MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Over compression problems


Hello,

I join you about two months into a full 1275 midget engine rebuild and was hoping for a spot of advise. To get you up to speed this new engine is to the following spec:

Over bore +040
Crank bore +020
Flat top pistons
286 scatter kent cam
Kent vernier adjustable timing
Cooper S gas flowed head
LCB exhast
Single SU on a tricky dicky mini manifold
Petronix electronic ignition

The car is running and sounds wonderful (as you'd imagine) however it appears to be pinking when under load such as when pulling away etc. which results in a loud clicking/rattling noise.

Compression test revealed it was compressing to a wapping 17 BAR! I take it that this would cause the aforementioned noise and pinking issue? If so, does anyone have any suggestions of how to get the compression down without changing the pistons?

On a seperate note, does anyone have a distributor advance chamber or even a whole distributor they would like to sell?

Cheers guys

Jonny






J Price

As the usual pistons have about 11cc of dish on a 1275 with 8.8CR. Your unswept volume is 40.86cm^3.

Now if you run with flat tops on a +040 I make that ((1310/4)/(40.86-11))+1 =11.97CR which is where your 17bar is coming from! So unless your using Vpower 99 octane fuel all the time (and even then) I'd expect this engine to pink. It will probably go like stink aswell! but good fuel and retarding the ingintion may help.
Nick

Nick,

Thanks for the maths, hadn't got around to doing the sums myself! You think it'll go like stink in a bad or good way?!

How about a thicker head gasket or a couple of head gaskets?

Jonny
J Price

I don't know where half my post went! The maths was just the first bit!

I'd imagine a 12:1CR engine to go really really well, but it might be too much for a road engine. To get the CR back down you can try opening out the head chamber as per a forced induction design or as you say something with the gasket. I've heard that fel-pro make a thinker head gasket for turbo'd minis but i dont know who sell them. I do rember they were pricey though.

Other than this you could try and get your engines volumetric efficiency (VE) down - i.e to lower the effetive CR, by say using a shorter duration cam or losing 1.5 rockers if you're running them. How this would affect the characteristic of the engine though - I don't know.



Nick

Id say you have 3 basic options

1. get the head re-worked having the chamders in the head milled out...

2. run multiple head gaskets...or take a HG to a machine shop and have them mill you a nice thick one out of copper and use the special head glue with a good new gasket.

3. do nothing to the car,,,,and go to your local airport using airport transfers tonbridge, with small propellor planes and buy airplane fuel...(***NOT JET FUEL***) Air plane fuel, will be ither blue or green in color. IIRC the blue fuel is 105 to 108 octane, the green is around 120 octane....but that was 25 years ago....expect to pay some bucks per gallon...most likely you will also have to grease some palms, if the airport is upscale,,,,usally the airports in dirt water little towns in the middle of nowhare with grass run ways, will be your best friend,,,granted the fuel maybe a bit old, so Id strain it and add fuel stablizer before using it...and check the color of the fuel.

BTW ...as to why NOT JET FUEL, un-like the movies where a small spark from 1000 meters away can detonate an under ground storage tank of the stuff,,,its actully the oppisite,,,,its ungodly hard to get lit, all most need a welding torch....its a super clean karosene...(PARIFIN) with a lower octane then common house hold K1 you would use in a space heater or fuel lamp...granted "IF" you can get it to burn, it would be a plesent heat source cause its so clean, but will not work in any combution engine, thats why flight 810 from new york to paris several years ago was such a joke, cause there in no way 26-28 gauge sparking wires can detonate a fully filled gas tank of the stuff....my therory...some 3 dollar an hour illegal mexican ailien line-man screwed up and put 2-3 gallons of airplane fuel into the Jet linner and didnt tell anyone of his mistake before it took off. that would do it alright, with no trace of the mistake ever happening. and there is no way an airline or the FFA for that matter, wants to admit they got that type of chessy work enviroment.

prop
Prop

if pinking is the mixture self-combusting because it gets hot when squashed, I thought the solution was to advance the ignition spark, not retard it, so the mixture burns before exploding? Or have I not really understood the IC process all these years?
David Smith

I think you have misunderstood all these years then David. Advancing the spark would make the detonation worse, in fact racers ought to be setting their maximum advance at probably 28 to 30 degrees and make it up with as much CR as possible. With advance on the timing and extra heat from the mixture compression, the flame front moves quickly and high pressure gasses occur far too early in the cycle and cause heat and detonation.
To combat this we can as has been suggested use HIGH octane fuel (in the UK it is illegal for airports to sell "avgas" to the general public who do not have a petrol engined aeroplane). This will require more heat to ignite it so it actually appears to retard the timing. The opposite was true years ago when we changed to low octane fuels. These required the timing to be retarded a couple of degrees to account for the earlier lower temperature ignition and an effective advancing of ignition timing, hope you are all keeping up.

Nick just a small point to effectively reduce the CR at lower revs one would need a high overlap cam with poor trapping capabilities at lower revs therefore the cam already in this engine would be the best solution for not pinking at low RPM.

Normally an engine with a 286 installed would pink at about 3 and half thousand revs in 3rd gear going up hill under full load, as I understand this case the car is pinking in most gears irrespective of going up hill and effectively loading up the engine fully. Thus it must have super compression as you have described Jonny.

I would not want to argue with the maths by Nick, but having run A series engines with a 286s and 12 to 1 C/R they would not pink like this one and would not record that sort of pressure on a Compression test.

Unfortunately it is my opinion that you must get the C/R down by measuring accurately and getting a cylinder head to suit. A Series machinists can enlarge the combustion chamber to achieve this but I am not sure they could do enough. Until you do an accurate measure then everything else is speculation
Bob (robert) Midget Turbo

must just correct my grammar

""in fact racers ought to be setting their maximum advance at probably 28 to 30 degrees and make it up with as much CR as possible""

Should have read

IMO racers ought to be setting their maximum advance at probably 28 to 30 degrees and make it up with as much CR as possible

:-))
Bob (robert) Midget Turbo

Has the distributor a tailored advance curve (eg from Aldon Automotive, APT in the USA) to suit the spec of the engine?
Daniel Thirteen-Twelve

It is NOT illegal for airports to sell avgas (probably 100LL) for use on the public highway unless there has been a legislation change in the last 5 years.

It is however ILLEGAL to use aviation kerosene, AVTUR, JET A1 etc in diesel vehicles on the public highway.
Daniel Thirteen-Twelve

Well we will have to beg to differ on that Daniel. The use of high LEADED fuel in a road going engine is I believe against he law. I think the first thing is that AVGAS has a different duty applied than Road Fuel and is ONLY for Aeroplanes. If an airport sells it to someone in a jerry can or other container and not directly into an aeroplane then I believe they are commiting an offence. If not then why would they not sell it? The way AVGAS gets it octane rating up is by high lead content and as we are all aware now, only SELECTIVE outlets are allowed to sell leaded fuel, unless the law has changed for me?
Bob (robert) Midget Turbo

I'd agree with Bob that a long overlap cam would have poor VE at low revs (as some of the incoming charge can go out with the exiting exhaust gas) but not at say 3.5K upwards (as they're holding that inlet open longer and higher so the cylinders get a good fill!) NO?
Nick

Also, just thinking about bobs comments about the 12:1 and 286 running well.

My maths only stacks up for a robored standard engine with flat tops. If your block and/or head has also been skimmed then thats going to add to matters, another back of the beermat calc tells me you could be up to 13.5 ish if the pistons were decked to the block too.
Nick

Yes at 3.5 K upwards you are correct nick the trapping abilities of a 286 come into their own and as I stated above that is the kind of area that we would expect to see detonation when under load, ie throttle fully open allowing maximum effective CR.
Bob (robert) Midget Turbo

David S,
Yes, you are slightly mistaken ;-)
"Pinking", or "pinging" as the USians call it, is indeed caused by the mixture self-igniting from the heat of compression. What you are hearing, however, is not the sound of the combustion, but the sound of the piston trying to push the crankshaft backwards because it has not yet passed TDC. No need to point out that this is a situation to be assiduously avoided...

Advancing the spark will not help this. Retarding the spark won't really help you either, because it is happening without the spark. Pinking caused by too much advance would benefit from retarding the spark, but not pinking caused by self-igniting.
David "slightly retarded" Lieb
David Lieb

Bob,

The different rates of duty between AVGAS and road use petrol is NOT an issue and does not create an offence (the last time I checked a few years ago). You may have a point about the lead content in AVGAS though in recent years even AVGAS is low lead content. I'd be interested in what legislation prevents aviation petrol with high(ish) lead contents being used on the public highway. At a push (and not for several weeks) I can find the legislation that makes clear that AVGAS can be used on the public highway is legal (by omission).

David,

I do not believe we have established beyond doubt that the ignition curve is incorrect for the modifications undertaken to the engine. It's possible then that the spark is in the wrong place at the wrong time.
Daniel Thirteen-Twelve

Manchester airport sells AVGAS for £1.51 a litre + VAT = £1.77 a Litre. I think that might preclude the use of it in a road car legal or not.

What about octane boost? After the distributor advance curve has been checked...
Daniel Thirteen-Twelve

A couple of things on that Daniel.
If it were available to use on the road then why would it not be permitted in a clubman racing event?
Further do you not agree that only certain outlets are allowed to sell leaded fuel?
The reason why Road diesel and "red" diesel is not to be used in road vehicles is that the duty is different, I believed perhaps wrongly that that is the same premise in the petrol debate.

when I was buying the stuff a good number of years ago I was actually buying it at about the same price as normal unleaded fuel Great. We measured it in our lab at Lindsey Oil Refinery (Knock engine) and it came out at 103 octane. Now with that stuff you could run very high compressions
Maybe thats why I won a good few times :-0)
Only kidding!!!
Bob (robert) Midget Turbo

Daniel,
You are right. We have NOT established that it is pre-ignition vs poor timing curve (or static timing for that matter) causing the pinking. I suppose Jonny could back off his timing a bit to determine whether he has too much compression for his octane or whether his timing is off. For that matter, his sparkplugs could be too hot for the application as well, I suppose...
David "speaking theoretically" Lieb
David Lieb

Bob,

The MSA blue book stipulates what fuels can be used in clubmans events and in fact over the past year fuel has been an issue in motorsports in the UK.

Red diesel has a specific set of legislation around it which is quite different from the legislation for AVGAS. I know nothing about the legislation for selling leaded fuel whether AVGAS low lead or whatever.

I think it would be cheaper to use octane boost than AVGAS to get similar octane numbers.
Daniel Thirteen-Twelve

I guess the one thing you could do, to (decrease) pinging as defined by david....

"""caused by the mixture self-igniting from the heat of compression""

would be to have the piston tops ceramic coated...as well as the face of the clyinder head, this would spread out the heat more evenly. and remove alot of various hot spots on all the sharp edges that no doulbt are contributing to pre-ignition....dentonation....


prop
Prop

Jonny

At 12:1, your CR is too high for the timimg of the std dissy. The short reason for this: high => high swirl => fast transport of flame front => rapid burning; which means you need less advance.

You can try retarding the ignition in 5 deg increments, and the detonation will go away; but you'll lose the power of the engine.

The only solution is a re-mapped dissy curve - normally stronger springs - that cuts the rate of advance rise to give more optimum timing.

I used 10.5:1 happily with an Aldon 100AV-Y (IIRC).

You should also use super-unleaded; and maybe an octane booster - from this point on.

A
Anthony

No such thing as over compression, just the wrong advance curve ;-)


Like Ant, I have 11.5:1 and have no issue but I do have a fully adjustable advance curve which is heavily retarded compared to a 'normal' 45D or 25D.
Toby Anscombe

10.5 = 11.5 but <> 12.0?
British maths seem a bit odd ;-)
David ".9999999999999999bar = 1" Lieb
David Lieb

Picky...

I will rephrase and say...

In a similar vein to Ant I also have high compression....but not that high!
Toby Anscombe

Thanks for your thoughts guys!

I think the first port of call is going to have to be a compression test with two head gaskets. If we I can reduce the compression by a couple of bar I may then just get high octane fuel in with a 3 point boost, time it up again and see how it goes.

How essential do you think the advance vacuum is in producing a solution to this problem? I'm pretty certain mine is buggered. Also, would you have thought that the shabby state of the distributor is contributing towards the pinking?

I suppose a burning question is what would be the more favorable solution should it come to it; digging out the head or replacing the pistons?

David Lieb:

Thanks for the sound diagnosis, you don't think this will have caused any permanent damage to the crank do you?

Cheers

Jonny

J Price

If your dissy is ""used up"" and you have put this much into the engine, I would defintaly get a new dissy, an aldon, a 123 (maybe one in the same), a duel point mallory, Id even look into the new pertonix dissys that have recnetly come out ...if for know other reason just good ol fashion reliablity...after all 1/3 of a good high performance engine is ignition,----(ignition, carberation, compression)

before Id have the heads dug out Id get the spec sheet from the manufatucture of the pistons....Ill bet ten to one you can get the pistons tops shaved to get your compression down....if you do, Id still look at having them and the head surface ceramic coated....and it could well be that just having the piston tops, valve tops and head surface as well as the inside of the exhauste manifold, ceramic coated and adding a couple of HG will do the trick...

Id have the heads dug out only as a last resort, due solely to the complexity vs. the capablitys of the machine shop...if the machinest has done this 20 or so times to an A-series head, then go for it, but the artiest of machine work is a dying breed....now we get fresh kids out of tech school making $15 per hour to push some bottons. on a big enclosed machine...which passes for a machinest...progress you got to love it.

I just hate to see you loose such a lot of potential by going backwards...as nick commented...you get this pinking issue figured out w/o going backwards....its going to go like stink...(well I took artistic liebertys on what nick really said...but its the same meaning)

prop
Prop

Some pistons come with deliberately thick heads to allow for a certain amount of shaving. Of course, this will affect the balancing of the reciprocating assembly...

Non-functional vacuum advance should not be an issue here. Its purpose is to ADD advance under no-load conditions in order to enhance mileage and smoothness, etc. Your problem was under low-vacuum conditions (high load), so the VA should not have had an effect either way.
David "a bit of a vacuum here" Lieb
David Lieb

You can drop the compression to 8:1 and run 105 octane fuel

BUT

If the distributor has the incorrect curve for the engine it will still pink.

Daniel Thirteen-Twelve

I dont know why some are going overboard with advance curves and the like when it is obvious that the CR on this car is well above 12 to 1 and thus will not run correctly no matter what flippin curve the dissy has. Even those of us with Programmable ignition would struggle to curve that baby!

do yourself a favour Jonny, take the head off and get an accurate measurment of C.R. and then ask again on how that can be reduced.

If you then can get a sensible C/R of no more than 11 to 1 and you then experience detonation at that point the other things mentioned above can be sorted.

I can assure you with a pretty standard dissy curve for an a series 12 to 1 CR and a 286 cam running 95 unleaded you may detonation at around 3.5 thousand RPM under full load, ask me how I know this? because I spent thousands of pound and hundreds of hours doing the tests. I do not guess.
Bob (robert) Midget Turbo

Jonny,

Have you used several compression gauges to check the engine? If my math is right, 17 bar is 250 psi, which starts to approach the lower range of compression for a diesel engine.

If it were mine, I would not pull the head based on one compression gauge.

Charley
C R Huff

seeing as nobody else has asked, did you do the compression test wet or dry?
David Smith

Agree with Nick's calcs that the CR could easily be 13:1 if the head or block has been planed significantly, which would be consistent with the compression reading.

We have run 11.5:1 successfully on a range of A series road engines with quite mild camshafts (around Kent 276). I agree with others above that the key to this is modifying the advance curve to cut back advance in the mid range while retaining about the same maximum (when the revs get high, volumetric efficieny drops off). This can be seen, for example, in the differences between the ex-factory advance curves of the 1275 Midget (8.8:1) and the 10:1 Cooper S - simlar start and finish points, but a few degrees less at 2-3000 revs.

Higher octane (~98)is also needed - I wouldn't even bother with 95.

The other critical factor is that as the CR rises, so colder plugs must be fitted.

I also have run 12:1 on a road spec MGB engine (which are usually touchier than the A series) for years (again, higher octane) with appropriate mods to chamber shape to reduce squish, the same treatment for the advance curve and much colder plugs.

However, above that, we haven't managed to tame it other than by having a ropey cam profile of 300 deg++.

The symptoms occur for all the reasons stated above;

- spontaneous detonation
- too much advance for compression ratio
- plugs wrong heat range

but the key to solving it is to identify which are a problem and which aren't.

I'd work through it like this:

1. Fit BP8ES plugs or equivalent such as N6Y (we've found that to be about right for high compression engines in good order).

2. Follow David Lieb's suggestion of retarding the distributor and seeing what that does to the midrange. Drop it back to around 12-15 deg at 3000 RPM. It will go sluggish but prove the point one way or the other and is much easier to do than fitting another gasket (even simpler than re-checking the compression). If it fixes or improves it, go to step 4, if not then there is probaly a detonation problem so do step 3.

3. Re-check the readings, if correct try another head gasket.

4. Once around 15-15.5 bar, that's the stage at which it's worth working on the advance curve. Easy to do - I've described how to do it at http://www.mgparts.co.nz/Setting_up_Ignition_timing.html. This uses the B as an example, the Midget is similar but establish the advance at 3000 and 4000 RPM as well as those listed.

My suspicion from the description of the symptoms is that it is a detonation problem, but steps 1 & 2 above are so simple that it's worth carrying them out to confirm
Paul Walbran

What a help ful post Paul.

Johnny - you got your work cut out feller, but the answers are here!
Nick

Jonny

Re vac advance:

Removing the vac advance (ensure there's no air-leak into carb/inlet) will have some benefit for part-throttle, by eliminating any advance due to vacuum in the inlet. This should benefit all situations but WOT - to the detriment of fuel cons (but I don't suppose that's a worry at this point).

A
Anthony

hey paul,

What a great site you have, I loved the above artical, just schemed it but looking foward to really reading it later tonight..

prop
Prop

I think it boils down to the fact that I believe in knowing what to do and not hoping. All of the above can help to suppress detonation to some degree. However I do not see it as a difficult job to simply remove the cylinder head measure the CR and then KNOW what you need to do.

I have to say though that with programmable ignition timing higher than normal CR can be used because it can retard the timing significantly in mid range and thus utilise the CR where its needed at high revs without pinking at mid range. This can make big gains in power. If the CR is so high it may be nice to instal a programable system and have a play around with curves to see what can be done.

Paul BP8s are fine in summer and possibly all year round in New Z, but here in Blighty could be a problem in winter. (6 months a year :-()
Bob (robert) Midget Turbo

Bob

Agree with your comment about winter if CR around 10.5 - once around 11.5 it should be OK. Even after a night in parc ferme out in the snow (it does happen in the far south) they were fine.

Key point is that higher compression usually requires colder plugs (all other things staying the same). BP8's are a good starting point for very high compressions, but if they prove to be grumbly when cold in winter in any particular engine, work back from there a step at a time.
Paul Walbran

Jonny
If I were you I would be removing the cyl. head and getting your combustion space (inc. head. gasket and piston to deck space)measured up acuratley to work out exactly what comp. ratio you have. THEN you will know if it's too high or not to run whatever fuel you want to run . After getting the compression ratio correct is when to start playing with ign. timing etc otherwise you are playing in the dark. Pull it apart and do it properly before it pops. Gary
Gary

Gary, The first part of your advice is ok but the 2nd part isn't.

With a cam change to the Kent 286 the distributor must be re-curved to get the timing correct, irrespective of what the CR is. Having a distributor re-curved is NOT the same as playing about with ign timing.

You re-curve the distributor and then play with the timing and after time on the rolling road may even have the curve adjusted.

The CR and the curve has to be correct.
Daniel Thirteen-Twelve

While we are on the topic of re-curving dist.

I have a mallory duel point, im going to install on my high performance engine(someday, in the next 30 years lol)....I can buy a curve kit for it that has differant springs and degree keys to (advance the curve and what not)

Is this kit doing the same thing as you are talking about when you suggest having the the dist. sent out and having it re-curved....or would more work have to be done to do a proper re-curve

my guess is YES to the 1st. part and NO to the 2nd part of the above, but wanting to make sure im not confused by language differances....aka (torch = flash light)as well as a general lack of understanding.


Im really enjoying this discussion on dist. and re-curves...as I dont know much more beyond high school basics on this topic....(thats embarresing to admit)

http://www.malloryperformance.com/pdf/29014.pdf

http://www.malloryperformance.com/pdf/29015.pdf

prop
Prop

Well, somebody set me straight here if I have it wrong, because I have messed up the math before. But, isn't it correct that there is no need to tear the engine down to calculate the compression ratio. In fact, if it were torn down and measured, we would only have the mathematical compression ratio and not the effective compression ratio. The mathematical does not take into account cam effects, volumetric efficiency, etc.

So, since Jonny's engine is already together, and he has run a compression test, the effective compression ratio should already be known by calculation from the measured compression.

Is it so that Pressure 1 x Volume 1 = Pressure 2 x Volume 2? (P1V1 = P2V2)

If so, isn't the effective compression ratio 17:1 ? Volume 1 and Volume 2 are unknown, but P1 x V1 is known and P2 x V2 is known. If P1 is atmospheric pressure, it is 1 Bar. So, 1 times Volume 1 = 1. And P2V2 is 17 Bar as measured. It is the 17:1 CR that makes me think the gauge is suspect.

So, what have I got wrong here? I know I've screwed this up before. Maybe we have to subtract 1 Bar for the difference between absolute and gauge pressure? I may have the math wrong, but I'm sure the concept is right, that there is no need to tear the engine down to know the compression ratio.

Charley
C R Huff

you've forgotten about Volumetric efficiency. When the piston hits BDC it hasn't filled the whole cylinder with air/fuel mix, also for quite a few degrees ABDC the inlet is still open so you're not compressing anything yet.

So, if you had an engine with 100% VE and no cam overlap which has 17bar on a compression test then you could say that the CR is 17:1. unfortunatley things arent that simple.

Nick

OK,
Here is another question on compression ratios.
If I do a compression test on my engine, WOT, 10 revs with the starter motor I get consitant 195 - 200 lbs / sq inch.
I believe atmospheric is around 13.5 so does this mean my CR is 195/13.5 = 14.4:1 If not, why not? Seems a bit high!

Guy
Guy Weller

Guy, isn't atmospheric 1 bar i.e. 14.9psi?
That would give you 13.4:1
But a compression check is also affected by sealing of the valves and piston rings.
Tarquin

Tarquin,
I obviously live in a more rarified atmospere than you over in Dense Wakefield. ;-)
But your comment about valves and rings would make one think that the real CR was even higher! Cannot be!
My guess is that the compression is climbing as it doesn't return right back to atmospheric between strokes
Guy Weller

Atmosphereic pressure is near enough to 1 bar, 15 psi.
One thing the above calcs based on Boyle's Law overlook: It assumes constant temperature. As we all know, during the compression stroke the temperature rises substantially which of course is part of the whole problem with CR - that's why there is eventually a CR at which the fuel spontaneously ignites. (In fact, deisels depend on it).

This makes the pressure/volume relationship depart from the simple Boyle's Law situation as the pressure rises in response to temperature as well.

For that reason, the rough rule of thumb is that cranking pressure on standard-ish engines is 20 times the compression ratio.
Example: Standard MGB or Midget with just under 9:1 standard give 170-175 psi on cranking (WOT, hot engine and good nick). There are plenty of other examples.

Delayed inlet valve closing (by longer duration or retarding cam timing) reduces this cranking pressure for a given CR. For example, at around 300 deg duration, it would take around 10:1 to get 170-175 cranking pressure. The difference is very apparent in cranking the engine with the starter after such a change in cam profile (which I have done overnight between events on many occasions when I was young & stupid). For a 286, it would be about half a ratio different.

To go back to the beginning, my take on 17 bar on a 286 is about 13.3:1. So I agree that the engine is probably headed for self-destruct if nothing is done.
However, it is always useful to test what you can simply do prior to dismantling, it helps with the interpretaion afterwards - especially if, as sometimes happens, you don't find what you were expecting. (Another lesson learned the hard way)
Paul Walbran

If the original reading of 17 bar was faulty (it can happen), then it's a completely different scenario of CR.

The pinking symptoms of this engine also occur if an engine with original CR 9:1 retains original advance curve and spark plugs at a CR of around 11:1, yet with proper setting up 11:1 is very useable. That's why it is useful to do the simple checks first.

It takes only 10 min to change the plugs, crack the timing back a few deg and do a single test run and get a helpful insight on what you might find if you strip the engine.
Paul Walbran hit the wrong key

Nick,

I may not have it right, but I don't think I’m wrong because of forgetting about volumetric efficiency.

In fact, I am saying quite the opposite. I am saying that the compression test accounts for the volumetric efficiency, at least at cranking RPM, while the mathematical CR ignores it.

Charley
C R Huff

Paul

You explanation is fine (PV/T = constant). May I add... you are referring to an adiabatic process whereby no heat is lost from the system. However, this is not quite the case.

The faster the engine cranks over, the less time there is for heat losses to the piston crown and head, so the temp / pressure rise (and apparent CR) are high.

If the engine cranks over slowly, there's more time for losses, so there's less temp, pressure and apparent CR.

This is also true for air leaks from the system, e.g. past the rings.

The diabatic process inside the cylinder also limits the engine's ability for economy at low revs (when there's more time for heat loss (ie pressure fall) compared with the excess pressure doing work to move the piston down).

BTW - good to see you at 'stone last month.

A
anthony

Anthony
Agree 100% with your fuller explanation re the varying extent to which (and when) the process is adiabatic. I was trying to be simple (= lazy) and stay in the context of taking compression readings, where enough heat is retained to affect the pressure recorded.

Good to see you at Sstone too - I was going to try & catch up after our brief chat but the weekend went so fast ... and now I'm back in the winter. I'll just have to come over again :-)

Paul Walbran

This thread was discussed between 15/07/2008 and 19/07/2008

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.