Octane ratings & performance
Discussion
OK, I know this is covering old ground but my searches through the forum has left me more confused than confirmed!
Sorry for all the questions below - any help will be most appreciated!
As I understand it, octane ratings on fuel basically explain how much the fuel can be compressed before it detonates. So, higher octane fuel should be used in higher compression engines.
Does running a higher octane fuel increase performance - I can't see how it would unless you tune the engine specifically for that fuel?
The RV8 (in particular the 4 litre which I'm buying) is not a particularly high compression engine, correct?
So, running 97 or 99 octane fuel - unless the timing has been set for it - is a waste of money and could cause problems.
Without knowing what the timing has been set to, what would you run in the car?
Also, would you alter the timing to run super or 99? If so, is this easy to do yourself with a timing strobe or better for a garage?
Sorry for all the questions below - any help will be most appreciated!
As I understand it, octane ratings on fuel basically explain how much the fuel can be compressed before it detonates. So, higher octane fuel should be used in higher compression engines.
Does running a higher octane fuel increase performance - I can't see how it would unless you tune the engine specifically for that fuel?
The RV8 (in particular the 4 litre which I'm buying) is not a particularly high compression engine, correct?
So, running 97 or 99 octane fuel - unless the timing has been set for it - is a waste of money and could cause problems.
Without knowing what the timing has been set to, what would you run in the car?
Also, would you alter the timing to run super or 99? If so, is this easy to do yourself with a timing strobe or better for a garage?
Hi Craig, my undrestanding of using high octane is that it allows you use a greater degree of ignition advance timing before the onset of pinking (engine knock) becomes apparent. If you have access to a
strobe light then it is easy to adjust ign timing, you will need a 9/16" spanner for the dizzy clamp.
It is adviseable to check that the pulley wheel timing marks are where they are meant to be, as some
owners have found errors with this. Alternatively, you can use a trial and error approach by tweaking
and test driving to listen for pinking, and setting at just before the onset of pinking as many owners seem to do.
When I first borrowed a strobe I found the timing to be quite retarded (2 degrees atdc IIRC) at tickover,
perhaps Fernies, where I bought the car, wanted the newbie TIV owner to be broken in gently with a tamed performance. HTH
strobe light then it is easy to adjust ign timing, you will need a 9/16" spanner for the dizzy clamp.
It is adviseable to check that the pulley wheel timing marks are where they are meant to be, as some
owners have found errors with this. Alternatively, you can use a trial and error approach by tweaking
and test driving to listen for pinking, and setting at just before the onset of pinking as many owners seem to do.
When I first borrowed a strobe I found the timing to be quite retarded (2 degrees atdc IIRC) at tickover,
perhaps Fernies, where I bought the car, wanted the newbie TIV owner to be broken in gently with a tamed performance. HTH
macdeb said:
SILICONEKID340HP said:
Weird thing is i can`t tell any difference between supermarket uleaded to the high octane stuff ..
Daz, the latter is nore expensive 
MAC don't start it him off on fuel prices FFS at least your getting out in yours Daz 
meanwhile I am stuck at work Polybushing a 67 Triumph Spitfire 
Edited by Simon says on Tuesday 5th June 15:14
I would not try the listen for pinking test- too damn noisy TVR's to pick up minor knock with the ears. I even tried wiring a knock sensor into an amplifier and headphones (A knock sensor is just a tuned microphone) but still could not pick it up- just odd looks from the locals as to why I was wearing headphones driving up and down the road!
fausTVR said:
It is adviseable to check that the pulley wheel timing marks are where they are meant to be, as some
owners have found errors with this.
Yeah, that's what worries me! What's the easiest way to check this? I know how to use a timing stobe, but I assume checking the timing marks means establishing if the engine is ACTUALLY at TDC when the marks are aligned.. which I'm unsure of the best way to do.owners have found errors with this.
Q: Does running a higher octane fuel increase performance?
A: No
Q: Does running a higher octane fuel increase performance if you adjust your ignition timing to make benefit of the higher octane fuel?
A: Yes
Q: Can I run higher octane fuel in my car with the ignition timing is set for lower octane fuel?
A: Yes (but you will see no performance benefit)
Q: Can I run lower octane fuel in my car after I have adjusted the ignition timing for higher octane fuel?
A: No (You will promote detonation and engine damage may follow)
Q: How can I run both lower & higher octane fuels without adjusting my ignition timing?
A: Set it for the lower octane fuel or run a much more modern ignition system that utilises a piezo crystal knock sensor that delivers a "Retard Ignition" signal to the ECU when detonation is detected
In the case of a standard distributor system you really need to choose what fuel you want to run and then set your ignition timing to suit.
If you are running your timing at a level that works best with the lower octane fuel and expect a gain in performance from simply adding a higher octane fuel you will be wasting your money.
Set your timing to extract the benefits of a higher octane fuel and you will notice an improvement, just don't put a lower octane fuel in or you are asking for trouble.
The bottom line is our cars just aren't sophisticated enough to alter the timing on the fly to compensate for changes in octane rating.
In the 4.0 litre Rover V8 my advice is set your timing at 32 degrees at 3800 RPM with the VAC advance disconnected.
Then run it on 95 Ron and save your money.
At the end of the day, it's a pretty low compression engine by modern standards.
Please note that setting your timing based on audible detonation is a highly risky practice.
Maximum power & economy are extracted from the fuel a few degrees before the onset of detonation.
The onset of detonation is inaudible unless you are a Mouse or a Dog.
A: No
Q: Does running a higher octane fuel increase performance if you adjust your ignition timing to make benefit of the higher octane fuel?
A: Yes
Q: Can I run higher octane fuel in my car with the ignition timing is set for lower octane fuel?
A: Yes (but you will see no performance benefit)
Q: Can I run lower octane fuel in my car after I have adjusted the ignition timing for higher octane fuel?
A: No (You will promote detonation and engine damage may follow)
Q: How can I run both lower & higher octane fuels without adjusting my ignition timing?
A: Set it for the lower octane fuel or run a much more modern ignition system that utilises a piezo crystal knock sensor that delivers a "Retard Ignition" signal to the ECU when detonation is detected
In the case of a standard distributor system you really need to choose what fuel you want to run and then set your ignition timing to suit.
If you are running your timing at a level that works best with the lower octane fuel and expect a gain in performance from simply adding a higher octane fuel you will be wasting your money.
Set your timing to extract the benefits of a higher octane fuel and you will notice an improvement, just don't put a lower octane fuel in or you are asking for trouble.
The bottom line is our cars just aren't sophisticated enough to alter the timing on the fly to compensate for changes in octane rating.
In the 4.0 litre Rover V8 my advice is set your timing at 32 degrees at 3800 RPM with the VAC advance disconnected.
Then run it on 95 Ron and save your money.
At the end of the day, it's a pretty low compression engine by modern standards.
Please note that setting your timing based on audible detonation is a highly risky practice.
Maximum power & economy are extracted from the fuel a few degrees before the onset of detonation.
The onset of detonation is inaudible unless you are a Mouse or a Dog.
Edited by ChimpofDarkness on Tuesday 5th June 18:23
CraigAuk said:
fausTVR said:
It is adviseable to check that the pulley wheel timing marks are where they are meant to be, as some
owners have found errors with this.
Yeah, that's what worries me! What's the easiest way to check this? I know how to use a timing stobe, but I assume checking the timing marks means establishing if the engine is ACTUALLY at TDC when the marks are aligned.. which I'm unsure of the best way to do.owners have found errors with this.
SILICONEKID340HP said:
What you can do for power is to loose the vac pipe then set your timming at 3000rpm ..More power but uses more petrol and crap mpg ,been down that route .
The vac advance does not influence max total advance so best left connected for improved MPG and low speed throttle response 
I have yet to see a Serp RV8 where the timing marks are off 
and if they are the outer pulley as crept on the rubber insulator (harmonic balancer) so its scrap anyways 
Edited by Simon says on Tuesday 5th June 20:32
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