difference in a 4 stroke is because the exhaust stroke is a completely parasitic process in that it USES energy. In a single cylinder, the only thing that makes the exhaust stroke happen is kinetic energy stored in the rotating mass of the crank and the piston and the flywheel. A 2 stroke also requires this same kinetic energy, but in a 2 stroke, immediately after the exhaust stroke is another power stroke. After the exhaust stroke in a 4, there is a whole other stroke, including the power hungry compression stroke, so it makes sense to lose as LITTLE energy as possible on the exhaust stroke, thereby having less back pressure to let the exhaust out easier. Having a slightly restrictive exhaust provides more low-end torque, generaly, because the back-pressure leaves some pressure in the cylinder to help push the piston back down for the beginning of the intake stroke, right before the intake valve opens.
2 Strokes don't respond better because less restriction lets IN more fresh air, if that were true, NO exhaust would be the best! A good 2 stroke pipe actually pulls exhaust out of the cylinder, and fuel/air in by utilizing the shock wave of the exhaust expanding outward into the cone shaped chamber. A 4 stroke doesn't benefit from this because the cylinder is a closed chamber during the exhaust stoke, with no way to "pull" anything in.
For high revs and top end, 4 strokes need zero back pressure, that's why at the drags you see the hottest rigs (dragsters and funny cars) with super short straight pipes right out of each cylinder.

Does that make any sense? It's late, so I may have rambled some...

 

Or to look at it another way: Lets say you have a toilet and you have a turd going down the pipe. With one or two turds, it's not going to be a problem. With 8,000 turds a minute, the turds are going to start to clog up the pipe and start backing up. If you have a bigger sewer pipe the more turds you can get rid of before they start clogging up the pipe.
The faster you can get rid of the waste the more power your engine is going to make. Like kybabyn said, the better the entire exhaust system, the better your engine will breath and the more power it will make.

 

So in theory what would a FMF Ti-4 type(pro-circuit 4, etc) pipe style be best for...I am thinking the higher speeds and more towards WTO.
Givin that, what would be the best low speed technical torque producing pipe/style?
Based on a Honda 400 EX.
Regards, Please reply.

 

A novel theory but I have to disagree. Except for very specialized circumstances, virtually all 4 stroke motor cams are made with a degree of 'overlap'. This means that the intake valve is opening before the exhaust valve is closed. This is to take advantage of the inertia of the intake charge, just as is the fact that the intake valve closes a certain number of degrees past the time the piston has reached bottom dead center and is on the way up again. The inertia of the air/fuel mix flow keeps it going. This valve opening overlap, however, means that there is no opportunity for any exhaust to remain in the cylinder to push down on the piston at this time. The intake valve in fact begins to open before the piston even reaches TDC of the exhaust stroke, by the amount, given in degrees which figure can be read on the individual motor's cam specs. If there were any expansion going on in the cylinder with the intake valve open then it would interfere with the incoming fresh charge and be counter-productive.

 

Maybe someone else can talk specific brands. I can only talk generality.
In general, if you want low end torque, you want to go small(er) on the pipe and induction system.
If you want top end power, you want a big(er) pipe and induction system.
Like aksafari pointed out, this will push your torque curve around.
If you have a small pipe you have a lot of flow (energy) coming out of that pipe. This will draw the next exhaust pulse out of the cylinder when the valve snaps open. The draw back is the turds get backed up on top end.
If you put a big pipe on your engine, the turds won't get backed up when your WOT but you will have less draw on the cylinder when the valve pops open. This will leave a contaminated cylinder for the next charge.
So it's a trade off game. Same way with the induction system. You can put a big hogged out intake manifold on your engine but you will loose low end power until you get the RPM's up.

BTW, I agree with Glenlivet on the piston thing.

The only time you will see zoomies on a car is when they are running a blower. Then they don't care about exhaust scavenging because they have twice or more exhaust flow, and want the waste out pronto. Look at Pro-stock, or any other class without blowers, they still run 4 into 1 headers.

 

I knew Glenlivet would weigh in. Even if I said the sky was blue, he'd come back with an argument against. hee hee

Like my disclaimer at the end said, it was late, I wasn't clear. Doctor Turbo seemed to put it well, though. Exhaust restriction TO A DEGREE affects low end torque in a positive way. Then, at higher revs, affects power in a negative way.

2 strokes are MUCH more scientifically designed and depend heavily on expansion chamber angles, size, and baffling.
What really boggles me, however, is why all the snowmachines I ever rode seemed to be more forgiving in the power band than the lt250r and the Banshee I rode. Don't lecture me about CVT clutches. That's not what I'm talking about. My AC Panther 500 and El Tigre ran very well at lower R's, as well as at WOT. Maybe stock carbs and reeds were the reason? I know the two 2-stroke quads I rode were modified. My Arctic Cat's were VERY reliable. Seems like the 2-stroke power band would be very advantageous with a CVT system... why isn't there a utility quad with a 500 twin 2 stroke?
That would rock!

 

I never really thought about it. I just figured you needed more air and fuel to make more power. I assumed you have a pipe that would let more gas out, so you can now put in more fuel.
Aksafari, What kind of El Tigre did you have? Did it have a tuned pipe, or a canister? My El Tigre hits the hardest at higher RPM's but have a very good midrange as well. What was your clutch engagement rpm? Mine will pull hard from 4,500 to the very top.