RussP

Hi Folks

What with all the sti engine problems of a couplke of years back (mine included!) I thought this might be a good place to start.

I'm having problems with an engine I've imported from the USA for use in single seat hovercraft. Its basically a tuned Briggs & Stratton commercial genrator engine - twin cylinder 750cc four stroke. The original puts out a throbbing 22bhp, the tuned version approx 40bhp.

The problem I've got is that we can't stop them pinking. The supplier in America uses them extensively for 'Legend' style racing in little race cars called 'Bandaleros'. Now he claims that they have absolutely no problem with them whatsoever, yet as i understand it, standard pump fuel in the states is just 83 octane. We always run them on Optimax at 98 octane.

On ititial pick up, and under load, the things really knock, pretty badly.

Is the US Octane rating the same as ours? Is the fuel fundemantally different in any way, more stable? Less prone to pre-ignition? Or can someone offer another possible cause?

We've already knocked the timing back, but we quickly lost power - we were back at 22bhp by the time we'd stopped the knocking.

Any advice welcome.

Thanks

Russ

Jye

Is it a new engine?

JackClark

I thought the US ratings were lower than ours, but am told they're higher. 100*RON* is available all over.

RussP

They are brand new, yes. I'm not sure what fuels are available over th epond, but the supplier says they ony use 83 octane.

Markus

I was under the impression that the RON rating over here is not the same as the UK rating.

Certainly at the Canadian pumps we have 83, 87 and in some places 93 RON fuel.

I would pop over to http://forums.nasioc.com/forums/ and ask on there for the definitive answer.

Jye

One factor sometimes overlooked is ignition crossfiring. This can be caused by induced voltage in the plug wires, especially if they run next to each other over a long distance.

Have you modified anything to suit the hovercraft application?

Lum

The yanks rate Octane in PON, we use RON
http://www.btinternet.com/~madmole/R...RONMONPON.html This site has a comparison chart.

gareth123

Hmm. I put 91 octane in my WRX here in California, which is the highest grade available (87 and 89 being the others). I thought this was the equivalent to U.K. regular 95 stuff, i.e. U.K. superunleaded 98 is higher octane than U.S. premium. Premium in other states can be 92 and 93, but I'd be surprised if your engine couldn't run on CA 91 petrol, if Ferraris can I'd expect a lawnmower to be able to

Lum

Ferraris sold in the US are tuned differently in order to run on crappy US petrol. This is probably also why the US got a 2.5 litre STi way before Japan or the UK did.

gareth123

Yes, I know, my point was if U.S. petrol is good enough for U.S. spec Ferraris it should be good enough for U.S. spec generator engines too. Which is what the OP has

Lum

Ahh ok, I take your point.

Perhaps the octane rating is too high? Octane rating is to do with resting ignition IIRC? maybe it's not firing at all sometimes?