Dynojet run at the Meltdown

I decided to have my bike dyno tested at the Meltdown Vintage Show in Hendersonville, NC on April 27th.
The engine is a detuned Combat. Flat top .040 pistons, .125 thick copper head gasket, mild head work. Webcam 312a advanced 5 degrees.
Amal premiers, (Jetted rich as you can see.) Pazon ignition. 22 tooth front sprocket, stock geared chain primary.
It will run 70 all day at the rpm I wrote in on the dyno report. It is still under 4500 at 80 mph and feels a bit more strained at that speed but it will do that hour after hour as well.
As you can see, the horse power fell off after 5700 rpm. In real life it will only go about 103 mph. I'm pretty happy with the performance and now need to work on the jetting. I'm thinking I lowered the compression too much so I'm thinking about putting in a thinner gasket.

For some reason the dynojet would not pick up the RPM so I calculated it off of wheel circumference which I measured carefully. I was also disappointed that torque did not show up. What you see there is two runs, both measuring horsepower.
It was a fun day.

I wouldn't mess with the compression ratio. Get the mixture right and you will see a big gain. Jim

comnoz said:

I wouldn't mess with the compression ratio. Get the mixture right and you will see a big gain. Jim

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PS, without an rpm reading the graph is in the area -at best.

That's actually a nice flat AFR line but its too rich. Try up to the 12 - 13 mark.

Stick your rpm and hp into a spread sheet and you will get a good graph to work with. Add torque as well.

A good street commando will run around 48 bhp rear wheel.

I think I was able to come up with the RPM pretty accurately by using the Miles per Hour and the wheel circumference as well as my primary and final drive gear ratios.
Does any one know the difference between Dynojet horsepower readings and Brake horsepower?

motorson said:

I think I was able to come up with the RPM pretty accurately by using the Miles per Hour and the wheel circumference as well as my primary and final drive gear ratios.
Does any one know the difference between Dynojet horsepower readings and Brake horsepower?

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That depends on the individual machine and operator but it's usually considered to be between 10 and 15 percent.

Of course figures from your rpm/speed calculations may easily be that far off since you don't know the wheel or clutch slip or the circumference change due to tire deflection. That is usually considerable. Jim

If you are using a .125" head gasket on an otherwise stock dimension Combat head/cylinder/piston combination, you basically have the compression ratio of a stock Commando with a .085" head gasket. That's almost three stock head gaskets. Way too low.

Ken

Yeah, after I thought about it I would have to agree with Ken. Unless you have done other machining to the head or barrel or are using unusual pistons you would likely be better off with about half that .125 thickness.

johnm said:

A good street commando will run around 48 bhp rear wheel.

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John, what do you mean by ‘good street Commando’?

Do you mean: that’s what they were from the factory...
OR
That’s what you CAN get with some mods, tweaks and careful assembly?

Plenty of folk on here with good stock bikes have posted less than 48 rwhp.

I believe most 50 year old stock Cdo’s are closer to 40 rwhp.

We had a standard 750 tested last July, very little changed on, only like for like so no trick parts, and that produced 48rwhp, the poor relation of the 850 would be nearer 40 in standard trim I suspect.

Nez from Classic Cycles in Upper Hutt NZ used to have a list of dyno results on the wall of his workshop office. And the only Commandos over 48 bhp were ones modified for racing. The best well prepared standard bikes ran around 48 bhp rear wheel. Both 750 and 850. So I would say a standard stock Commando in excellent shape would do about 48bhp rear wheel. My 500 Donnie with a lot of work ran 52 bhp on the same dyno but the Commando curves were a much better shape

Interesting, I’m gonna try my hardest to persuade my mate to come along on his low mileage, never stripped, MK3 next time I go to the dyno.

Time Warp said:

I seem to remember Dodge / Mopar doing a lot of testing producing formula calculations to what was needed to obtain certain quarter mile times etc and they were very accurate... If you didn't meet the goal you didn't have the hp no matter what you thought you had........

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Yes, they did! You could build a competition Mopar -1/4 mile or Nascar - using their "Direct Connection" manuals which included Nomographs for 1/4 mile ET/and trap speed. They were great and told you immediately what was right or wrong with your setup, including what your ET SHOULD be based on your trap speed. The trap speed and your weight gave you HP. FWIW, trap speed is the useful tool for determining power, not the ET.

Interestingly, we sometimes found very noticeable differences between Dyno HP and Track HP using the nomograph. In most cases it turned out to be airflow/carburetor-related. What SEEMED like a good intake setup - a large hood scoop feeding the carb(s) directly - kind of a ram-air thing - actually disrupted the airflow as the vehicle speed increased. Running the scoop to an airbox, ensuring a smooth entry into the carbs eliminated the problem. In one case there was a drop of over 60HP between the RWHP the engine produced on the Dyno and the HP generated on the track. The figures were correct in both cases and once the airbox was added, the trap speed (HP) went up to essentially the same figure as the Dyno. The Mopar engine/car building data kept you from chasing the wrong stuff when setting up a performance car. The DC manual was as thick as a NYC phone book and covered every single aspect of building a Mopar for 1/4 mile or Nascar-type competition use.

Aren't Dynojet numbers notoriously high?
This one doesn't seem to be. Perhaps no corrections were used.
The car world seems to think that with the proprietary Dynojet correction, the numbers given are representative of crankshaft HP, even though they are taken from the back wheels. One operator stated that the correction factor used for drivetrain loss was 1.18 plus there are corrections for temp, humidity and altitude.
I wonder if much of the apparent discrepancy in numbers comes from application of the corrections. If the operator uses all of the corrections, he is giving you a very optimistic crankshaft number. If he gives you an uncorrected number, it could be even less than rwhp.
Im only going by info on the net, I have no idea if this 1.18 mechanical multiplier is in fact part of the program.
If so it would really confuse things as it seems to be up to the operator to choose the corrections.
Big number, happy customer. Small number, unhappy customer.

Wanna sell some tuning?
Free dyno day, no corrections, you all have loser motors.
Get our tuning done, we apply corrections, your are a winner with a very fast steed.
Take this invoice to the cashier at front please.




Glen

What would the consensus be about my top speed? I'm a bit of a big guy for a commando (235 Lbs.) and the bike will actually go 103 mph at the indicated 5700 rpm. How much power does it take to do that?

I think thats where a lot of these bikes will run with a full sized person on there.
It's not so much the weight as the extra width and height of the bigger body that pulls top speed down.

On my Thruxton R I checked top speed once and hit 220 kmh and the rev limiter.
About a year later on a lonely road I tried the same with a very small set of panniers on , 15 litres per side. It would only pull to 200kmh.

What sprocket are you running?

Glen