Thread: A P38s' preferable octane

What CR number is on the block near the dipstick? If it has CR 8.36:1, it's low compression and 91 RON is fine.

Originally Posted by Sidewinder76

Hi all.

Which octane is best for a '98 4.0? 91, 95 or 98?

Thanks.

Check your compression ratio, its next to the engine number, next to the dipstick tube. If it's a standard 8.13 compression just use 91 octane but steer clear of ethanol containing fuels if you can.

Edit: As Ron said

Originally Posted by bee utey

f it's a standard 8.13 compression

I thought it was 8.13:1, too. In fact, I'd typed that then decided to check my engine(s). Here's a closeup of another engine and like the one above, it definitely reads 8.36:1

Odd.

In the Owner's Handbook LR recommends 91 as a "MINIMUM" RON for the low comp engine . I generally run 95 (and give up drinking red for a week to make up the difference in $$) There's less tendency for it to ping with the higher RON fuel, esp. on hot days.

I always run 95 in mine, had no choice but to use 91 at the Palmer river roadhouse, got better highway economy on it than I do on premium, however I did notice a much smellier exhaust. Mine's a 99 Bosch 4.6 (unsure of my compression ratio).

Cheers
Keithy

Always ran 95 in my 4.6 MY98. Went much better than on 91

Gary

Originally Posted by Hoges

(and give up drinking red for a week to make up the difference in $$) .

I saw an episode last year on a motoring show where they did a live test using the three types of fuel. As it turned out, the cars economy improved with the more expensive types so much so that the price difference was negligible. I would be interested in personal experiences...

I saw an episode last year on a motoring show where they did a live test using the three types of fuel. As it turned out, the cars economy improved with the more expensive types so much so that the price difference was negligible. I would be interested in personal experiences...

Horses for courses
A low comp no knock sensor engine will get no benefit from higher octane.

A high compression engine with knock sensors tuned for say 95 or 98 octane (eg BMW , VW ) will get significant benefit from the octane it was designed to use as the timing will be retarded using lower octane .

A clean Gems 8.36:1 or even Thor motronic 8.36 .1 should gain no benefit, although a carboned up Motronic may due to more advance being tolerated without Knock.
Regards Philip A