Balance pipe and Dyno Hill

[worntorn]

Even though I said I wouldn't, just had to give it a try.
The pipes were off the MK3 850 for exhaust port repair, so it was easy to turn up some alloy plugs to run with the balance pipe removed.
With the plugs in, so simulating regular twin exhaust, I did 4 runs on Dyno hill using the GPS for a speedo.
All runs gave the same result, 113 km/hr at top of hill marker, 100 km /hr start on hill then full throttle in top Gear.
A real midrange test.

With the plugs removed and balance pipe back in the first three runs netted 110 at top. The fourth run gave 111 but we'll throw that out as an anomaly.

This was very surprising and not at all what I expected.

At 1000 rpm idle the sound level from 1 pace away was 68 db+- for both the plugged setup and the balance pipe setup.
Also not what I was expecting.

Glen

 

[rweb]

Very interesting. Who'd a thought?

 

[MexicoMike]

Interesting, it contradicts actual dynomometer tests from back in the day that were published in Norton tuning tips in the early '70's which is the reason Norton switched to the balanced pipes.

They are used in virtually every dual exhaust car for the same reasons - more power and reduced noise. Maybe they don't really work!

 

[worntorn]

It doesn't work on my bike and it's a nearly standard Mk3.
I have some 1 1/2 " non balance pipes coming for the project bike, will likely try them on the MK3 and see what the results are.
When time permits, I will try again with the plugs back in these 1 3/8 stock pipes to see if the speed goes back up to 113.
I'm quite confident that it will.

Glen

 

[comnoz]

The crossover only works if your using the restricted mufflers. Either the small hole peashooter or the black cap muffler. If you were running either of those mufflers you would see a big difference.

As for the 1-1/2 pipe you will likely loose power unless you have a pretty well built up engine.

 

[rweb]

I wouldn't have been too surprised if when using low restriction mufflers there had been no change between crossover and non crossover pipes. What's surprising is that the crossover hurt performance. Never heard of that happening before.

 

[comnoz]

I wouldn't have been too surprised if when using low restriction mufflers there had been no change between crossover and non crossover pipes. What's surprising is that the crossover hurt performance. Never heard of that happening before.

Actually the single exhaust with a low restriction muffler does benefit the motor because it acts as a tuned exhaust.
The crossover is not in the right place or of the right size to help with exhaust extraction. It is only a way to reduce the backpressure when using restricted mufflers.
It de-tunes the motor a bit when used with open mufflers.

 

[MichaelB]

Very interesting, I would have lost that bet....

 

[comnoz]

Increasing the pipe diameter by 1/8th inch raises the tuned RPM of the exhaust of a Norton sized motor by 800 to 1000 rpm. [with the same length tube]
Note: that does not mean the motor's torque peak will raise 800 to 1000 rpm. That would require that the volumetric efficiency is maintained at the higher rpm also.

For a road going motor best overall performance is normally obtained when the exhaust pipe ID is close to the exhaust valve diameter. That gives adequate velocity in the headpipe to help extract the exhaust gas over a wide rpm range.

This is not going to give a high peak horsepower number but it will create maximum midrange torque and the "most area under the curve" if your looking at a dyno chart.
On a short road course it will also get you from one corner to the next the fastest.

 

[Triton Thrasher]

Did you change the exhausts without checking or altering main jet size?

 

[worntorn]

Yes.

Mainjets have been optimized for the balance pipe exhaust some time ago.
Removing the balance pipe and plugging the stubs made it go a bit faster with the same mainjets.
It's possible that a bit more could be gained by trying different mainjets with the non balanced setup.
The extra acceleration from 110 to 113 is quite a bit considering the grade the bike is working against is 12.5 %. This is over 50% steeper than the steepest highway climbs I've ridden in BC and the Western US.
Most of those are 7%, a few are 8.

My father built this section of road in 1963 using an International TD24 crawler and an Allis Chalmers TS 260 motor scraper.
The Municipality wanted the gravel trail that switchbacked up the hill made into a straight , paved road.
He told me that he and his employee wanted to spend more time and get the grade down closer to 7%, but the Municipality only had a small budget. They ran the machines for 3.5 days and that's when the money ran out .
I'm so glad it did, 12.5% is ideal for a dyno hill!
I have the bill for the road building job from 1963, found it in the piles of records I had to go through when my Dad passed away.

Glen

 

[Fast Eddie]

Great test Glen, thanks for doing and for sharing.

As Jim has said, the “more power, less noise” claim is clearly relevant to restrictive silencers, which does then seem logical.

The tuned length of correctly dimensioned pipes must be what’s in play here.

The noise at tick over is interesting too, but my guess is the balanced pipes might be quieter when ‘on the cam’.

Please do test the 1.5” pipes! I was told by a racer from back in the day that 1.5” pipes is what liberated the power from 850s and until they did that, they’d been really struggling to make the 850s go like the 750s had.

But when I dyno’d 1.5 inch pipes on my 850 the results were negative. I will try them out on the 920 one day though.

 

[Danno]

I'd like to see the test performed with the individual pipe setup jetted down to about a 230 from the 260s the crossover 850s came with. Maybe some of that lost power would be restored.

 

[kommando]

My secondhand MK2a came with crossover pipes, peashooters (instead of the std black caps fitted when it left the facory) and 230 jets, within 2 weeks I had ditched the TT100's to stop the whitelining and upped the 230 jets to 260 jets to restore WOT performance.

 

[baz]

This is a disappointing finding for me
I literally just fitted a pair of balanced downpipes on my 750
I have stainless straight through pea shooters these I believe have a perforated tube through the centre rather than a fluted one,but certainly no resistance there
I was hoping to quieten the bike down just a little and maybe pick up a little power, certainly not losing any!
The other thing I was planning was to fit the bung for my gas analyser into a spare balance pipe
Rather than welding into the down pipe on only one side as I had done previously
Oh well I'll give it a try when the minor rebuild of the rest of the bike is finished

 

[worntorn]

I'd like to see the test performed with the individual pipe setup jetted down to about a 230 from the 260s the crossover 850s came with. Maybe some of that lost power would be restored.

Not sure of your meaning. There wasn't any lost power to restore with the individual pipe setup, just the opposite.

Glen

 

[maylar]

My only experience with/without balanced pipes is that I lost a bit of midrange torque and about 5 mpg when switching to unbalanced. This was with the factory pea shooter silencers, too long ago to recall if they were straight through or not.

 

[WZ507]

Glen - just curious, what rpm range does the dyno hill test encompass going from 100 to 113 km/hr?

 

[MichaelB]

My only experience with/without balanced pipes is that I lost a bit of midrange torque and about 5 mpg when switching to unbalanced. This was with the factory pea shooter silencers, too long ago to recall if they were straight through or not.

Do you still have the originals?
The original Mk2 peashooters were restrictive with built in mutes.
Your results are consistent with Jim's explanation.

 

[maylar]

Do you still have the originals?
The original Mk2 peashooters were restrictive with built in mutes.
Your results are consistent with Jim's explanation.

This was almost 40 years ago. I've been through a couple sets of silencers since.