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MG Midget and Sprite Technical - Over Compression Problems Update: It lives!

Hello guys,

Firstly, thank you all for your suggestions and help in the last thread, it all came in very useful in understanding what is quite a complicated and delicate process. I measured the capacity of the head at 24 cc per chamber and after doing the mass calculated that with 2 head gaskets we should be operating with 10.8:1 compression ratio.

This should be sufficient with some tweaking and is certainly preferential to major machining work! Currently this is the action that has been taken:

- Added a 2nd head gasket making it 2 x 3.4 cc copper gaskets with high temp sealant in between and torqued down to 55nm (40lb/ft)

- Running 99 octane fuel with octane booster

- replaced distributor with another one I had lying around with an 18 degrees centrifugal advance (I take it that is what the number on it means). It also has a working vacuum advance on it though I am not sure how to optimise this.

- Timed with stroboscopic light from 14 to 32 degrees at 4500 revs. Have I got this right. I, like many am a novice to advance curves etc.

The results are as follows:

- The original pinking problem has ceased.
- Between 2500 and 4500 revs the car feels like its got a 5 litre v8 in it!
- noticeable power increase and notably more gutsy engine.

Problems:

- There is still a small amount of pinking when under full load between 2200 and 2700. This is not an enormous problem as the majority of the time there is no problem, but this is not right. Any suggestions on how to overcome this last hurdle?

- Also, the car does not particularly enjoy idling and often needs a blip of the throttle to stop it dying. Again any suggestions, is this just the 286 cam?

- On a side issue, how best can I optimise the vacuum advance?

Thanks again for your thoughts. When this car is fully useable I shall be sure to come to the next major meet so you can put a face to it!

Jonny
J Price

286 cam is probably going to want to idle around 1200rpm or so and will be "lumpy".

The proper solution to your current pinking is to re-curve the distributor to have less advance at that band of the rpm. can be done with springs, probably best done with a rolling road. Could be a good application for the 123/Mini distributor as it probably has a curve to suit. Meanwhile, I would suggest backing off the distributor a bit to avoid pinking altogether even at the cost of some performance at other rpm. Pinking is bad for the engine!

Vacuum advance is not a major tuning issue. You want to be sure that the vacuum advance module on the distributor in question is connected to the vacuum source that it expects. IE, it might want port vacuum off the side of the front carb near the throttle plate or it might want manifold vacuum. The two have to agree.

The 18 on the mechanical advance arm in the dizzy indicates the number of cam degrees (double it for crankshaft degrees) it allows. Might be a bit much for this application.
David "a little dizzy" Lieb
David Lieb

I'd expect there is still time to fit a distributor with the correct curve for the cam before you put a hole in a piston.

David is right you have 32 degrees of ignition advance and the cam wants less.
Daniel Thirteen-Twelve

I wouldn't want 32 degrees of advance @ 4.5k. No more than 30. In fact I timed my race engine up to 28.

Please keep us informed how the two gaskets trick works long term. I've never seen it actually done...
Max T

The cam actually wants more!
It is quite happy at 32 degrees *full in* as you have said what the problem is however is that the advance is too much at lower revs. Thus if the dissy had MORE advance then you could start at say 10 degrees at idle and still finish at the holy grail of 32 degrees at maximum advance. Thus effectively the advance in the 2.5K range would be also less.

But considering everything at the moment I would knock it back to probably 30 degrees at max, thus hopefully this would back off the advance at 2.5 and should stop the pinking. This will not lose you much in performance but may make the tickover a little worse.

Most engines I have had with a 286 fitted have needed to have idle speed of no less than about 1000 to 1100RPM.
I believe your calculation must be a little out as with only that amount of C/R and 99 fuel WITH OCTANE boost you should NOT have a problem with pinking!

I am pleased however that you like the performance, a sign of plenty of SQUEEEEEEZZZZZEEEE..
Bob (robert) Midget Turbo

My dyno experience is that a 286 cam doesn't want more than 32 degrees - at least not to produce more power. In fact it produces full power with a lot less.

The problem here with pinking is about where I'd expect the distributor to be at full dynamic advance so the engine needs a lot less advance.

Retarding the ignition by swinging the distributor is simply moving the curve and thus moving the problem and that's if it can be moved enough.

It's worth pointing out that pinking and detonation are not the same thing. You can hear pinking you can't here detonation. On the dyno the power readout will show if yo have detonation.
Daniel Thirteen-Twelve

I did not say it wanted more, I said the dissy needed more so that the lower rev advance would effectively become less. Your going to have to be more specific Daniel.
The engine needs between 28 and 32 degrees of maximum advance to be best. I do agree the difference between 28 and 32 is minimal but it does need it to be absolutely perfect. However if the dissy does not have alot of advance then it needs a lot of static to get up to 28/32 degrees at higher revs. Then you end up with too much at low to medium revs and experience detonation/picking to my way of thinking I can always here it and the results are always the same call it whatever you like. :-)

Here is a case in point that this engine is pinking at 2 to 3K RPM and therefore is too far advanced at that figure, the answer as always is to have MORE dynamic advance as you call it so that 32 degrees can be attained with a lower advance figure at low and medium RPM. I do not see what is difficult with this explanation?
Bob (robert) Midget Turbo

>>>'The cam actually wants more!'
I can see how Daniel thought you said 'the cam wants more'....
David Smith

Yes I can see that Dave which is why I tried to ask Daniel to be more specific.
The whole thread changed slightly from how to stop detonation/pinking at about 2.5KRPM to one of what Daniel suggested as how far does the engine need to be advanced with full dynamic advance.

I like Max have run a couple of race engines at 28/30 degrees advance simply to run the engines "safe" and the result was about 1 BHP if that from a normal figure of 32 so why not? Thus I tend to agree with Daniel that 32 degrees is not absolutely necessary for engines although in a normal road car the extra advance at the bottom end and especially tickover can be helpful.
Bob England

Thanks for your comments.

Do you reckon that backing the distributor off a bit to get rid of the pinking (something that has been achieved before) is the best course of action to take? This is certainly the safer option and will result in only a negligable loss of power.

Also, as a side point, Does anyone know which timing tooth (on the timing cover) represents TDC when you have five of them?

I will certainley keep you informed of how the double headgasket works in the long run!

Jonny
J Price

Apart from the first pointer which is TDC the rest are all BTDC in 5 degree divisions. Looking from the front the engine spins Clockwise and so the last timing mark, that the mark on the pulley lines up with will be TDC. When these are positioned on the bottom of the timing cover it will be the one to the left as viewed from the front. Some timing covers have 3 points TDC, 5 and 10BTDC Whilst some have 2 further pointers which are 15 and 20 BTDC

In my opinion if you can back the overall timing off to no less than 29/28 degrees and stop the mid range pinking then that to me would be your best solution.

I am not confident of the 2 gasket method but who knows. :-)
Bob (robert) Midget Turbo

I'd definitely knock the timing back first. No disrespect Jonny, but when CR is that critical it's easy to have made a mistake in measurement or calculation. Therefore the first thing I'd do would be to retard the engine a little (it's free, it can easily be re-set etc etc) to see if it makes any difference.

THEN you can start playing around to try and restore the ooomph, but you've got some knowledge as to whether the pinking can be made to go away.
Max T

I will definately back it off a bit. The pinking is barely audible even without the boost so I reckon backing it off, running it in and then see where we are in 500 miles time. Worst comes to worst I'll put dish pistons in!

On the double head gasket: The car has been running for a week now and has done about 200 miles with no signs of any problems. Only the timing issue. I'll let you know if and when it blows but I'm quietly confident!
J Price

Just so you all can have a good laugh, back in the heady 70's I used to run the Frog on a 13.5/1 compression ratio. Of course the poor thing was plagued with all manner of terribe symptoms - overheating,mid range detonation and chronic running on being favourite. After melting a piston I took advice from Special Tuning and rebuilt the engine with 2 head gaskets - and carried on. The twin gaskets failed after about 1000 miles, so I had a special thick gasket made in the experimental workshops at work. Looked marvellous and that lasted a little longer but eventully burnt through between 2 and 3. Then the engine was detuned radically - the 649, solid rockers, flat top pistons and race head were changed for a spec similar to yours. And the car was far more drivable although with a heavy right foot a faint rattle was still there. So I learnt to ease up and not give it the beans at 3500rpm and lived with it. The one thing that remained largely unaltered throughout all of these escapades was the distributor. It was originally modified as per Clive Trickey with Cooper S points, and bob weights and springs and no vacuum advance. I had it checked on a distributor machine and the numbers all looked good, but still the advance felt wrong somewhere. After trying various settings on the car I settled for so much initial advance the thing would struggle to fire up at all. So I used to trick it by pulling on the starter and when it was all whizzing round hit the ignition. Then I changed it for a new Aldon distributor and the difference was a revelation. It would start immediatly, idle at 950 rpm, accelerate faster with no rattles and not run on - not ever. Goodness me I could hardly believe it. The point of this is getting a distributor with the right curve for your engine is all important, and although getting the idle or full advance can be achieved with any distributor it's actually the bit in between that really matters.
f pollock

Right at the end of this thread you announce 'without boost'. Did I miss something earlier? Is this a turbo engine? if so, doesn't the turbo distributor retard under boost?

F Pollack - what you say is what I've been saying all along. I've stayed away from this thread as I'm: busy, lost the distributor plot curve for the old distributor but known advance and while I have the distributor plot for the new distrib and engine it's not yet been run on the rollers.

It did occur to me that with the wrong advance curve in the distributor it could indeed be back off enough to run ok but that starting might be interesting.
Daniel Thirteen-Twelve

This is why I'm going to megajolt - you can write whatever advance curve suits your mechanical spec right into the control unit's map. No fussing about with springs or sticky advance weights...
Michael MacQueen

This thread was discussed between 06/08/2008 and 13/08/2008

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