100 octane petrol

[yankeedoodoo]

Soon I will have to face the california DMV to register my Caterham. I am wondering if using 100 octane gas would make its breath any cleaner than our standard 91 crap pump gas. Does anyone out there have thoughts, ideas or knowledge of such matters? Are there other relevant considerations, positive or negative, to consider? Thanks in advance for any and all replies. Will share beer on the USA tour in return for shared knowledge.

 

 

[V7 SLR]

Can't imagine how changing to higher octane will improve your emmissions unless you redesign the engine for higher compression and run it hotter to burn off more fuel.

 

Do I qualify for that beer? smile.gif

[Simos]

You could tune for emissions rather than performance or economy (economy might = emissions) if you really thought you'd have a problem, then re-tune when you're finished. A friendly tuner will understand teeth.gif "see ya tommorrow to put it back as it was" kind of thing...

[V7 SLR]

Hmm, I suppose you have a point. Now that I've got a user-programmable ECU is there any mileage in my asking Dave Walker to map V7 normally, and then offer me a map which provides good emmissions for the MoT (fist MoT due in December - didn't that come round quickly)?

 

If you lean-off the fuel delivery (I presume that's the way to make emmissions better) then will I risk damaging the engine, and does a properly mapped engine only have one "proper" map and anything else is a dangerous fudge?

[yankeedoodoo]

V7: Name your poison!

 

 

[Simos]

There must be a broad range of tune for any engine. Optimum mixture for economy is around 16:1 (20:1 in lean burn engines) whereas the optimum mixture for power is 14.7:1.

 

This'll vary at differnet revs and atmospheric conditions but basically there must be quite a lot of "give" in the map...

[V7 SLR]

Yankeedoodoo... I'll take a nice cold beer please mate. We're having quite decent weather over here for a change and my thoughts turn to skimpy dresses (observing rather than wearing, but hey, I'm not too bothered) and beer gardens.

 

Simos, does an economically tuned map have a knock-on for better emmissions. It sounds like it should but I haven't a clue.

[Peter Carmichael]

Economy/efficiency. vs. what?

 

Fuel economy is desired power achieved per unit of input fuel/second.

Emissions are based on desired power achieved per unit of reject fuel/second.

Power is achieved where the limiting factor is air/second.

 

You can go as far as 12:1 in the search for more power. 14.7:1 is stoich (lambda). Max power is usually richer than lambda, but no point in going there until you are running out of throttle opening.

 

US pump fuel is labelled with its MON (Motor Octane Number) rating. This is based on a higher speed test than the RON (Research Octane Number) and gives a lower value for equivalent fuel. UK 95RON fuel is closely equivalent to US 91MON. As octane goes up, the fuel company is mixing in more additives so that in any given gallon, there are fewer fuel molecules - the only way this makes more power in any given situation is if the thermodynamic cycle is more efiicient. Thermodynamic cycles based on high compression engines are more efficient thermodynamically.

 

If octane is the limiting factor on an engine, the engine will ping as you crank up the advance, while the power is still rising in response to the increased advance. Increased octane will give better efficiency of the burn, outweighing the reduced calorific value of the high octane fuel.

 

If you have a healthy margin between the advance you run which achieves optimum power and the onset of detonation, then a higher octane fuel will give you less power (or worse economy) because there is less *fuel* in the injected mass - in order to get the appropriate amount of *fuel*, you need to inject more. Seeing as most economical running is done at part throttle where detonation is unlikely, emissions are better with low octane fuels. This is why we have seen a gradual creeping decline in pump octane levels.

 

The only way this logic could be turned on its head is with very low capacity engines, where the desired power for general running is close to the maximum power available - figure on a gutless car with a maximum speed of 70mph. With this sort of vehicle, a high compression engine with high octane fuel can have better economy and emissions.

 

For a high octane fuel to be worth anything other than safety margin, the engine needs to have a higher compression and needs to be running an appropriate map.

 

 

Edited by - Peter Carmichael on 23 Apr 2002 17:23:08

[TomGaval]

Peter, My engine which was built by QED came with a note to use 97 octane miniumum, which I suppose is due to the 12:4:1 compression ratio. I've been using Sunoco's 100 octane GT race fuel. Based on the different measurement of octane ratings in the US and UK, could I assume that I could use 93 or 94 octane here in the states and still meet the minimum UK recommendation of 97? It's about a three dollar a gallon price difference. Tom

[Peter Carmichael]

97RON is the "Super Unleaded" rating in the UK. Its MON rating equivalent will be a lower number. I think your QED engine will be quite happy running the better pump fuels available - its been a while since I have been in the US, so I forget what the generally available grades are, but you shouldn't have to use a race fuel generally.

[yankeedoodoo]

V7: I've got just the place for you. Beachfront location, reasonable beer selection, women in bikinis, 70 degrees, California sunshine, and live entertainment in the form of walkers/runners/skaters/bikers etc. A morning mountain blat and some afternoon consumption!

 

 

[Simos]

Peter,

the question (or answer given) was never economy Vs anything but tuning for emissions which was one proposed answer to the original question asked. The comment about economy or power is to illustrate the fact that there ARE different fuel/air mixtures and they DO have different results. Whether you'd want them.... well.

 

Clearly tuning for economy in a 7 is pointless, go buy a diesel micra instead. Tuning for efficiency as you rightly point out must take into account the environment and workload intended for the engine.

 

However within the confines of simple tuning of an engine (timing mixture etc) and excluding those external factors there are a variety of combinations which will yield different fuel usage/emissions at different revs. Again whether these are ultimately useful is a separate issue.

 

The problem posed is one of passing an emissions test, which, and I confess I don't know the US tests, probably means no-load, arbitary revs (idle or 1500rpm in UK?) and tests for CO, Hydrocarbon, NOX and maybe some others.

 

I'm suggesting that a visit to a local tuner who knows the regs will yield a map or jets and timing that at the given target will better able the engine to pass than the "max power at 8Krpm full load" tune that is probably currently on the car. (I also proposed changing it back afterward to what we assume is an optimum tune, or at least what Mr Yankeedoodoo can live with).

 

I realise while writing that with your Emerald ECU you can probably have a map that will give an emmissions pass at idle AND give max power at 8K, BUT, with my carbs and dizzy solving for both is by no means achievable. One or t'other - easy, both - difficult, so for me retuning would be a requirement if I thought the current tune would not pass. I've never built a MAP for an ECU so I concede that you may solve for both targets by default in your map. However, Since I don't know Mr Yankeedoodoo's engine I suggest it as one possible alternative.

 

My second response was to V7's question about there being more than one "correct" map for an engine... Even if, as we all know he doesnt give a stuff about consumption and would sell his strangely shaped potato collection for another 10 hp, I stand by my assertion that there is an "economy" map for his engine that will yield him better mpg without blowing it up. Implicit there is the assumption that all other factors remain unchanged and as you point out, would have to be taken into account when arriving at such map. If he were to do so of course, we'd have a better chance of keeping up with him teeth.gif

 

V7 Hope I've answered above, tongue in cheek at all times please smile.gif

 

Cheers, Simon.

[V7 SLR]

What? Sell my strangely shaped potato collection for a power increase? Never..!!

[Simos]

teeth.gif