Building new short stroke engine

Fast Eddie · Nov 15, 2014

Kvinnhering said:
Fast Eddie said:

Kvinnhering said:

You must have a few more km on it now that summer is
Why are you going for the big valves on a 750? Is it because its a short stroke?

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Click to expand...

Yes. Factory short stroke also used the larger intake valves (some told me). Jim's C solution is to use larger intake valves, but with unchanged diameter of the inlet channel! This leads to higher flow and higher air velocity.

It will be very interesting to see what you think to the head.

What CR did you end up at? I recall you were struggling with this.

Are you going to carefully re-check the cam timing this winter too, I think we discussed comparing notes on this in an email didn't we?

lcrken · Nov 15, 2014

Kvinnhering said:
Yes. Factory short stroke also used the larger intake valves (some told me). Jim's C solution is to use larger intake valves, but with unchanged diameter of the inlet channel! This leads to higher flow and higher air velocity.

Factory short stroke heads (RH7 and RH8) had same 32 mm port size (at the manifold face) as other Commando heads, except for the RH1 and RH10 heads with 30 mm ports. Note that the actual port diameters are smaller than the nominal 32 mm and 30 mm in the specs. Is that what JIm is doing for your head, or are you starting with a 30 mm port head? All the standard heads, both 32 mm and 30 mm, had the same port diameter at the valve pocket. Only the diameter at the inlet is different. The short stroke and AMA heads did have larger intake port dimensions at the throat to accommodate the larger intake valves.

Back in the day, it was common to open up the intake ports to 33 mm or 34 mm at the inlet, with tapered manifolds to accommodate 36 mm carbs. Some of the factory tuners believed in doing this, and some didn't. Now most tuners are more aware of the need to keep the port velocity up, and like Jim, do not hog the ports out like in the bad old days. The big valve head that was on the factory short stroke dirt track bike I bought had even larger intake and exhaust valves, but still kept the original 32 mm port size from manifold to valve guide, and it seems to work very well.

Ken

Hobot · Nov 15, 2014

I'm confused [shushup] on advantage of Norton short stroke if redline was limited to 7500 which many standard stroke cast iron flywheels exceed. About only thing I can think is bigger valves in bigger bores while retaining race rule displacement, but don't know if short strokes turn fast enough to get much if any more head flow. Its said the short short slower piston decent don't draw in as well as faster long stroke decent, but don't know if applies to Norton.
In old Chevy V8's that are similar rpm limited its the long stroker torquer's that improve them most. Set me straighter please.

lcrken · Nov 15, 2014

hobot said:
I'm confused [shushup] on advantage of Norton short stroke if redline was limited to 7500 which many standard stroke cast iron flywheels exceed. About only thing I can think is bigger valves in bigger bores while retaining race rule displacement, but don't know if short strokes turn fast enough to get much if any more head flow. Its said the short short slower piston decent don't draw in as well as faster long stroke decent, but don't know if applies to Norton.
In old Chevy V8's that are similar rpm limited its the long stroker torquer's that improve them most. Set me straighter please.

Steve, I think you'll find most of that question has already been answered on this thread by John, SteveA, etc., but there is an even simpler answer. Serious Norton tuners in the last 40 years have found they can get more horsepower from the short stroke 750 than from the long stroke 750. We could discuss the technical reasons forever, but the bottom line is they make more HP, and don't we all want more HP? For a street bike, it's probably more cost effective to go to more displacement to get more performance, but if you're sticking to 750 cc, either for race rules or personal preference, and want max performance, shorter strokes are the way to go.

Ken

acotrel · Nov 15, 2014

lcrken said:
hobot said:

I'm confused [shushup] on advantage of Norton short stroke if redline was limited to 7500 which many standard stroke cast iron flywheels exceed. About only thing I can think is bigger valves in bigger bores while retaining race rule displacement, but don't know if short strokes turn fast enough to get much if any more head flow. Its said the short short slower piston decent don't draw in as well as faster long stroke decent, but don't know if applies to Norton.
In old Chevy V8's that are similar rpm limited its the long stroker torquer's that improve them most. Set me straighter please.

Click to expand...

Steve, I think you'll find most of that question has already been answered on this thread by John, SteveA, etc., but there is an even simpler answer. Serious Norton tuners in the last 40 years have found they can get more horsepower from the short stroke 750 than from the long stroke 750. We could discuss the technical reasons forever, but the bottom line is they make more HP, and don't we all want more HP? For a street bike, it's probably more cost effective to go to more displacement to get more performance, but if you're sticking to 750 cc, either for race rules or personal preference, and want max performance, shorter strokes are the way to go.

Ken

I don't need more BHP by moving the power band up, with the commando engine I've always concentrated on improving on improving midrange power - TORQUE. And you don't get that by enlarging inlet ports or shortening the stroke. With me it is about improving the cam timing and jetting in conjunction with the exhaust system, then raising the gearing to utilise any increase in torque. What I've found is that the need to increase the overall gearing is not obvious. You might believe the bike is achieving all the acceleration the motor can deliver, however if you raise the overall gearing and use the close ratios to get it to climb, it often performs better.
If you move from racing a bike which has almost zero torque to a commando engined bike, the difference is amazing. What really impresses me is that the motor is fast enough to win races with almost no modification. I've led a couple of races with my 63mm stroke Triumph, the commando engine bike makes it look silly. There is a substantial capacity difference, however the way the power is delivered is much more important.

acotrel · Nov 15, 2014

Our local circuit at Winton is only 3 Km around, and I am very familiar with it. If you stand in the main spectator areas you can see all but about two of the nine corners. If you watch a fast guy when he is practising, in the slow corners he will be using second gear and from there you can count his gear changes, and put your head where his is located. The differences in gearing on many bikes around the same circuit are quite substantial.

Dances with Shrapnel · Nov 15, 2014

acotrel said:
I don't need more BHP by moving the power band up, with the commando engine I've always concentrated on improving on improving midrange power - TORQUE. And you don't get that by enlarging inlet ports or shortening the stroke.

In the context of a 750cc motor, this bit about shortening stroke reduces torque is a fallacy. Let me repeat, this bit about shortening stroke reduces torque is a fallacy.

You can loose torque if you botch the tuning but you can do that with a race tuned 89mm stroke Norton if you muck around. The 750 ultra short stroke Norton (75mm stroke) has as good a mid range torque profile as some of the best tuned 89mm stroke engines.

acotrel · Nov 15, 2014

I suggest that shortening the stroke without maintaining a suitable rod length to stroke ratio reduces torque. The other thing is that if you reduce the stroke, you can get the same or more power output by revving the tits off the motor. The bike just becomes more difficult to ride on short circuits. To my mind a 75mm stroke 750cc commando or nourish engine would be great.
In commando engines, the standard rod length probably suits the (82mm ?) short stroke 750 engine better than the long stroke (89mm ?) engine. In the long stroke engine, if the rods are slightly too short, the leverage is better ?
I suggest that is the reason that the factory 500cc Daytona Triumph engine would have been much better than my 500cc converted 650 Triumph engine which still had the long rods. I gave up on developing it when I couldn't shorten the standard barrels any more, I'd already cut 12mm off the top and couldn't deepen the stud holes more.

acotrel · Nov 15, 2014

What bore size is the max. for standard commando barrels and what stroke would be needed to make a 750 using them ? We could use Jim Schmidt's long rods with the lightest standard length pistons.

Dances with Shrapnel · Nov 15, 2014

acotrel said:
The other thing is that if you reduce the stroke, you can get the same or more power output by revving the tits off the motor. The bike just becomes more difficult to ride on short circuits.

What you loose in mechanical advantage of a longer stroke you gain in greater piston area in which the combustion pressures act upon (ie. more rod force). Furthermore, the larger bore allows for larger intake valves which allows for better breathing which allows for greater combustion pressures.

This "cartoon" of revving the tits off the motor to get the same power is just an old cliché with little basis in this instance. This also tells me that if that is what is needed then the motor is not correctly tuned.

Kvinnhering · Nov 15, 2014

Some feed back.
Fast Eddie.
I opted for 9.33: 1 and it functioned fine, but ideally it should be lifted slightly. I will consider this when I install the Fullauto head.
When it comes to cam timing, I have reduced the advance somewhat. I feel this was better. More power from 5000 to 8000 RPM without losing to much torque.

Dances with Shrapnel
I will support you in that there is plenty of torque and mid range of 750 short stroke too. Maybe not on the level with a potent 850.
In the spring we get the answer when the bike will be tested on the dyno bench.

Ken.
If I understand Jim Comstock right, so is the diameter of the port unchanged with the valve guide and increase to 34 mm on carb flange. The diameter at the valve guide and down toward the valve is increased to reduce the speed and thereby reducing air resistance when the air is going into the cylinder.

Rohan · Nov 15, 2014

Why are doing all these changes to the motor before doing the dyno testing ?
Doing the dyno testing and then seeing how much it improves - or not - would be more instructive ?

lcrken · Nov 15, 2014

acotrel said:
What bore size is the max. for standard commando barrels and what stroke would be needed to make a 750 using them ? We could use Jim Schmidt's long rods with the lightest standard length pistons.

81 mm bore and 75 mm stroke gives you 773 cc, and is a combination that we used back in 2000 for the AHRMA F750 class, which allowed a .060" overbore. 81 mm is pretty much the practical limit for the Norton bore, unless you use the wider bore spacing from the 1007 kit, which will allow 83 mm bore. In any case, the 75 mm stroke seems to be a good choice. As you can see from the dyno chart below, it is lacking in neither torque nor horsepower. That's almost 15 years ago, so I suspect that there are short stroke 750 engines out there now that are even better.

Also, from your comments I suspect you have not noticed that the factory short stroke 750 already has longer rods than the 89 mm stroke engines. The stock Commando alloy/steel rod is 5.875" center to center, and the short stroke steel rod is 6.200". Jim's long rods are 6.400", and would probably work well in a shorter stroke engine.

I wonder if your animosity towards building Commando engines for high horsepower stems from the sort of tracks you raced on. We race on some very fast tracks in the US (Daytona, Road Atlanta, etc.) where there really is a need for top end horsepower. In spite of what you seem to believe, it is quite possible to build a Commando engine for top end horsepower and still have great mid-range power. As John has pointed out repeatedly on the subject, if you are losing mid-range power in the process of building for high horsepower, you are doing something wrong. To be fair, I have to admit that it is possible to build an engine for something like landspeed racing, where max horsespower is paramount, and end up with an engine that would be useless on a road race track. I'm building one of those now. But that's not what we do in building high horsepower road race (and street) engines.

Ken

acotrel · Nov 15, 2014

Sounds to me as though it would be sensible to build an 80 X 75 motor if the class is 750cc max. plus tolerance. I'm not intending to take the 850 to Phillip Island. If I was going to do that my outlook would be considerably different as far as the stroke is concerned. I don't believe in riding at a faster speed than I'm prepared to crash at, considering the circumstances. A get-off at the Island is always too fast. From the conversation on this topic, it looks as though building a 75mm stroke 750 might be worth doing. I like the fact that the stresses on the cases and bearings are lower, and Jim's pistons look very light.
Has anyone fitted the short stroke Triumph Thunderbird crank into a commando engine ? What crank is in the modern Triumph twins ?

Fast Eddie · Nov 16, 2014

Rohan said:
Why are doing all these changes to the motor before doing the dyno testing ?
Doing the dyno testing and then seeing how much it improves - or not - would be more instructive ?

Rohan raises a very good point Kvinnhering.

I wanted to 'baseline' mine before I started on it and didn't, now kick myself for that as I lost so much information and it would have taken so little effort!

If your bike is still running, please take the time to Dyno it before hand... I guarantee you'll regret it if you don't!

Jagbruno · Nov 16, 2014

I just had a strange epiphany...my Laverda 750SF would qualify as a 'short stroke' compared to a regular Norton...

Obviously a very different animal, with a VERY heavy crankshaft (15kg/35lbs +). Lot's of torque and screaming hi-end power (with my 10.5/1 Asso forged pistons). My Norton (850 PW3 in Slimline with Dunstall exhausts and lightened crank and rockers) will easily out-accelarate the Laverda...but of course the Norton is a good 70kg (150lbs) lighter...

8)

SteveA · Nov 16, 2014

Kvinnhering said:
You must have a few more km on it now that summer is over......how is it going?

And what is the long winter plan?

SteveA

Hi Steve.
The bike is going very well. Coming on cam from 4500 up to 7000 rpm. It's also have good torque. I'll take it on the dyno bench to the spring and get it fine tuned and verify power and torque.
In winter I will mount FA head with larger inlet valves and Jim will also clean up the ports. I will also mount MSD ignition system and head steady from him. Carburetors will be replaced of PWK 34mm. I hope the gearbox will take the torque :?

I have fuel/air ratio sensor on each cylinder connected to two instrument. These will be replaced with wideband type for more accurate measurements. I use these is to adjust the carbs one the road.
I hope to change the first gear to the standard ratio. The first gear is to high ratio on close ratio cluster from RGM for my taste.

A case in the end, the engine will not idle below 1000-1100 RPM !! Is this normal for short stroke ??

From what I read here concerning cam wear, your high idle will help protect the cam a little, and I suspect it is normal to need to set a higher idle due to the lighter flywheel.

I understand the comments guys are making regarding before and after dyno testing, but I also recall it always was your intent to fit the big valve FullAuto, so I suspect you are not that interested in the current performance figure, it is just a number, and a step on the way, but I imagine you will want to be optimising the final set up with the dyno runs, with new carbs and ignition you will have a lot to dial in.

As for the gearbox, your way forward is the expensive one, you can fix your first gear ratio and go bulletproof with a TTi 5 Speed road box, the following winter maybe?

Rohan · Nov 16, 2014

Jagbruno said:
I just had a strange epiphany...my Laverda 750SF would qualify as a 'short stroke' compared to a regular Norton...

Yes, I paused and reflected for a moment when a roadtest of a new Duc said the motor "has fairly long stroke dimensions - at 88 x 67.5

Ee by gum...

acotrel · Nov 16, 2014

SteveA said:
Kvinnhering said:

You must have a few more km on it now that summer is over......how is it going?

And what is the long winter plan?

SteveA

Hi Steve.
The bike is going very well. Coming on cam from 4500 up to 7000 rpm. It's also have good torque. I'll take it on the dyno bench to the spring and get it fine tuned and verify power and torque.
In winter I will mount FA head with larger inlet valves and Jim will also clean up the ports. I will also mount MSD ignition system and head steady from him. Carburetors will be replaced of PWK 34mm. I hope the gearbox will take the torque :?

I have fuel/air ratio sensor on each cylinder connected to two instrument. These will be replaced with wideband type for more accurate measurements. I use these is to adjust the carbs one the road.
I hope to change the first gear to the standard ratio. The first gear is to high ratio on close ratio cluster from RGM for my taste.

A case in the end, the engine will not idle below 1000-1100 RPM !! Is this normal for short stroke ??

Click to expand...

From what I read here concerning cam wear, your high idle will help protect the cam a little, and I suspect it is normal to need to set a higher idle due to the lighter flywheel.

I understand the comments guys are making regarding before and after dyno testing, but I also recall it always was your intent to fit the big valve FullAuto, so I suspect you are not that interested in the current performance figure, it is just a number, and a step on the way, but I imagine you will want to be optimising the final set up with the dyno runs, with new carbs and ignition you will have a lot to dial in.

As for the gearbox, your way forward is the expensive one, you can fix your first gear ratio and go bulletproof with a TTi 5 Speed road box, the following winter maybe?

To my mind a number off a dyno doesn't mean much until it is put in context with handling and gearing , suspension and tyres. It's easy to build a fast bike which is horrible to ride. What might theoretically be an improvement can sometimes be a step backwards. These was a comment about my short stroke 500cc Triumph being poorly tuned and that might have caused it to be difficult to ride. The motor was on methanol - how can you get that wrong ? Over a 12 year period, I raced it about 4 times a year and I believe I tried every possible combination. I finally got sense out of it, however it was never nice - just vicious. However I learned a lot. I enjoyed racing it even though it was much smaller capacity than all of the other four strokes - it was still competitive to a point. The T250 Suzuki I built made it look stupid, however as soon as I started racing it I lost interest.

Kvinnhering · Nov 16, 2014

Hey guys.
Thanks for the many interesting feedback. I am sorry that I answer so slow, but I've been busy this year and is going to be busy at work and at home in the winter too.

Roban
I agree that it would be interesting to test before and after the conversion and preferably testing with same dyno bench. It 's one guy who lives 5 minutes away so have this as a hobby, But he's busier than me!! :roll: Tried to test my bike this fall, but we did not have time.
The engine was already dismantled when I decided to build bulletproof short stroke.

Fast Eddie.
Maybe I should test it before I put on Fullauto head. Will see how it goes throughout the winter.

Building new short stroke engine

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