This ? may alienate a few of you but...

[poadeleted20]

you are right, there are several STC s that require higher oct than 87,

Glad you realize that now.

But you are not going to find a reliable source of fuel supply. because most of the auto fuel suppliers use alcohol to gain higher oct ratings

No, Tom, they don't use alcohol for that reason, and it doesn't accomplish that purpose.

First you must comply with the STC requirements, but there is no problems mixing 100ll and the proper auto fuels, but trying to mix 100ll to meet the engine requirements isn't a method approved any where as you have pointed out.

Glad we agree.

my point, is when you can find the auto fuel that meets the requirements of the STC there is no problems mixing it with that fuel.

Again, glad we agree.

or did the statement of a properly applied STC escape you?

You're the one who said all mogas STC's required only 87 octane, not me. But I wasn't aware that by "properly applied STC" you meant "abiding by all the operational restrictions in the STC" as well as having that STC properly accomplished by the mechanic performing it.

 

[Capt. Geoffrey Thorpe]

Ethanol does increase the octane rating. But you don't want it in your aircraft fuel system.

 

[N801BH]

I'm not sure I've ever seen detonation that cracked the piston head before the ring lands, especially with an octane problem. Are you sure that isn't preignition? It does appear as though it hasn't melted.

Detonation is not only caused by poor gasoline. A really obvious source for severe detonation is a turbocharger failure, especially a stuck wastegate. Oil consumption can do that as well, but it would foul the piston head.

It's true that flycuts are the weak spot in the piston head, but did the valve look OK? That kinda looks like a valve contact, which can happen in an interference engine with a sticky valve.

Octane ratings are regulated, so if you have proof that 91 is not 91, contact DOT to spare the rest of us from that.

I have been a SERIOUS motorhead for 45 years and all my turbo motors are free spool units.. I don't run wastegates... I limit boost and detonation with water injection and the throttle... Those are land based toys though... The plane is naturally aspirated and is SO noisy inside I never heard it detonate.. As a disclaimer I was on a test run, as I usually run 1725f EGT but on this flight I pushed it to 1775f. No doubt the 300 hours of running over 1700f altered the aluminum properties of those racing pistons. I had another set built with thicker decks for added durability, and I now limit the EGT to around 1650f max..

The valves looked great and since it is a full roller motor, valve float at take off rpm's of 4400 is not gonna happen.. This failure was at 3100 rpm.

This Windsor motor has very small combustion chamber heads on it so a localized detonation event is a possibility.. At sea level it is a 10.4-1 motor but this failure happened at just under 10,000 msl.. That puts the motor in the 8.2-1 or so range.. That alone leads me to believe it was octane related... It was on straight mo gas labeled as 91.. Since then the plane gets a75% mo gas and 25% 100LL cocktail fuel blend for my piece of mind. I don't want to limp back to the airport on 7 cylinders again....

Moral of the story is..... Do NOT trust the labeled octane number displayed on the pump.

 

[timwinters]

most of the auto fuel suppliers use alcohol to gain higher oct ratings

No, Tom, they don't use alcohol for that reason, and it doesn't accomplish that purpose.

Ethanol does increase the octane rating. But you don't want it in your aircraft fuel system.

As Light and Sporty Guy pointed out, yes, ethanol does indeed increase the octane rating of MoGas. 2 points for 10% here in Mo.

Typically, here in MO, regular is 87, mid-grade 89 and premium 91.

Used to be, we got 87 at the pipeline. 10% ethanol was added and it became 89. All the stations (or, at least the smart ones) shut off their blender pumps and you got the same gas (89 octane) whether you pumped regular or mid-grade. You typically paid 10 cents more for mid-grade but it was the same gas.

Then, about 18 months ago, the petro companies started shipping "sub-grade" gas up the pipeline. It's only 85. So, now it takes the 10% ethanol to make 87 and everyone had to turn their blender pumps back on to dispense 89 mid-grade and their cash cow is on vacation.

 

[Sac Arrow]

I don't know why it's not. Most gas stations these days have "blender pumps" and only two storage tanks, regular and premium. When you pump mid-grade you're getting a 50/50 mix.

It may not be so exact to allow him to have confidence in a 90/10 mix but 80/20 would surely exceed the requirements and be more stable than straight mo-gas too.

Ethanol does increase the octane rating. But you don't want it in your aircraft fuel system.

Yes ethanol does, and it also affects the "effective octane" which can be higher than the rated octane, particularly in high compression, high rpm engines. Ron is correct in that the principle reason for adding ethanol is not to boost octane, however, ethanol is factored in the mix when rating fuels. It displaces higher cost and higher energy content petroleum fractions.

That's another thing to think about if you run an ethanol blend in your airplane, even if your fuel system can handle it. Your range will be reduced, and that reduction won't be reflected in your performance charts. It could be on the order of 10% or more.

 

[Skylane81E]

Ethanol does increase the octane rating. But you don't want it in your aircraft fuel system.

Very much so, all the local boost junkies love E85 as they can crank the boost and timming way up

 

[txflyer]

I always get a chuckle when someone refers to my 470 cubic inch engine that generates a whopping 230 horsepower as "high performance".

And in the same statement refer to a turbo'ed auto engine producing 3 or 4 times more horsepower per cubic inch than my airplane as "low performance".

(and turning 3 or 4 times the RPMs too!)

Yeah, what's up with that? I have a 470 too. My God, I just watched a youtube where they took a 500cu.in. Caddy motor and stroked it, flowed it, roller rockers and the works, and they got 600h.p. and almost that much torque at first turn of the key.

Why are these boxers so anemic?

 

[Tom-D]

No, Tom, they don't use alcohol for that reason, and it doesn't accomplish that purpose.

You are disagreeing with every distributor that blends alcohol to the Oct required.

You need to actually read what I wrote in my first post here, rather than spin it to fit your beliefs.

Originally Posted by Tom-D View Post
When you have an auto gas STC properly applied to your aircraft, there is no reason you can not mix 100LL and auto.

 

[MAKG1]

Yeah, what's up with that? I have a 470 too. My God, I just watched a youtube where they took a 500cu.in. Caddy motor and stroked it, flowed it, roller rockers and the works, and they got 600h.p. and almost that much torque at first turn of the key.

Why are these boxers so anemic?

The low RPM gives you a factor of at least two immediately.

That Caddy engine isn't making 600 HP at idle, nor at 2700 RPM (a somewhat high design cruise for a street car) even at full throttle. Maybe half that. Depending on peak power RPM, it might be a lot less.

And if they stroked a 500 CID engine, it was more than 500 CID when they were done.

 

[Skylane81E]

The low RPM gives you a factor of at least two immediately.

That Caddy engine isn't making 600 HP at idle, nor at 2700 RPM (a somewhat high design cruise for a street car) even at full throttle. Maybe half that. Depending on peak power RPM, it might be a lot less.

And if they stroked a 500 CID engine, it was more than 500 CID when they were done.

Bingo, HP is a factor of RPM. Hold the RPM down to a reasonable speed for prop efficiency and you loose power. This is why we have engines with G prefixes and why rotax gets so much power out of such a small package.

 

[rbridges]

Bingo, HP is a factor of RPM. Hold the RPM down to a reasonable speed for prop efficiency and you loose power. This is why we have engines with G prefixes and why rotax gets so much power out of such a small package.

that's also why turbocharged 2 liter cars that make 300HP at 8000RPM aren't necessarily great at the 1/4 mile strip.

 

[Skylane81E]

that's also why turbocharged 2 liter cars that make 300HP at 8000RPM aren't necessarily great at the 1/4 mile strip.

My 2 liter car only makes 205 (says GM odds are more like 230) but does it at less than 7000rpm, good for mid 14s. Not bad for a stock Saturn.

 

[Henning]

Most of the kids get " liters " more than Cubic Inches these days.

"The O-470 is a 7.7L straight six cylinder that only puts out 230 horsepower at sea level." usually surprises the gear heads. Heh.

They ain't efficient. That's for sure.

They are more thermally efficient than most auto engines. HP/Displacement is not really a marker of efficiency; HP/Fuel is. Look at the RPM difference that a small car engine operates at vs an aircraft engine and what the HP values at those RPMs are.

 

[Henning]

Yeah, what's up with that? I have a 470 too. My God, I just watched a youtube where they took a 500cu.in. Caddy motor and stroked it, flowed it, roller rockers and the works, and they got 600h.p. and almost that much torque at first turn of the key.

Why are these boxers so anemic?

Because they aren't geared down to keep the prop at an acceptable and efficient RPM. If you ran a 2.5:1 gear box and flowed the same amount of fuel as they ran through that Caddy, you could make 600hp as well. The GTSIO-520 turns 3600 RPM IIRC and puts out up to 475hp.

 

[Henning]

I have been a SERIOUS motorhead for 45 years and all my turbo motors are free spool units.. I don't run wastegates... I limit boost and detonation with water injection and the throttle... Those are land based toys though... The plane is naturally aspirated and is SO noisy inside I never heard it detonate.. As a disclaimer I was on a test run, as I usually run 1725f EGT but on this flight I pushed it to 1775f. No doubt the 300 hours of running over 1700f altered the aluminum properties of those racing pistons. I had another set built with thicker decks for added durability, and I now limit the EGT to around 1650f max..

The valves looked great and since it is a full roller motor, valve float at take off rpm's of 4400 is not gonna happen.. This failure was at 3100 rpm.

This Windsor motor has very small combustion chamber heads on it so a localized detonation event is a possibility.. At sea level it is a 10.4-1 motor but this failure happened at just under 10,000 msl.. That puts the motor in the 8.2-1 or so range.. That alone leads me to believe it was octane related... It was on straight mo gas labeled as 91.. Since then the plane gets a75% mo gas and 25% 100LL cocktail fuel blend for my piece of mind. I don't want to limp back to the airport on 7 cylinders again....

Moral of the story is..... Do NOT trust the labeled octane number displayed on the pump.

And if you don't like the price of 100LL, you're gonna **** when you see the price of VP 104UL.

 

[SbestCFII]

You have the choice of running mogas in the 150 HP 0-320 engines but not the 160 HP 0-320s. You could run mogas in the PA28-151 or any of the Cherokee 140/150s, but the octane is insufficient for the 160/161 crowd and can result in detonation and, of course, the valve wear without the lead. I wouldn't buy a plane and then purposely destroy the engine to save a little money.

 

[Henning]

You have the choice of running mogas in the 150 HP 0-320 engines but not the 160 HP 0-320s. You could run mogas in the PA28-151 or any of the Cherokee 140/150s, but the octane is insufficient for the 160/161 crowd and can result in detonation and, of course, the valve wear without the lead. I wouldn't buy a plane and then purposely destroy the engine to save a little money.

Why would no lead cause valve wear?

 

[Skylane81E]

Why would no lead cause valve wear?

Because TEL isn't just there to boost the anti-knock properties of the fuel..

Oh, it is only there for that reason?


Never mind

 

[Henning]

Because TEL isn't just there to boost the anti-knock properties of the fuel..

Oh, it is only there for that reason?


Never mind

Exactly, lead just causes valve sticking.

 

[MAKG1]

Why would no lead cause valve wear?

The conventional explanation is "microwelds" between the valve and valve seat, leading to exhaust valve recession. It's negligible with hardened valve seats.

Some of us have dealt with cast iron heads from pre-1975 cars using unleaded fuel. The exhaust valves are always highly recessed, even with modest mileage. That is, until the seats are replaced with hardened inserts.