920 lightweight

Status
Not open for further replies.
W

worntorn

VIP MEMBER
Joined
Dec 22, 2006
Messages
8,149
We have sunshine and temps in the mid 50s f here today and for the next week or so.
I put insurance on the 920 and went on a shakedown ride today.
There are a couple of items that will require a change, but most things seem OK.
Could not resist going for a run up dyno hill.
That will be the one and only run up there with this bike.
100 at bottom gets 127 at top, too fast!
The 850 manages 113 at top when everything is clicking.
The 650ss just holds 100.
The BSA drops a fair bit, so it needs third to hold 100.
I can't recall what the Thruxton R hit, will look it up. It was also too fast for the hill test.

 
worntorn said:
We have sunshine and temps in the mid 50s f here today and for the next week or so.
I put insurance on the 920 and went on a shakedown ride today.
There are a couple of items that will require a change, but most things seem OK.
Could not resist going for a run up dyno hill.
That will be the one and only run up there with this bike.
100 at bottom gets 127 at top, too fast!
The 850 manages 113 at top when everything is clicking.
The 650ss just holds 100.
The BSA drops a fair bit, so it needs third to hold 100.
I can't recall what the Thruxton R hit, will look it up. It was also too fast for the hill test.

Click to expand...

Superb!
 
It would be better if I hadn't looked up the Thruxton R speeds. (85 kmh at bottom, 140 kmh top using top Gear) I used the lower bottomspeed after trying the usual 100 kmh bottom speed then hitting 160 kmh part way up the hill and having to shut down.
Those speeds are all too high really.
It's only a 50hp hill, good for a standard Norton , Vincent, BSA or old time Triumph.



The 920 won't be going to those Thruxton speeds without nitromethane or a supercharger, maybe both. The Thruxton 1200 is probably one of the worst bikes on the planet to go against on this type of test. I know it blows the 161 bhp Daytona away on-top Gear roll on. So I'll forget that bit of wishful thinking for the 920!

Still, 127 vs 113 is a good result and the handling lightness of the bike is really something special.

Glen
 
Last edited:
What percentage are those grades? I need a hill with a bit steeper incline, although this one is already twice that of the big Mountain passes in US and Canada.
I'm pretty happy with the 920 result. It will have really good passing power on those long 8% mountain grades. I might squeeze a bit more out of it with tuning.
Correction, Dyno hill is 12.5%, so about 50-60% steeper than those 7-8% mountain pass climbs.

Glen
 
Last edited:
If you wheelie all the way up the hill it will slow it down some. ;)

Very nice. Sounds great, and based on the description of the hill run, it runs great too.
 
Real nice job (as usual) Glen. Great vision and excellent mechanical skills.
If you ever get to Watkins Glen NY, 2nd Street has a 18 degree hill leading out of town. might be a good test.
 
Deets55 said:
Real nice job (as usual) Glen. Great vision and excellent mechanical skills.
If you ever get to Watkins Glen NY, 2nd Street has a 18 degree hill leading out of town. might be a good test.
Click to expand...
18 degrees or 18%?

18% would be just over 10 degrees. 18 degrees would be around 32%

Can you be specific on the location? I might be able to give it a try sometime.
 
Out riding yesterday I realized its smoother than the 850 below 3000 rpm. Both are good above that point but the 850 has annoying vibration around 2500 rpms. The 850 also shakes it's front wheel at idle, made worse if the idle is set high. The 920 doesn't seem to shake at idle even with the current 1200 rpm setting.
Both bikes have the same standard weight pistons and rods but the 920 has much higher compression, which should add to vibration, if anything.
The 920 crank was dynamically balanced, that might account for the difference in vibration levels.

Glen
 
2nd Street right in the middle of the town of Watkins Glen. It’s residential and has a flat at every intersection. Not really a good place to do what Glen does.
My Jeep has an off road display in the radio/Nav screen. It showed 18 degrees on the angle-meter (not a real word) . It’s probably the steepest paved road I have ever been on.
 
I found a cross section of it on Google Earth.
Yikes, that is steep!
As you say, too built up for a speed run.
We encountered a B road in the UK with a sign at top that claimed a 25% grade. We stopped at the top and read the sign. I recall thinking that there had to be a math mistake there.
Once on it I realized that 25% was probably correct. To make matters worse it terminated in a T intersection with a busy road. On the other side of the T there was a stone building to run into if the brakes failed!

Glen
 
The plugs looked a bit lean with the 260s so I tried 270 jets. This is not what was expected as the common wisdom is that, all else being equal, the bored out engine should like slightly smaller jets.
The CR has been raised quite a bit, so that may be part of it.
After waiting until the road was empty, I gave it a run up the hill.
This time it clocked 131 kmh at top, so will have to try 280s!
Its a bit tricky at the top as the road flattens off and the acceleration increases quite a lot with this bike.
I realized that the acceleration on the hill with this 920 is more than double the acceleration of the 850, which isn't a slouch.
One day it would be interesting to add ballast to the 920 to give it the same load as the 850 has.
That would show how much boost is coming from the 920 and how much is from the weight reduction.

Glen
 
Last edited:
worntorn said:
The plugs looked a bit lean with the 260s so I tried 270 jets. This is not what was expected as the common wisdom is that, all else being equal, the bored out engine should like slightly smaller jets.
Click to expand...
You sure about that?

I thought the opposite.
 
Got to ask... are those barrels bored out standard/sleeved/A.N. Other?

Lots of 25% gradients over here... including my driveway :rolleyes:
Hardknott is more challenging due to the hairpin bends!
 
  • Like
Reactions: MIK
Fast Eddie said:
You sure about that?

I thought the opposite.
Click to expand...
So did I until encountering this when tuning the 1360. An old time engine builder and racer told me that the overbored Triumphs and Nortons generally used smaller mainjets than the standard bore bikes, all other things being equal. I found his info to be correct for the 1360.
Later on I realized this is part of the Bernoulli Principle on Venturi effect.
The flow through a Venturi ( mainjet) increases at the square of velocity.
The larger engine with the same inlet tract and carbs has to have higher flow and velocity at a given rpm as it has a larger cylinder to fill in the same amount of time.
The larger engine needs a proportionately larger amount of fuel and air. It gets that with the air but gets more than that with the fuel because of the exponential relationship between velocity and Venturi flow.
That was how it went with the 1360. It required mainjets that were smaller than used on the 1200 cc version of that motor.

For some reason this motor liked the 270s. I will try 280 s today.
 
Last edited:
B+Bogus said:
Got to ask... are those barrels bored out standard/sleeved/A.N. Other?

Lots of 25% gradients over here... including my driveway :rolleyes:
Hardknott is more challenging due to the hairpin bends!
Click to expand...
The barrel was supplied by Ken Canaga, Left Coast Racing. It's an alloy barrel, one of a batch he had made up in the UK, if memory serves. It was a nice looking barrel until I started in on it!
I bored it out on the mill to fit the RGM 920 sleeves.
The boring process was going to create some daylight thru places so I added Tig weld in those areas before boring.
That caused some warpage so the head and base surfaces required re-machining.
It was a lot of work and learning. So far it seems to have worked out ok , that is, it is producing very good power in that 3-5 k range that gets used so much.
It will be interesting to see how it holds up.

Glen
 
worntorn said:
It's part of the Bernoulli Principle on Venturi effect.
The flow through a Venturi ( mainjet) increases at the square of velocity.
The larger engine with the same inlet tract and carbs has to have higher flow and velocity at a given rpm as it has a larger cylinder to fill in the same amount of time.
The larger engine needs a proportionately larger amount of fuel and air. It gets that with the air but gets more than that with the fuel because of the exponential relationship between velocity and Venturi flow.
I ran into this with the 1360. It required mainjets that were smaller than used on the 1200 cc version of that motor.

For some reason this motor liked the 270s. I will try 280 s today.
Click to expand...
I can remember a mate of mine having to go down a jet size on an xs1100 when fitted with drag pipes and K&Ns
And that's with CV carbs
I think he shimmed the needles as well, can't remember
 
Status
Not open for further replies.

Similar threads

Fast Eddie
JS lightweight pistons and rods
Replies
13
Views
926
Hortons Norton
Hortons Norton
Fast Eddie
Norvil lightweight twin disc kit feedback.
2 3
Replies
41
Views
2,953
Fast Eddie
Fast Eddie
961al
2019 LIGHTWEIGHT TT
Replies
9
Views
2,099
TonyA
TonyA
jseng1
  • Locked
Lightweight pistons, Ignition timing and skirt length
Replies
19
Views
3,082
Nater_Potater
Nater_Potater
jseng1
  • Locked
Leak down test for lightweight pistons in Cmdo motor
Replies
10
Views
967
Hobot
Hobot
Back
Top