OZ ATV :: The Australian ATV Forum

[skidplate]

If running on 98 octane pump petrol, what is the highest compression ratio before it detonates?
I read few posts but they do not seem to agree.
Is 13:1 safe?

[bullet]

A number like 13:1 represents your 'static' compression, but not the actual achievable cylinder pressures.
Go too high on your cylinder pressures and you will get good old destructive detonation.
The number you want to get right is your 'dynamic' compression.
You can not measure this number by reading the packets of differnet products on the shelf.
Factors like cam timing play a big part, generally more aggressive cams lower cylinder pressures because the valves are basically kept open for more of the engines cycle.

I've seen 13.2:1 work fine with 91 octane fuel, but seen 13:1 detonate on 98 octane.

I think most modern multi valve engines can be conservatively built with a 13:1 piston, very safely.

It's probably fair to guess than most of these engines would be in detonation territory if you went over 14:1.
So somewhere upward of 13.5:1 is starting to push things, and you need to start asking yourself why.
The power gains are minimal above that and all the other stresses on your engine start to go through the roof.
Engine life is dramatically shortened at that end of the scale, as well as the fact that these parts are usually built for people chasing the highest performance regardless of cost (maintenance).
The actual piston skirts and wear areas of the piston are physically reduced, and piston life is dramatically reduced.
This generally makes these very high comp engines less reliable too.

hope that helps, and does not add confusion
_________________
Can-Am X Team
Bullet #Y12. MrsB #Y18.

[skidplate]

Thanks bullet.
I read your post a few times now and I think I'm starting to understand it
Since the dynamic pressure can't be measured anyway, then are there knock sensors I could install ?
I want the engine to survive Finke and some prerunning.
So I guess to simplify the question, what static compression would be better 12.8:1 or 13.1:1 as these are the two kits I have been looking at.
And would changing of cams be mandatory?
And another question: When is the moment that the standard LTR computer would stop coping with changes.
So far I have had perfect reliability from this motor and would like to keep it this way.

[JONO]

Guessing this is being ran in an lt-r, you can get up around 13.8 on 98 octane with no detonation at all and still keep it fairly reliable, but best bet is around low 13's and you'll get great reliability and plenty of power.

[bullet]

I'd be with Jono on that one.
Go for the 13:1 kit and stock cam and you'll be safe.

Sometimes there are fuel mix ups at Finke and other times you might even be unfortunate enough to have to borrow some generator fuel from a spectator or something
At 13:1 you could still run 91 octane if you knew not to load the hell out of the engine.
The LTR's have good grunt off the bottom and through the mid range, so you don't really have to chase super high compression too much anyway.

Australian fuels are actually very good.
Manufacturers all build their engines for the lowest common denominator fuel in any country.
The U.S. pump fuels get down to 87 octane and i can imagine that there are other countries that might be worse than that.
That makes it very sensible for every manufacturers standard products to be safe on at least 87 octane.

So in Australia, if you are running a mass produced quad, it's probably pretty fair to say that any quad would benefit from a half a point compression increase even on our 91 octans regular unleaded.
Like from 12:1 up to 12.5:1

There is a bit of a miss conception that EFI bikes computers tune themselves to different conditions and engine modifications.
They don't.
They measure engine temp and run cold start functions and a few over temp cuts and things like that, but definitely on wide open throttle you are just running on the map which is 'set' in the ECU.
This is why the LTR uses a CHerry Bomb, and the Can-Am has a 'pink wire cut'.
They both change the 'set' maps for the normal de-restriction modifications.
Neither of their ECU's self adjust to meet these conditions.

David Wade from Colac MC's would be the best for you to talk to about Suzukis.
And if you do change your engine more drastically, get it on to QuadSquads dyno for setting up.
Adam has built a heap of very strong LTR speedway engines.
_________________
Can-Am X Team
Bullet #Y12. MrsB #Y18.

[Linc]

bullet wrote:

Australian fuels are actually very good. Manufacturers all build their engines for the lowest common denominator fuel in any country. The U.S. pump fuels get down to 87 octane and i can imagine that there are other countries that might be worse than that. That makes it very sensible for every manufacturers standard products to be safe on at least 87 octane.

Don't get suckered into comparing US to Oz fuel. Octane is measured differently in the US and their "low" octane stuff 'aint as bad as you think.

The amount of compression an engine can handle is all down to the efficency of the combustion chamber. Our modern 450 engines are all pretty good but for safety I wouldn't go over 13:1 on any of them. We ran 12.8:1 in our LTR and it ran great in all conditions.

Linc

[skidplate]

Thanks , I think I got it.
Now the money spending part