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Post by gotsron on Dec 18, 2023 14:06:44 GMT -5
One of our past contributing members had strong opinions about how the internal diameter of the head pipe effects exhaust scavenging. He had articles and mathematical constructs to support the opinion as well as racetrack evidence. In essence, smaller can be better within limits.
This has intrigued me and last summer I dug out my old original SR headpipe to give it a whirl, but unfortunately the interior of the pipe is all rusty and needing cleaning to really see about flow characteristics. I'm not sure how to go about cleaning the inside of the headpipe, so any suggestions would be accepted.
It will take some adapting to fit the original headpipe to the current Delkevik canister, and I wonder if the interior small diameter headpipe needs to be a bit longer for this application. This is nowhere near a measured study, and will only be a seat of the pants trial. I've always had some trouble with over-rich mixture when opening up the throttle in high gear at over 4500 rpm, so that is my target (to see if this gets better with this change).
The current headpipe came with the bike and is a Triumph Bonneville headpipe mated to enough of the SR original pipe to get use of the reduction ring where the internal pipe begins in the factory SR headpipe. I am running an 18" Delkevik canister as you see in my avatar and using the "baffle" supplied, which still is a little noisy during full acceleration when the street cam hits it's stride.
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Post by stevep on Dec 19, 2023 2:43:42 GMT -5
Not sure how to go about cleaning the inside of the original pipe to any satisfactory degree, but why not consider inserting a torque cone (also known as anti reversion cone) at the head end of the existing pipe? Restores the back pressure lost when using a single skin pipe like you have now.
Just Google searched 'sr500 torque cone' and our old friends at Vintage Spoke are selling it through ebay for $67, but you may find cheaper. (Weirdly, if you go on eBay site to search it says no results, but if you Google search, you find the eBay hit. I can never understand algorithms.)
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Post by gotsron on Dec 19, 2023 11:47:56 GMT -5
Steve: " but why not consider inserting a torque cone (also known as anti reversion cone) at the head end of the existing pipe?"
You have a good point there, the pipe I'm using incorporates several inches of the original SR headpipe including the exhaust flange and the internal reduction(of diameter) of the inner pipe (about a quarter inch of that before the Bonneville pipe begins). That was a 1980s way of dealing with this portion and no doubt there are improvements built into the current solutions.
The anti reversion bit as I understand it is effective at lower engine speeds. The explanation I got in the 60s and 70s is that it provided a little back pressure to keep from dumping fuel/air mix into the exhaust, but the name "anti-reversion cone" would imply more that it would keep exhaust from crowding back into the cylinder at lower engine speeds... well, no reason it can't do both I suppose.
The spot I am aiming at though is top gear at over 4500 RPM. The theory goes: The SR can get more effective exhaust scavenging with a smaller diameter headpipe. This effect is gained because the speed of the exhaust gases going out is greater in the smaller diameter pipe, and this improves scavenging at higher engine speed/higher demand. The most efficient diameter (and length) are governed by a number of factors including the engine size, RPM and the amount of air it is capable of running through the engine per unit of time. There are limits built into these formulae , but possibly there is room to improve what I have.
I have gone after this goal in the past by going leaner in main jets and needle jets, changing to a less restrictive silencer, and finally using a 3.0 slide in my VM 34. Now I just have a tendency to burp when backing off the throttle, instead of a stutter on acceleration. But my feeling is that I've lost a little of the mid-range "WOW" factor, and if this experiment works, maybe I can go back to the 2.5 slide?
I am leaning toward using wire brushes and naval jelly tor cleaning the rust scale inside the SR downpipe. Maybe mount the brushes on a "roto-rooter"?
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Post by gcrank1 on Dec 19, 2023 12:54:21 GMT -5
I submit you need a smooth pipe and the work put into the old hunk-o-pipe wont be satisfactory. Think of why we port & polish heads and why the skin of any speed-craft is baby-butt smooth; we dont generally want turbulence kicking around at the edge.
Iirc the smaller than commonly used headpipe (mine is an old White Bros) transitioned up incrementally down the length but the target rpm/power point desired is critical to pipe dynamics. That looked good to me for a high rpm engine but on consideration I concluded that I spent little time there. What I wanted, and got, was a clean pull to redline and mid-rpm strength and responsiveness for street use.
That old long SuperTrap c/w some 'extra plates' really worked. Nothing has worked as well since I 'blew it off' at speed one fine day.....
Btw, an 'open megaphone' SR500 on the run is an awesome thing!
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Post by gotsron on Dec 19, 2023 18:05:15 GMT -5
Gcrank: I submit you need a smooth pipe and the work put into the old hunk-o-pipe wont be satisfactory. Think of why we port & polish heads and why the skin of any speed-craft is baby-butt smooth; we dont generally want turbulence kicking around at the edge.
Those were my exact thoughts as i drove home from the hardware store today with wire brushes and steel wool ! I couldn't agree more. On the other hand this is only intended as a trial to see if this is a path I might want to pursue, and I can pretty much do this in my garage with the help of a band saw and a mig welder.
I am interested in the theory behind your White Brothers pipe, why it incrementally sizes down, what it's end ID is , does it use an anti-reversion cone, since I am seeking the same thing you get from that pipe. I do believe that your motor is modified some, more than mine is, which makes a difference too. (nope, just checked, we both have basically stock motors, yours has a larger VM carb than mine, I have a mild street grind cam from WEBCO)
Gcrank: Btw, an 'open megaphone' SR500 on the run is an awesome thing!
Did you look in your mirror when you lost the plates off your SuperTrapp, by instinct, expecting to see a crop duster bearing down on you, ready to pass?
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Post by gcrank1 on Dec 20, 2023 12:23:59 GMT -5
When I lost the ST it was the whole guts out of the meg can, not just the end baffle plate; I was hunkered down winding toward redline in 4th on a long straight anticipating 5th. All of a sudden the sound changed Big Time! What happened was the mounting screws points at the tail end that hold it all together to the ss can broke out of the can. Took part time searches over 3 days to find the guts in the ditch. I intended to get some ss welding done on the can....intended.... The shorter cheap Emgo expedient 'muffler' made it ridable but the high end isnt there, plus I put on a higher geared rear sprocket and lost some torque (but easy living rpm's for street).
To be clear, my White pipe is the std one, what, 2" (and Ive always wished they would have made em at least 4" longer). My guess is that for an average SR on the street it was 'good enough' but the only way I got the high end run length I wanted (then) was to use the long ST. But hey, 2" is what HD Sportsters used for straights and an SR is half a Sportster, lol.
Ive not been in the engine but it cant be stock. Ive had a stock '76XT, '78 and '81 SR's and this ('81) puppy has way more beans. When I had the stock gearing it broke the ton with 200+ # of me on (speed on Garmin); a stock SR wont do that! The chap I bought it from brought it back from California, the home of SR hot-rodding.
Iirc there were good points made that a VM34 was a better match for a stock to mild engine; my VM36 seems to all get used.
My comments about the incremental upsizing of a 'smaller dia headpipe' came from my recollections of the long-gone, in-depth thread on pipes (toward the end time of the old forum). Someone with knowledge and skills was really working it out. Seems to me the pipe started at the exhaust port dia and ran that to about under the front engine case, upped dia some to about the rear of the cases then upp again to 2"?/can inlet dia. at the end run. I dont remember where he wanted the power band to be.
And that is the point for me.....define just whachwant in a powerband and tune for that; a track guy has dif needs than a street rider.
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Post by gotsron on Dec 23, 2023 10:35:50 GMT -5
Ive not been in the engine but it cant be stock. Ive had a stock '76XT, '78 and '81 SR's and this ('81) puppy has way more beans. When I had the stock gearing it broke the ton with 200+ # of me on (speed on Garmin); a stock SR wont do that! The chap I bought it from brought it back from California, the home of SR hot-rodding. Iirc there were good points made that a VM34 was a better match for a stock to mild engine; my VM36 seems to all get used. My comments about the incremental upsizing of a 'smaller dia headpipe' came from my recollections of the long-gone, in-depth thread on pipes (toward the end time of the old forum). Someone with knowledge and skills was really working it out. Seems to me the pipe started at the exhaust port dia and ran that to about under the front engine case, upped dia some to about the rear of the cases then upp again to 2"?/can inlet dia. at the end run. I dont remember where he wanted the power band to be. And that is the point for me.....define just whachwant in a powerband and tune for that; a track guy has dif needs than a street rider. I did have an opportunity to put several hundred miles on a friend's SR which had porting and higher compression and a good cam. I recall the first time I twisted the throttle at highway speed, that I had to hang on with both hands, quite a difference from mine. On my SR I hope to get it to climb smoothly to redline in high gear on a flat stretch of road. It will do this now, but there is a spot where it seems to struggle slowly to gain RPMS and opening the throttle further gets no response at all. Once through this spot, it picks up again and by then the gauges are a little hazy with vibes so i can't say precisely at what speeds or RPMs this all happens, but it begins at a little over 65 mph so it is in the "fast traffic" realm where I'd like to have better access to the power. I can make this better by going leaner on the jets and throttle slide, so I think there is still room for improvement there. |
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Post by skyshadow on Feb 27, 2024 11:11:24 GMT -5
Does the SR500 have valve overlap to allow scavenging? If so, is it enough (above 60 degrees of rotation) for you to benefit from scavenging?
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