Bike engine devellopement
Turbo Mini Forums

Hi everyone,

Don't know if I can sparkle your interest on this as it's not Mini, not turboed, not british and not even a car.

So here we go.

I have bought a beemer.

It's a a R65 so 0,65 liter, twin cylinder.

I was comparing it to a 1300 mini engine.
It is the same cylinder capacity and only two valve per cylinder with pushrods and it's N/A.
But it is also aircoolded, the head is hemispheric, it is short stroke and sport a carb for each cylinder.



From what I have red here and there, a racing A series on a 296 pulls 120 hp with a less efficient inlet tract.

So how do I get those 10 missing ponies on my bike ?

What bugs me is that it is using a 308° single pattern camshaft so pulls out 50 hp a 7250 and it's peak torque is at 5500.

Sorry but I am not going turbo on this

Valves are 40 mm inlet 36 exhausts. Carbs are 32 mm.

I don't need power at 7500, I want more only down the rev range.

The only other camshaft available is a 284.

The way I see it is I twin plug it, up the CR to 10,5, maybe change to a 284 with hi lift rockers to get the same area under the lift curve.
That is never gonna give me 10 hp.

I'll even be happy with keeping what is standart ...

But will I gain torque ?



What do you think ?

Edited by Advantage on 15th Feb, 2010.

Rusty by nature

one thing you havent mentioned is cooling,

I'd imagine as its air cooled it would be more prone to det, could that account for the 10hp?

Cooling means lower CR have to be used I guess.

But specialists sells 10,5 cylinder kits.

I blame the short stroke because it pulls air gentler / piston speed is slower and as the camshaft holds valves open longer than a day without bread (do you use an expression like that in english ?) the actual work the expansion stroke performs is very low.

In fact there is a assymetric cam that is used for better torque and what it does do is reducing exhaust timing and lift.
But the timing is reduced only on the beginning.
=> It allows the expansion to last longer / artificially adds stroke

To sum it up, I would like to have those same 50 HP at 6500 and peak torque at 4500

Barrels are also Nikasil plated, it's all aluminium and cylinder are well in the air pattern so cooling is not really be a road block.

At standart with the 308 camshaft, CR is 9 to 1.
And petrol from 1981 was of poorer quality than today's, wasn't it ?

Rusty by nature

You might want to look at adding fuel injection and mapped ignition, ram tuning too.

News, the proper benchmark for this build will be the Alfa Romeo / Fiat twin cam 1300 as it have the same chamber shape

The 1300 puts out 78 wich will be 39 in my 650 world but it is in standart trim.
I bet ther is no 308° camshaft on that so now I am gonna look for racing devellopements of the 1300 and what it can be expected to pull

Rusty by nature

There is no problem with air cooling. Look at Porsche flat6.
Short stroke is the best for the power at high revs. The air speed depends on the volume flowing through the valve, not the speed of the piston. Take a 1m piston and a 1mm valve and you'll get any air speed you want.
Alfa engines have similar heads, yes. Like many many others. But there are lots of details that matter. You can not compare engines on the basis of advertised cam timings and head shapes. BTW 308 cams were quite common on street Alfa engines even in the Injection era. These engines were long stroke. On the picture I'd say you've got 45 degrees between valve stems, which is much better than 60 degrees of the 1300 Alfa. 45 degrees Alfa heads were twin sparked. There is a nice squish surface, there was none on Bialbiero.
I think your main pb is you want power at low RPM on an engine designed for high revs. Start analyzing all the data: bore, conrod R/L, cam lift, true cam profiles (and not advertised and often wrong numbers), all the air restrictions, intake/exhaust manifold design. This should help you understanding the engine.
If you want torque with big valves you need less lift and high turbulence. This is against high revs the engine was built for.
Expect 100hp/L at 7000rpm on race classic Bialbiero (Alfa) engines you are looking at. Long stroke, 60 degrees between stems, single spark plug. I'm sure you can do better on a bike engine, but it will give nothing under 3000 RPM.
Hope this helps.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

100 hp/ L will set me at 65 wich would be excellent.

Given the power to weight ratio of a bike, I expect a powertuned engine is not gonna suffer as much as in a car.

I must have misunderstood velocity and piston velocity.

My understanding was based on Vizard writing a 1100 can stand cams a little hairier than a 1000, I assumed it was more due to the greater momentum the faster travelling piston put in the inlet tract rather than the bigger capacity

Apparently I was wrong ...

Also, there is the ennemy sister : Guzzi V65 wich with parallel valves and flat head are doing 52 @ 7000 if I recall correctly.

Parallel means smaller valves are used
And they also sport smaller carbs (to match the smaller valves ?!)

They also feature a 10 to 1 compression ratio wich without the cam charachteristics is meaningless.

But all things being the way they are, they must be more efficient in the combustion department to do more at a lower RPM ...


Last question for today. What if I replace the CV carbs by some pumpers of the same size ?
Less loss in the inlet tract due to the obtrusive piston ?

Rusty by nature

The piston speed could help at low RPM or light loads to better start chamber filling. But what matters is that long stroke makes more torque because of better mechanical efficiency. The air momentum is much more related to the inlet port dimensions.
Parallel valves mean worse flow in the chamber but sometimes it helps, especially with very compact bath chambers (old US v8 engines are quite efficient up to 4000 RPM). Hemi heads are the best for a lot of reasons: cross flow, actual flow but also thermal efficiency. Vast discussion, better read a true engine book like Heywood's "4 stroke engines', sorry I don't appreciate much Vizard talk, I've seen other books from him.
Smaller valves will make more turbulence -> better combustion. But the flow is limited by the smaller size as you say.
I don't know about carbs, not my department. Start measuring pressure drop at every intake stage: after the air filter, after carbs. You'll see where you are loosing pressure.
Then there are manifold acoustic properties that you can estimate quite easily. Use Powerdyn like software (or accelerometer) to get the torque curve to know where the engine makes the best torque. There are lots of things to do before deciding to change engine parts.

Edited by alpa on 3rd Mar, 2010.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

Rise from the dead ...

Bike has been down since April.

Ignition issue.

And also a growing exhaust valve clearance requiring some attention and possibly the heads being refurbished.

I'll use the opportunity to up the CR and add a second spark plug as it's what I was willing to do for a while.

Rusty by nature

instead of a second spark plug , you could add material to the combustion chamber in the area opposite the existing plug , this would increase CR and add a squish area .