TRX250R.ORG
Workshop => Engine and Bottom End => Topic started by: rablack21 on February 03, 2014, 10:31:50 AM
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I really like the idea of building a stroker motor, especially the extra bottom end power. I'm a little concerned about issues with the frame though. I know several people that have stroked big bores that have had issues with breaking motor mounts.
I also wonder if the price difference of building a stroker is worth the performance gain. Does it make the same kind of difference that a big carb or pipe makes? more? less?
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I'm a little concerned about issues with the frame though. I know several people that have stroked big bores that have had issues with breaking motor mounts.
There is some misinformation here. A balanced/trued stroker crank isn't going to cause damage to a frame that would otherwise be fine. I could see some issues with an improperly balanced crank with a huge piston on it but I have never had issues with any of the 4mil hot rod stroker cranks I've used.
As far as performance gains - I'll leave that to guys that have built enough similar motors with and without stroker cranks (example: 76mm piston X 76mm stroke versus a 76mm piston X 72mm stroke) and can really compare the two. Unless you're looking at a big Puma motor, typically you don't need to balance or change anything on the hot rod 4mm - so the incremental cost is ~$300 ($250 crank and $50 to weld the tins).
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IMO its best to keep the bore and stroke close to the same or even slightly more stroke than bore, and seams to be a proven combination over the years.
The longer stroke allows the transfers to be shorter, allowing the exhaust port area above the transfers to be increased without raising the duration numbers as high as a short stroke setup with a equal exhaust port area.
The exhaust port size above the transfers and its ability to remove the exhaust gases before the transfers open will make be one of the largest impacts on the output of the motor and its ability to have a good over-rev. If the exhaust gas pressure's are not brought down lower than the pressure in the crankcase the exhaust gases do not care which way they go and will travel down the transfers until the pressure drops low enough to allow flow to change direction and back into the cylinder. Every cylinder has this condition at some point in the rpm range but the better your exhaust port flows during the "blowdown period" the better the power will be at high rpm.
The problems with stroker cranks are piston speeds are increased along with friction and wear and port timings do increase on both the transfers and slightly less on the exhaust port, so if you do not have a stock like port timing, your port timing may be to aggressive for your style of riding. But some of that is offset by the fact that you have more cc's and with more displacement you normally have more lowend power
Hot rod has brand new long rod +4 cranks for 250.00 and that is a great deal so for the money its hard not to justifiy not buying one.
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Is it necessary to weld the tins?
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Is it necessary to weld the tins?
I know some builders remove them but my experience with the 4mm strokers is that under 370cc you can just tack weld in a few places. For bigger motors often times they are fully welded.
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from what I have been told the newer hot rods have better crimps on the tins so its less likely to have a issue, but it never hurts to be safe and have them tacked.
I think its a RPM related issue not so much a motor size problem with the tins coming loose or rotating from their orig position
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from what I have been told the newer hot rods have better crimps on the tins so its less likely to have a issue, but it never hurts to be safe and have them tacked.
I think its a RPM related issue not so much a motor size problem with the tins coming loose or rotating from their orig position
would it be good to remove the tins for reliability? i have heard that crankcase volume should be a consideration.
john
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Don't know at what point the tins should be removed but @363cc's my tins are still on my Hotrods +4mm crank.
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have not measured the crankcase compression but i will guess that is not something to worry about until you are in the 400+cc range and even then most guys still have the tins
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Another thing to consider when building a stroker engine, is the rod ratio. It can play a big part on piston wear.
IMO, 130.3mm is good for a stock stroke but gets undesirable when you start adding stroke to the crank.
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If the Hot rod crank is built with a 130.3 rod, you are not doing yourself any favors dropping it in, IMO
once you get into changing rods for a better ratio, it really bumps up the overall price of the crank
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unless ur building a drag 431 puma or larger don't trip
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u never mentioned the topend ur using aswill
there's plenty of builds up to 370 and above that don't have welded tins ,some are removed and some are not,depending on motor setup and compression
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IMO, I think the 130 long rod is fine for a +4 stroke, the cr250 ran the 125 rod until its last year, adding 5mm to the rod length and +4 in stroke over a proven setup is within reason.
The KTM rod up grade that is 132.5mm long is done more for the rod strength rather than the rod length
But once you get to the 78mm piston size the KTM rod should be considered very seriously for long term reliability and performance
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rsss396
cruching some quick numbers....the 76mmm stroke with a 130 rod gives a 1.71 ratio.
stock ratio -130.3 is 1.80
125 short rod is 1.73
adding a +4 stroke with a 130 rod makes the ratio worse than the short rod at 1.71
IMO this would make for a very high piston speed (F.P.S)
If I am wrong please correct me
Thanks
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how about de-stroking?
john
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Sound good info in this thread guys!
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rsss396
cruching some quick numbers....the 76mmm stroke with a 130 rod gives a 1.71 ratio.
stock ratio -130.3 is 1.80
125 short rod is 1.73
adding a +4 stroke with a 130 rod makes the ratio worse than the short rod at 1.71
IMO this would make for a very high piston speed (F.P.S)
If I am wrong please correct me
Thanks
your numbers are correct and here is a little more to expand on those
calculated @ 8700rpm
125.5mm x 72mm = 1.743 rod ratio w/ avg piston speed of 20.9 meters per sec and a peak of 76.3 mps
130.3mm x 72mm = 1.81 rod ratio w/ avg piston speed of 20.9 meters per sec and a peak of 75.5 mps
130.3mm x 76mm = 1.71 rod ratio w/ avg piston speed of 22.0 meters per sec and a peak of 81.0 mps
132.0mm x 76mm = 1.74 rod ratio w/ avg piston speed of 22.0 meters per sec and a peak of 80.7 mps
my Liger
144.0mm x 83mm = 1.73 rod ratio w/ avg piston speed of 24.1 meters per sec and a peak of 88.1 mps (present setup)
144.0mm x 85mm = 1.60 rod ratio w/ avg piston speed of 24.7 meters per sec and a peak of 90.9 mps
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your numbers are correct and here is a little more to expand on those
calculated @ 8700rpm
125.5mm x 72mm = 1.743 rod ratio w/ avg piston speed of 20.9 meters per sec and a peak of 76.3 mps
130.3mm x 72mm = 1.81 rod ratio w/ avg piston speed of 20.9 meters per sec and a peak of 75.5 mps
130.3mm x 76mm = 1.71 rod ratio w/ avg piston speed of 22.0 meters per sec and a peak of 81.0 mps
132.0mm x 76mm = 1.74 rod ratio w/ avg piston speed of 22.0 meters per sec and a peak of 80.7 mps
my Liger
144.0mm x 83mm = 1.73 rod ratio w/ avg piston speed of 24.1 meters per sec and a peak of 88.1 mps (present setup)
144.0mm x 85mm = 1.60 rod ratio w/ avg piston speed of 24.7 meters per sec and a peak of 90.9 mps
I am still very new to engine building / porting on my own stuff, so I am always trying to learn as much as I can.
Can you explain why you have used a 1.6 rod ratio before? To me it seems like that ratio would put an extreme side load on the piston
Thanks
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1.6 is not a ideal rod ratio for sure, and the very reason I shortened my stroke on my bike.
Wear on the stock cr500 cylinder was very bad but then so was the stock 79 stroke due mainly to the large intake opening and half moon cut away on the pistons. Wear in the bridged intake Liger cylinder was much better but still more than I liked.
The problem with 85 stroke is the rod choices, the cr500 144 rod is the longest rod with smaller sized crank pin.
Arctic cat has a 146 and I think even a 156 but have very large crank pins and then the crank physically runs out of metal when you start pushing the stroke
I think Pete at Hybrid did a cr500 liger with a stroked crank and AC rod and had to do all kinds of welding and moving bolt holes on the cases because of the rod big end size, then stating he would never do it again.
sometimes you build things in compromising ways simply because of resources but I will say while the +6 stroke crank was hard on pistons it lasted 4 hard seasons and is still together.
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got a spacer plate and +10 rod ready to go in a 300 when i get around to it. thought of using the cr500 rod but i dont think i will
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Many moons ago when racing was more popular, the max displacement for the 250cc class was 265cc (to allow for overbores).
However, many of the top racers used to STROKE their engines to 265cc rather than bore them, even though boring would easily be the cheaper option.
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Stroke it ryan!!!
There are some killer kits right now with it all including the case bore mod required for the 330/350.