Jump to content

exhaust system air velocity (dr650)


Recommended Posts

So Im reading a post about a guy doing the header grind and it seemed to lose low end power. Easy I thought, more air flow means he leaned it out and he should just have to richen it up. Then Rob drops a bomb on me and now I'm wondering....

I removed my header weld also. I thought increasing potential volume was a no brainer for better effeciency. Now Im understanding the role exhaust air velocity plays (to a point) and I'm wondering if it wouldnt be worth experimenting a bit with head pipe restrictions to find the perfect match. I noticed that I don't have the low end grunt I thought I would or that I seem to remember on my last DR. Maybe it's all in my head but I'm going to experiment anyway.

Any ideas on the best way to go about re-restricting a head pipe? I'd want to be able to use very small changes. IE reducing the diameter 1/32-1/16 at a time. And would reducing the diameter at one point be enough or would you want to change the diameter of the entire pipe (or section of pipe) for better results?

My thought is to take several thin disks of metal the exact same diameter as the crush washer and have a machine shop drill out the middle to various opening sizes from 1/32 restriction to 3/16 restriction in 1/32 steps. Then place the disks (one at a time) on the INSIDE of the crush washer and tighten the header back on. Dyno and repeat with a tighter size and so on. This would increase air velocity as close the piston as possible.

Obviously there is a point where you will increase restriction enough to suffer negative results. My goal is to see where this point is, as well as to see if it is possible to gain any hp or torque.

Any thoughts? Am I wasting my time? Is there an easier way?

Link to comment
Share on other sites

Turbulence can destroy flow... the idea is not a new one as Procycle mentioned... the best way to do what you are talking about would be to have an insert that starts out the same diameter as the DR's port and then tapers out to the pipe ID. A direct jump in size does not promote flow as the negative pressure caused by the exhaust rushing past the step causes the air to recirculate behind the step and therefor disrupts flow... although the restriction may provide a benefit at a particular RPM it could be much worse at others. I've been thinking about a slip in insert for the stock head-pipe for some time now to address this issue... well, at least it's an issue in my mind anyway. 😏 I plan on machining one up before I hit the dyno for the FI VE table. Should be interesting to see what happens. It might be like the velocity improver(s) I machined up for the FCR's. Yes, there was a noticeable improvement in the dyno run but in the grand scheme of things the improvement was not worth the investment of time... (HP to $$ ratio). I have one on my bike along with a PowerNow. Another HP to $$ ratio ruiner. Still, worth every penny to me. 😏 Or the insert could be a useless exercise of wasted time providing only minimal entertainment. :cheers:

Link to comment
Share on other sites

All, The cone in the link above is actually an anti-reversion cone and is used by NASCAR racers in their exhaust headers. It helps prevent the positive pulse coming back up the pipe from going into the cylinder and pushing the incoming charge back up into the carb. It works! I've done it. I had a 79 KLX250 that would stall out at slow speeds and spit back thru the carb. The cone helps a great deal... at certain rpms. As mentioned above, the power now has a similar effect but works on the carb side. Is it worth the trouble... its a toss up. My suggestion would be to go with a slightly smaller dia head pipe and longer runner for torque, or a shorter and larger dia pipe for max power, and stay away from the inversion cone, just not worth the trouble. My best experiences on 350 size motors has been to use a Supertrapp muffler with discs that I can take out or add to help fine tune jetting issues. I ran 8-12 discs and added 8-10 inches of pipe length which really helped on low end torque. The larger 650 would probably need a 1 5/8" to 1 3/4" dia pipe.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

×
×
  • Create New...