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Fuel Question

W

WobbleWobble

Diamond Member
So I recently bought a 1999 Honda Prelude, great condition and I have a few questions about fuel.

Do I really need 91 octane as specified in the manual? What are the consequences if I use regular 87 octane?

I have a few places to buy fuel from.

Shell, Esso and Petro Canada have 87, 89, 91 but Chevron has 92 instead of 91.

Husky/Mohawk have 90, 92 and 94, but with 10% ethanol. Their 90 is the same price as 87 at other places.

Is the 90 octane with 10% ethanol good enough? My manual says I can use up to 10% ethanol blend, but is it truly rated 90? Or does blending it with ethanol artificially inflate its octane rating?
 
Does the manual say "91 octane required", or just "recommended"?

Either way, use the specified grade. You probably won't damage your engine by using a lower grade, since the knock sensor will retard the timing, but you'll lose power and in general won't be using the engine to its full potential. If you're too cheap to use a higher octane, buy a lower-performance engine.😉

Yes, ethanol does increase the octane rating. If the octane rating is 90, with 10% ethanol, then if you were to use the same gasoline blend without ethanol it would be lower octane. The rating you see on the pump is the true rating, after all factors have been accounted for.

Just get the 92 or 91. It's only a few cents more, and your engine will thank you. If you *have* to save your pennies, get the 90 octane with ethanol over the 87...but I really think you should get the higher octane stuff.
 
I'll have to double check what the manual says.

I spend about $200 on gas a month, so going from 87 to 91 will cost me about $360 more in a year.

The 90 with 10% ethanol seems like the best option as it's the same price as regular gas elsewhere, but would I be losing performance using an ethanol blend? Would I be losing fuel economy over non-ethanol blended 90 octane?
 
Again, if you're concerned about the cost of fuel, you shouldn't buy a car with a higher-performing engine😕

IIRC ethanol blends do yield slightly lower fuel economy than regular gasoline. You could try it yourself, of course...fill a tank of 91, calculate your MPG at the next fillup, fill a tank with 90+ethanol, and calculate your MPG at the next fillup, then take it from there. Ethanol *may* end up costing you more per mile than 91, once you factor in fuel efficiency.
 
IIRC the Compression Ratio in the Prelude's H22 is 10.0:1 (for the SH I believe it is 10.6:1)...that's not super high like some smaller Honda motors, so if you really wanted to take a small performance hit I'd say you could run regular 87 in it without too much of a problem. The ECU should automatically adjust timing, etc if it detects that you are using regular so that knocking does not occur.
 
The Husky 10% 90 for the same price is the best option.
By going to 87 you might get engine knock which will damage your car over time.
Then again you might not 🙂
But I wouldn't pay for the premium, any mid-grade should do if your not a hard onvehicles kind of driver.
 
If the octane is too low, the engine will knock and the computer will retard the timing to prevent this. This will rob the engine of power. As long as there is enough adjustment in the system to prevent knocking you won't harm the engine.
 
Not only will you have less power but lower fuel economy as well which means you will need more of the 87 than the 91.
 
I actually didn't know that it used premium fuel when I bought the car. I bought it from my aunt/uncle who kept it in great condition, had low KMs and was serviced regularly by the dealer. I looked in the manual and it said that I could use lower octane fuel if I can't find 91, but I would lose power.

Now I'm wondering, would running 90 with 10% ethanol cause me to lose enough fuel efficiency that it would have been cheaper to use normal 91 octane? It's about 15% more expensive, but will it give me 15% better fuel economy than the ethanol blend?

Also, will using 94 octane give me any benefit? 😀
 
Originally posted by: WobbleWobble
I actually didn't know that it used premium fuel when I bought the car. I bought it from my aunt/uncle who kept it in great condition, had low KMs and was serviced regularly by the dealer. I looked in the manual and it said that I could use lower octane fuel if I can't find 91, but I would lose power.

Now I'm wondering, would running 90 with 10% ethanol cause me to lose enough fuel efficiency that it would have been cheaper to use normal 91 octane? It's about 15% more expensive, but will it give me 15% better fuel economy than the ethanol blend?

Also, will using 94 octane give me any benefit? 😀
Click to expand...

A 10% Ethanol blend shouldn't cost you the full 15% difference in mileage.

If the engine is set up for 91 octane, 94 octane will give you zero benefit.
 
Originally posted by: Tarrant64
Putting 87 in a Mazda that requires 91 or higher will kill the car.

So I'd say if it says 91, do 91.
Click to expand...

Either the electronics were failing, or this is just flat out wrong.

Higher octane fuel is needed for high compression engines that need the higher octane to prevent detonation (pre-combustion) in a cylinder. Even if you use crappy fuel, a knock sensor will detect pre-detonation and retard the timing through it's electronic distributor to prevent the knock.

It will cause no permanent damage to the car, albeit low gas mileage and reduced performance.
 
Originally posted by: Nyati13
Originally posted by: WobbleWobble
I actually didn't know that it used premium fuel when I bought the car. I bought it from my aunt/uncle who kept it in great condition, had low KMs and was serviced regularly by the dealer. I looked in the manual and it said that I could use lower octane fuel if I can't find 91, but I would lose power.

Now I'm wondering, would running 90 with 10% ethanol cause me to lose enough fuel efficiency that it would have been cheaper to use normal 91 octane? It's about 15% more expensive, but will it give me 15% better fuel economy than the ethanol blend?

Also, will using 94 octane give me any benefit? 😀
Click to expand...

A 10% Ethanol blend shouldn't cost you the full 15% difference in mileage.

If the engine is set up for 91 octane, 94 octane will give you zero benefit.
Click to expand...

Are you sure? I don't know about 99 but current Honda engine's can advance the timing all the way up to take advantage of 100 octane race gas. At least that is true of the accord derived 2.4 I4 in my TSX.
 
http://www.rx7club.com/showthread.php?t=505887

LS1 Rotary
Left Front lbs. 684 718
Right Front lbs. 695 718
Total Front lbs. 1379 1436

Left Rear lbs. 705 660
Right Rear lbs. 645 618
Total Rear lbs. 1350 1278
TOTAL Weight 2729 2714
% over Front 0.505 0.53
% over Rear 0.495 0.47

*my conversion deleted AC, PS and ABS.

now, remove all the same stuff on the 13b, as well as a better exhaust manifold and a single turbo(which will remove weight) and you will end up with a even greater weight distribution. as someone else put it later on. if your going to put as much money into replacing the 13b-re with the modifications to change to a ls1, you could have put it into a 13b, made it an even better car, with even more power and just end up generally lighter.
 
as your ecu will retard the timing till after the piston passes tdc for knocking/pinging running lower grade fuel will only cause less power, thats it. octane count only tells you how high compressability the fuel your using is. nothing more.
 
Originally posted by: Dman877

Are you sure? I don't know about 99 but current Honda engine's can advance the timing all the way up to take advantage of 100 octane race gas. At least that is true of the accord derived 2.4 I4 in my TSX.
Click to expand...

The TSX engine has a compression ratio of 10.5:1 , that can't make use of 100 octane regardless of any amount of advance timing. It could exceed 93 octane by a small amount ( say 95-96 octane at most ) but won't get close to using 100 octane.

A regular Honda engine will need even less octane so even with advance timing 93 octane would be a bit of a waste.
 
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