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MG Midget and Sprite Technical - compression

well to day i finaly got everything in
omega 73.5mm pistons and "new" head.

35.7 in 31 ex valves not the best portjob ever but not to bad either
fine for my butget (for now).

So it wil have to go to the machine shop this week.

But now i need to put vizard in my lap get that burete here and start mesuring and calculating.

I always run ron 98 fuel or higher and the engine will get my old 731 cam (that butget again)
According to a little chart in vizard i could run an 11.5-1 compresion and be ok
But that sounds a bit high to me.
i want a high compresion engine but i do not want to blow it all to pieces!

so anny advice on this?

i will post my calculations later so i can be double checked

Onno Könemann

I have run an engine quite happily at 11.5 to 1 with a 286 cam and 98 octane fuel. With a more moderate cam and 95 octane fuel I would run 11 to 1.
Robert (Bob) Midget Turbo

ok so with my 731 (less wild than the 286) aiming for 11.5 to 1 is not that crazy

than i will calculate one cylinder head volume with that to
Onno Könemann

Mine is 11.1 with a 276 cam I use which ever fuel is at the pump (my local petrol station has stopped selling 97 octane. Assuming that my compression ratio is actually accurate. I don't seem to have any problems at all.
G Lazarus

ok here we go!

I need to know the cylinderhead volume

we want an 11.5/1 cr
the swept volume is 345cc (V)
and we need to know the combustion chamber volume.

the formula is:
v/(cr-1)= chamber volume

345/(11,5-1)=32,9

but now the important bit

the combustion chamber is:

ringland volume: ???
piston dish: 7cc
gasket volume: 4,2cc
chamber volume: ???

so now i have to figure out the ringland volume but i do not think it is verry mutch

so without the ringland subtractet the volume of the cilinder head is 32,9-(7+4,2)=21,7cc

anny comments or thoughts
ringland numbers? or a way to calculate it?


thanks for anny help
Onno Könemann

Try this Onno
http://www.csgnetwork.com/compcalc.html

or for a more sophisticated Excel calculator, send me your email address and I'll forward you a copy. It's meant for A-series, and gives a default 1275 ring land volume of 5cc. You really need to measure the head chamber volume to calculate CR.
Steve Clark

Could i have a copy too please Steve.
Brad (Sprite IV 1380)

onno could you please forward the xcel calc please (if steve doesn't mind ofcourse)
Alex G Matla

Mail on the way

I need to calculate the ideal chamber volume first

then i can measure the head and determen wether i need to have the head milled or increase the volume (or live with a higher cr)

after the work has been done i will measure again to check and calculate the final cr
Onno Könemann

hmm when mesuring the "new" head i get 19.5cc
witch would make it a 12-1 cr
that sounds a bit high to me....
Onno Könemann

Onno I have not seen anything in the calculation for the piston height in the bore. Is it flush with the top of the block (decked)?

If you are happy running with 98 fuel and it is indeed 12/1 CR then I would be tempted to pop the head on and try it. If it suffers slightly from detonation then remove the head, (easy job) and get the chambers machined out by a cc or 2 before refitting. You may find out that it will run quite happily as is.
Robert (Bob) Midget Turbo

i have not mentioned the piston height because it is indeed 0

i must say i am tempted as well but i a bit paranoid about damaging my new engine

the head still has to be skimed so cr will slightly increas

might just stick the dremel in that chamber to enlarge a little bit
Onno Könemann

2 gaskets?!

A
Anthony Cutler

I have just CC'd a block and piston on a 1380 engine using the trusted Perspex plate and burette method, I got a reading of 14.5cc, I have a head with the same volume (14.5cc) and I use 3.8cc for the compressed gasket. Different folk have different views on gasket volume, but between 3.8 and 4.2cc is usually somewhere close, using the lower volume (3.8cc), this adds a small safety margin into my calculations.

So, 345cc Swept vol Plus 14.5cc Piston and land vol plus 3.8cc gasket vol. plus 14.5cc combustion chamber volume all that divided by Combustion Vol of 32.8cc gives me a compression reading of 11.52:1

That is:

SV + CV
_______ = C/R

CV

Hope this helps a little.

Mark.
M T Boldry

mark you are confusing me
are you just showing your calculation or are you trying to make a point?

anny way i am not to fond the 2 gasket idea
seems like 2 times the chance of blowing

but i do have a choise of gaskets!
orderd a competition one from Peter May
got one with the parts pile of the car from the PO
And orderd a cheap gakest kit for all the other gakets (witch turned out to have the same gaket as PM sent me?!)

so i have 2 copper colored with insert ones and one black with inserts (see pic)

the copper ones ar uncompressed slightly thicker (0.07mm)

witch one to use


Onno Könemann

Mark -

Do you seal round the piston ring with grease?

Tom
Tom Coulthard

Can't comment on what PM states he has had great results in competition so he must be doing something right.
However we used to use the copper gasket for competition but I and I believe many others have changed to using the black gasket that is the Payen composite one which seems to be just as good if not better. Is your black one a payen or is it just a cheap copy?
Robert (Bob) Midget Turbo

i do not know for sure
i got it from the po but it looks realy nice

i think i wil have every thing machined and then try it if it detonates i can grind some more out of the head
or mil something of the pistons? (1mm wil get some thing like 2cc)
Onno Könemann

Tom.

No, I find by using Paraffin as the medium, it doesn't creep past the rings for some time, perhaps 5 mins or so, so plenty of time to take a reading.

Onno.

I am showing my way of calculating C/R. I don't use two gaskets, nor have I mentioned two gaskets, all I have said is that some use the figure of 3.8cc and others use the figure of 4.2cc, the difference of 0.4cc doesn't make too much difference in the calculation.

Bob.

I now ALWAYS use the 'Black' Payen gasket, I have good results with these.

Mark.
M T Boldry

the way to calculate cr is clear to me but thanks annyway

the 2 gaskets comment was directed at Anthony

thanks for al advice so far
Onno Könemann

Onno, my input;

Burette the block, piston height, piston dish and ring land voloume. It's much easier and reliable than measuring. Then calculate the gasket as a cylinder. Add to you head chamber numbers and run the formulae. I would also check all the head volumes separately as the volume relies on an as-cast surface were casting core-shift can change the values. Also if a 2nd hand head, be wary of valve seat recession, as the valve recede unequaly the chamber volumes can also change unequaly.

I used a CR of 10.8, which gave me 190psi on a warm compression check with a proper screw-in gauge (all other plugs out to get the cranking speed up). I run a 288 deg cam (BMC 544) timed to the book figures given in Vizard.

I run on any pump fuel and do suffer with a bit of detonation and 'run on'. I've retarded the spark to give static timing of near TDC (to avoid the det), which means I fall short of full advance the engine needs for high engine speed of around 34 degs.

Head cooling is another key factor in successfully running high CRs. I run a standard thermostat, but have recently though of going cooler. I have also started running a cooler plug the BP7ES (i.e. one cooler than standard) in order to attempt to reduce combustion chamber hot spots.

I also run a lead additive, which doesn't (knowingly) change Octane value.

There's a v. good book on ignition timing "How To Build & Power Tune Distributer Type Ignition Systems" by Veloce Books, which explains why not to go too high on CR, and how to set up your own system. A real hands-on book, v useful.

Good luck.
J E G Eastwood 1

Sorry that meant to read;

Burette the block, piston height, piston dish and ring land voloume in one go, with every thing assembled. (I used engine oil to preven th erun thru the rings).
J E G Eastwood 1

-How To Build & Power Tune Distributer Type Ignition Systems" by Veloce Books
Thats a good book, i have a copy too.

I just use the metro turbo head gasket now, expensive but it seems bullet proof.
Brad (Sprite IV 1380)

Handy spreadsheet http://www.taratec.co.uk/Mini_engine_sizes.xls

One sheet gives bore/piston sizes of all common combinations (and some weird ones)..

The other is a compression calc that will also work out what chamber volume for a given compression.

Rather useful!


Ign systems book is also top notch!

I like the copper gasket and ran 11.5:1 with no real problems other than a bit of run on...
Toby Anscombe

I like that, thanks Toby.
Brad (Sprite IV 1380)

Not a problem; work has sort of buggered a load of stuff up for me (lack of internet access, no free time etc) but first thing in the morning and last thing at night are do-able.


My contract runs out in about 8 days so that means lots of garage time :-)
Toby Anscombe

Lifted from post by a well known Engine Builer AC Dodd on the minifinity forum.....

A successful engine needs the cam an octane rating matched. New cams such as the SW5 and Morspeed ph2 work best with a much lower CR. In my own engines I use 9.5 to 9.75:1. As a result I get higher mileages more power under the curve and a much nicer less fussy engine. I also get good fuel economy. Older cams use much more lift on overlap together with a slower opening rate as such the engine needs a higher effective CR to compensate for the increased overlap lift. These cams are best used with a higher static CR and perhaps more cam advance giving more effective CR which brings up the low speed torque (road engine use. It is quite common as I have seen looking at others dyno plots on this very forum, my own builds tend to produce more area under the curve. My testing indicates that running too higher CR and artificially retarding the timing to get the engine to run clean results in less overall performance. The best engines run more timing and enough CR verses octane of the fuel used to get the best from the fuel without pinging it off in the cylinder. Those who chase CR in the quest for power will be dissapointed in the long run, its not the way to build a long lasting road based engine. The maximum CR I would use with the 98 RON fuel and the SW5 cam in the application above would be 9.75:1.
The same fuel with a 286 cam I would use 10.25:1. Change the cam to a
649 and I'd head toward 11.25:1. Push the octane to 105RON and I'll be looking at 12.5:1 to 13:1. No octane no CR.

Oh and a further point I have not built a road engine for more than 5 years that uses more than 10:1 CR!

(http://www.minifinity.com/distribution/viewtopic.php?f=5&t=81735)
Dean Smith ('73 RWA)

hmm i think i need to ignore that last post because i had just mad up my mind ;)
Onno Könemann

It is an interesting post I must say.

In the vast amount of time I spent on rolling road testing in the early nineties I found exactly the opposite to be true. Indeed I found that retarding the max timing from 32 degrees to as low as 28 degrees reduced power by only a mere 1 BHP or so. Advancing or retarding the cam by a degree made hardly any measurable difference in the power curve.
I must admit that each 1/2 point increase in measured c/r will release subsequently less power increase, however up to 12 to 1 the increases in power were excellent with a 286 cam. I also tried that kind of C/R with a more subtle cam and still gained good power although with the better trapping capabilities of a lesser cam timing either had to be retarded or octane raised. However the overall package was still of overall benefit.

Not trying to convince anyone but myself perhaps if I wanted only to use a 266 cam or equivalent then I would probably limit myself to about 11/1 and use pump fuel of 95 octane (far less expensive) A 286 I would go to 11.5 or slightly more and use 95 octane.

All that said I am now into programmable ignition which when fited gives the ability to taylor an advance curve to suit any situation. This enables the advance to be retarded in the mid range (pinking) area and advanced at higher revs to have the best of all worlds and allows 12/1 C/R to be used with 95 octane fuel without issues or cam choice
All IMO of course, take it or leave it!

Robert (Bob) Midget Turbo

what kind of programable ignition are you running?

i was thinking that a mega jolt would be a nice solution to pinking problems
(have diffrent maps for track days with octane booster)
Onno Könemann

For various reasons I ended up going for the Weber ignition although they must all be about the same.

I turboed my engine a couple of years ago whilst still on distributor ignition. Hence I reduced the C/R to a mere 8.3 / 1 which allowed me to use max boost of about 15PSI at 5KRPM. With a standard turbo head this gave me 115BHP at only 5500rpm. Now I have the Programable ignition I wanted to port the cylinder head and raise the basic C/R and suppress any detonation on the ignition map. Unfortunately I can not afford to do the cylinder head development at the present time so I can not make full use of Ignition maps. But if you are fitting it Onno then I am sure you will have no worries with any C/R you care to choose. Therefore if I were you and had PI fitted I would go to 12/1 without hesitation. A couple of degrees less advance at midrange I believe is not worth the worry.

Always remember we have these cars for fun, where is the fun in a conservative approach to engine power?

Secondly when you do the work you know the answers. Good luck
Robert (Bob) Midget Turbo

Most electronic / programmable systems have a 3D map, so you have a 'spreadsheet' with revs vs throttle posn on the 'axes' and a number in each intersection to represent ignition advance. see pic...

This allows ign to be optimised for all engine speeds and load.

For comventional ign, the bob-weight system provides a ~3 straight lines to match the ideal curve; then you have the vac adv superimposed on that... always approximation based on a compromise.

A


Anthony Cutler

well if i have anny problems with pinking i think this will be the way to go and replace the 123ignition with programable ign
Onno Könemann

now we (me and an mg friend) are having a bit of a discusion about pinking.

wat is it
is it premature ignition du to compresion?

if so how can retarding the ignition (letting it spark later)help to cure the pinking??

Onno Könemann

A 4-stroke or ‘Otto Cycle’ engine goes through four distinct 'strokes' as the pistons move up and down the bores. Simplistically it is as follows:

1. Induction Stroke: The Inlet valve opens, the piston moves down the bore causing a vacuum and sucking in the Fuel/Air charge.

2. Compression Stroke: The Inlet and Exhaust valves are closed, the piston moves up the bore thereby compressing the Fuel/Air charge.

3. Power Stroke: The Inlet and Exhaust valves are closed, the spark plug fires, the Fuel/Air charge is burnt producing expanding gasses that push the piston down the bore.

4. Exhaust Stroke. The Exhaust valve is open, the piston moves up the bore forcing the burnt Fuel/Air charge out through the Exhaust Valve.


Therefore the output (Torque) of any given 4-Stroke motor is directly proportional to the pressure exerted by the expanding gasses, produced by the burning Fuel/Air charge, upon the piston pushing it down the bore and, hence, causing the crankshaft to rotate.


Now, ideally, you want the expanding combustion gasses to start pushing upon the piston when it is at TDC. If they start pushing when the piston is still coming up the bore on the Compression Stroke (ie BTDC) then they will be fighting against the piston resulting in a reduction in torque output (and, hence, a drop in power). If they start pushing when the piston is already going down the bore on the Power Stroke (ie ATDC), then, again, there will be a loss in torque (and hence power) because they are pushing into a space that is expanding so the pressure felt by the piston will be less.

In an ideal world, therefore, you would have the spark plug fire giving an instantaneous ignition of the Fuel/Air charge at TDC, with an instantaneous burn, and a concomitant instantaneous expansion of the combustion gasses.

Of course, we don’t inhabit such an ideal world and it takes finite amounts of time for the spark plug to fire, for the flame front to expand across the piston face, for the charge to fully burn, and for the combustion gasses to exert pressure on the piston.

Therefore we fire the spark plug before TDC such that the burnt charge exerts its maximum pressure on the piston at TDC (in reality this maximum pressure usually occurs slightly after TDC for a number of reasons, not least detonation. (qv later). This is known as Ignition Advance.


As engine speed increases (ie rpm's rise) there is less time per cycle for these processes to occur so, for maximum performance, you require increasing amounts of Ignition Advance. This is known as an Ignition Advance Curve and is controlled by the distributor (early cars) or the ECU (modern cars).

In fact distributors give a fixed (mechanical) advance curve, ie they are 2D and map purely Ignition Advance versus rpm, whereas an ECU controlled system can be mapped to give variations in the curve at different rpm's to take account of such things as changes in cylinder burn rates caused by differing engine loads (they will assess Throttle Position and/or Manifold Air Pressure), ie they are 3D. In fact the ECU can also alter the fuelling (on fuel injectef cars), thereby giving a more efficient burn throughout the rpm range than can be achieved by a distributor/carburettor set up.

On cars fitted with distributors and attempt at 3D can be made by using a Vacuum Advance signal taken from the inlet manifold (essentially this mimics what a MAP sensor does), albeit this is rather crude owing to the limited nature of the distributor response (you aren't really getting 3D as such, you are merely changing the point at which the distributor 'reads' the advance curve).



'Pinking' is the early onset of Detonation.

OK so what is Detonation?


To understand this we have to take account of a piece of Physics known as 'Boyle's Law'. This states, for any compressible fluid such as the Fuel/Air charge in an engine, that:

[Pressure x Volume] / Temperature = Constant

That is to say that if you have a fixed volume of gas (the Fuel/Air charge), if you compress it, it will heat up.

So, on the Compression Stroke, what we are doing (aside from the heat soak from a hot motor) is heating the Fuel/Air charge as it is squeezed by the rising piston.

Now, obviously, the Fuel/Air charge has got fuel in it (petrol) so there will come a point whereby the compression induced heating effect will cause the charge to self ignite (in fact this is how diesels work – they don’t have spark plugs, but rely upon compression to ignite the Fuel/Air charge).

In cases of detonation, what happens is the spark plug fires (as we said, BTDC) and the flame front starts to travel across the piston face. If then the fuel also self ignites we have two flame fronts that must eventually collide. Now, while I said it takes a finite amount of time for a flame front to travel, the fact is that it is traveling at supersonic speed. So, when the two flame fronts collide, they do so with massive force, resulting in a sudden huge pressure 'spike'. Owing to Boyle's Law this pressure spike causes an enormous temperature spike sufficient to burn through to the piston face / cylinder head face causing pitting, or (at worst) a hole in the piston crown.

(Note. There is another effect known as pre-ignition whereby something glowing red hot in the cylinder, such as a spark plug that is of too soft (hot) a grade for the motor or an incandescent carbon deposit ignites the Fuel/Air charge before the spark plug fires. The effect is the same – two (or more) flame fronts colliding.)



So, what puts your motor at risk of detonation?

1. Running too much Ignition Advance. If we ignite the Fuel/Air charge too early on the Compression Stroke then the charge will start burning before the piston reaches TDC. If this happens the expanding gasses are fighting against the rising piston causing increased cylinder pressure (ie more heating) to the point where detonation is likely to occur.

2. Too high a Dynamic Compression Ratio for the fuel being used. Static Compression Ratio (CR) is the ratio between the cylinder swept volume and the volume of the combustion chamber. However Static Compression Ratio takes no account of the fact that the valves are open during much of the engine's cycle. On racing engines with long duration (ie the valves are open longer), wide overlap (ie both the Inlet and Exhaust valves are open together) camshafts, the Dynamic Compression Ratio is far lower than the Static Compression Ratio so overall higher Static ratios are used as well as greater amounts of Ignition advance. For example our racing midget uses 11.75 : 1 Static with 29 degrees advance BTDC at 3000 rpm. However, the higher the Compression Ratio the closer you get to the detonation line.

3. Too lean a Fuel/Air charge. Leaning out the mixture causes it to burn hotter and faster. On the other hand running a slightly rich mixture will mean that not all the fuel is burnt and the unburnt fuel can act as a 'heat sink' absorbing the heat and carrying it away through the Exhaust Valve (this is why Nitrous motors are set up to over-fuel slightly to keep them safe from detonation). However, too rich a mixture will result in a drop in power, cylinder bore wear owing to the petrol removing the oil film, increased emissions and, ultimately, catalyst failure.

4. Running an engine too hot, causing excessive heat soak into the Fuel/Air charge.



So how do we prevent detonation?

1. Don’t run too much Ignition Advance. This is where Knock Sensors are helpful since they signal the ECU to retard the Ignition if detonation starts to occur.

2. Don’t run lean.

3. Don’t run hot.

4. Use a fuel that is detonation resistant.




Detonation resistance in petrol is indicated by its RON. The higher the octane, the more detonation resistant it is. In the old days of leaded fuel, Tetra-Ethyl Lead was added to prevent the fuel detonating (as well as to protect the Exhaust Valve seats). However, Lead is a toxin.

Once Unleaded fuels became the norm, initially benzo-phenolic compounds were added (although these were highly carcinogenic). Other additives included Methylcyclopentadienyl manganese tricarbonyl (MMT) (a powerful neuro-toxin) and Methyl tertiary-butyl ether (MTBE) which pollutes ground water supplies.

These days the trend is to add Ethanol (albeit at levels over 5% this can exacerbate fuel evaporation) and Ethyl tertiary-butyl ether (ETBE).
Deborah Evans

for the more simple-minded instead of suck - squeeze - bang - blow we want suck - squeeze - burn - blow.
David Smith

thanks Deborah.

that made some old sience clases come up in the brain.

What you are in fact saying is that to cure the pinking (or worse detonation)you first have to asses why it occurs!

And in some cases a mapable ignition can be the cure.
And in others refuge should be sought in other solutions (fuel ot temp)

so the advantage of a lower cr is the fact that you can play with octane and ignition figures

where with a higher cr the demand is there for a higher octane number!

all of the pieces of theory was there (in my brain)just never put them together!
Onno Könemann

Onno sorry not to have replied before but have been on holiday. River Nile cruise, not too bad but could have been better.

Anyway back to pinking. to simplify and add to Debs point may I say the following.

With our engines normally we can set the ignition timing well enough to not cause problems of detonation so generally it is caused by modifications we make. Namely high Comprssion ratios. High Compression ratios do cause pinking because as Deb described more PRESSURE results in more heat and therefore preignited fuel can be the result.

However with a normal ignition system this pinking will normally occur at mid revs because due to gas flow dynamics we generally end up with a higher effective Compression ratio at mid revs. Normally at higher Revs the Effective (actual) Compression Ratio is reduced due to poor high speed gas flows and therefore pinking will not be a problem at these higher revs.

Therefore if we can "back off" the ignition timing we can run higher CVs. Unfortunately if we back it off too much we will begin to poose power due to retarded timing. So generally it is a compromise between high CR and retarded ignition.

Now if you can have programable ignition you can back off the ignition timing at mid range to protect from pinking at mid revs, then bring the ignition timing back to optimum at higher revs when effective CR is lower. Thus you can have good optimum ignition timing with high CR.

I ran a 1400cc engine in my Midget for a year or so with 12.5 to 1 CR on a 286 cam and normal fuel (95 octane). Then engine would pink badly between 3K and 4.5K RPM but would be fine above this. So for a year I simply backed off the throttle slightly at mid range to prevent pinking, Not a great solution but the engine performance above 4.5Krpm was staggering and well worth the effort. However if I had had programable ignition I would have not had any problems at all.
Robert (Bob) Midget Turbo

All good stuff above (or below). But I like to go back to the basics to make things even simpler and show a picture.

Why does the ignition spark need to be advanced? So that the spark occurs at a suitable time such that the main burn and pressure increase happens as the piston is beginning to move down the bore (as already said).

What happens when the plug is fired? A plasma very quickly builds around the plug tip that ignites the gas in the region; the spread of the flame front is relatively slow – the flame front is carried by swirl (gas movement).

Swirl begins when the gas moves along the inlet, into the head and past the valves. Swirl is promoted by the speed of compression (ie engine speed) and amount of (real) compression related to compression ratio, gas temperature increase, cylinder filling efficiency, and speed of piston movement to create swirl.

At very low engine speeds, we need to advance the ignition purely to offset the slow swirl… and around 5-8 degrees for most engines gives enough time for full burn to be in process as the piston starts to drop.

As the engine speeds up, we need to increase the amount of degrees of advance to keep the amount of time between spark and burn constant. Very simple.

Except that increased engine speed has 2 main effects: 1. more energy in the swirl makes it travel more quickly; 2. less time for the heat in the gas to dissipate into the head/piston, so the gas stays hot and gives higher compression (as Bob says); process becomes more adiabatic.

Both these effects mean that the rate of advance needs to be lower (as engine speeds increase).

If we are WOT, we have good gas filling, faster swirl, and need less advance at any rev. compared with standard.

As revs become high (esp with WOT), the swirl becomes increasingly fast such that the amount of advance (in degree terms) flattens out; in terms of time, we actually need less advance but of course we always stay in the degrees measurement mode. For the A Series, this is around 32-34 degrees.

So you can see, higher compression means faster swirl and less advance needed compared with standard (as stated). Better filling (e.g. though better inlet design) means less advance needed compared with standard (as stated). If higher compression created by smaller combustion chamber, then there's less gas volume to be burned, so the flame front reaches the extremities sooner so less advance needed. If you can arrange in the inlet (runners, ports, valve seat, combustion chamber) to maximise swirl as the gas arrives, you need less timing advance compared with standard.

As Bob says, you can't match the relatively simple timing requirements vs revs described above by changing the 2 bob-weights and 2 springs – the 3 straight-lines (degrees timing vs revs) you obtain from the conventional mechanical system has to be compromised over the whole rev range. For a racer, this means optimising for 5 to 7K plus. For the road user, the rev range is wider, so the compromise can be less satisfactory.

If we back off from WOT, we have less gas in the cylinder, and so lower effective compression and lower swirl speeds; so the optimum spark needs to occur earlier… hence the need for vacuum advance.

Too much text already… here's a pic from BRB's Emerald system (same data as earlier spreadsheet view). Load axis is accelerator depression: 15 is WOT; speed axis is RPM (divide the number by 2); ignition advance (the vertical scale) is in degrees.

For WOT (load site 15, the nearest line), we can see static timing around 10 deg (the shenanigans around idle is to stabilise the idle speed without the need for an inlet air control valve); speed sites 4, 5, 6 shows the ignition advancing quickly with revs 2-3K; then 7 to 9 it flattens out (3.5 to 4K) due to good filling (as Bob mentions), then around 10-12 (5-6K rpm) we see increase in advance again (inlet now probably providing all the gas the engine would like); finally, flattening out around 13-15 (6.5 to 7k rpm) as swirl dominates and no more advance increase needed with 32 degrees max.

If any time we lift off the throttle, (assuming engine speed stays the same) the advance increases to compensate for lower flame-front/burn speed; we can have another up to 20 degrees of advance added (to make the engine fuel efficient) – follow any rev line 'towards the back wall')

As Bob says, a mechanical/vacuum system can't get close to this.

Sorry that's so long. Hope I didn’t over-bore you.

A
Anthony Cutler

... and the pic...(!)

A


Anthony Cutler

Allow me 1 mistype corection:

(inlet now probably NOT providing all the gas the engine would like)
Anthony Cutler

Anthony and Bob thank you verry mutch for all of the info.

I think that i am going to start saving for an ignition (and maby efi)after my engine if finished.
Onno Könemann

This thread was discussed between 28/03/2009 and 14/04/2009

MG Midget and Sprite Technical index

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