Compression and Ignition

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Rohan, tell us about the races YOU have won. Better still tell us how many race rides you've had and how many fast bikes you've built.
 
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Apart from a bit of street racing, I haven't officially raced or won anything.
But then, I'm not telling silly stuff about how you can guess the right settings either.

I have been involved with engines on the dyno, and what it can show up and set up.
Only small engines, but the procedures and stuff is the same for bigger engines.
Have fitted some big bore kits into a few engines too, only for road use though.
Not british but...

I do like my road bikes to get there AND back though, and the practice is none too different ?
Going with what works for other folks doesn't seem like such a bad idea here ?

We haven't actually heard details of any of yours yet ??
 
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Rohan, I raced my triumph for 12 years on methanol and when I ran my commando engine, it took me almost no time to get to a really good power setting right across the throttle opening and rev range. I did not use the amal needles and needle jets, they were simply absurd even though the normal methanol kit for commandos. I use mikuni petrol needles, and there is probably opportunity still for some gain. You've been rabbitting on about stochiomethric mixture - that is the explosive mixture (detonation), race engines run richer than that. How much richer do you recommend ? Tell me this : Mikuni needles come in three taper stages and a large variety of adjustment. How do you choose the correct one using a dyno when the needs change according to the circuit and gearing ? Getting the main jets correct is the easy part, good midrange is difficult to get right . Sooner or later you have to include the rider in the equation.
 
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In fairness, I should point out that a dimensional change in a methanol jet has half the effect it does in a petrol jet. In the 50s our guys went to the UK and found that the Pom's bikes were as fast on petrol as ours were on methanol. However because of our use of methanol, our guys were used to the speeds. Getting jetting right for methanol is as difficult as it is for petrol, but most of our old racing bikes aren't going as fast as they really can - running rich.
 
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Rohan, If you are getting involved in street racing , you might be better to try some form of classic racing ? You've obviously got the necessary thought processes, and it would be a matter of you finding an appropriate class in which your efforts would be cost effective. It looks different from inside the fence.
 
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My first time on a track was 2 years ago, joined the local motorcycle club, did some rider training days at the track, l1 and 2 Superbike School on a Triumph Thruxton, built up the BMW and went classic racing. Awesome fun, great people, I miss wandering around the car park looking at bikes though.....
Its sort of become a bit of an obsession.... mind you last time I followed motorcycle racing was when Wayne Gardner was on a 500 and last one I went to there were Norton Rotaries winning! :mrgreen:

I looked at all the classic options:
Ducati ( bevel)- expensive, parts are expensive,
Triumph- OIF in Bonnie.....getting collectable, mostly worn out.
Norton- same as Triumph
Guzzi- expensive here.
BMW- underdog- they built lots so parts are easy and cheap to get,easy to work on, built to last..... slow.....maybe...people laugh at you....not so much any more...
I watch my mate buy alloy cylinders,fancy pistons, head work, dual brake set ups for his T140 ....not having it running for the last 3 meetings.......
If they did racing using the new Bonnies that would be fun.
 
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I've only won about four races in my whole life, the first two were on two strokes, the last two were with the Seeley 850 in historic races and there was nobody there. When I raced the Triumph it was 15 years older than most of the other bikes in Allpowers C Grade, and I never got to race it against other 500cc fourstroke twins or singles in a class defined for them. In the end, the competition were riding H1 and H2 Kawasakis, Z900s, and RD350 Yamahas, and I was still competitive i.e. in the lead group of most races . I was never fussed about winning, if you race and don't control your ego, you get hurt. My whole life both professionally and in motorcycle sport has been about development. The most fun you will ever have is when you make a change and it enables you to improve your performance against other riders on the same type of bike. Your BMW would be a real buzz, however the gearbox and diff, would be a turn-off for me. If I owned it I would make a replica Zoller blower for it and turn it into a retro 'Rennsport compressor'. If race meeting officials complained, I would paint the supercharger black, - that usually fixes all eligibility problems.
 
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Yes a compressor BMW would be a hoot....they are banned here, shame as it could make some interesting machines.
I have a road R90/6 as well and its like chalk and cheese, with the late model flywheel and clutch the gearbox is very good. I used to notice the 'jacking ' effect on my R65 as they have shorter swing arms but on the race bike its not there.
Currently in the Classic Racing Register they have a no Japs, pre 76 rule....also a very declining membership.
Your right about making changes you can notice, I'm busy working out how to stay ahead of my mate on his R90s..... all I have at the moment is the ability to brake later, go around the outside and cut him off on the inside....also bearing in mind we both have to go to work on Monday....The guy on the meth Triton rides really hard...but I have seen him crash a few times.....
I can make a bigger motor but its fun eeking power out of what I have for the moment.
apparently quite a few Aussies coming over in October for the Barry Sheene Oceania Challenge.
http://www.barrysheenetranstasman.co.nz/
Cheers
 
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I would never go for making my motor oversize, or dramatically increasing compression ratio unless I was using methanol. And bigger ports aren't so smart either. The three angle valve facing is important, and the camshaft is the most important, but it can all be stymied by the gearbox if you destroy the midrange. If you are permitted to use methanol, I wouldn't hesitate to go that way. About the blower, I was being sarcastic about painting it black - would they really stop you from racing if you arrived with it, and both it and the carbs were neatly tucked away behind a fairing ? How perceptive are their scrutineers ? anyway I'd be working to get permission to race it, if I wanted to go down that path - perhaps they might allow you to run it with the superbikes?
 
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I was just looking at the photo of your bike - I would be proud to own it especially if you are getting good rides on it. I believe that boxer BMWs are very under rated. The Rennsports were not too bad back in the early sixties, when they were unblown and Duke and Dale rode them. There is one genuine solo here in Australia owned by Jack Forest who rode for the works back then, and one sidecar which is being converted into a solo (sacrilege) .
 
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About increasing compression ratio. You need to consider what fuel you would use. Methanol has unlimited antiknock, however normal hydrocarbon based fuels dictate limits to comp. ratio, and the amount of advance which can be used without causing detonation.
 
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Thanks, I'm not increasing compression but was wondering if using the std advance of 30 degress it was now firing too early. I look at the plugs and they are nice with a hint of tan.
I spoke to dyno man and have booked it in, there is a 4 week lead time..so after the next race, I should probably leave it alone till then and get a before and after print out.
 
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It is not the plug colour you should be looking at, but whether the 2mm wide black ring is present on the porcelain right down inside where it meets the metal. If that disappears you are either too lean on the main jet or your timing is wrong (both can have the same effect). If you are too lean in the midrange, the motor will cough as you ride it around the twisty stuff - lift the needles one notch. You can't tell mid-range leaness by plug colour. Good luck with your racing, I love to see guys so enthusiastic.
 
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72Combat said:
Thanks, I'm not increasing compression .
You could - 1000cc top ends are supposed to just be a bolt on fit....
11% more go, just bolt up...
 
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X-file said:
You'll know when it's going well,because the clutch will begin to slip when you reach about 125 mph.You might reach 130 mph,with the clutch intermittently slipping (until it gets hot).
The clutch arrangement got better after about '79.I think you need to change the flywheel to fit the later stuff.
Where did this info come from ?
A quick search on airheads forum didn't find much on this...
BM clutches back then may be heavy, but unless they get oil on them , they could haul a truck.
They got lighter after 79, but stronger ??
The Butler and Smith racebikes didn't use anything so special in the clutch dept ?
 
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acotrel said:
I was just looking at the photo of your bike - I would be proud to own it especially if you are getting good rides on it. I believe that boxer BMWs are very under rated. The Rennsports were not too bad back in the early sixties, when they were unblown and Duke and Dale rode them. There is one genuine solo here in Australia owned by Jack Forest who rode for the works back then, and one sidecar which is being converted into a solo (sacrilege) .
That sidecar "Rennsport' isn't the fake one, with the bolt on bevel drives that go no-where, is it ?
Even looking at the pics, you can see its just for show....
 
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Rohan said:
Where did this info come from ?
A quick search on airheads forum didn't find much on this...
BM clutches back then may be heavy, but unless they get oil on them , they could haul a truck.
They got lighter after 79, but stronger ??
The Butler and Smith racebikes didn't use anything so special in the clutch dept ?
It comes from experience with one fast,but almost standard (we used standard parts,slightly massaged),R100 S .You can get 10.5:1 just by leaving out the head gasket,but even with the gasket it went well.

Have a look in the manual or parts book at the early clutch.The wrong part of the diaphram spring is pushing on the clutch plate,and it doesn't exert much force.
It got corrected on the later clutch.

If it goes well,you need the later clutch.
 

johnm

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Hi John,

Congratulations on your results. Your doing very well to start racing and have consistent finishing. I raced for about 10 years in the NZCMRR 500 clubmans class first riding myself and then after a painful crash having someone else ride my bike. My first few years were terriable for non finishes but we sorted it in the end.

Going back to your original post.

I have no experience with BMW or your carbs and ignition type but for what its work here are my ideas.

Firstly you are in general correct - if you increase compression you should retard ignition. Paul Dunstal for example recomends coming back from about 30 deg to 28 degree advance when increasing CR to 10.5 plus in the 750 Norton. The best theoretical discussion together with lab results I have seen on the subject is in Gordon Blairs book Design and Simulation of Four Stroke Engines.

http://www.goodreads.com/book/show/4537 ... ke-engines

However this is a book for engineers and most definitely does NOT give a cook book answer to your question. He uses an engine test bed to investigate the burn time inside engines for various fuels and compressions.

In practice CR, head design, inlet tract efficency, mixture, plug location, fuel atomisation etc etc will all impact to determine the ultimate curve so theory is only going to help you to understand and direct your experiments ( unless you have accccess to some very serious computing and sofware programmes !!!)


A practical if expensive approach is to follow the methods described in G G Bells books. Run your engine on a brake dyno where you can control the load on the engine for incremental steps of rpm. At each point determine your best carb and ignition settings and then programme your ignition curve.

This is expensive but if you can get a brake dyno this is the way to do it. Unfortunatly getting a brake dyno at a viable price WITH AN OPERATOR WHO KNOWS WHAT THEY ARE DOING!! is almost impossible.

Viable alternatives.

-I have read about onboard data loggers. This might be good but I have no experience.

-Getting ignition right by reading spark plugs. Gordon Jennings wrote some excellent articles about reading spark plugs. For carburation AND timing.

Read this. This is very very very good!

http://www.strappe.com/plugs.html

Jennings says most riders have too much ignition advance , jet to rich on mains and have too cold a plug.

I do not feel qualified to comment on the heat range statement but based on what I have seem and heard in the pits at meetings 80 % of NZCMRR bikes are too rich and too advanced. The size of mains some people say they use are incredible.

Based on this Jennings paper in about an hour I changed my clubmans bike from petrol to methanol . Adjusted the ignition by looking at the plug from two test runs and ran it in the 2009 or 10 ?? 500 Senior TT. It came second with a one minute 11.2 sec lap of Puke.

So if you dont have a dyno then read this paper several times , buy a magnifying glass and a box of new plugs and try testing. Jennings really knows what he is talking about.

Last thing. I like your use of Commando mufflers. I tested hollow ones on a dyno with my bike and they gave nearly the same performance as a full race megaphone system - just a bit heavy. Plus you might want to try cutting off a bit of the reverse cone.

good luck

John
 

johnm

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Hi again

Just re read some of your posts above and see you are booked at a dyno.

I advise you to think carefully through what you want before you go.

Record everthing you do making sure your run number and the dyno operators run noes match. It is really bad to get home and not be able to match the changes to the run results. Get the output in some sort of file you can put into a speadsheet so you can overlay runs. Im a big dyno fan but it is very easy to get home with a pile of paper, a box full of jets and no clear picture of what change produced what result.


I think you can use methanol in this class corrrect?. If you can then convert to methanol and away you go. "Pour in hp" -increase of about 5 % hp without change of CR. You need to change carb settings, needles, jets etc, put about 3 to 6 degree more advance, steel tank, open up all fuel lines, taps etc for fuel flow because you need about 2.3 times as much methanol as petrol, get rid of plastics and dont drink it because it is very poisonous.

John
 
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Thanks Johnm, lots to think about there.

Re the Dyno I am going to leave it as is and as you say record all the settings, then go from there. I watch Team Trident when they race, drain the tank, measure what was used, then refill. The way I tell the serious ones is the tyres....if they have treaded Avon race tyres they are serious.
The heads are where BMW seem to limit power, the way the inlet tracts turn in for the riders legs, forcing the mixture to go around a bend.
BMW went down the Combat route with hogged out inlets and 40mm carbs. The later heads have a D inlet port that works better. I have a set of these heads so may look at that in the future.
At the end of the day its more about just being out there on the track and the buzz. Many of my friends have talked about it for years....but as time goes on the years get less... :D


Note to the other chaps comments:

Running later post 1981 flywheel/clutch with HD diaphragm.
 
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