What Petrol.

[Highroller]

Hey Guys,
Now that i am getting my van back no doubt at some point I will need to put some petrol in it. After reading through some posts I have been left a little confused as to what type of petrol to use. Some say 95 ron and some say 97 ron, what would you say is the best for a 2.1 DJ on twin carbs? The engine has had a relatively fresh comprehensive rebuild.

[AdrianC]

The fact that somebody's downgraded your engine to carbs instead of the injection that it originally had shouldn't make a difference to the theory - the 10:1 compression in a DJ means 98RON. IF it's timed right, it'll run happily on 95, though, without pinking.

Assuming the rebuild didn't replace the DJ 10:1 pistons with lower compression ones, of course.
And that's on the original injection. On carbs? <shrug>

[1664]

I was under the impression that the timing had to be adjusted to run on 95RON although I admit this is just what I've read/heard over the years on 80-90.

[AdrianC]

I was under the impression that the timing had to be adjusted to run on 95RON although I admit this is just what I've read/heard over the years on 80-90.

Sorry, bad phrasing. I should have said that the timing can be set to run on 95 without pinking.

[Highroller]

The engine was rebuilt with the intention putting carbs on it. I may have have to check the timing to see what it's set for.

[mark]

you can set it to 5degs for 95ron and 10degs for 98ron, mine runs very well on bp super unleaded 97ron timed at 10degs......this is how I choose to run mine and is in no way a reflection on those that don't. this is an interesting read

mark
https://club8090.co.uk/forum/viewtopic.php?f=4&t=85055" onclick="window.open(this.href);return false;

[California Dreamin]

DJ's have a 10.5:1 compression and need the higher octane to prevent pre-ignition (premature detonation/pinking). Retarding the ignition by 5 degrees makes the engine run a little cooler but also lowers power output, although, by how much is unclear.
Premature detonation over a long period can be quite damaging to an engine, however, it is clear from the many DJ uses on here, that this particular engine is fairly robust when run on lower octane fuels with the necessary timing adjustments.

In direct reply to the question, 97Ron would be better for your DJ, it will allow correct factory timing settings and keep the 'pinking' at bay. That's not to say many use 95Ron very successfully with the slight adjustment.

And, at this point, lets shatter that age old misconception that higher octane equals more power.....the fact is the calorific value of both these fuel ratings are the same, it's just the higher octane has an increased resistance to detonation. Higher performance engines inherently have increased internal temperatures so the fuel is more prone to ignite before the designed spark ignition point......Interestingly, octane ratings as high as 150 were formulated for the Merlin & Griffon supercharged engines powering Spitfire planes in WWII, Engineers desperately trying to eak out every BHP advantage over the Messerschmitt 109's but at the same time trying to ensure the engines remained reliable.

Martin