Dr. Blair on Dyno Hill- Place your bets!

1 1/2“ pipes had a negative effect on my 850 according to the dyno. So I can’t imagine 1 5/8” would be a step forward either.

1 1/2 " also slowed this 850.
The speed at top was 113 kmh with 1 3/8" and 109 to 110 with 1 1/2".

Glen

Since we're talking exhaust,

Many years go I was talking with Kenny Price of Samson Exhaust about baffles for my Shovelhead.
He had his standard and his performance baffle. The fluted was the standard, quieter one, perforated was the performance, louder.
Explained how the perforated one performed better than the flute at higher RPM. About the same down low.
This matched my own experience on the Norton years earlier, where the stock, fluted Pea Shooter would hit a wall, but the Dunstall Muffs on the same balanced headers would run out further and smoother. Same 260 mains. Stock motor.

I often wondered if some of the current Pea Shooters may be flowing better than the my original 73 factory ones.
Mine had the removable mutes, which were removed.

Don't know all the evolutions of the Pea Shooters, but I'm sure there were several.
I recall reading something somewhere, the best flowing stock exhaust was the original S type. (Brian Slark maybe?)
It was also the loudest. Don't know if was fluted or not.

Not saying Kenny Price is the consumate exhaust expert, but the muff on my tuned, 400/453 KTM is perforated....

I tried two sets of silencers of the straight thru perforated type. These both use fibreglass packing to subdue noise.
One set was short, one long.
Both were much louder than the open peashooters. In fact they were quite harsh sounding. OK for a hill run but not for travelling.
Both produced less power than the fluted peashooters.

Glen

Interesting......

Fast Eddie said:

1 1/2“ pipes had a negative effect on my 850 according to the dyno. So I can’t imagine 1 5/8” would be a step forward either.

Click to expand...

I had big bore down pipes on a Commando about 30 years ago and they done nothing for performance at all.

The bike is back in the free-flowing 113 kmh state today.
A test run was done to verify. The expected result was achieved, 113 kmh at hilltop.
Next I added weight to the bike to change the power to weight ratio to the same as with the restrictive Blackcaps.

I started with 100 lbs. My first thought after loading the weights on was that it was likely too much weight.
The bike managed just 102 kmh hilltop with the Blackcaps, so an 11kmh loss from the freeflowing 113kmh exhaust combo.
To my surprise the result with the 100lbs added ballast was only a 5kmh drop in hilltop speed, 108 kmh.
Based on that it will take 215lbs ballast to slow the bike the same amount as the Blackcaps did. Unfortunately I cannot put that much weight in the bags. A large passenger would suffice and I will make that test one day. In the meantime, we can calculate based on the 215lb number.

The 850 engine produces right around 40bhp at crank at 4000 rpm. That would be with a non restrictive exhaust.
By my calculation the Blackcaps chop power to 31bhp at this rpm, a 9 bhp loss.
Loss will be much greater at full rpm.

Glen

Damn.

worntorn said:

1 1/2 " also slowed this 850.
The speed at top was 113 kmh with 1 3/8" and 109 to 110 with 1 1/2".

Glen

Click to expand...

Stock 850 motor, 32mm Amals, no carburetor tuning effort, right?

Another dumber question:
Is that 1 1/2" inside or outside diameter?

My headers are 1 1/2" outside diameter, so must be 1 3/8" or close to it on the inside. Funny thing about satin black pipes is they look bigger than chrome pipes of the same diameter. I always thought mine were larger than stock Commando pipes, but apparently not.

Yes, stock motor
Various mainjets tried with every test except with the Blackcaps. For those I fitted 230 MJ's, same as the factory supplied with the Blackcap bikes. Needle also in the factory position.

The header pipes that made the greatest power are the 1 3/8" OD stock pipes, separate ( or blocked balance pipes, I tried both, same 113kmh)
The RGM " big bore" pipes measure 1 1/2" OD. They knock off about 2.5 HP at 4000 rpm vs the skinny stock pipes.

Glen

worntorn said:

Yes, stock motor
Various mainjets tried with every test except with the Blackcaps. For those I fitted 230 MJ's, same as the factory supplied with the Blackcap bikes. Needle also in the factory position.

The header pipes that made the greatest power are the 1 3/8" OD stock pipes, separate ( or blocked balance pipes, I tried both, same 113kmh)
The RGM " big bore" pipes measure 1 1/2" OD. They knock off about 2.5 HP at 4000 rpm vs the skinny stock pipes.

Glen

Click to expand...

OK

I'll be darned, I do have larger headers then. They work. However, it wasn't plug and play to make that happen.

Thanks for putting in the time to perform the tests and post the exhaust result info.

It's interesting reading the period tests. The loss in performance with the A bikes and MK3 is mainly blamed on weight gain, which is a fallacy.
The 750 Commando Interstate was weighed in an earlier test.
With a full tank of fuel it weighed 460 lbs, same as an 850 Interstate +-5lb.
The MK3 is heavier by the weight of the starter mechanism, but that 25-30 lbs cannot be felt as far as acceleration is concerned.
I could barely feel any difference in acceleration with the 100 lbs added. I could not sense any difference in hill power, the instrument had to tell me about it.
Some of the testers blather on about a weight gain that doesnt exist while the whole performance problem was the silencer restriction.

Glen

I think it’s as we’ve discussed before. The lack of oomph would easily be translated by the arse dyno as the engine struggling to move a heavier plot.

NVT probably accidentally contributed to this by telling the world how wonderful their anular discharge silencers were. Thus, convinced that the silencers were the best thing since sliced bread, testers would inevitably look for other factors to blame.