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YZ426F - SuperTT lobe centres
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Posted by: jescombe---------------------
I'm hoping someone (Rich?) can give me a pointer for improving the cam timing for mid-top power on my Supermoto (SuperTT) bike.
It's a '00 YZ426F with a 98mm JE piston, 1.1mm squish, flowed head, oversize inlet valves, and Falicon camwheels. I'm currently at 103deg for both inlet and exhaust, which is giving a good spread of power and 52.5hp peak with the stock pipe.
The cam timings were done on a dyno, but largely by trial & error, tweaking the camwheels and comparing runs.. I figure software might provide a better answer than burning more dyno hours?
Thanks in advance,
Jon Escombe.
Posted by: SFO---------------------
Who did your head?
Where did you get a 98mm piston?
Is it a sleaved kit?
On my 97mm motor I made 54 dynojet rwhp on nutec fuel with an Arrow pipe nand header.
I was running 105 intake lca and 106 exhaust lca measured at .010" lift.
Posted by: Rich Rohrich---------------------
Jon - A couple of questions:
- How much air (in cfm) on the intake side does your head flow at max lift (~9mm)?
- At what rpm were you looking to set the torque and horsepower peaks?
- What is your measured compression ratio?
- What type of fuel are you running?
SFO's LCA numbers have proven to be good all-around with the stock cam but tend to lower the torque and HP rpm peaks. With a 97 or 98mm piston this isn't such a bad idea. A replacement cam with a more agressive profile is the logical place to turn. The stock pipe is great but it's ID is a bit small if you are looking for mid-top rpm power with a 98mm bore. Something on the order of a 1.75" ID with a tuned length similar to OEM might better suit your needs.
Posted by: jescombe---------------------
SFO/Rich,
The 98mm piston is a custom JE part, I believe it's made from a TL1000 blank. It weighs approx 7% more than the stock piston, and was a slight pain to fit as the bottom of the piston hit the crank webs at BDC before I took some material off. The barrel was bored & replated, I use custom 98mm gaskets from Cometic.
My head was done by Slipstream Tuning in the UK. The guy there is Martin Sweet, and although I knew he'd never done a 426 head before, he has a very good reputation with the Yamaha 5-valve heads in general. (Has run everything from R6's to the works MuZ 660 singles raceteams). I'm afraid I don't have any flow figures, or access to a flowbench to get them. FWIW, the gas-flow didn't seem to do a lot on the dyno[1], but the valves (which were done later) appeared to make a couple of hp through the mid-range.
I'm running 97 octane unleaded pump fuel, peak torque is at 6.5k rpm and peak hp is at 9k rpm. My power curve flattens out around 7k, with maybe 5 hp difference over the next 3k rpm. I'm happy enough with a 9k power peak but feel that there is more top-end potential if I could stop the curve flattening off so early.
I've never measured the compression ratio. Presumably I need to measure the head volume & piston crown volume, and then use this figure added to the volume of the squish band and the swept stroke?
I also have a Thunder Alley pipe (a reverse cone megaphone), which gives great bottom-mid but loses a couple of hp off the top. I've considered going up 1/8" on the straight section which is only 1 5/8" all the way back to the meg but not tried it yet. Akrapovic are just releasing a Supermoto pipe for the YZF which I should be able to dyno next month.
Thanks again,
Jon Escombe.
[1] In fairness, the 'after' runs were done with a new rod that threw the squish out to 1.6mm, so may have been a little unfair.
Posted by: jescombe---------------------
Ok, I think I've figured out the compression..
Swept volume is 453.33cc, 1.1mm of squish gives another 8.3cc, the head volume is 23cc, and the piston crown volume is 12cc (according to my best efforts at measuring!). Which by my reckoning gives an 11.47 / 1 ratio.
Cheers, JonE.
Posted by: fastkevin---------------------
Martin is the man!! I raced an MZ(in California) and pcked his brain a few times. You're lucky to have him so close to you.
Posted by: Rich Rohrich---------------------
Jon - I'd recommend sticking with the LCA numbers you generated with your dyno testing. The simple fact is I don't think you have enough compression or cam to reach your goals with a 98mm bore. I think you'll be hard pressed to make a big difference by fiddling with the LCA or the head pipe.
Posted by: jescombe---------------------
Thanks Rich, hope you don't mind another couple of questions?
Cams - been looking through the archives & found your posts on HotCams, will you be sharing dyno results from a big bore motor?
Compression - What compression ratio would you suggest for 97 RON fuel?
The only way I can think to increase compression, without ditching my 98mm barrel & piston, is to machine down the piston crown to reduce the depth of the cut-out (and deck the barrel to match of course). Is this a reasonable approach? (I know I would need to re-check the valve to piston clearances).
Assuming a target compression ratio of 12.5 / 1, I'd need to remove something in the region of .75mm (which would bring the crown volume down to 8cc). The flat part of the squish area (at the piston edge) would also increase from 8mm to about 10mm - don't know if this would be good or bad..?
Cheers, JonE.
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