15 to 1 Compression on Alky

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jseng1

jseng1

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Just found out that Robert Hood had his 920cc Alky side car racer set up with 15.5 - 1 CR. Thats got to be a record for Nortons. He's rebuilding the motor now and will be changing to a Maney Stage 3 head with larger combustion chamber so it will drop to 13.8 -1. Before this I've only heard of Nortons with 12 to 1 CR on Alky.

15 to 1 Compression on Alky
 
I ve been on this monster at a track day, an absolute beast!

Rob and his swinger Mark have won many titles with this set up.
 
Commando engines love methanol. Mine is still on standard comp, but it is very quick. At 15 to 1, the ignition is probably retarded by about two or three degrees. On mine, it is advanced 4. With methanol, you have unlimited anti knock. But at very high comp., strange things start to happen to the pistons. My friend's Manx was on 14 to 1 comp. and the pistons used to collapse under the inlet valve. however we have a 600cc Jawa on 17 to 1 comp. , but that is on speedway where the races are much shorter, so you don't get the heat build-up.
I have bought a nice new Elf racing fuel can, in case our historic people ever decide to ban methanol. I'd never race a four-stroke using petrol - it is all too difficult.
 
Its been reliable with over 90 races on it until the Maney cylinder bolts broke. Now he's using custom ARP fluted reduced shank bolts for the cylinders and head so they stretch along their full length instead of breaking at the threads.

15 to 1 Compression on Alky
 
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Our earlier builds of the 500cc ultra short stroke ran a compression ratio of 13.5:1. It was a major chore to get that CR with a 59.6mm stroke. It seemed to run happier on pump gas.
 
Another email from him today:

Just did The Festival at Pukekohe in February. That was after the cc halving at The Cemetery Circuit last December. Three races, two thirds and a first. That was with the standard valved head. Also had the ignition timing in the low twenties.

The races above were with the compression at 15.5 to one. Since alky burns faster its best to retard the ignition a little (low twenties).

The 81mm domed piston he's using is similar to the 850 domed piston below (same dome) but the increase in displacement brings the compression up to 15.5 to 1 with a stock head.

CR would only be 11.4 to 1 with a 73mm bore, the same dome and a stock head.

CR would only be 12.3 to 1 with a 77mm bore, the same dome and a stock head.


15 to 1 Compression on Alky


CR over 12 to one is risky with gas. Bores larger than 77mm have increased CR and you can get up to 13 to 1 and risk overheating/failure (not a problem with alky). So far the pistons are doing fine - even with only .150" thickness at the dome. The area to watch out for is the intake valve pocket near the ring - you don't want it too thin.
 
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Dances with Shrapnel said:
Our earlier builds of the 500cc ultra short stroke ran a compression ratio of 13.5:1. It was a major chore to get that CR with a 59.6mm stroke. It seemed to run happier on pump gas.
Click to expand...
Another interesting note on the 500 build was that it did not lose anything when we dropped the CR to 11.5:1. A lot of factors were involved. The motor performed very well with relatively mild cam profiles.
 
Dances with Shrapnel said:
Another interesting note on the 500 build was that it did not lose anything when we dropped the CR to 11.5:1. A lot of factors were involved. The motor performed very well with relatively mild cam profiles.
Click to expand...

There could be a trade off of too high a dome inhibiting combustion efficiency and diminishing returns with higher CR. But for Alky you want it higher yet.
 
Dances with Shrapnel said:
Our earlier builds of the 500cc ultra short stroke ran a compression ratio of 13.5:1. It was a major chore to get that CR with a 59.6mm stroke. It seemed to run happier on pump gas.
Click to expand...
My 63mm stroke Triumph engine used the normal 650 cylinder head and pistons which give 12 to 1 in the 650, so the effective comp. was about 10 to 1. I still used methanol. But the motor was so radical that it was simply nasty. I rode the Triton again long after I had sold it back to the guy who originally built it in about 1957. It was running petrol, had a 5 speed box and decent rubber. It was almost sane. But it still went sideways in one corner.
 
jseng1 said:
There could be a trade off of too high a dome inhibiting combustion efficiency and diminishing returns with higher CR. But for Alky you want it higher yet.
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It is much easier to get the tuning right if you use methanol. That is the reason most bikes are faster when it is used - it hides-up the tuning errors. What a lot of guys do not realise is that running it rich takes the edge off the motor. If you want the best out of methanol, it is as difficult to tune as petrol. In the 1950s, when Australians went racing in Europe they foiund the Poms got their Manxes to go as fast on petrol as we could on methanol. However what it meant was that the Australian riders were accustomed to the high race speeds. Also our Manxes had Symco con-rods which allowed them to be revved to 8000 RPM.
There is a silly thing which happens - a lot of guys copy the jetting for high comp use of methanol and try to use it for low comp use. The needle jets in a 650 Triumph engine on 12 to 1 comp. are usually 0.120 inch. At 10 to 1 comp. 0.117 inch is usually slightly too large. Also at 12 to 1 comp., petrol ignition timing is normally used. For higher comps - less advance, for lower comps, more advance.
With methanol, it is the higher latent heat of vaporisation which gives the supercharging effect. The cold cause the air density to be higher, so more fuel is used. With high comp. you also use more fuel to keep the combustion conditions right. Methanol's calorific value is about 20% less than that of petrol, but you use much more of it.
 
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I am never afraid to find out how lean you can run methanol. You usually get plenty of warning before you do damage. With two-strokes, if they start to really fly, you need to take care - that is the sign that they are about to seize, but it is also close to where you need to go..
 
When I first built my Seeley 850, I bought two 34mm MK2 Amal methanol carbs from Mick Hemmings which had the correct low running jets and the right floats and needles and seats. But the needles and needle jets were ridiculous. So I made my own needle jets out of brass hex and used 6D Mikuni needles, to get the mid-range jetting lean enough for use at 9 to 1 comp. You do not really need high comp. The motor is usually fast enough without going there. When you are close to getting the mid-range jetting right, a difference of one thou of an inch in the needle jet is very significant. If you raise the needles one notch in my carbs, the motor becomes slightly sluggish. If you lower them one notch, the motor coughs. If you run at high comp. with methanol, it is doubtful that you would be able to buy the correct needles for best power.
 
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jseng1 said:
Just found out that Robert Hood had his 920cc Alky side car racer set up with 15.5 - 1 CR. Thats got to be a record for Nortons. He's rebuilding the motor now and will be changing to a Maney Stage 3 head with larger combustion chamber so it will drop to 13.8 -1. Before this I've only heard of Nortons with 12 to 1 CR on Alky.

15 to 1 Compression on Alky
Click to expand...
 
To Jim’s question. Pertaining to gasoline (not methanol). From discussions with Norm White, he called for 12.6:1 CR with a full hemisphere combustion chamber. I recall others of the era bantering around the 12.5:1 CR as a happy max. Going much higher would certainly boost the heat load yet from first hand experience I know a Commando with iron barrel can handle a lot of heat.

With quality alloy race barrels available today I wonder if a Commando (for racing) could safely handle more than 12.5:1 CR?
 
Heat build-up in a limiting factor with four-strokes, but aluminium barrels with methanol fuel is probably not a good idea. You might start breaking pistons due to piston-slap, if you cannot get the motor warm enough before racing. With four-strokes, how many modern engines can run petrol at above 12 to 1 comp. without getting problems with detonation ? A Commando has squish - that might help stop detonation through better mixing, but the combustion chamber is not like the ones that have four valves. The 500cc Paton twin is probably on the limit.

P.S. methanol burns SLOWER than petrol in motors - that is what anti-knock is about, and with methanol you have unlimited anti-knock - it does not detonate, you usually get the seizure first in two-strokes. To get it to detonate in a two-stroke, you usually need a defective ignition system.
Methanol is a far superior race fuel than petrol. It works through latent heat of vaporisation which freezes the incoming charge, so the air density is greater needing more fuel to get the mixture right. Methanol has a lower calorific value than petrol, but you use almost twice as much of it - so you get more power. In a race on the IOM, you would probably have to refuel 3 or 4 times.
 
With methanol - as the comp. ratio goes up, the jets get bigger and you need more retard. My 500cc short-stroke Triumph motor was ion 10 to 1 comp. and I used 44 degrees of ignition advance with methanol. If it had been on petrol, I would have used about 38 degrees advance. With my Commando 850 engine, it is on 32 degrees advance with methanol at a comp. ratio of 9 to 1. It is actually fairly irrelevant - what you end up with is a balance between ignition advance, comp. ratio and jetting. The higher the comp., theoretically the more fuel you use to get the balance right. But it is deceptive - methanol still works extremely well at low comp and more ignition advance.

Judging from what a few of my friends have done with methanol in four-strokes - use standard petrol timings at 12 to 1 comp. Add 4 degrees for 10 to 1 comp. and take off 4 degrees at 14 to 1 comp. - then jet to that.
(two-strokes on methanol are very different)
 
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jseng1 said:
Its been reliable with over 90 races on it until the Maney cylinder bolts broke. Now he's using custom ARP fluted reduced shank bolts for the cylinders and head so they stretch along their full length instead of breaking at the threads.

15 to 1 Compression on Alky
Click to expand...

That is a novel way of preventing the bolts from breaking.

Re, “Its been reliable with over 90 races on it until the Maney cylinder bolts broke.”

I would think that the lifespan has been pretty good for these, replacing them with new ones say, 80 every races or the equalvant miles should have been the norm. Also threads that are rolled rather than cut would have been better.
 
acotrel said:
P.S. methanol burns SLOWER than petrol in motors - that is what anti-knock is about,.....
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“burns SLOWER” is not what anti-knock is about. The air-fuel mixture should burn or deflagrate rather than detonate which is a somewhat different phenomena than detonate. Yes, for a given fuel, detonation phenomena is much faster than burning (deflagrate) phenomena.

One could make the argument that a slower burning fuel would leave a motor more prone to detonation.
 
Bernhard said:
That is a novel way of preventing the bolts from breaking.

Re, “Its been reliable with over 90 races on it until the Maney cylinder bolts broke.”

I would think that the lifespan has been pretty good for these, replacing them with new ones say, 80 every races or the equalvant miles should have been the norm. Also threads that are rolled rather than cut would have been better.
Click to expand...

Those are rolled threads on the ARP bolts in the picture. Steve Maney's original bolts were also rolled threads.

Ken
 
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