Sprint Octane Requirement

[GrahamFountain]

As a consequence of the replacement tank, which came with a sender installed that I didn't want to replace in case it worked, I'm filling the tank a bit at a time to calibrate that replacement sender. As a result of that, I'm going to the nearest garage and using their BP 97 RON rather than the Shell or Tesco 99 RON I normally use.

Anyway, at 10 degrees BTDC static, and an unskimmed head, that's not giving any pinking I can hear.

So, does anybody have an opinion on what lowest RON we can use in relation to advance and skimming?

Graham

[Galileo]

Unskimmed head to my knowledge, but I've always just used super unleaded of whatever variety is available, which is not always, so occasionally a splash and dash of unleaded to somewhere that has, which is always noticeable.

[cleverusername]

I suspect the reason you're not getting many replies is because most are simply experimenting and setting the timing for what works for them.

Old four stars, which is a bit before my time, apparently had an octane rating of 98, which means you do need super unleaded for a Sprint. Personally I would am going to retard the ignition so I can get away with using the cheaper stuff.

However if maximum performance is what you want, buy the highest octane fuel you can find and set your car up to run on that. That is the beauty of older cars, it is really up to you how you set it up.

[tonybsa]

I use only Shell, or Tesco super unleaded if I can.
I have the timing at 8 degrees static.
Brand new unskimmed head fitted in 2008.
Car runs well, is quick, pulls cleanly, and ticks over nice-on points.
It will run exactly the same on 95 unleaded-but runs on all the time, and a touch hotter as well.
I suspect as the Sprint motor was designed in very early 70s,its actually designed to run on 5 star rather than 4?

[GrahamFountain]

I suspect as the Sprint motor was designed in very early 70s,its actually designed to run on 5 star rather than 4?

The conversion of the slant-4 to 2Ltr 16-valve config. was designed around 1970 by Coventry Climax (Harry Mundy), under the oversight of Charles Spencer King for the Specialist Division of BLMC. That was specifically so they could take the Dolomite racing in Group-1 (Series-Production Touring Cars), which needed 5000 in a year to qualify. Group-1 homologation only allowed/demanded modifications from the production spec. for safety.

So I reckon it was designed with little regard to what was sold in pumps, but only to what was available in the pit lane for group-1 use, and timing on the race cars set to whatever that would allow. That, careful assembly, blueprinting, and a bit of engineering to spec. limits, etc., presumably, accounts for how they got well over 150bhp on HS6s and a standard spec. cam and pistons. I believe there's a figure of 175 quoted somewhere, but I can't find it at the moment.

As I understand it, the Group-2 use (Touring Cars with "modifications aimed at making them better suited to competition"), which allowed different carbs, inlet and exhaust manifolds, and other mods from production spec, came later. I understand there were a couple of external firms developing the Group-2 (and Group-4 TR7) engines, and they gave around 250 bhp. That's stunningly close to what Costain-Duckworth's twin cam BDG engine gave, which was developed from the Kent, presumably on a much bigger budget (alloy block and such). But, as I understand it, Ford/Cosworth never made enough to gain Group-1 approval - the RS1800 was a Group-2 car, but re-homologated into Group-4 in 1977 as the 2ltr Escort RS, because of the changes the FIA made to the rules in Dec. 1975, with Appendix J to the International Sporting Code for 1976. That was mostly dropping the 100-off rule that had got some Group-2 modification approved on 100 kits of spares made available to buy, not production of homologation specials like the TR7 Sprints of 1977.

Graham

[tonybsa]

When you start messing about with octane rating it affects a lot more than timing. If you are running proper race fuel, or military spec Avgas(also used in stunt planes),you are looking at around the 130 octane mark, normal Avgas that I use in the race bike is around 105.

[GrahamFountain]

When you start messing about with octane rating it affects a lot more than timing. If you are running proper race fuel, or military spec Avgas(also used in stunt planes),you are looking at around the 130 octane mark, normal Avgas that I use in the race bike is around 105.

I don't think the FIA let Group-1 cars run on special fuels, even in 1974. But it probably could be the best premium petrol you might get at the pumps. Whereas the road cars would have to allow for the variations and opportunities for additional profits in a commercial distribution system without too much risk of the engine being killed.

Graham