23 replies to this topic

[alpder]

Getting pinking on a recently built engine. 1310 A+ / 12g940 lead-free / HIF38-red spring / Kent 246 - which I believe is not much different from an MG Metro cam? The engineer who did the rebore and skim told me he would aim for 10.5:1 (and as a complete noob I said "whatever" - should've said aim for 9.5 or 10 I guess.)

 

I can drive like a granny, and get no pinking at all. So if the only answer is head off then I can leave the fix until salt-on-the-roads winter-time. Right now just want to get some ideas about what's the best answer, please.

 

Symptoms are: floor it coming off a roundabout (especially uphill or when towing)... and when the rpms hit 3500 the pinking starts just like a switch has been pressed and continues all the way up the rev range. Ease off slightly and the pinking stops. Floor it again and after a second or so (the delay caused while the carb piston lifts?) the pinking starts again.

 

What are my options...

Better fuel... doesn't seem to make any difference using E5 or E10.

 

Change ignition timing... already tried a 2-degree retard and makes no observable difference.

 

Try to richen the needle at the thin end? (This needle was hand-made on a rolling road so it's not a case of 'just' buying a different needle.)

 

Have the distributor curve changed or buy a more appropriate distributor?

 

Or is the only possible answer to have the head off and increase the volumes?

Edited by alpder, 02 June 2025 - 09:26 AM.

[68+86auto]

What is the current timing and advance curve?

[ACDodd]

Too much compression. Artificially retarding the ignition to ‘get around that’ is futile, it looses more power than having compression ratio a little too low and using more ignition timing.
That said, what ignition timing are you using across the reve range, maybe you are just using way too much anyway?

Ac

Edited by ACDodd, 02 June 2025 - 09:40 AM.

[alpder]

Sorry - should have said. I believe it's the original Metro 65DM4. Mileage unknown. Very unlikely to have been modded or swapped-out - the rest of the engine was standard and equally "patina'd" with a layer of oily muck when acquired.

 

Timing was set by the rolling road crew. When I got home I checked it with a basic strobe at idle (no vac) I saw they'd set 8 BTDC. I can't measure rpms but picking up the throttle a smidge doesn't change that reading.

[alpder]

Too much compression. Artificially retarding the ignition to ‘get around that’ is futile, it looses more power than having compression ratio a little too low and using more ignition timing.
That said, what ignition timing are you using across the reve range, maybe you are just using way too much anyway?

Ac

Thanks AC... figured this was the most likely answer I'd be hearing :-(

 

Head off this winter, then. What CR do you reckon I should aim for, if E10 is my preferred fuel?

[ACDodd]

9.75:1 max.

Ac

[alpder]

Aim for 9.5 then.

 

A quick calc tells me I'd need +4cc (per cylinder). Is this easy to do in the chambers without - say - reseating the valves deeper or drastically changing the gasflow in the chamber? (Unfortunately - and I have no idea why - the engineer skimmed the block, not the head, when increasing the CR. I just went with what he said as a complete noob. So now just slapping a spare head onto the engine won't undo the change.)

 

I notice +4cc is almost exactly what I'd get with two gaskets. (Cue a fiery debate and possibly even my expulsion from the forum for just mentioning it...)

Edited by alpder, 02 June 2025 - 11:03 AM.

[ACDodd]

What pistons are fitted?

Ac

[alpder]

What pistons are fitted?

Ac

 

Nural 87-524111-00 ... which MiniSpares say is equivalent Rover TAM2050 / AE21963

[ACDodd]

Good news can redish the pistons, easier here to rehone the block, fit a new set of pistons with custom dishes to get the cr where you need it. 4cc is likely a bit too much to come out of the head.

Ac

Edited by ACDodd, 02 June 2025 - 03:56 PM.

[lsto]

Aim for 9.5 then.

A quick calc tells me I'd need +4cc (per cylinder). Is this easy to do in the chambers without - say - reseating the valves deeper or drastically changing the gasflow in the chamber? (Unfortunately - and I have no idea why - the engineer skimmed the block, not the head, when increasing the CR. I just went with what he said as a complete noob. So now just slapping a spare head onto the engine won't undo the change.)

I notice +4cc is almost exactly what I'd get with two gaskets. (Cue a fiery debate and possibly even my expulsion from the forum for just mentioning it...)

I know it's not the right way to do it but I have used the twin HG option in the past to reduce CR. It got me out of trouble for 6 months until I could afford to get a new head done and the car was fine with it.

[nichmoss]

There is a company called Cometic that make composite head gaskets in various thicknesses to reduce CR. They have a very good reputation in old Jaguar circles as many of the aluminium heads have been skimmed so a slightly thicker composite gasket is often specified. This may not be the most elegant solution (clearly, ACDodd gives the best advice) but it's a lot easier than stripping the block. Have a look at their website and you'll see A series gaskets in various thicknesses.

Good luck, Chris

[alpder]

Thanks everyone. For now, I've retarded still further - we'll see how that goes. And I'll keep the lead out of my foot until the car's off the road for its winter break. And fill with E5 when available. I had hoped for better news about head machining options.

First thing to do, clearly, will be to have the head off and measure the actual theoretical CR. Then I'll decide what to do from there.

 

If I haven't started (yet) another classic project by then, maybe I'll have sufficient 'thusiasm to haul the engine out of this one and extract the pistons for machining. More likely, I'll give a Cometic gasket a go.

 

Also currently waiting for a reply from Mick Tanski, regarding decompression plates. I wouldn't fancy fitting one against the block (I'll bet it's no longer absolutely flat around the studs - and taking it out for skimming would be no easier than taking out the pistons for machining). Maybe one could be fitted against the head, though.

[Earwax]

Just as another thought.   

 

 

  Octane booster could be worth a try. On our fuel out of the bowser here in Australia (98 RON 90MON) a CR of 9.75 works.   I also still advocate for the true fix , which is good CR and good timing curve, but I don't like the idea of detonating in the meantime.

 

I don't know if Boosters are legal or even which ones are snakeskin oil and which are as claimed. Some may also achieve the octane boost in a way our old engines don't like ( Oxygen enriched???) 

 

When Av gas was taken off our approved list in Historic racing here , there were lots of engines tuned to 100 octane Low lead AV GAS and I have to say the plugs running AVgas were things of beauty ( light gray and a hint of lube) and some changed to different fuels and used boosters until a more permanent solution was found.

 

NOTE AGAIN   I have done this on a race car and think I used VP racing octane booster ( long time ago) BUT  do your own research I dont want to steer you into a blown engine, It may be a silly idea 

[timmy850]

I tried everything for my mini:
Colder plugs
Mixture set by AFR gauge
Timing lowered at problem RPM
Octane booster + 98 octane fuel
Colder thermostat
Nothing helped remove the pinging at 3000rpm

Then I checked the compression ratio and the machine shop had set it at 11.4:1 (when I picked it up he’d said it was a bit over 10:1)
There are lots of solutions to lower the CR, as you’ve mentioned getting more volume from the head and/or pistons are the best, but as a trial I decided to use 2 head gaskets. It was instantly much better afterwards and I could add an extra 10 degrees at 3000rpm, and more at other rpm, and no more pinging. It’s now down to 10.4:1 which is manageable for the moment, and when I have the chance to have it off the road for a while I’ll get a more permanent fix done

Also worth noting that a decompression plate needs a head gasket on either side, so you need to factor in 2 x head gaskets + the decompression plate in your calculations