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Long Rod or Short #798035 02/10/20 9:56 am
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PEH Offline OP
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Im about to start putting together a Trackmaster framed classic flattracker, with the intention of competing in the Uk dirttrack vintage championship. I had the idea of running a long rod 650 bonneville engine with option of fitting a routt 750 kit if needed, After a chat with a prominent racer of the day who had used most of the Bsa/Triumph big twin options at the time, his preference was for a short rod 750 T140 engine, he felt he had better acceleration out of the corners than the other options, The big spoiler in all of this is the tracks we use in the Uk are short, mostly 300/330 meters, with a loose shale top surface, so grip and drive is the top priority. I have both engines in stock, i am looking for some input from experienced riders/tuners as to which option to go for, i intend to use T140 inlet and a 3134 profile exhaust cam unless there are good suggestions for alternatives, i also have single and twin carb cylinder head options. I would also like to hear about inlet port filling if there's anyone out there willing share the tricks of the trade. regards. Paul (Harry) Harrison

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Re: Long Rod or Short [Re: PEH] #798052 02/10/20 7:09 pm
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For my sins, I'm going to give you my non-racing view on what I would try... With such short tracks I'd go for a short rod TR7 top end with a single carb and a pair of E3134 cams.
That should give good low end and a beefy midrange, while a T140 inlet cam would push the happy zone up to 5000. The competition is gone by the time you get up there.
I'd try different size carbs to tailor the throttle response to suit the different tracks and traction requirements.
Standard ports should be near perfect for that particular application I think. Power may tail off above 6500 or so but on a short track the 4000 to 6000 rev band is probably what you need.

But what do I know, I've trashed these things hard over the years, but never in (legal) competition... Some real competitors will assist you very soon of course, with some real info.

SR

Last edited by Stein Roger; 02/11/20 10:44 pm. Reason: typing error.
Re: Long Rod or Short [Re: PEH] #798057 02/10/20 8:32 pm
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It makes perfect sense SR . The Bulmers Triumphs where all TR7's i believe. They were used in a dirt track Uk v Us challenge way back, on our speedway tracks, Similar to the Trans Atlantic Challenge on tarmac. both of which i was lucky enough to see and hear, the sound of 14 BSA/Tri 3's coming out of Gerrads Corner on full throttle has never left me.!

Last edited by PEH; 02/10/20 8:45 pm.
Re: Long Rod or Short [Re: PEH] #798139 02/11/20 11:08 pm
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I remember the Bulmer bikes from the UK weeklies back then. We got them a week late, but what the heck, everything was slower in those pre-internet days!
Lucky you to have heard, and seen them!

Please take my "insights" with a pinch of salt, but I can't see how you could go flat out and exploit a 5000 to 7500 rpm power band on a short speedway track.
In fact I wouldn't be surprised if T110 cams (E3325) would give you an edge, with a nice low end to control the traction from start.
I have convinced people with Bonnies and Tigers to use an E3134 inlet with an E3325 exhaust, like the Bonnies from 1959 through 1966, and they all loved it.
I gave a TR7 I built for a friend over 2 decades ago this combo, and it was great. I had the opportunity to ride it again last summer, and it didn't disappoint, it had BIG power from down low and to over 6000 rpm.
It would make a nice engine on the dirt I believe. Or the cinders.

SR

Re: Long Rod or Short [Re: PEH] #798163 02/12/20 10:21 am
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That is the kind of insight i am looking for SR, Aggressive power is a waste on our tracks, I have won several heats and a couple of finals on my Moto Morini 350/410 with 28hp. My latest project i manage to get on track at the last meeting of 2019 is a honda Xl350/412 single in a C&J frame. That feels much better, with big flywheels and a proper frame. I beat the European champion and the German Champion in that first heat. That bike has 30 rear wheel hp 24 ftlbs torque and is light. only when i got off the bike did i realise i had broken the swing arm bolt, so that was the end of my day, I will look into the 3325 cam. Im not sure i will be faster on the Triumph but it will be nice just to feel what a Trackmaster big twin is like on track. I may be 64 but i can still give the young guns a bit of a battle.

Re: Long Rod or Short [Re: PEH] #798219 02/13/20 12:30 am
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I dirt tracked a Trackmaster for several years with a short rod T140 Bonneville twin carb engine and 34 Mikunis and Megacycle 1065 cams, and Kibblewhite valves, a pretty standard trim for vintage dirt track racing, and it was plenty fast, but I was always told by dirt track folks in the US that back in the day in the 70's they pretty much all preferred the long rod, because it got better drive/traction off the corners. For what it is worth. You might think a little about what pipes you put on it, 43 inch (measured around the outside) TT pipes usually work best on short stuff. Not megaphones.

Last edited by linker48x; 02/13/20 12:34 am.
Re: Long Rod or Short [Re: linker48x] #798230 02/13/20 10:17 am
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Thanks linker48x, Thats what i had read a few times, Mike Sullivans observations threw me off a bit, i do have some TT pipes in stock, I will measure them today.

Re: Long Rod or Short [Re: linker48x] #798279 02/14/20 7:35 am
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Stein Roger Offline
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Originally Posted by linker48x
I dirt tracked a Trackmaster for several years with a short rod T140 Bonneville twin carb engine and 34 Mikunis and Megacycle 1065 cams, and Kibblewhite valves, a pretty standard trim for vintage dirt track racing, and it was plenty fast, but I was always told by dirt track folks in the US that back in the day in the 70's they pretty much all preferred the long rod, because it got better drive/traction off the corners. For what it is worth. You might think a little about what pipes you put on it, 43 inch (measured around the outside) TT pipes usually work best on short stuff. Not megaphones.

From the horse's mouth etc, great info. As I've said, I'm guessing. Takes experience to know.
Long rods makes better power at higher rpm as they dwell longer around TDC, consequently they make a little less at lower rpm, as the combustion pressure peaks before the rod to crank angle is optimal.
I guess that would soften the power enough to let the rider find more grip on the dirt.
This points to one of the advantages that can be had from modern engine management systems, they juggle injection and ignition parameters, or even the cam timings, to suit the crank angle and rpm, among other things.

As an aside, not at all pertinent to the Original Post, modern engines tend to go with smaller bores and longer strokes, for reasons of fuel economy and compactness, not unlike a century ago.

SR

Re: Long Rod or Short [Re: PEH] #798301 02/14/20 4:56 pm
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Yes indeed. I guess its make my mind up time. Deep thought over weekend. Thanks for the input. I have to say i did hope more folks would chime in, but it seems plug leads is the hot topic at the moment.

Re: Long Rod or Short [Re: PEH] #798336 02/15/20 12:26 am
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I rode both a short rod and long rod Triumph in a Trackmaster frame on short track and 1/2 mile. There was quite a bit of difference in how the 2 engines were built. I would give a slight edge to the long rod, only because that engine was not as serious in spec as the short rod engine. The long rod was a bit easier to ride on a short track. On a 1/2 mile, there was no contest, the short rod engine was much faster.

Keep the carbs small, no bigger than 32mm, stock crank weight, heavier than stock is better, for short track, likely some of the suggested cams will be fine. Good crank case breathing, ignition can be simple points with the advance unit locked for full advance. Total loss works. Play with timing, going beyond stock full advance livens up the engine. I ran 38 deg. advance, played with 40 too.

Clutch needs TLC, it can be a weak link, especially the key on the shaft. Run a return line oil filter.


Never underestimate the human ability to elevate stupid to a whole new level!.
Re: Long Rod or Short [Re: Rich B] #798368 02/15/20 7:25 am
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Many thanks Rich. Sums up what i have in my head and a few other folks have mentioned. Better get cracking.


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