91 octane or 98

Rwilloughby

Hi guys need some help i have a 83 B23e kjet engine and its a high compression motor 10.0:1 now im trying to figure out if its ok to run on 91 octane fuel or 98 octane is better for it

Ex850R

tony93

I ran 91 on my 89 240 without any problems
I put 98 in a few times didn’t notice any difference power or fuel economy wise
So I kept running 91 till I put the turbo Motor in now I’m running 98 at all times

egads (she/her)

It really depends on the car, I have had a couple of kjet 240s and one needed 95 + to not ping likened and one ran great with anything. I think @Spac did long term comparison across a few kjet 240s too and found similar variations

Ex850R

My kjet went like the clappers with 98,even better with 70% 98 +30% E85

Dauntless

The manual calls for 96RON or higher.

Spac

As the newest 240 is now 25 years old, there's multiple possible motor/cam/induction specs and very few of them are in excellent, totally unmodified condition, I don't think that anyone can make blanket statements that are useful.

Based on my past experience, I would run an 83 model 240 on United E10 (95 octane).
91 octane will almost certainly cause it to ping.

96 octane E0 would be worth considering if you are allergic to E10, but in my experience will just cost you more.
98 octane is likely to just be a waste of money.

Of course, if it has had the head skimmed multiple times in the past, it is entirely possible that the compression ratio is now high enough to need 98 to avoid pinging.
Occasionally you will find a car that has been ridiculously detuned to run on 91 - retarded timing and/or running rich. I have never seen this work decently - they always give up so much performance and economy that you would be better off tuning it properly and running 98.

Max

In my experience, lower comp motors use lower RON.
Lower comps 'burn' the fuel more than the explosion required for high compression.
I think of a lazy 5:1 bike I had once and how it had time to ignite all the fuel/air mix.

JakeNZ

I have a B23E, only petrol at pumps here is either 91 or 98...

I ping every other day if I drive up a hill on 98, need more octane!

jamesinc

JakeNZ;c-152785 wroteI have a B23E, only petrol at pumps here is either 91 or 98...

I ping every other day if I drive up a hill on 98, need more octane!

You might need to adjust your distributor if that's the case, it sounds like you might be too far advanced

Dauntless

jamesinc;c-152788 wrote
JakeNZ;c-152785 wroteI have a B23E, only petrol at pumps here is either 91 or 98...

I ping every other day if I drive up a hill on 98, need more octane!
You might need to adjust your distributor if that's the case, it sounds like you might be too far advanced

Yep, set base timing with vacuum advance disconnected. CO setting might need adjustment too.

JakeNZ

Yea I know it’s got problem, runs way rich too which I suppose dosnt help. The brass lines from the bottom of the intake manifold have been sealed and clamped shut by the previous owners mechanic or something, I think they are supposed to route into the vacuum system somewhere - so would mess up the vacuum advance.