Leaded fuel and pump petrol

owene, Sep 24, 12:10pm
I have a vehicle running a 350SB chev which was last running 10 years ago on leaded fuel. Can anyone explain what's involved to make it safely run on unleaded or is it likely to be easier to simply chuck the lead additive in each time I fill it (it;s a fun car so will be doing bugger all miles). It is running fairly low compression, from memory something like 7 or 8:1. Is this merely a valve replacement excercise or is more involved!

morrisman1, Sep 24, 12:18pm
I dont have any experience with that chev engine but in the morries you could just run unleaded no problems. I did for 4 years in a daily driver and discovered no issues.

If you still wanted wanted lead in your fuel you could go to the local aero club with a 20L container and fill it up with Avgas 100 (octane 100 under lean and 130 under rich conditions) but it has a lot of lead in it, apparently much more than what the old leaded fuel had.

If you put a litre or two of this in each tank it should see you right.

owene, Sep 24, 12:24pm
Yep that's interesting as I run avgas in a sporting type engine but as its 100 octane may cause other probs. The Chev motor came out of a 76 Corvette and was from the days when emission reduction was at its best thus the very low compression. I guess worse case scenario, it might shag the valves/seats if I over-work it but then I'd be forced to fix the issue anyway. Is it only the valves/seats that this leaded or unleaded problem relates to!

morrisman1, Sep 24, 12:38pm
as far as I know its just a valve seat issue.

A litre in each tank will not cause any issues with fouling plugs but if you were to run 100% avgas yes it probably would cause a bit of trouble in a lower performance engine. I have read that a few people find that their aircraft engines run better on pump gas rather than AvGas which is interesting, I think it was the cessna 152 in question which only has about 110hp out of a 235ci 4 cylinder with compression ratio around 7.5:1. AvGas is the fuel it is because it needs to run in some pretty high spec engines, like supercharged radials which can run up to 60 inches of mercury continuously (about 15 psi at sea level) with minimal risk of detonation and also low chance of fuel vapourising at high altitude.

In short, it is not really a road going fuel and is more suited to high compression/boost engines which run in those conditions for long periods of time. Bring on the good diesel engines for aircraft!

owene, Sep 24, 12:44pm
Yep, but certainly food for thought - many thanks.

pettal, Sep 24, 6:52pm
Throw a splash of two stroke oil in ya tank every now and then to help provide the valve seat lube .

clark20, Sep 24, 9:12pm
As stated above, just a small amount of race gas on every tank will do it. Lead stays on the seats for a while, however if you give it a valve grind then its all gone, and problems are after that. It "may" already be built for unleaded (hardened vavle seats)

owene, Sep 25, 6:10am
Yep, it was a UK-new motor and they still had lead when I bought the thing out of there in 2002. But I guess that worse case I may end up buring a few valves if I overdo it here. Certainly some AvG now and then will be going in.

dave653, Nov 23, 1:45pm
I fitted a Fuelstar (wait for it.) in '96, removed the head in '08, the valves were as good as the day I fitted the new head in '90. High comp Holden 179. Stopped useing that valvemaster shit!