What is the difference between a O-540 J1A5D and a O-450 B4B5. I have a 1989 M 7 235 hp.

Aside from what Jeremy wrote in the last thread, I usually reference (of all places) wikipedia.

The quick and simple answer is that the J series (what I have) has the dual magneto, cannot run on mogas and spins at 2400 RPM.
The B4B5 has two separate mags, can run on mogas via STC and spins faster like 2650 or 2700 RPM's or so.
Both are good!

Rez, that's 2575 rpm for B4B5 and lower Compression ratio than J1A5D

What is the difference in the J1A5D and the W1A5D? I have the W1 in my 1990 M-7-235.

0-540J is carb. IO-540W is fuel injected

Thanks, Gary.

However that begs another question:

Rezrider said above that he could not run Mogas, but I thought all carb. engines could, and injected engines could not.

I'm confused.

Only the low compression O540 235hp B4B5 and O360 180hp C1F and C4F can burn non ethanol autogas in Maule aircraft.

Thanks, Jeremy.

maules.com wrote:Only the low compression O540 235hp B4B5 and O360 180hp C1F and C4F can burn non ethanol autogas in Maule aircraft.

I've flown an MX7-160 that uses autogas.

Not to hijack the thread but I've read articles that say if you burn 91 octane unleaded (non-ethanol) autogas in an aircraft engine that's been modified to take it, there's about a 25% power reduction due to the lower octane (100 vs. 91) and lack of lead that boosts combustion effectiveness. Has anyone with the autogas STC actually experienced this? If so, is the 25% reduction about the same for the O-540 and O-360?

Hi Andy.

My understanding is the Octane rating is related to the fuel's ability to resist knock, not the energy contained.

See: https://www.exxon.com/en/octane-rating

The question would be would a low compression engine produce less power? The answer to that probably would be found in the details of the HP rating.

If engine X produces 235hp at 2400 rpm and engine Y needs 2550 to produce the same HP, then the question would be if they both ran at 2100 would they produce the same or different HP. My guess is, no.

Then again I am a biologist and chemist, not an engineer.

Tom

I occasionally run 93 octane mogas in my O-360 MXT-7-180. The STC requires 91 octane minimum, but we can only get 89 and 93 octane. I have not noticed any reduction in anything, except my fuel bill.

Here is my understanding, and some possible insight into the confusion regardlng less power from lower octane fuels. I'm open to being corrected on any of this.

Octane rating is a measure of RESISTANCE to the fuel igniting spontaneously, i.e. detonation, or knock. Naturally, there is a relation to resistance to normal burning as well. The higher the octane number, the more resistant the fuel is to igniting.

Higher compression equals, among other things, higher heat. Higher heat means better likelihood of sponteneous combustion (knock).

Engines with high compression ratios typically specifiy higher octane fuel, to resist the tendency to knock under the higher heat created by the greater compression of the incoming fresh fuel/air charge.

Back in the days before electronic controls in cars, people probably came to associate higher octane with more power (and therefore lower octane with lower power) because high-performance cars (which partially got that performace through higher compression ratios) needed higher octane fuel. The relationship was there, but folks might have understood the causality backwards.

These days, if one is driving a car that specifies high-octane fuel, that car might indeed be more powerful with that fuel than with a lower-octane variety. That's because modern cars have knock sensors, which will cause the computer to retard the ignition timing slightly if the engine knocks, thereby lowering combustion temperatures and ending the knock. It goes something like this: Owner puts 85 octane fuel in a 200 hp car that specifies 91 octane. He mashes the gas pedal to pass that hippy bus, the engine begins to knock, the knock sensor tells the computer about it, the computer retards the timing, and now his engine is only making 170hp, but it's no longer knocking. This all happens in fractions of a second. All the driver feels is a slightly less peppy engine. So this dynamic coild definitely cause someone to think that lower octane fuels produce less power. Again, connection, but the causality is backwards.

So in our aviation engines, with no knock sensors, lower octane will not lower the power output. I suppose it might make them more susceptible to knock, but even "high compression" aviation engines are very low compression compared to modern cars and motorcycles (which can be up to 13:1 or so), so I would suspect that. 91 octane would be just fine. That said, I haven't run auto fuel, so I am just theorizing on this point.

TomD wrote: If engine X produces 235hp at 2400 rpm and engine Y needs 2550 to produce the same HP, then the question would be if they both ran at 2100 would they produce the same or different HP. My guess is, no.

Assuming engine Y has lower compression, then I'm with you. At 2100 the high-compression engine (X) will produce more power than the low compression engine (Y). The whole reason Y has to spin faster to produce the same horsepower is the lower compression ratio.