Retard settings for 5.0 litre on 95 RON?

[bhpjon]

Happy New year everybody!! Can anybody help me out with the correct retardation settings necessary for the timing to use 95 ron rather than the usual 98 as im going to czech in the summer and 95 seems to be the only regularly available option and the octane boosters are so pricey! cheers Bhpjon

[tahiti450]

Typically something like 6 degrees, but many (most?) TVR RV8s run quite happily on regular 95 octane unleaded without adjustment. Its only really going to be an issue if you - for some perverse reason - like slogging the engine at low revs in too high a gear, when you will get pre-ignition (pinking). You can provoke this with the standard set up on 97/98 octane, but really unless the engine is otherwise sick it will only do it if subjected to what I'd consider abuse. I have on several occasions had to use reg unleaded (no octane booster in the boot whilst in Eire for example) and frankly i've not noticed any pinking to speak of, but then I change down and let the thing rev.
High coolant temps will have a significant effect on the knock threshold also, so make sure its not running hot, ideally 75 - 80 but no more than 85. Most engines are set up from the factory with very conservative ignition advance (and so missing significant potential GGs)specifically in order to allow for the vagraries and abuse of the buying public, so used sensibly this shouldnt be a big issue.
Anyway if the worst comes to the worst and you get a bit of pinking on acceleration just back off on the throttle a bit... Equally in the unlikely event of pre-ignition at cruise you could always disconnect the vacuum advance (plug the hose tho) might hurt your part throttle economy a bit but, you won't melt pistons...

[GreenV8S]

The difference between 95 and 98 is quite small, it only needs a couple of degrees to compensate. But depending how your engine is set up currently, you may not need to adjust it at all. Best bet is to try a few tanks of 95 before you go, and see whether it pinks. It may only happen in hot weather, or when you get a particularly poor tank of fuel, or it may not happen at all.

If you do suffer from pinking, just ease off the throttle to stop it, and avoid full throttle afterwards until you can get the ignition retarded. You can easily retard the ignition yourself if you're willing to get your hands dirty, or any competant mechanic should be able to do it for you in thirty seconds.

[bhpjon]

Thanks for those thoughts guys, i have tried running on 95 before and it manifests itself in stalling when you go to tickover sometimes which doesn't happen with 97/98 but as you say 6 degrees is probably just enough to correct this (i understand that the 4 litre's are less temperamental than the 5's in their needs)

[GreenV8S]

6 degrees of retard is masses, it'll cost you a lot of power. I doubt you will need anything like that much.

[tahiti450]

GreenV8S said:

6 degrees of retard is masses, it'll cost you a lot of power. I doubt you will need anything like that much.

I think Peter is quite right, 6 is probably over the top. I'm surprised to hear that you are getting idle issues as i wouldn't expect a small octane change to affect the idle at all. Have you checked out the stepper motor, coil / ignition and for induction air leaks etc (if you are getting stalling from idle).

[rev-erend]

Is the tickover too low to start with ?

[bhpjon]

No i haven't checked any of the above as it has only been happening with the 95 but as soon as it comes back to me from the garage i shall look at all of the above .I've just read about the stepper and it's maintenance as my christmas reading was the bible cover to cover, as i said it was throwing me because it was only happening when i had to use 95 but maybe it's on the verge even with the 97/98! Thanks for all the tips chaps!