info on BMC 950cc tuning

[marjon]

going to try racing front engine form junior. So on setting the ignition timing,on a F/F 711M
the ignition timing is set at >38 deg @3000 revs, what would be the setting on a 950 early sprite engine with no vacuum advance.

thanks John

[Bob L.]

Always used 30 total on my 948, H cars.... 14-15:1.... more advance made less power on dyno.

Another point: you have to take some mechanical adv. out of distributor (most Lucas distributors have a 15 deg cam, 30 at crank) >>> reason: to have enough static adv. to get engine to refire when hot. Drop the mechanical to 10-13 (at distr.). You can do that by putting sleeves over posts (used copper tubing from hobby shop), or welding/brazing the arm.

Cranks are the most critical item in build. Give plenty of side clearance on rods (#3 usually first bearing to die). Have block bored for 3 cam brgs (only one brg. on all by late 948/Mk2 Sprite blocks). Proper prepped stock rods will work just fine...used to turn mine to 9.5K with confidence and zero failures.

Finally, if you want help building a decent engine, try Mike Miller at Comptune in SC. He has all the go fast stuff and background to help. The books by Dave Vizard are good references (those showed up after we found most of info on our own....info a bit dated).

Bob L.

[Brian]

It totally depends on the build spec of the engine. The last Formula Junior A-series engine I built had around 13;1 compression, a lot of cam (it was a custom Elgin grind, around 300 degrees duration), and a distributor that I built the advance curve on. It was dyno tuned and the advance was around 33 - 34 degrees. That will vary based on the fuel used, and air temperature, and a bunch of other factors. If I don't have a dyno handy, I use 32 degrees as normal on A-series race engines. Bob's advice on tuning above is very right, I was running less compression and probably less cam than he did.

As an aside, the engine I built was based on a 1070 Cooper S engine that I made a new rear main cap for. Given that I didn't have access to a line bore or hone and had to cut the cap to the block, it was about the most fraught and precise machining I ever had to do.

The reason formula ford engines can run such highly advanced ignition timing is mostly down to the cam timing in my opinion. Not at all the same thing as a purpose built race engine.

Brian

[rp]

i have a friend who has a lot of 948 sprite parts if anyone is interested.
rp