14.7:1 comp ratio for ultimate 98?

[slowpoke]

Engine guru's req'd:

Can a modern 1000cc 4 cylinder race engine with a 14.7 compression ratio run on Ultimate 98?

Assume it has race cams (longer duration for lower dynamic compression ratio) not just high compression.

Informed opinions from guys who have actually played with/built up race engines would be much appreciated. Thanks.

[pete376403]

Water cooled or air cooled? My speedway Jawa was around 14.5:1 and required methanol - on petrol (even 100 octane) it was rumored (but never observed first hand) that the engine would detonate to pieces in a very short time). A water cooled engine *might* last a little longer.
Also I had an 1100 GS suzuki (air cooled) that had h/comp MTC pistons - compression not measured but the piston crowns were a lot higher than the standard 8:1s - on pump gas that detonated very badly and seized after a fairly fast run. Fortunately it un-seized after it cooled down a bit but the rings were toast and it used 11 litres of oil getting to and from the Brass Monkey.

So with that in mind I'd say your c/r would be unfeasible on petrol, especially the swill available at the pump.

Eit - I see you wrote "modern" so obviously water cooled

[slowpoke]

The speedway stuff is pretty interesting. Just goes to show how far you can improve a basic combination with constant evolution.

[slowpoke]

Hmmm, BMW's S1000R uses a 14.0 - 14.1 CR in WSBK, form now to find out what fuel they use in WSBK.....

[slowpoke]

.....good God, methyl tertiary butyl this, ISO5164 that, but basically it looks like 95 - 102RON. You'd have to assume they build their engines to take advantage of the upper limit.

[FruitLooPs]

I've heard that 13:1 is getting toward the limit you would run on high octane pump fuel, on older oil cooled GSXR engines.

No idea for the newer, but it sounds very high.

[Ozzy27]

Engine guru's req'd:

Can a modern 1000cc 4 cylinder race engine with a 14.7 compression ratio run on Ultimate 98?

Assume it has race cams (longer duration for lower dynamic compression ratio) not just high compression.

Informed opinions from guys who have actually played with/built up race engines would be much appreciated. Thanks.

Hi Spud,
14.7:1 is too high for 98 pump Gas!
You might get away with it if you had dynamic ignition control (knock control) And not too small a squish for localised deto! Also Cast stock pistons tend to start fracturing ring lands from Detonation if you are running on the limit. Forged piston are better is this regard but can have other issues.
As to a bottom line number around 14:1 is tops but will vary from engine to engine.
Cheers
Ozzy

[slowpoke]

Thanks Chris, exactly the sort of info I was after.

I stumbled a cross an attractive race engine package in the 'States, and the alarm bells were ringing as soon as I saw the comp ratio....although there are a couple of head gaskets for sale on TM at the moment.....

Nooo! Begone evil temptation!

[Warr]

........
As to a bottom line number around 14:1 is tops but will vary from engine to engine.
Cheers
Ozzy

Just for clarification I would guess you are talking about this engine being married up with a completely computerised injection system.

I once had a 351 Cleveland V8 and put the Auzi 302 heads on it.
Pinking all round. I CC'd the chambers and bodgy as it sounds gained the extra room by fitting an extra head gasket.
Had a 4 barrel 780 holley carburetor cams etc etc.
From memory I managed to get the compression ratio down to less than 12.5 which sorted the pre-ignition issues.

[SS90]

Hi Spud,
14.7:1 is too high for 98 pump Gas!
You might get away with it if you had dynamic ignition control (knock control) And not too small a squish for localised deto! Also Cast stock pistons tend to start fracturing ring lands from Detonation if you are running on the limit. Forged piston are better is this regard but can have other issues.
As to a bottom line number around 14:1 is tops but will vary from engine to engine.
Cheers
Ozzy

The most I have personally run on a four stroke ILF was for a friends Kawasaki ZXR400 about five years ago.

We where experimenting with compression ratios and at the old Taupo track running 13.8:1 there was signs of detonation after the meeting.

Of course there is always discrepencies with octane ratings (as you should know, Octane is simply a measure of anti knock value) and you can indeed find one batch of fuel is different to the next.

But for certain, anything more than 14:1 is not suitable for "pump gas", and even "Av Gas" may struggle to surpress detonation.

[F5 Dave]

I do note some people comparing 250cc cylinders with those 2-3 x the capacity. Spark lead will be considerably longer than even a short stroke GSXR, esp compared with modern designs.

'course I've run 19:1 in one of my engines, but I'm special. . . . So I'm told.


There will be a stage where an active ignition will cope with higher than optimum compression, but cope doesn't necc mean the most efficient combustion of the gas used. Pumping losses will hurt and ignition timing will have to be backed off silly amounts + chamber shape will likely be compromised.

[pete376403]

'course I've run 19:1 in one of my engines, but I'm special. . . . So I'm told..

Wasn't a diesel was it? Or was that the theoretical c/r, not taking into account port closing or something?

[F5 Dave]

Velly small engine.

[sinfull]

Thanks Chris, exactly the sort of info I was after.

I stumbled a cross an attractive race engine package in the 'States, and the alarm bells were ringing as soon as I saw the comp ratio....although there are a couple of head gaskets for sale on TM at the moment.....

Nooo! Begone evil temptation!

I thought about getting a faster bike too, but it sorta defeats the purpous when i need to learn to ride it faster first !

[slowpoke]

I thought about getting a faster bike too, but it sorta defeats the purpous when i need to learn to ride it faster first !

Goodonya Bill, throwing a cold bucket of reality on my hot 'n sweaty dreams.....

I know exactly what you mean mate, and it is a very valid point, more so in my case than yours I fear