850 Dyno Run

Stock '74 850 Commando with Premier carbs and Tri-Spark. Engine has 16,500 miles and I am pretty certain the head's never been off.

Ignition timing was set at 31 btdc, although I've never checked the veracity of the timing marks versus TDC and how it relates to the timing scale on the primary.

The big dip down low was from too much throttle at once.

On the first runs, I'd set the spark pickup on the dyno at 360 degrees since Tri-Spark fires both coils all the time, but when I was doing pulls I only got the right rpm reading when I went back to 720. What was great about those first runs reading half the correct rpm was that the torque was around 89 ft.-lbs...

Horsepower was 46.59 at just under 6000 rpm with a nice curve. For torque, it's easy to see why these engines feel so good on the road. It's making near-peak torque well under 3000 rpm and doesn't dip much before the "second' peak of 44.34 at 4600 revs.

I didn't rev it to redline because, well, it clearly wasn't doing any good to keep flogging it.

Sure makes a lot of hydrocarbons compared to a modern bike! Luckily, the ventilation is great on this dyno.

Good effort, nice run - no point flogging the old gal too hard, she gave what she had.
About the "hydrocarbons" - did you get a stoichometric ratio [fuel/air] readout, she`s maybe running rich? - Or was she smoking like a stoner & dyno guy didn`t want his probe getting filthy from ramming it up her fundamental orifice? - [Like a 2-stroke...]

The way it ran it seemed in the ballpark mixturewise. On the road, it's running about 50 mpg so the pilot certainly is in a good spot. Didn't have the sniffer available. I'll see if I can get that for the next run. I'd like to experiment with timing a bit, too. There was a bit of oil smoke on overrun from the guide seals but otherwise it was pretty okay.

New bikes are just exceptionally clean and that's mostly what I've been around for runs.

Thanks for posting that, it is very interesting. I would have thought a standard Commando would produce about 55bhp. But 46 is about whhat good Triumph Bonneville 650 would give, and the Norton 850 would be doing it easier. How reliably can the torque measurements be made ? My interest is always in improving that aspect for racing.

acotrel wrote;

I would have thought a standard Commando would produce about 55bhp.

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Maybe 55 bhp at the crank. There are the usual losses through the drive train which IIRC are 10-15%? A friend of mine had his Combat tested recently and it managed 38rw bhp, although it was running a bit rough. He was really *issed off, although on the road it goes quite well.

My 920, which has a Maney stage 2 head and PW3 cam, managed 61 rw bhp. I remember reading an article in one of the classic magazines a few years ago and it tested a few Commandos and they were making rw bhp typically top 30's to low 50's IIRC?

I put mine on a dyno about 10 years ago and got similar readout!

Factory claims I have seen for 850 have been between 53 and 60 (65 for Combat) Typically drivetrain does take 10-15 percent. Or, putting it this way, rear-wheel numbers are usually this degree less than factory claimed amounts. Which gear you run can also have an effect. Some bikes have no "direct" now, all ratios being under- or overdrive. I experiment with which gear on new bikes. I ran the Commando in fourth gear only, since that is direct and should be most efficient.

I had about 35 psi in the rear tire. The chain was lubed with that great, black sticky stuff by PJ1 (my fave). I have found on modern bikes that a quick shot of thin spray lube (WD-40, etc.) on the chain gives you some fraction more power; on a 150-horse bike you might pick up 1 or 2 hp.

The torque reading is accurate, so to speak, but this is an inertia dyno and these numbers were obtained in that way. I'm not a tuning or engine-builder dyno expert, but some of the guys I know in modern racing say that a pure inertia dyno doesn't tell them what they need to know. I need to follow up on those conversations.

For general purposes and if I were to go racing, I would certainly be happy to tune on this dyno. It's a Dynojet 250i, and it does have a brake also, but I have only used it to slow the bike down after runs. You can use it to vary load at certain rpm to get throttle in a position where you may be having jetting trouble, etc.

I was going to experiment with ignition timing a little to see the difference between 28 and maybe 35 max. I would be really happy if I could program the curve. I've done quite a bit of recurving on old British cars and there is good room to play there. Also sort of wish I could have vacuum advance. My best fuel mileage has come when I over advanced (accidentally!) and was running way ahead of normal at 3000-3500. Lots of cars pick up 10-12 degrees in high-vacuum (small-throttle-opening) cruise conditions and it really helps fuel economy.

I wanted to do full-throttle plug chops to see what was what but it was so busy I forgot. Barely made it home in time for the 9-hour, apple-wood-smoked pulled pork! Which may mean I'll need to emphasize more torque if I ever mod this motor...

http://www.youtube.com/watch?v=ZeIpr32reQ0

Here you see an 850 engine that I built for a customer a couple years ago - they tested it on a dyno at a Ducati shop and were highly impressed. I think the numbers are a bit optimistic.....

Cheers

Hartmut

Whitworth Ranch said:

I was going to experiment with ignition timing a little to see the difference between 28 and maybe 35 max. I would be really happy if I could program the curve. I've done quite a bit of recurving on old British cars and there is good room to play there. Also sort of wish I could have vacuum advance. My best fuel mileage has come when I over advanced (accidentally!) and was running way ahead of normal at 3000-3500. Lots of cars pick up 10-12 degrees in high-vacuum (small-throttle-opening) cruise conditions and it really helps fuel economy.

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Slightly off topic, I fitted a Ignitech ignition to my BMW racer so as I did not like that the Boyer just kept on advancing....Had to make up trigger plate but these can be used on any bike with 1-4 cylinders and they are a nice bit of kit.http://www.ignitech.cz/english/aindex.htm
You can view and alter the curve live and its a easy to upload and download as pics on the net.


Its now crank fired instead of cam fired eliminating chain slop.
I have not investigated using one on a Commando....anyone done one?

i would be interested in the set up with the ignitech on the crank for my Bike #2 project
Anyone out there tried this?
Regards Mike

Hartmut, any chance you could share your rebuild specs to gain that sort of HP?
PM if you don't want to post public!!!
Regards Mike

Its a good Commando that make more than 50 bhp rear wheel.

But dynos are notorious for different estimates so absolute numbers cn be misleading anyway.

Thats a big dive in the power curve at around 2500 rpm.
Standard carbs and airbox/ aircleaners ??
Carby setup should be able to improve that...

Rohan said:

Thats a big dive in the power curve at around 2500 rpm.
Standard carbs and airbox/ aircleaners ??
Carby setup should be able to improve that...

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As I said in the first post, the dip was too much throttle at once. It pulls cleanly when done smoothly and the curve runs straight. I picked this run to save because it had the best peak.

If its jetted and carbed correctly, it shouldn't bog down like that with too much throttle ?
Standard air box ??

Standard airbox. Runs beautifully on the road, but I typically don't snap the throttle WFO at at 2200 rpm. There no accelerator pump and I've had experience like this previously with slide carburetors. I think the opening gets too large for the air velocity and there is no signal to the jet. Hence the dip. Happy to hear from someone with an opinion on this if I am not correct.

Brooking 850 said:

i would be interested in the set up with the ignitech on the crank for my Bike #2 project
Anyone out there tried this?
Regards Mike

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I ran an Ignitech box on my Yamaha SRX 600cc road racer and loved it. You can program the advance curve with a laptop, although the curve Ignitech set up on mine worked great right out of the box. If you tell them what you want when you buy it, they will program the curve to suit you. Great people to deal with, even if they are a continent or two away from me.

Ken

Almost every engine in cars or bikes or boats with carbs w/o accelerator pumps one needs an educated throttle opening at off idle or pretty low rpm of it just sucks air w/o enough fuel to make power to speed up or can even stalk the fire out. I've had lawn mowers I could snap throttle on off idle and they'd pick right up and my chainsaws are pretty responsive right off idle but even they can bog for a instant before kicking in. So both views are correct. One the fuel curve is way off w/o an accelerator pump to fill it in and two with a bit smoother inital throttle it'll pick right up w/o bog but not as quickly till air flow picks up its full fuel load.

@ Whitworth Ranch. Not an expert on low speed street tuning but if you really have a need to whack the throttle open without a stumble my hunch is a lower cutaway on the slide will help.

Shrapnel, I am running #3 slides already and normal response on the road is fine. Just way too much way too low on the dyno.

This was the kind of snap open that EFI (which is almost exclusively what I dyno) doesn't mind at all.