Compression and low octane petrol (revisited)

[Itchy]

High compression or highish say 11:1 runs badly on low octane petrol right? something like a CBR with 12.2:1 would run like an absolute dog on non 95RON petrol right?.

Low(er) compression say 8.5:1 runs "better" on low octane petrol right?.

With 95 Octane we have in the UK you can run anything, however as said I may encounter lower octane petrol 60-85 , or should I just carry around a tin of octane booster / benzine etc, and do such boosters actually work?

Peter is trying to hawk me a heavily modified KTM640 LC something or other pre switchable CDI which has a 11.5:1 compression ratio , where as the Africa twin is 9.0:1 , and the other bike Peter is trying to Hawk me an XT600 is 8.7:1...

Chris Scott's adventure motorcycling book states that you can alter the compression some how by adding gaskets and ignition timing , but this is way beyond my megre ability, and I glazed over for this section.

But then surely you get the opposite problem if you reduce the compression right in that the RON is too high for it?..

Any thoughts on this?.

Thanks
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[virus]

carry octane booster, It would be so much easier than playing with the base gasket thickness etc etc



Cheers
John
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[tafflade]

during a talk that I attended with a Shell boffin dude, he said that in general the octane boosters that you can buy over the counter in halfords are largely ineffective, this is due to how toxic the stuff that really makes a difference is, you'll never be able to buy it.

as a side note, the RON rating on the fuel pump is the minimum you'd expect and in reality what comes out of the 95 RON pump will usually be higher than 100.

the other thing that I noticed the other week while watching the Long Way Down was that ewan and Charles apeared to be pumping petrol from some right dodgy sources and i think those 1200's are supposed to have 'super unleaded' 98RON

[Kickstart]

Hi

Err, big post and hope you don't glaze over.

Use too low an octane fuel and if you are very lucky it will run like a dog. It is entirely easy for it to burn a hole in a piston. You are right that lower compression ratio generally doesn't need such a high octane rating.

Octane rating is the fuels ability to resist igniting itself. Before the piston reaches the top the fuel / air mix is ignited. When it is ignited it starts to burn and increase in volume, and for a short while it is still being compressed by the piston. This if you retard the ignition you have less of this overlap. Similarly you can reduce the compression ratio to ease the problem.

How an engine runs on fuel of too high an octane rating is not really certain unless you know what fiddles have been done to reach that rating but basically you will likely lose some power. For example avgas tends to be very high octane rating but also very slow burning, being intended for slow revving long stroke engines.

There is more than one way fuels avoid causing problems by igniting itself . It can either actually be more resistant, or it can burn slower (so that the increase in volume from the fuel burning while the piston is still going up is less of a problem, and also gives the piston a chance to go down quickly enough for the pressure not to increase too much ).

There are 2 main ways of measuring the octane rating resulting the RON figure (used in most of Europe) and the MON figure (difference is details in how it is checked). Rule of thumb the RON figure is 10 points higher than the MON figure for the same fuel so 85 MON could be the same as 95 RON, but the MON figure is probably more of an accurate indication of the fuels resistance to detonation (it is entirely possible for a fuel to have a better or worse performance in one test than the other). America uses the PON figure, which is just an average of the RON and MON figures. Basically check which figure is being used.

From an Autodata book, the CBR600 until 2001 was designed to run on 91 RON fuel. That should give you slightly more leaway than you thought.

You can lower the compression ratio a few ways. Simplest would be to use a thicker base gasket (less stressed than the head gasket so easier to do successfully), but think the CBR has the barrels cast with the upper crankcase so you cannot do this. You can use lower compression pistons but this would not be cheap. You could use a thicker head gasket but you would probably have to get one made. One problem is that an important part of how well an engine copes without detonation is the shape of the combustion chamber. Using a thicker head or base gasket could dramatically increase the squish band, and quite easily give you a lower compression but more detonation problems.

Either way you could have issues reducing it enough. Eg, if you have a 12:1 ratio and want to reduce it 8:1 (random figures to make the maths easy, no particular meaning in them) you would need to increase the combustion chamber volume from (assuming your CBR600) from 12.5cc to 18.75cc. Think you have a 65mm bore so that works out at using a head gasket that is 1.8mm thicker. Which is a pretty substantial amount.

Modifying a bike to run on lower octane fuel will loose you power. Reduced compression or retarded ignition will both cost power.

Altering the ignition timing might be fairly easy to do crudely. You can buy ignition advancers for many bikes, which basically just move the trigger point round a bit. You could probably modify one of these to retard the ignition instead.

All the best

Keith
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[Itchy]

No no no , its just that I've been offered multiple bikes various high compression ones ie:

11.1:1 the KTM 640 adventure

or something based on the XT600 which is 8.5:1

And thus I don't know which one to get since the Africa is now restored and being sold off to partially fund buying something more adequate for adventure touring or rather something better than an Africa twin at the muddy stuff.

I'm going to start applying for Visas in March for Turkey , Kazahstan Russia and Mongolia and thus I may encounter v low octane petrol there, no way I'd take me CBR out there into the wilderness , it would die horribly or I would be restricted to sealed roads which isn't what adventure touring is about.
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[Itchy]

[Kickstart]

[iooi]

[hmmmnz]

i think i would almost go with an old 2 stroke with a few sets of spare rings a couply pistons and a spare barrel,
ive seen vids where they manage to get an ols 2 stroke to run on a 50/50 mix of shitty fuel and diesel, if they can run on that they can run on anything
plus its a doodle to change a piston or rings, and not really to much of an effort to even do a bottom end bodge,

i managed to strip and rebuild an old ag175 in about 2.5 hours with no previous experience and no special tools, although it was interesting splitting the crank, with a hamme and a drift and then re-assembling it with just a hammer (not recomended, but that bike is still running 10 years on with that hammer beaten crank )

go smokey, im convinced its the smart choice, albeit the slow one
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[Itchy]

This is unbelievable I've found a DRZ400 for an excellent price low mileage (its not even been run in yet) and its got a smegging 12.2:1 compression ratio.

I assume it will run like an absolute dog on 60 octane petrol right? the stuff found in mongolia?....


Erm does anybody actually know of an octane booster that actually works and isn't snake oil?. I need to carry enough for 3000 miles...
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Spain 2008France 2007Big one 2009 We all die. The goal isn't to live forever, the goal is to create something that will. In the end, your life will flash before your eyes. Make sure it is worth watching.

[Kickstart]

Hi

Think you are getting a bit too concerned with compression ratio. It alone is a factor with detonation, but not the only one. And if you have a serious problem with detonation then it running like a dog would be the least of your problems as it would likely kill the engine.

Most octane boosters work, but how much varies with the fuels you use. As they might use an additive already used by the random fuel company your current tank of fuel came from, and as the effects of the additives are not linear the same amount of octane booster can have very different effects of 2 different fuels of the same nominal octane rating (and using a different octane booster could give a different effect on the same 2 fuels).

At the very least you would need 3L of octane booster (based on 3000 miles = 300 litres of fuel with a 1% octane boost), but without knowing the exact characteristics of the stuff not much you can say. Eg, Silkolene Pro Boost works effectively from 1% to 3%, with a claim of a 0.5 to 2 point increase in octane rating for a 1.5% mix. So on there figures (which will be based on normal fuels) the best you could get would be 4 points increase (nowhere near enough), and would require you to carry 9 litres of the stuff around/

From an Autodata book the DRZ400S only wants 91 RON fuel, while the other DRZ400 models are listed as 95 RON.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[chillyman0]

didnt really read all of the thread but if you use too high an octane fuel there is a chance of 'pinking' or pre-det, the same goes with increasing the compression ratio... if you vastly increase the CR and keep the same octane fuel (or even increase the octane) there is a chance that the fuel will pre-ignite before the spark comes in because of the increased temperature from compression

[Itchy]

[Kickstart]

[Kris]

Dude, just get the DR650.

Last thing you want to be worrying about is blowing up the DRZ engine in Mongolia. I reckon Suzuki wrung every last hp available out of those anyway.
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[G]

[Itchy]