Compression and Ignition

I'm hoping to pick the brains of the vast engine knowledge on this forum

I race a 1973 BMW R90.
The engine was rebuilt with new rings, bearings, timing chain.
Heads have had 1mm taken off them and the piston pockets enlarged slightly.
Carbs are 36mm Dells with 137 mains ( a stock R90s runs 155)
Ignition is Ignitech with full advance at 30 Degrees.
Cam is a BMW sports cam.
Pistons are the stock 1973 9.0:1 originals.
Exhaust is a 2 into 2 with Commando mufflers ( I like the sound)
I have done over 30 races on it and it runs well enough for what it is, recently the ring gear came off the flywheel and it looked pretty flogged out. My mates rollers would start it when its warm but not when cold.

My question is ( finally):
Having raised the compression could/should I be able to drop the advance back from 30 degree?
Also I can alter the advance curve to suit so how do you determine a suitable curve?
Thanks John

72Combat said:

Also I can alter the advance curve to suit so how do you determine a suitable curve?

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This is usually done by putting it on a dyno, and watching the power output while monitoring the exhaust temps and cylinder head temps and O2 and CO levels with an exhaust gas analyser.

What is a "BMW Sports Cam" ?

If its getting hard to start, sounds like something is getting a bit worn.
Done a compression test lately.
Or checked the condition of the valves and valve seats ?

Running it with mufflers would do nothing for the power output, what do other bikes run ?

It's easier to tune for decent horsepower with a straight-through (absorption type) muffler than with no muffler.

grandpaul said:

It's easier to tune for decent horsepower with a straight-through (absorption type) muffler than with no muffler.

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Do peashooters meet that standard in any respect though. ?
Unless they are those big bore 'race' ones ?

If mainjets are smaller than stock, then its not even making stock power ?
Fast R90S's can use 40mm Dellorto pumpers ?

Hello , I quote this from a BMW forum

The 336 cam increases valve lift (IIRC close to 1.5mm greater than stock) and duration/overlap

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I find it has more top end than the standard cam which is all in by 6k, the 336 runs to rev limiter of 7500.
It starts tine on the electric start but not on rollers or pushing.
I could try the R90s main jets of 155.
Probably needs to go on a dyno, finding one with an operator who likes old bikes is the challenge.
thanks for the replys

Rohan said:

If mainjets are smaller than stock, then its not even making stock power ?

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I don't agree.There could very well be a higher gas speed in the intake port,and more suction on the main jet.Total fuel flow is a combination of jet size and the amount of suction on the jet.
It's often a good sign,if the main jet can be reduced.It can happen with a well ported head that makes more power.

Less advance is usually good if the cylinder pressure is made higher.Run it at full throttle between maximum torque and maximum power,and see what gives the best results.A little less advance is always safer,like 1% less than best power.If the engine runs hotter at some time,you'll still be safe.The advance curve probably won't need changing.You're likely to be running at full advance most of the time.If the timing is right when it's fully advanced but you're getting detonation at full throttle at lower rpm,then you cold have a problem with the advance curve.

If you're good at reading spark plugs,you can check the plugs after about a minute of full throttle to see if the advance is right.http://www.strappe.com/plugs.htmlLook at the ground strap on the plug.

Many bikes run faster when the carburation is leaned off from the stock settings. Timing, jetting and compression ratio work together and what you set them to depends on the torque you require in various situations. A high compression motor is likely to require slightly retarded ignition to stop detonation, and if the mixture is lean and the ignition is at stock timing , combustion temperatures can be high enough to start burning the combustion chamber internals. Changing any one of those three variables can present similar symptoms.
The worst case situation exists with two strokes on methanol fuel. We usually use the stock petrol ignition timing with methanol fuel although it is more knock tolerant. to get the most out of methanol or petrol for that matter , it must be run as lean as possible right through the carburation range, without doing motor damage or causing missing or detonation. If the ignition timing drifts, you are in business - it becomes expensive.
I am sceptical about dyno tuning, unless you are able to get reliable torque readings across the usable rev range. With race bikes it is easy to get and increase in top end power, and destroy the midrange necessary for riding the twisty bits of the circuit. Torque wins races, and a lot depends on the gearbox. My friend races three BMWs of various historic eras. Changing gearing between venues is more difficult than with most other bikes, so you are pretty much behind the eight ball. Of course you could always use a big dose of nitro and blast off in a straight line forgetting the corners ?

My approach would be to take either of two paths. Use standard fixed petrol ignition timing and compression ratio, and adjust the carburation to it. Then adjust the gearing to suit the torque. Alternatively buy a good quality programmable ignition system, do research about the most suitable advance curves and gearing for the types of circuits you race on, and jet to them as constants. (Politely ask your local racing superbike riders how their bikes are set up for your playground. Speed is only relative.)

acotrel said:

Many bikes run faster when the carburation is leaned off from the stock settings. ?

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Not for long though !!! :mrgreen: :mrgreen:

Might be OK for a few short laps, but engines ultimately have to obey something called stoichiometric ratios - air:fuel ratios.
On the dyno, on petrol, it can be demonstrated that around 12:1 gives sustained maximum power, and 14:1 gives general good running,
although the acceleration will be a bit weaker.

Above 14:1, and the cylinder head temps and internal engine temps begin to climb.
Setting these on a dyno can get things quite well sorted.
Far better to do this by dyno - than by the seat of the pants, and melt a few pistons along the way ?.

Fuel injection can have mixtures as low as 24:1 still firing, but this has to be on a very light throttle setting -
and the computer monitors the temps, and the knock sensor, to keep things from going belly-up.
This is fuel-saving mode. Race engines prefer to waste some fuel keeping the engine internals cool.

I think I'll find a dyno so as to benchmark what I have currently.

Rohan, If a motorcycle manufacturer is building a bike for road use in various conditions, do you believe the jetting will be set for maximum power when the bike leaves the factory? If the bike is to be used in extremely cold conditions and it is not jetted rich, there is a danger to the piston. The factory cannot know where their bike will end up, so I suggest you get a 'one size fits all' answer to jetting. Because you lean your jetting off it doesn't mean instant destruction, especially if you take care when doing it. On racing two strokes such as TZ Yamahas the best power is achieved well beyond what you would consider to be a 'safe' plug reading . In the 70s riders had to go there to be competitive, that is why those bikes were so expensive to race. One thing that my brother and I always do when jetting our methanol fuelled Kawasaki two stroke triples, is to sneak up on the jetting. Even then we usually chicken out on going too far in the lean direction. The final jetting is arrived at over several meetings which are usually held at night - so the air temperature doesn't change much. For daytime meetings, the higher temperatures means the jetting richens up - the 'safe' direction.
I can tell from a lot of your comments that you have never raced, and that's OK. However when you race you become accustomed to sensing when a change has given an improvement. Most of us have experienced the big bang, so we work around that - it is always there waiting. We got caught out recently when one of my brother's bikes popped an ignition lead, and he plugged it back in and kept racing. It obviously affected one of the boxes in the ignition system. The result was a cracked head and a piston had to be replaced.

'Race engines prefer to waste some fuel keeping the engine internals cool.'

I would never race a four stroke using petrol - always methanol, unless it was a very modern bike.
The combustion stoichiometry is irrelevant to this discussion. As an industrial chemist, I can tell you that you cannot predict an outcome from that. It may be possible to correlate maximum power from very responsive exhaust gas analysis, that still would not tell you what gives maximum torque across the whole rev range, or the shape of the torque curve. And even then the optimum is dictated by your gearbox. Every racing motorcyclist should enjoy the experience of riding a bike with an extreme top end motor, and not enough gears . I always get a bit of a laugh when guys exercise their bragging rights based on a dyno number, it demonstrates an ignorance which means they can be defeated.

So tell us about some races you've won ??

Dyno shows what you have. And any subsequent improvements - or backward steps.
You seem to be scattering disconnected comments all over the place.

Although methanol fuel certainly does keep things cool.
Saw a 650 triumph iron engine somewhere, where the finning was shielded somewhat, to keep the heat in for shortish races...
BM R90S does have an all alloy engine though, and as well air-cooled as you can get....

Rohan said:

So tell us about some races you've won ??

Dyno shows what you have. And any subsequent improvements - or backward steps.
You seem to be scattering disconnected comments all over the place.

Although methanol fuel certainly does keep things cool.
Saw a 650 triumph iron engine somewhere, where the finning was shielded somewhat, to keep the heat in for shortish races...
BM R90S does have an all alloy engine though, and as well air-cooled as you can get....

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My mate took a clip of my first meeting in Feb 2012 :mrgreen:


I only won two races out of about 20 with 1 DNF in last years series. However due to the lack of reliability of some of the others the points accumulated.

Here is a pic of the bike.

The original question was about retarding the timing when compression is raised.
The bike performs well enough, just need a few more ponies to keep ahead of my mate on his R90s. the R90 and the R90/w are nearly identical other than the R90's have 0.5 more comp as std and 38mm Dells.
Found a guy here who does Dyno tuning so he can have a look at the jetting and the programmable ignition.

My comments about race wins were actually directed at Aco.
But thanks for the updates John - looks like you've been having fun.
Look forward to later updates about where this goes.

P.S. How do you find those front brakes ?
I had a BM with them (just one), it didn't excite as the most powerful brake around.

Later they went to brembos, which had considerably more bite.
Are you allowed to use brembos ?

It looks like you've got enough space to take advantage of a longer intake,something like 13" or 14" from valve to bell-mouth entry.Rubber mount the carbs on rubber hoses.You'd need a short spigot in the carb,2" or 3" long,with rubber hose connecting that to the rest of the extended intake.The bell-mouths look a bit large in diameter to match up with 34mm carbs.Ram intake works best with a small radius on the bell-mouth entry,not bigger than 1/4" radius.

Hi X-File- The Carbs are 36mm and solid mounted as per the Ducati they came off...yes , longer intake with rubber mounts sounds like a plan.

Rohan- The ATE brakes can be made to work very well, not Brembo well but good enough.

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.

72Combat said:

My question is ( finally):
Having raised the compression could/should I be able to drop the advance back from 30 degree?

Click to expand...

The simple answer is - it depends. Yes changing the compression will likely change the "happy spots" for the ignition.

72Combat said:

Also I can alter the advance curve to suit so how do you determine a suitable curve?
Thanks John

Click to expand...

As mentioned earlier on this thread, it's best to use a dyno (with eddy brake) to see where ignition needs to be set at for the various rpm.

I think the boxer is an easy candidate for dual plug heads. Dual plugs will help your mid range (and maybe top end) and will require less ignition advance - always a good thing.