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MG Midget and Sprite Technical - Benchmarking your CRs and Ignition timing

I'm running my 1380 (288 deg cam, flowed standard valve head) on a burette measured CR of 10.3:1. I'm getting 190psi of compression when hot. The head is not unleaded so I'm running with the correct quantity of lead replacement, and I'm using standard UK pump unleaded fuel.

Initialy I tried running a full advance of 34 deg (according to my ignition book), but it knocked at full load. Having read the midget dyno shoot out article in Safety Fast most peopel were nearer 30 deg full advance, so I've tried that, I still get some knock and it still runs on when I turn ignition off. If I back off the ignition further I lose (percieved) torque above 5500rpm.

The question is what are your typical values for Calculated CR, hot compression in PSI, static timing and fully advanced timing. Bottom line is do I simply have the CR set too high for modern fuels, or do I need to continue to experiment the advance curve?

Thank you. James

James Eastwood

hmm 12:1 cr with a 731 cam (something like 270 deg)here
ignition @ 32 deg max advance and only up hill fully packed with camping gear and 2 people in it knocked a bit
put it on 30 deg max advance an no more knocking.

i do run premium fuel (ron 98) if i use ron 95 it knocks like a mad donkey

i do not know the curve of my ignition it is one from a 850 mini :P
cr will go down soon with a diffrent head and ignition will be changed for a electronic 123 with lots of curves to chose from
Onno Könemann


12:1 v interesting that would imply I should be able to make my set up work, thanks. Do you have pressures from a hot compression test?

James Eastwood

Hi James something does not sound right.

10.3 to 1 is not excessively high and should run very happily with 32 degrees max and 95 octane fuel and a warm cam (250/260). However with a 288 (hotter) I would expect the trapping to be a bit worse and pinking should be no where to be seen.

Questions need to be asked, does it pink at all RPMs or is it just at midrange 3.5Krpm for example?
Are you 100% confident in the calculations?
What plugs are you using?

On top of that it is worth noting:
I have tested some high CR engines and run them at 28 degrees without a noticable reduction in power and torque
Some distributors give a little too much mech advance at midrange which causes problems
I fear the only way to sort this problem is to get to a rolling road and sort out the advance curve.
I always used to keep knocking the max advance back on the rolling road to see what I could go back too without affecting top power and torque, sometimes you can be very surprised
Robert (Bob) Midget Turbo

Id think at 5500 rpm, you would be running out of torque

to make things really fun and complex, have you tried 2 differant tension springs inside the dissy, or bending the spring perch tabs to provide diffent tension to the springs.


Bob, many thanks.

Typically the detonation would be very low speed i.e. 1500rpm, then go away as speed increases but reoccur around peak torque 4-4500rpm, then go away towards peak rpm. I'm confident in my calculations, I burette measured the head, and the block/piston dish at TDC, and added the gasket volume. Plus the guys at work helped me with a Polytropic compression calculation to convert my measured hot compression test back to CR and that tallies almost exactly. (The calc is P2/P1 = [V2/V1]^1.32, or 1.30 for a cold engine. I can post more details on how to do this, as it appears very useful).

I'm using one grade hotter plugs i.e BP7ES.

I think what I take most importantly though is you think that a 288 cam and a 10.3 CR looks ok, as does Onno also. So I need to look in more detail and try some more advance changes, possibly more spring tension. I have a 5 speed pillar drill I though I could use to map outthe advance curve. Long winded but at least scientific. Yes I could take teh rolling road approach but I'm stubbornly grass roots when it comes to finding solutions (I find it difecult to ask for directions also..)

Prop, thanks for that additional degree of freedom, just in case I didn't have enough to think about! I have two different springs currently, but they effectively act in parallel, so I don't think the solution lies there. Ref torque at 5500rpm, don't forget this is a 288 degree cam, it will rev through to an indicated 6800rpm on a gear change.
James Eastwood

What's the mixture strength like? What are the carb(s) settings?

You should be ok with at least 10.5:1 on a 280/290 cam IF correctly set up.
Daniel Thirteen-Twelve

"" sod the rolling road!!!"

LOL love your style James sounds like my attitude when I was a bit younger.

Obviously cam C/R and full mech advance sound cosher so I think the mech advance curve is causing unecessary problems. (I operate with programmable now so don't have such problems)

You obviously understand the mech advance issue, seems to me like initial advance is too much (pinking at 1500RPM this also means that total mech advance is not enough and the start point is therefore too high) also pinking at 4.5K also means that full mech advance comes in to early. so I am sure you can sort this so we can only wait while you get to work and mend it!

Looking forward to a great result.
Robert (Bob) Midget Turbo

btw my cr will go down to something like a 10,5:1 cr because ron 98 is getting harder to get in holland
and with only a 27 liter fuel tank going to the next fuel station is not always an option.

and the other head i have has way better port work and nice rimflow valves

you set up should indeed work fine and i suspect a faulty distributor or other wise your timing light is off?

no compression figures they say nothing with diffrent cams and specs.
And each time i have mesuared them there was something wrong (old tired engine's) so now with a new properly rebuilt one i just assume is fine.
a bit superstisious about that now
Onno Könemann

Dan - I'm using a fully ported set of 1.25 inch SUs, with all the butterfly mods. Needle is a no.3 (rich) which has also had flat filed on the reverse by a previous owner/set-up. I've tried the carbs on an unmodified No.3 and standard needles (because I had them) but it runs best on the mod'd No.3's.

I think having read the input I need to go over the whole problem in a super logical manner check every thing, trial some further timing changes and report back!

Onno - the thing about the polytropic calculation is that it does take into account cam timing, i.e. trapping, as the V2 value is the volume above the piston at the point where the intake valves closes, which you need to calculate back from your cam timing.

It's long winded but I'll copy the whole meail from my cooleauge to exaplin how the calc works;

The pressure ratio corresponding to a volume ratio can be calculated by assuming a polytropic compression process, given by P2/P1 = [V2/V1]^n

(^n is the exponent, i.e. "raised to the power of n")

A typical polytropic index for a gasoline engine is 1.32.

Hence, assuming compression from BDC right through to TDC, a compression ratio of 10:1, a polytropic index of 1.32, a pressure at BDC of 1.0 bar and no leakage losses from the cylinder, the pressure at TDC would be 20.89 bar

In practice, inlet valve closure occurs after BDC so at lower engine speeds the compression doesn't start until after BDC (at high engine speeds the compression will effectively start much closer to TDC because there is less time for backflow through the intake valve, also will tend to have wave tuning/momentum effects). There will also be losses through the intake system, in addition to throttling, as well as residual gas effects, that will reduce the volumetric efficiency and hence pressure in the cylinder at the start of compression, also atmospheric pressure will vary day to day.

In the case of a motored test (crank speed), main effects will be inlet valve closure, leakage past valves and piston rings and increased heat losses due the loss speed and cold engine.

The "effective" compression ratio takes into account the inlet valve closure (IVC) angle and (although I don't know the actual valve timing), I would estimate an effective compression ratio of 9.5:1. The polytropic index will be lower to account for the cold engine and lower engine speed (heat losses from the hot compressed air to the wall will be higher), say 1.30. Pressure in the cylinder at IVC could be assumed to be 0.90 bar to take into account manifold resistance (assuming throttle is wide open) and hotter clearance volume gases.

The calculation now gives 0.9*9.5^1.30 = 16.8 bar (absolute)

This is close to the measured value (not sure if the value quoted by James is absolute or gauge), any difference can be accounted for by small changes in the assumptions.

The main purpose of the motored compression test is to check cylinder to cylinder differences, the absolute values are generally less important, and will vary day to day due to ambient conditions.

James Eastwood

James, to concur with the others my race engine is 1380 / 286 cam / 10.75CR calculated / Aldon-PMay race dizzy set to 31 deg total / hiratio rockers and running on Tesco's 99 I've never knowingly had pinking or knocking.
David Smith

thanks for that formula.... vary cool



My website has a number of pages on combustion issues/ignition timing. Many of the points repeat the many very useful comments above, but there may be one or two other ideas not yet covered so it may be worth having a look. It is mainly written for the B series which is a lot more touchy especially with the petrol we have here, but it applies equally to A series.

are probably the most relevant.

We had a 1275 A series with a mild cam, LCB and 9.6:1 CR. I found it needed BP8ES plugs (but see note above re petrol) A good test to see if the plugs are running to hot is to find a long hill and climb it at full throttle in the highest gear you can. If there is no combustion niggles initially, but it sets in (or if a misfire develops) after a while, you may have plugs which are running too hot. repeating the test with a cooler grade of plug will confirm it or not.

There is one remaining peculiar circumstance in which pinking can occur. If the engine has the wrong vacuum unit there is a distinctive symptom of pinking on part throttle which disappears or diminishes on full throttle. I have yet to find any other cause of this symptom (though there is always a first time!). I have another page for my site waiting to be posted on this which I could email if relevant.
Paul Walbran

James, I can't see if you#ve mentioned what it's like with higher octane super unleaded. Have you tried it yet?

Mine is 1380, 10.25:1 (CC measured by prof engine builders, CR calc by me), Morspeed PH3 Cam (276/286ish) with an Aldon Yellow dizzy. Haven't had any problems with knocking but don't know what my max advance or hot compression is. Be a few weeks before I'll be running again.
Dean Smith ('73 RWA)

does it pink from cold?


A huge thank you to every one for posting CRs together with cam timing, plug recommendations etc..

Shawn - unknown if pinking when cold, I haven't run full load before it warms up.
Tarquin - I haven't tried Super Unleaded (but I'm also a miser!)
Paul - thanks for links will be reading tomorrow.

I just measured the timing, unfortunately I couldn't rev it high enough to get the full adavance, as concerned over noise considerations re. neighbours. But the results are as follows, see 'Meaured' line in graph.

Now I will repost it and show where it pinks (once weather clears), but for now any feedback on my advance curve is appreciated. In particular it appears to advance immediately from idle, should the advance potentialy be held until ~1200rpm then advance? I have the very good and inexpensive book "How to modify and power tune distributer type ignition systems" which describes a typical advance curve being constant until 1200rpm is reached.

Thanks James

James Eastwood


Our 1380 has a 276, flowed 12G940 'big valve' head, very similar CR (10.4), same plugs and 1.5 rockers.

Using an Aldon 'Yellow' dizzy, with vacuum, set at 15° BTDC at ~1,000 idle, results in around 32° maximum advance, which with super unleaded gives no pinking at all. Similarly with 'ordinary' 95 unleaded, but always use the super when available.

Even with a Metro anti-diesel valve fitted, we still get run-on occasionally.

What distributor are you using?

Richard Wale

This thread was discussed between 23/02/2010 and 01/03/2010

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