Topic: Exhaust tuning

[scottly]

I've tried some different exhaust systems on my favorite dyno-mule, the FT 500, AKA Simon's favorite "ditch-pump".
What surprised me most was the lack of difference between the different lengths of primary pipes, ranging from 22" to 34". The first test on the chart was with 26" pipes, the second was with 34" pipes, and the third was the 22" pipes. Oddly, the 22" pipes had a slight torque advantage at mid-range, while the 26" pipes had a slight HP advantage on the top end. I would have expected the opposite?? Johno, does your software have provisions for 4-valve per cylinder heads?
   

[turboguzzi]

interesting stuff scottly. john robinson (sort of UK's kevin cameron....) once wrote that during the 70s-80s, japanese engine designers did everything they could to actually destroy any resonance tuning to get flat torque curves. If your FT's cam has little overlap/shuts EX early and restrictive porting, then there's little that the pipe can do all alone... i would say that with a sharper cam you would start seeing bigger differences. your test almost confirms the JR's comment.

[simon#42]

i would have expected a much bigger difference  in torque , as tg said i would suspect the cam is masking your results .

i read all the robinson books and must admit thought they were mostly rubbish with a few interesting fact thrown in , having said that im not very impressed with kevin cameron either .

[scottly]

The cam is not stock. The two 32mm ID head-pipes appear to act like an equivalent length of 45mm pipe. Both of the shorter pipes in previous tests had a 40-41mm intermediate pipe between the collector and the muffler, while the 34" long pipes dumped from the collector straight into the muffler. The next try will be with the head-pipes merged together as close to the head as physically possible, with a 42mm down tube.
 

[turboguzzi]

mmm... dont know if it's any help, but state of the art in current supermono road racers as well as Moto 3 GP 250 singles,  is two separate pipes AND two silencers.... Just a few merge the two headers into one silencer but they do it very close to it, i.e. still long separate pipes.
now that i am thinking of it, you might have seen bigger impact changing your single header "primary" lengths rather than the "secondary" collector....

 

[scunny]

John  Britten merged the 2 cylinders into a sort of catch tank, which then branched back out into 2 exhausts. looking at some of his prototypes earlier in the year and it looks like he played with different primary lengths before said catch tank.
catch tank my words not his.
got some pics here somewhere before my camera turned turtle.

[scottly]

If it were a 2 cylinder motor, separate systems would make sense, but there would be a weight penalty; in my case over 500 grams, not counting the extra silencer.
Scunny, I've thought of using a sort of "expansion chamber" between the silencer and the 42mm pipe. I've gotten the merged head-pipes and down pipe welded together, as well as a couple of bends that I can clamp together for quick changes. Sorry, no pics, as after doing blacksmith work in 94* heat I was beat and forgot...

[calj737]

500 grams is barely over 1lb. Not a significant amount of weight penalty in exchange for performance (if it yields more performance).

[Old Scrambler]

Scotty...............I like the 'expansion chamber' idea...............sort of a megaphone with a reverse cone and then an extended pipe past your foot with a 'baloney-cut' to help extract the flow.

The best performance increase I saw from a stock FT500 motor was to re-install the stock muffler and do the suggested carb mods provided on the yahoo-groups page................one tooth larger on the front sprocket allowed me to cruise on the x-press with some passing power.

I have observed several examples of 600cc FTs on the AHRMA race circuit and most used a megaphone-shaped muffler on a double-pipe with a 'squeeze' section at the joint just before the mega.

[scottly]

Dennis, were you using the stock header? The inner tubes are only 22.8mm ID, with a funky  32mm transition from the 25.4mm exhaust port to the entry of the inner pipe. The first header with the 26" primary pipes with an ID of 32mm resulted in a 7% HP increase over the stock header. The header on the left is an FT, while the one on the right is from an XR. The XR pipe is only slightly larger than the port.

[scottly]

This is the newest pipe mocked up, almost ready for testing.

[bear]

Me thinks you have way to much time on your hands Scott.

Cheers, Brian

[MRieck]

Me thinks you have way to much time on your hands Scott.

Cheers, Brian

You think???

[scottly]

  I stole this book from the high-school library about 45 years ago, and have finally gotten the chance to test the formulas; so far they don't work very well.
 

[Old Scrambler]

I had a stock header............but your smaller pipe ID is the way to go.  I saw one example of 'spigots' inserted into the head and extended to the pipe-bend to transition the flow. Doing it by the book is a good way to get started on the fundamentals..............but doing it with a large bank account gets better results.

I'm in the middle of making (I research, Dave welds) a megaphone for my CBR salt flats bike. It is meant to work best at WFO and I'm hopeful that it will help transition me from 4th at 11,000 rpm to 5th beginning slightly below 9,000 rpms............

[scottly]

.but doing it with a large bank account gets better results.

So far my only expense has been $7 for the 18" length of tubing, other than my time, which I apparently have too much of.  

[bear]

 

[ivanhoew]

i would say with a single  its the distance to the end of the silencer that is what matters here scott . try changing that by the same step measurements and see what happens ?


robert.

[simon#42]

.but doing it with a large bank account gets better results.

So far my only expense has been $7 for the 18" length of tubing, other than my time, which I apparently have too much of.  

$7 scottly ............ i think you have just doubled its value

[scottly]

You're mean, Simon.
Robert, thanks for the input. The newest pipe has the shortest overall length so far, and I can shorten it by 3 or 4 inches at the most. I'll start here, then I can easily lengthen the pipe before cutting anything. (I cut it twice and it's still too short?) I did get a chance to ride it a little bit today, and the exhaust seems to have a crisper note to it.

[scottly]

I ran the bike on the dyno with the newest pipe with the very short primary pipes, then extended the overall length from the head to the tip of the muffler by about 5". The chart shows the first experiment with 26" primaries compared to today's experiments. Not a lot of difference between any of the long primary combos and the short primary combos. All of the experimental systems have used nearly the same primary and secondary pipe IDs, and it would be a PITA to change the primary pipes to a smaller size. Perhaps instead of an expansion chamber, I should insert a strategically placed restriction at some point in the secondary pipe? 

[ivanhoew]

hmmm, seems a lot leaner on the latest tests ?

[scottly]

I didn't bother with the A/F probe on the latest tests.. I've decided to pursue the smaller head-pipes, salvaging what I can from an XR that was ridden hard and put up wet. It won't be pretty, but it should provide at least some data with regards to primary pipe ID. 

[scottly]

So much for the small primary pipe ID. The graph compares the last three pipes tested.

[turboguzzi]

well, even more than before, it looks to me like your restriction to power is elsewhere and the pipe cant do much about it.

how difficult would it be to just retard the cam a bit with a slotted sprocket?