leaded to unleaded

Submitted: Thursday, Jul 24, 2003 at 08:45
ThreadID: 6144 Views:2307 Replies:6 FollowUps:1
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just after some information in regards to converting the old 83 swb mq to run on unleaded instead of leaded. From what I can understand it's only the valves that are the problem, burning them out or something, so does this mean that i can get the head off a 86/87 mq L28 engine and just change it out or do I basically just change out the whole engine. another question is about tyre and wheels from what i can read on my tyre placard i can change my tyres from original to 15x7jj rims and run a 10r15 tyre does this mean a 31x10.5x15 or does it mean a 31x10x15
any help would be muchly appreciated
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Reply By: Steve from Drive Systems Victoria - Thursday, Jul 24, 2003 at 10:48

Thursday, Jul 24, 2003 at 10:48
Correct, it is the valves and valve seats that will give trouble on unleaded. The tyre placard shows an old designation of "10R15" which has been replaced with 31/10.5x15. It is the same size tyre for all intends and purposes. Given the 2.8's torque characteristics, it is about as large a tyre you should run for comfortable, economic motoring. So, provided you are not following in the wheel tracks of a big block beast running 38" mudders, they will do everything you will want in 4x4 driving.
AnswerID: 25761

Reply By: Member - Willem- Thursday, Jul 24, 2003 at 10:57

Thursday, Jul 24, 2003 at 10:57
I rebuilt my 1978 G60 engine(4 litre) at 380,000km( with the help of a qualified mechanic).The pistons and rings were replaced and we also fitted new bearings.

We put the head through an acid bath, machined it and then changed the valves to suit unleaded petrol. The mech also fitted new valve stem liners and valve springs.

I have been running on unleaded now for the last 70,000km and the motor runs sweetly. You should get out of it for around $400 including a gasket kit if you take the head off and refit it yourself. My total engine rebuild was $900 with me doing most of the lackey work.

Cheers, Willem
Never a dull moment
AnswerID: 25766

Reply By: howesy - Thursday, Jul 24, 2003 at 19:51

Thursday, Jul 24, 2003 at 19:51
Take the head offf and put it in for a full job and tell him to make it an unleaded. The unleaded heads have hardened valves and valve seats and they also have bronze guides and run more clearance (valve to guide) the reason for this is that lead and its substitute apart from having an affect on the "ron" rating, also lubricated the valve and stem. Without the lead to lubricate the stem you can get sticking valves, hence the slightly larger clearance and the use of bronze guides. The absence of the lead coating the valve and seat also is why it leaves them prone to burning out valves and valve recession. Thats why they are hardened items in an unleaded head. Depending on the compression ratio of your donk you may have to run premium unleaded if the car pings as modern unleaded motors run lower compression but if your motor is older this may not even matter. On an environmental note , to run unleaded fuel without a catalytic converter puts considerably more cancer causing nasties in the air so dont let your kids go sniffing the fumes. It's an easy job, go for it. Happy motoring.
AnswerID: 25829

Reply By: Member - Oskar(Bris) - Thursday, Jul 24, 2003 at 21:06

Thursday, Jul 24, 2003 at 21:06
Does any one have any experience with the "Prozone" type fuel catylyst. They promise that no damage will come to a leaded engine if you switch to unleaded with one of these gadgets??
I had a student last year who fitted one to his HQ holden and after one tank of super with the thingo he ran it exclusively on ULP thereafter. He was very happy with it.
They claim fuel savings and more power (of course).
For about $130 it might be an option.
AnswerID: 25843

Reply By: Member - DOZER- Thursday, Jul 24, 2003 at 21:08

Thursday, Jul 24, 2003 at 21:08
I would convert it by pulling up at the unleaded bowser and trying to run it on some first. Unleaded doesnt do too much good to the carbi rubbers/seals, especially if it has Ethanol in it. Compression ratios were probably reduced in the unleaded motors aswell. Best to get one of them from a Skyline/280z.
Plenty of motors around to convert that Datto, i believe a 3.0 VL motor will fit to the 2.8 gearbox aswell, so maybe you can source a wrecked VL 5 speed and get all the stuff you will need to do the deed:-) Injection will give you some economy back too.
I ran a L300 on unleaded for 18 months before selling it, and it didnt need a thing done to it, so its up to you, i dont think the super you are buying has any more lead than the unleaded does.
Andrewwheredayathinkwer mike?
AnswerID: 25844

Reply By: Hugh - Friday, Jul 25, 2003 at 01:01

Friday, Jul 25, 2003 at 01:01
All the comments above are correct for engines designed originally to run on leaded fuel only. Many pre '86 imported Japanese vehicles used leaded fuel due to lack of unleaded supply but were actually designed for unleaded fuel which was available in Japan along time before Oz. In other words they already have hardened valve seats, etc.

The following link is a useful guide to find out if your lead fuelled pre '86 engine will be OK on unleaded fuel.


The MQ L28 is safe to run on unleaded fuel without modification. One thing to be mindful of though is that unleaded fuel has a lower octane rating that leaded fuel (RON ~ 91 for unleaded c.f. ~ 96 for leaded). This means that ignition advance may need to be retarded slightly when switching to unleaded. You'll notice the engine pinging under load if too far advanced i.e. combustion detonation. This may not occur because a lot of Japanese imported vehicles maintained the spark advance settings for unleaded fuel, even though they used leaded fuel.

I hope this helps and saves you an engine change.

Regards, Hugh
AnswerID: 25867

Follow Up By: mq swb - Friday, Jul 25, 2003 at 10:33

Friday, Jul 25, 2003 at 10:33
Thanks for that Hugh, just checked out the web site most excellent that should save some money and a head overhaul, with regards to timing mine is already advanced a little so I don't go from 110kph down to 80 kph up hills so some retarding is due. and once again many thanks for all the help from yourself and all the others who responded.
FollowupID: 17569

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