The building of my vintage racer

All the quickest airheads I've seen have the best riders.

Agreed!! When you look at the flash across the piston with a small bore motor a single spark plug is fine. I have been building 750 race motors for over 20 years with results in the 60HP+.

Does anyone have single/dual plug comparison dyno charts to post up? Over on the Norton Commando forum there is a guy who has posted up lots of head flow charts...seeing is believing.
Some interesting results on port sizes, lie the standard head flows better than the factory ported one :huh
I twin plugged an R100 20 years ago,as was the fashion but I think it was the lighter flywheel and 40 mm carbs that made it go well,

Dyno and flow charts? The problem with both is that the numbers all depend on the dyno and flow bench and how they are run. Huge differences can be had with both depending on a lot of variables definitely including the test equipment itself. The numbers can be manipulated on purpose too! Making fair and objective comparison runs is a skill and harder do do than most imagine. My advise is don't be a sucker.

Small bore? A 750 is still a short stroke, big bore engine. 82mmx70.6. 1.16:1 bore to stroke ratio. Not nearly as short stroke as our R65's and R100's at a whopping 1.33:1 ratio but . . . . 1.33:1 was considered a REAL short stroke for a long while!

The Dyno throws up some interesting results. I sometimes race my Sons Bucket Racer, a Suzuki FXR 150 DOHC former commuter bike, the 20 bike field has about 15 of these.

Piccy from last weeks meet.
Lots of mods are done and tried on ONE dyno by the same guys and they have tried almost everything to make these things faster...cams, carbs, pipes etc.
Latest one is the Ignitech programmable ignition that you can adjust while the motor is running, Ignition, flat slide carb and exhaust with no other mods turned out 18 HP. More than the bikes with fancy cams and pistons on the same dyno.
The guys who own the dyno do it free. Then the bikes are tried on the track measured with transponders.
I'll mention next month that their dyno is for suckers....

You need to read my words again. IF you come to understand them, run them by your friends. I am sure they would agree. Now that I think about it, you had better just have them read what I wrote. You are getting a pretty long history now of turning what I write on its head. You are still putting some pretty stupid words in my mouth but at least you aren't using quotation marks while you're doing it! What is with you guys?

I'm not disputing your power outputs, I'm asking if you have direct comparison dyno charts for single and dual plugs.

And how may plugs do these bikes have?

some single, some dual: it varies

I'd like that kind of power... :~)

Sorry, I do not. I have never been a fan of dual plugging.

It's not cheap.

I doubt you'd see much difference in HP just between single and dual plugged. The addition of another plug in and of itself doesn't really bump power up much if any. It allows a faster and more complete burn with less advance. This will result in an engine producing less waste heat and which will run cooler. Because it runs cooler it allows you to bump up compression with less likelihood of pinging. Bumping up compression WILL increase your power. So in short it's not the plug that gives the power, but the higher compression the additional plug allows that'll give you power. Also, as BMWeuro stated, twin plugging is more useful in 1000cc engines as the flame front has farther to travel in the wider bore of these engines and thus a second ignition point and two flame fronts will show a greater improvement there than in smaller 750 and 800cc bores.

And then a strict comparison of just the additional plugs wouldn't make much if any difference at all more than likely. The whole point of dual plugging is raising the compression even higher and getting the setup to make peak power running a more retarded ignition timing. Both generally net more power. Hell, if I set up a racing engine I would reverse offset the piston bores Smokey style for closely related reasons. It does work! But I would long rod one too. That works big time again for closely related reasons. It's granted that generally larger displacements have more detonation issues because flame front travel times remain relatively constant despite displacement but there are still advantages to be had there in a 750 IMO as well as many others but to each there own.

Is it any wonder that a guy who calls himself SuperShaft is such an expert on DP'ing?

Oh oh. FLAGGED.

As I understand it, there is less pinging because the twin flame fronts allow less time for the unburned portion in FRONT of the flame front to overheat and detonate versus burn. That unburned part of the mix in front of the frame front gets super compressed and the molecules break down. That is what causes detonation. That's why you can always see the piston melting around the very edge first. What's going on behind the flame front is a burn. You don't want the remaining fuel for that on coming flame front and burn to disappear in a bang right in front of it. It cuts way down on the push and PUSHING that piston is what makes torque and torque times rpm is HP.

I forgot to add ignition timing. That is a big part of the extra power too. You can generally determine the efficiency of a combustion chamber by it's peak power ignition timing. The more retarded it is, the more efficient it is. Peak power ignition timing drops dual plugged 750 or 1000cc's. Perhaps not as much with a 750 versus a 1000 but gains are there for the taking IMO.

I wish I had the time and money to test more of this stuff... I sure many of you do to. Last time I went from singles to dual plug it was also from worn out head to fresh seat and guides. Not much of a test. It is so very hard to truly do one thing at a time.

I have a hypothesis that dual plugging is more of an advantage at lower RPMs when there is less turbulence to spread the flame front then at high RPM where turbulence assist with rapid combustion. Couple this with the idea that spark advance is stopped at higher RPMs because turbulence takes over and spreads the flamefront more quickly for us.

I wish I had more to add than that, but that is what has been rolling around in my head as I think about buildin my next race engine.

Eric

I got that moniker from an old girl friend I had while I was working at San Jose BMW. Some of my friends don't believe me and I let them go on for a bit at my shop and then I take a well done drawing of a beemer off my shop wall and show them the back of it were I am fondly referred to as "Supershaft". The hand writing is obviously a women's. She is a tattooist. Anyway, they usually then go into ultra-denial. I got out of that relationship without a single mark!

The only way I can/could afford to do most of this stuff is doing it for hire. I have never had my own TIG welder, my own dyno, my own machine shop or my own real sheet metal equipment! Get out there and do some good work! Listen and learn along the way! And READ!

Nicely explained

more compression in a race engine only gives extra power up to a point, after that the engine just starts to get harsh. and as for twin plugs, i have never seen any gains, so i stick with single plug even on my bored and stroked engines.