Dyno - Firstimer

Having sorted some of the teething troubles with my JPN spaceframe replica , I thought I'd get it dynoed so we knew where we were up to and make a plan.


Information is King , or so I thought, That is if you understand what they are saying!
So if anyone out there understands what the results are saying then please explain.

I do know that something isnt right , the torque and power is in there but not in the right place!

Two things identified from the session 1. Initially the fuel /air ratio was showing to rich (240 mains) and ended up at 200 mains. 2. it was noticed that the rear tyre was expanding and rubbing on the swinging arm.

The first sheet is one of the later runs showing power and torque. Power is all over at 5,200 rpm , the second sheet was a speed run through the gears which recorded the highest power output at a nats under 60BHP but only recording a top speed of 117MPH
( I thought this would have been higher , due to no wind resistance etc).

Any one any ideas/thoughts

Like i know anything but the 5000-ish petering off power is suspiciously close to the top out of full advance of electro brain ignitions. I had event where pulled great till 5000 then pooped out on pull but still ran smooth. Discovered cam chain tensioner gone so advanced spark too much to pump against.

Like i know anything but the 5000-ish petering off power is suspiciously close to the top out of full advance of electro brain ignitions. I had event where pulled great till 5000 then pooped out on pull but still ran smooth. Discovered cam chain tensioner gone so advanced spark too much to pump against.

"I do know that something isnt right , the torque and power is in there but not in the right place!"

You are absolutely correct - the curves are invalid. Torque and HP curves on a dyno MUST cross at 5252 RPM.

The formula to calculate HP forces the curves to cross at 5252 RPM. If it doesn't, the machine is either running improper software or there is some other defect.
IOW, the torque/HP readings shown on that printout are not accurate and they will not correlate to actual performance or to a dyno running the correct software/calibration.

Remind us again - what engine, what head and compression ratio, what carbs, what ignition, what exhaust and what exhaust length and baffling,
what gearbox and what gearing.

You may need to just raise the gearing a tad, so that the torque peak happens at a slightly taller geared top speed.
Depending on track, of course.

Lovely looking cycle, you win just on looks alone. !

mike996 said:

You are absolutely correct - the curves are invalid. Torque and HP curves on a dyno MUST cross at 5252 RPM.

Click to expand...

Note that the scales for torque and hp are drawn differently, so the curves WON'T cross at 5252 rpm.
It is better if the scales are drawn the same, then we can be sure everything is drawn and compared equally.

If the scales are drawn the same though, then they do have to cross at 5252 rpm...

mike996 said:

"I do know that something isnt right , the torque and power is in there but not in the right place!"

You are absolutely correct - the curves are invalid. Torque and HP curves on a dyno MUST cross at 5252 RPM.

The formula to calculate HP forces the curves to cross at 5252 RPM. If it doesn't, the machine is either running improper software or there is some other defect.
IOW, the torque/HP readings shown on that printout are not accurate and they will not correlate to actual performance or to a dyno running the correct software/calibration.

Click to expand...

The torque and horsepower are not being graphed on the same scale. One scale on the right and the other on the left so they will not cross at 5252.

The horsepower is peaking way too early. It hard to say why as there is a lot of things that need to be working together to get the power right. After making sure the ignition timing and jetting is right then exhaust system and cam timing would be next things to work with. Jim

The bike has white plates so I assume it's a 750?

A bit more info on what state of tune the engine is in would be useful.
Mine has 10.5 compression, 36mm carbs and a PW3 cam and makes about 59 HP at 6400RPM, I like to think it makes a bit more, but the dyno tester wouldn't rev it any more.


As others have said. Check ignition, also if the curve is that far off, I'd check cam timing and exhausts. What length headers are you running? For a 2:2 I would expect to use maybe 25" headers.
It might be worth trying it without those megaphones to see what that does.

Thanks to everyone for the replies: The spec of the motor is:
850 Mk 3
Lightened crank to JS tuning Guide
Cyl Head is a RH 4 with the ports cleaned up around Guide ( nothing to radical)
Standard size Valves ( lighter ones from cyl head shop with 3 angle around valve seat)
Wiseco 10.5 : 1 Pistons
PW3 cam on standard timing marks
33mm Amal premier carbs c/w extended manifolds and bellmouths ( fitted length from head 10.5 ")
Boyer ignition c/w JS 680 ohm resistor ( tried it with and without , no difference to max power , did start power
slightly earlier)
exhaust is based on Norton tuning sheet, 1.5" I.D pipe 30" from head to meagaphone , 24" tapering up to 4" with a
3" dia reverse cone 1 5/16, silencing is by a 3" dia absorption tube the length of the megaphone packed with
fibre glass packing.
Hemmings 6 speed box , 21 gearbox, 44 rear

pommie john said:

It might be worth trying it without those megaphones to see what that does.

Click to expand...

Indeed - can those reverse cones on the end be removed to see what that does.
Only 200 main jets sounds like its baffled too much, or those reverse cones are influencing the breathing - badly.

When the PW3 cam was installed, did it measure up to the degrees quoted for it ?
Someone else here was having difficulty making a PW3 rev enough.
A Combat cam gives max power at 6800 rpm, after all....

I vote with the exhaust suggestions as had great power depression below even normal Combat tll accidentally getting the right opening on long megaphone on 2-1 headers then woke up so much I sought out hot shots on sports bikes as no acceleration advantage till 3rd gear 90 mph. I certainly did not delay the pack or get left behind. Got the basic guts for a tire burner so get back at it - again and again.

The theme of the replies is pointing at the exhausts! The original spec of the exhaust was determined by what they looked like on photos of the original and making something that would pass a noise test and remain period looking.
They satisfy the noise tester at 102 dba ( limit 105dba).

after reading the posts I've realised that the headers are 4 " longer than the stage 2 service release.

Ive a 2 day race meeting coming up next weekend , so before that I'll check the ignition timing , cam chain tension , cam timing
and remove 4" from the length of the header and raise the gearing, not much to do then!

Moto55UK said:

The theme of the replies is pointing at the exhausts! The original spec of the exhaust was determined by what they looked like on photos of the original and making something that would pass a noise test and remain period looking.
They satisfy the noise tester at 102 dba ( limit 105dba).

after reading the posts I've realised that the headers are 4 " longer than the stage 2 service release.

Ive a 2 day race meeting coming up next weekend , so before that I'll check the ignition timing , cam chain tension , cam timing
and remove 4" from the length of the header and raise the gearing, not much to do then!

Click to expand...

Keep us posted

One point of encouragement from me is I've had 4 engines that were modified to stage one or two levels that depressed with poor idle, sluggish low rpm response and annoying rough lazy near redline, till one little thing changed like cam degree, exhaust feature or intake change that both pleased the snot out me and won engine builder-dyno room bet it couldn't be done on size engine or carb or type fuel. So the worse it seems right now likely the better it will be once dialed in. Best engines are rather sensitive to little things - once that is figured out or just trial error like tuning headers that eventually became 'puterized programs.

One other thing: Exhaust lengths are often ( but not always) quoted as length from exhaust valve, not from where the header attaches to the head.


I'll measure mine tonight when I get home.

'Puter programs may measure from valve seat but good ole Nortons go by header flange to exit and almost always like 30-32". Sliding in/out of a megaphone don't seem to make much effect good or bad. Expensive best way is on dyno, slice, dyno slice dyno slice etc then buy new set cut at best lenght. Ya are out to get ahead aren't ya. Bet ya eventually get near 70 hp measured somewhere.

I would agree that the exhausts are too long. The muffler core will add to the length also unless it is considerably larger in diameter than the headpipe. Like at least 1/4 inch. If you can get a larger core diameter in the muffler it will help. Or shorten the core so the front part of the muffler acts like a megaphone and the core does not start till halfway to the rear of the muffler. It still needs to be bigger. Jim

Just measured my pipes. They are 26" from the flange ( where they meet the head) to the end. They work well with an Axtell #3 cam. I now have a PW3 but I haven't had time to dyno it yet.

I've been happy with Supertrapp silencers. They have a short megaphone hidden up in them followed by perforated tube and then their discs at the back end. The good thing about them for racing is that they are adjustable, so next time the ACU change the noise limits, you can change the number of discs or the end cap.

Moto55UK said:

The theme of the replies is pointing at the exhausts! The original spec of the exhaust was determined by what they looked like on photos of the original and making something that would pass a noise test and remain period looking.
They satisfy the noise tester at 102 dba ( limit 105dba).

after reading the posts I've realised that the headers are 4 " longer than the stage 2 service release.

Ive a 2 day race meeting coming up next weekend , so before that I'll check the ignition timing , cam chain tension , cam timing
and remove 4" from the length of the header and raise the gearing, not much to do then!

Click to expand...

DONT...do not shorten the headers right off by 4"....you are close enough....for now...or at least don't take 4" inches off the headers...to find you need to put some back....maybe take some at a time....but really....

The Mega's I think are too long and as Jim says you have too much pipe down inside....you are effectively running a 54" pipe

Works pipes were 28" to the point they welded on Megaphone, but according to Norman White had 2" inside the Mega! My NRP system for my 750 short stroke is not much shorter, maybe 29", and NRP has made exhausts for most of the top four strokes running in CRMC today, includingg Dave Watsons, Nortons, 750 and 1007.... decision on length came from a combination of NRP input, advice and figures from Norman White and my old experience.

I ran 28" pipes with open Mega's no problem, on an 850 with a short stroke head, works cam and 10.25 compression, and with a baffle that only ran from the reverse cone maybe a third into a much shorter Mega (17" I think).

You are killing it with what you have inside the mega....get that out first and work on cam timing, jetting and ignition....

Of interest one time I did run a very long slash cut open pipe, actually very similar to pipes seen in pictures of Dave Croxford in a Monjuich Park endurance race......it made torque like nobody'ss business....but ran out of puff up top.....on track it was quite quick out of corners but sluggish at the end of the straight....probably similar to what you have right now? But I actually got good lap times with it.....if there were enough twisty bits....

Two things here.....you are chasing dyno performance....what is the riding experience like?

"Two things here.....you are chasing dyno performance....what is the riding experience like?"

+1000 !!!!!

THAT is the most important question.