full exhaust question

[interceptorz28]

Hey all, What is the story with performance of a full exhaust on a 400ex?

Do they not create enough backpressure and do you lose low end?

I had a e series slip on and a rev box and jetted on my first 400ex and that was perfect. I did see some good prices on some full systems though. This '03 I just got, really needs something soon.

The bike is basic, not mx or a drag bike. Stock internals and what not.

[GPracer2500]

The goal is to balance backpressure with exhaust gas velocity. You want the least amount of backpressure possible that yields the highest exhaust gas velocity.

If you bolted up a huge header you could have little to no backpressure, but exhaust gas velocity would be slow. Slow exhaust gas velocity reduces or eliminates the scavenging effect of each "pulse" of gas coming out of the exhaust valves. High exhaust gas velocities will actually suck fresh mixture into the cylinder during the valve overlap period while also sucking out all of the previously burned mixture. The low pressure area behind each exhaust gas "pulse" is responsible for this.

So, it's a balance. You want the smallest header you can have (for high gas velocities) that doesn't create so much backpressure that flow is hindered too much. When it comes to headers, bigger doesn't mean better.

Personally, I would not spend money on a full exhaust system on an internally stock 400EX. Once you've got a larger bore and a camshaft, then a header starts making sense. The stock headers work rather well on a stock engine.

Here's a graph of an HMF slip-on vs. a WB full system. These are different bikes run on different days, but the stock runs for each bike were basically identical. I'm not saying this is the end-all be-all evidence for full vs. slip-on on an internally stock EX, but it's compelling evidence nonetheless. Had these EX's had big-bores, cams, porting, etc. I suspect the results could be quite different.

http://www.exriders.com/vbb/attachment.php?s=&postid=2400645
[graph and testing courtesy if MobileDyno]

I would LOVE to see an HMF slip-on vs. a HMF full system on an internally stock 400EX. I've never found such a test though.

If you search in the 400EX section for words "slip-on full system velocity" you should find some threads on this subject.

[250X_project]

Hey, nicely said. I personally would put an 87 or 89mm big bore in it and a cam then go full exhaust. I also would put a high flow filter in it (I like K&N) and then jet the carb to work with all this..