Bonneville America Forums Forums Technical Forums Performance Mods finally got a dyno run

after way to much time playing with my carbs and the half a#@ed gas analyzer I took the bike over to Waukesha and got a dyno run. no tuning, just the run. The dyno guy was very helpful , he made several sugeestions, all of which I followed and made the bike 'pep up' some more. He said getting the A/F closer would pick up 2-4 hp more.

this run was way too rich (sorry I did not scan the A/F chart) K160 mains, 45 pilots, 2.75 turns, 2 shims on thuxton needles.
Based on this it is now running with K 154 mains, 42 pilots, 2.5 turns, 1 shim - thruxton needles.

Very impressive numbers! Could you tell us a little more about your setup?

Nice,impressive

the A/F chart would be really helpful to show others how many of us (myself included) have tended to be too rich in out jet settings and now you are another one who have had this verified by your dyno.

Thanks for sharing the setup. Like I said, very impressive power there. What strikes me is that the stock carbs actually are capable of feeding enough air to achieve good hp and torque. Of course it's likely that you get more with even bigger flow, but prob. not that much more. But what do I know???

Have you gone to premium gas to also reduce the knocking?

I wonder how much the timing is retarded with the TPS hooked up but not physically connected to the carb? You would gain in terms of retarded timing but what about the top end? As you said, it doesn't appear to hurt output but... Did premium gas help with the knocking in any way with the TPS fully hooked up?

Pretty sweet power there, the high compression looks to be the ticket!

Quote:

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12:1 JE pistons and nikasil barrels

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always said this is the way to go with this mod .....

great number mag10

Ed

Hi Mag10, got your PM. First, is your Speedie one of the last 790's or an 865?

The rev limit on the early 790's was 7,500rpm and on the later 790's 7,400rpm. The 865's have always been 8,000rpm.

At what revs do you hear knocking?

An 865/904 with 790/813/Thunder cams doesn't run great with 865 maps. I have dyno developed a mapping on a 360° 865 with Wiseco HC pistons and 813 cams. Started out with 865 maps at 70.19bhp @ 8,500rpm / 49.07ftlb @ 6,200rpm and ended up with the custom mapping at 74.60bhp @ 8,420rpm / 52.28ftlb @ 6,200rpm with gains all the way up from 2,500rpm. (the dyno's and a writeup will be published on my site shortly)

Although the above development is on a 360° engine, it works the same on a 270° in the same tune.

The low rev igniton timing is very different on the 270° 790's and 865's. If I load standard 790 maps to an 865 with 790 cams, the normal overrun snap crackle and pop turns in to loud bangs which sets off car alarms and makes small children cry. This has been set right on the 270° custom mapping.

So as to your knocking. I haven't experienced or heard of a modded twin knocking, except on occasion in high temp weather, but there again I haven't worked with the JE pistons and your petrol, which I believe is slightly different to ours. TPUSA have much experience with BB HC engines and haven't suffered knocking with a modded igniter. It may be worth speaking to Carlos and asking his opinion.

On a side note while I'm here, sorry to go off subject, but with regard to the early 270° 790 igniter's that cut-out when you hit the rev limiter. With the one's I have modded the cut-out goes with the rev limit. So if the rev limit is raised to 8,500rpm for example, the cut-out doesn't happen until 8,500rpm and as useful power drops off before this level, it should virtually eliminate the problem. (unless you like the sound of a screaming 790 of course).

Sorry for the long post, but I don't get over here as often as I might like.

Pieman - thanks for the reply.

some details about the knocking. I had ridden over 400 miles the day it started (with 150+ to go)so the engine was very hot. I beleive that I got some questionable fuel at one of many stops. I was running with my 'highway gears' 19 tooth front sprocket whick allows my higher speeds at lower rpms. Giving it WOT in 5th gear under these conditions started the knocking.
Since, I have inspected both cylinders and valves with a bore scope and all is still good.
I mixed in some 110 octane fuel and put the regular gears back on, the bike really runs great now. Long term a better solution is wanting. In the mean time I have installed a colder spark plug to see it that helps.
As for your questions, the engine is an early 865. The ignitor is not the original one (it died) the original would not rev to 8000, the new one does.
Am I reading correctly that you have a re-map available or that Carlos does?

well I got the knock fixed or removed. I may have picked up some additional power as well ( can't get to the dyno until next week). After discussions with several folk that know this stuff better than me, it was suggested that I could retard the ignition timing by 8-9 degrees with a free flowing head and high compression. This turned out easier said than done. I had to make an adapter plate to relocate the pick up head relative to the rotor. I retarded the timing by a calculated 8.25 degrees. I put a couple hundred miles on since the mod. It runs great and pulls harder than ever, all on 92 octane gas. Feels like it winds out faster. Next tank I'll try the mid grade just to see how low I can go.
Although I must say that playing with racing fuel mixes for the last week has been a lot of fun, costly but fun. And not very practical either.

Thanks Mag10 for the update. Love to see those dyno results when you get them. Keep us posted.