750/850 weight comparison

concours · Oct 19, 2022

worntorn said:
I asked him afterward and he said he rolled it to full bore as soon as he got straight on the pavement.
I rolled it on about a half second later.
His bike has a quiet exhaust which might account for the power difference. When his bike zips by at the end it is still full bore. You can hardly hear the engine. I think he has an overly restrictive exhaust, but he's happy with it.

We wont do drag starts, no need and it's hard on the bike.
Get the clutch out with a few revs then wack the throttle open, simple.

Glen

"quiet exhaust" meaning? Standard issue pea shooters?

concours · Oct 19, 2022

worntorn said:
Acceleration test
I stopped at the top of 2nd as it was no contest.
That's with the 750 rider at 198 in gear and me at 240 in gear. We weighed that too!
So the MK3 is carrying about 80 lbs more than the 750 here.
Both bikes have EI. Both have 20 tooth cs.
Go figure.

I just watched the vid. Way lame. Clearly the guy on the right hasn't a clue/balls to wring the neck of a bike.

Mart UK · Oct 20, 2022

concours said:
I just watched the vid. Way lame. Clearly the guy on the right hasn't a clue/balls to wring the neck of a bike.

That's probably why he chose a 750, rather than the faster 850...

concours · Oct 20, 2022

Worntorn, Just to clarify, I LOVE the idea of your "dyno hill" and have been an avid follower.
Please provide us with MORE dyno hill challengers videos!

baz · Oct 20, 2022

concours said:
Worntorn, Just to clarify, I LOVE the idea of your "dyno hill" and have been an avid follower.
Please provide us with MORE dyno hill challengers videos!

I agree
Maybe running bikes up your dyno hill would make more sense

worntorn · Oct 22, 2022

750 Interstate with Alton starter , fuel 4 " down, 1" lower than the earlier bikes weighed.
471 lbs.

My MK3 Interstate with fuel 4" down 477 lbs.

So the lumbering Collussus of all Commandos, the porky 850 MK3, is 6 lbs heavier than a 750 with Alton Estart.

Glen

EstuaryBoy · Oct 22, 2022

worntorn said:
750 Interstate with Alton starter , fuel 4 " down, 1" lower than the earlier bikes weighed.
471 lbs.

My MK3 Interstate with fuel 4" down 477 lbs.

So the lumbering Collussus of all Commandos, the porky 850 MK3, is 6 lbs heavier than a 750 with Alton Estart.

Glen

You can get in trouble these days for: 'Body shaming'....

Mind you, you'd have thought a bigger hole would weigh less, two bigger holes even more so

worntorn · Oct 22, 2022

It just puts the lie to the myth that the MK3 has this tremendous amount of extra weight.
The added weight is just the Estart and a few pounds of well needed engine and frame strengthening.
Also, 750/850 kickstart bikes are essentially the same in weight, pick the one you like.
It isn't "nimble 750 and hefty 850."

Glen

worntorn · Oct 22, 2022

Here's the roll on
The 750 has a Jim Comstock ported head with D exhaust ports, bumped compression and an aftermarket cam similar to a 4s.

The 850 is a stock MK3 other than the silencers.

Total bike and rider weight is within 12 lbs.
I forgot to ask about cs sprocket, will check that sometime soon.
The bikes stayed in lock step until the top of 3rd where 750 pulls away. My first two shifts were ok but should have shifted from 3rd sooner. I think his would still pull away although once I finally realized we were going to keep going and got it in 4th, the spread stopped growing.
The original plan was to shut off half way thru 3rd, but apparently that plan was changed on the fly!

MexicoMike · Oct 23, 2022

Doesn't look like there was much difference; makes one rethink "performance upgrades."

worntorn · Oct 23, 2022

Not a whole lot in it. I would say the stock 850 has a bit more pull around 5,000 revs and the modded 750 has a bit more than the 850 above 6000.
I would love to get a proper go against a stock 750 but I don't think that will happen.
Again, the difference would not be huge.

Glen

Fast Eddie · Oct 23, 2022

Glen, much more interesting would be to get a volunteer pilot and show your own 920 and 850 back to back…

Best to mount the camera on the 850, otherwise we‘d likely not see much of the 920 !

worntorn · Oct 23, 2022

It would be fun to see that.
I could get Jim (the modded 750 rider) to ride my 850, he gets the throttle open!

Glen

baz · Oct 23, 2022

worntorn said:
750 Interstate with Alton starter , fuel 4 " down, 1" lower than the earlier bikes weighed.
471 lbs.

My MK3 Interstate with fuel 4" down 477 lbs.

So the lumbering Collussus of all Commandos, the porky 850 MK3, is 6 lbs heavier than a 750 with Alton Estart.

Glen

My 750 roadster weighs 442 lb with a full tank of petrol and Alton starter
I'm guessing a full interstate tank makes the difference

worntorn · Oct 23, 2022

Yes , that and perhaps the differing scales.
These weights are all on the same hanging scale.

concours · Oct 23, 2022

worntorn said:
Here's the roll on
The 750 has a Jim Comstock ported head with D exhaust ports, bumped compression and an aftermarket cam similar to a 4s.

The 850 is a stock MK3 other than the silencers.

Total bike and rider weight is within 12 lbs.
I forgot to ask about cs sprocket, will check that sometime soon.
The bikes stayed in lock step until the top of 3rd where 750 pulls away. My first two shifts were ok but should have shifted from 3rd sooner. I think his would still pull away although once I finally realized we were going to keep going and got it in 4th, the spread stopped growing.
The original plan was to shut off half way thru 3rd, but apparently that plan was changed on the fly!

There!

Now THAT was a horse race!

baz · Oct 23, 2022

worntorn said:
Yes , that and differing scales.
These weights are all on the same hanging scale.

I agree the scales can make a big difference
This was on two race car pads
I'd previously weighed it with bathroom scales
There was around a 15lb difference
Using just one set of scales you should at least get the correct difference

Schwany · Oct 23, 2022

The 850 sounded great. Didn't skip a beat.

If doing the 850 920 test, it might be nice to mount the camera on the back of the 920 and watch the 850 fade away.

worntorn · Oct 23, 2022

I might find someone to ride the BSA against the 850 first just so the 850 doesn't get too far down on itself.

Glen

worntorn · Oct 25, 2022

Another friend with a modified 750 would like to have a go at the MK3. His bike has a lightened crank and other enhancements.
I might do that for fun but it doesn't really answer the question " Is a stock 750 faster than an 850"
That is the claim I've heard over and over for years and I'm now almost certain it is incorrect, just as the weight claims (750 much lighter than 850) were incorrect.

We can see that the hotrodded 750 with raised compression , much modified head and hot cam is dead even with the Pig of all Pigs MK3 850 to about 85 mph, then has a slight advantage for a couple of seconds, then is even again in top gear.
So, assuming the power enhancements work, which the owner tells me they do in spades, it's probably safe to assume that a stock 750 is a bit less powerful than a stock 850. I expect in the real world the difference isn't great, the 850 would only pull away by a small amount.
But it is the reverse to the commonly held belief.
I think that myth came from those horribly restrictive black cap silencers that were on the later 850s.
With those in place, a standard 750 would slaughter the poor wallowing 850.

Glen

750/850 weight comparison

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