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MG Midget and Sprite Technical - Compression pressures

I checked the compression of my 1310 sprint engine yesterday for the 1st time in 5 years of ownwership and I wish I hadn't now!
Pressures (with the throttle wide open) were 250, 145, 250 and 145 psi (Cyls 1-4). Now I know that the indicated pressure may not be accurate - I am using a gunson gauge - but the variation is a puzzle as the head was modified by a proffessional only 6 sprints ago and there isn't a lot of crankcase pressurisation so I thought the engine was in good condition - it goes well.
What tests and checks can I do to determine the cause of the pressure variation?
Thanks in anticipation

John Turner
John Turner (Midget & MGB)

Try a leak down test. Easy to do if you have a compressor & the spark plug adaptor, otherwise your local garage should be able to do it.
Brad (Sprite IV 1380)

145 psi sounds OK to me; and 250 psi so high as to be an anomaly. The ways the compression pressure builds up is:

1. CR: that is 10:1 CR might cause 10x pressure build up (10 x 14 psi atmos)

2. Heat: as you compress the air, it heats up and therefore pressure goes up (vs heat loss through the piston and cyl walls)

3. Poor trapping of cam: with late inlet valve closing with piston rising on the compression stroke, some of the air that migh be trapped is pushed out before the valve closes and compression can begin

4. Fuel drops in the 'air' sucked through the carb become more vapourised and take up more volume (=pressure).

5. Speed of cranking: faster you crank, the better the trapping and the hotter the compressed air will be (=more pressure); less time to lose heat through piston and cyl walls.

Hence, no-one can give you a figure for your engine.

If you had 4 readings at 145, most of us would have said 'fine!' 180 would also be fine. But 250 in 2 of your cyls seems high.

A
Anthony Cutler

Anthony Your a genius,


I have always wondered what the compression on the gauge repersents, when it says 180, 190, 200 ect. like most I always figured it was just a number, as long as its consistant across all 4 cly.

So if a person want to know there CR, all they need to do is devide the tested number by there atomspheric air pressure at there elevation of sea level.

So if your tested number is 140 and the atmos. is 14psi, your CR is 10


Prop...I think My atoms is 17 psi....Prop
Prop

I'm seeing 210 across all 4 cylinders with a CR of 11:1. However, dividing 210 by 14.7 doesn't equal 11.
Daniel Thirteen-Twelve

Yeah,

I just ran some numbers also and came up high as well.

perhaps theres a part 2 to the math, that we need to take into account due to the valve timing and heat generated/lossed, during the compression stroke

prop
Prop

isn't the cam overlap part of the deal??

you are all talking about static compresion not dynamic (witch you are mesuring)
Onno Könemann

Compression readings are not the same as a compression ratio because of the volumetric inefficiency of the engine. Another way of considering it is that the compression does not drop back to zero (atmospheric) at the exhaust stroke - unless it is turning very slowly! So the next compression cycle starts at something above the previous one. As you turn the engine over the compression reading builds step-wise for a few revolutions. I always allow 10 compression strokes foreach reading, by which time it has pretty well stabilised to give a good measure. More important that you do the same for each cylinder to get consistency.


I wonder if your higher readings on cylinders 2 and 3 is an indication that the shared exhaust ports are not scavenging as well as they should?

Guy
Guy Weller

Hi Prop

If only it was that simple.

IF... your piston could pull in air all through its downstroke and the inlet valve shut immediatly at BDC (which it doesn't: it shuts AFTER BDC - some 45 to 70 deg or more later - that's say 1/3 of the way back up!)

IF... you could compress what's in the cylinder without it heating up

(probably one or 2 more IFs, incl no air leaks!)

... THEN your measured pressure would be somewhere near the CR x atmos

Unfortunately, we're dealing with all sorts of other factors as above that makes this rule not directly applicable.

Of course, your compression pressure rises with compression; in fact, taking into acount the additional pressure due to compression heating, you may do better than CR x atmos (as Daniel does).

But then the later the inlet valve closes during the compression stroke, the less gas you have (at least at low RPM) left to compress.

The slower the engine runs, the more time there is for the excess heat to dissipate, so pressure will be lower; and the more time there is for leakage of gas past the rings, so pressure will be lower.

In some ways, these effects can work in our favour. For example, if we installed high CR heads, our starter motors wouldn't be able to crank the engine due to the high pressure created during this process. Thankfully, a late closing cam reduces the pressure during cranking, so the engine will turn over fine.

And so on. In the real world, for every law we'd like to apply, there are 10 reasons why the law isn't telling the whole story.

A... I fought the law and the law won... nt
Anthony Cutler

Its not that simple....LOL.

is anything ever (:

I guess we can only hope one day

Back to the matter at hand.


Those numbers are SO consistant and patterned.... ( 250, 145, 250, 145 )You said you never checked the compression sence it was built,

Maybe 2 differant sets of pistons aka 21251s and 21253s if the head was shaved off a lot, that would account for the compression of 250 if those 2 cly. had 21253 pistons, after all the 21253s are rated at 10.0 CR right out of the box with untouched head and deck, some hard head shaving and you could easily see 11-12CR

Or 2 differant sets of rods (as in all 4 rods are the same) or the rods could be reversed (1 is 2 and 3 is 4)...but if that where the case you would hear the rods bashing into the crankshaft counter wieghts I would think, another thought surly the crankshaft cant be out that far out...or simple as something like 2 differant sets of rings

Im just thinking out loud...what throws me is the consistancy and pattern of the numbers, vary odd.

Prop
Prop

Ah Prop,
I thought that the two, 250 readings were for cylinders 2 and 3, but I mis-read that. Makes a nonsense of my "shared ports" theory.

Perhaps the pattern is a function of a scatter cam?
Guy Weller

>>>>>>>Perhaps the pattern is a function of a scatter cam? <<<<<<<<<<<<


That makes me go Oooouuu, good thinking. I wish I knew more about scatter cams to have an opinion beyoud Ooouuuu.

Perhaps we need a little more info on Mr. Turners engine build

to add to the pattern and consistancy, its also a differance of 95 psi from the low and high cly., thats almost a cly psi by its self.


I think your onto something Guy

Prop
Prop

Hi - again

Scatter pattern cams vary the timing between 1 and 2 and (by way of symmetry 3 and 4); the idea (on most I've seen) is to change the timing of the inlet lobe so that 2 might close early and 1 might open later to avoid 1 sucking out charge from 2 (since 1 will be on the downstoke sucking in and 2 will have started the compression stroke but the inlet will still be open). Robbing takes place across the valves/shared port.

It helps to think of the firing order being: 2-1 and 3-4.

The amount of timing difference is only 5 deg or so, and this would not be enough to provide such a big difference in pressure.

A
Anthony Cutler

Wow,

Lots of informed views but I feel the thread is getting hijacked into theory!
The cam is a 649, head gas flowed and large valves breathing thro a 45DCOE and exhausting into a LCB.
I've just rechecked with the engine hot (a colleague suggested that the piston rings might need some oil to seal properly), with the same (virtually) results.
I think the higher pressure is the problem as on my MGB I get around 160 on all 4 cyls. The question is still there - why such a high pressure showing in cyls 1 & 3

As always thanks for all the ideas

Regards

John
John Turner (Midget & MGB)

Forgot the 1.5:1 roller rockers!
John Turner (Midget & MGB)




Im likeing Brads idea more and more of a leak down test, and even a engine vacume gauge test to go along with That, those 2 test will tell alot.

So its not a scatter cam, You said its a 649 by whom? kent, piper, crane...ect.

My guess is this cam is somewhere between race and hot street...combined with 1.5 roller rockers (meaning the valves stay open longer), Im not sure what Im thinking at this point.

Prop
Prop

Hi John

What's the history of the engine?

Fresh to this thread I'd guess that you have 2 different types of piston - maybe different deck heights; or even 2 different con-rod lengths.

A
Anthony Cutler

Thanks Anothoy,

Being that your patting me on the back, Can you pat me a little harder to the left, and up a little, yeah, now tell me how sexy my hair is....LOl

Oh, Im on a roll...Is there anything I dont know about midgets and 1275s, OH Im so great, it just gives me a shiver down my spine....LOL


Prop
Prop

I'm beginning to favour the idea that the pistons/rods may be different in 1 & 3 cyls although with a reading of 250 I would have expected there to be problems with running the engine which there isn't. 250 is the best part of 80% greater than 145 which implies a huge CR - noting the discussions re. calculating CR.
I have found some info from the POs bills that a new piston was fitted (only one and don't know in what cyl).
For the present I shall leave alone and when I have to take the engine out next I will give evrything a good check through.

Thanks for all the info

Regards

John
John Turner (Midget & MGB)

John,

I wonder if you could check by looking down the plug holes with a good mini-torch?
The only way I think that compression could be altered would be by use of different shaped piston crowns, which you might be able to see, or maybe feel with a probe. The compression ratio of swept to unswept volume can only really be altered by changes to the unswept volume - essentially the cylinder head combustion chamber. Assuming that all pistons come pretty well to the top of their bores. The variation that you have might be given by having dished pistons on #2 and #4, and flat or even domed ones on #1 and #3. - ( No, perhaps not domed ones as I think these would contact the head). You could try to see or feel for any differences without doing any strip down.

Another thought - are all the plugs the same rating? I would expect the very high compression cylinders to run hotter and maybe need a different garde of plug.

Guy
Guy Weller

Guy - indeed. Additionally, each pair of cylinders would have differing timing requirements: the higher compression ones would need much slower advance rate and therefore be retarded wrt to the normal pair, esp for 2-4K rpm.

A
Anthony Cutler

I agree in principle with Guy, but note there there is more than one compression height in 1275 pistons, so the outer edge of the piston can vary between virtually level with the top of the block and at least 1 mm down. 1mm = a bit over 4 cc unswept voleme, which is the difference between 9.8:1 CR and 11:1 CR
Paul Walbran

Guy - I will certainly have a look down through a plug hole when it stops raining! When I last had the head off there wasn't any obvious difference in pistons but then I wasn't looking for anything in particular.
Paul - I am amazed at how much difference 1mm makes to the CR - when my head(?) was worked on last year I estimated the height of the piston below the block - I can see now that I should have been more accurate.

Will report back on what I can see through the keyhole!

JohnT
John Turner (Midget & MGB)

I seriously doulbt your going to glen much info from looking down the spark plug hole, or probing thur it, I think a better test is looking at the plugs after 5000 miles as guy makes note of...1mm is about the thickiness of the head gasket...the best way is to pull the head, and measure the dish and the space from the piston top to the deck.

Prop
Prop

I am re-activating this thread as I now have some more info which makes me even more confused!

On the advice of a friend I poured a small amount of oil into the cylinders with low pressures - the idea being that if it was a sealing problem with the rings then the oil would improve sealing, and test by turning the engine over, without plugs in.

Well, the result was that No2 cyl (which was showing 200psi (gauge)) now goes off the gauge - approx 300psi!!!! No 4 cyl (which was showing 190psi (gauge)) now comes up to 250psi - the same as cyls 1 & 4!!!!!

This all points to bore/ring wear but 2 things bother me 1. I don't get a lot of crankcase pressurisation which I would have expected due to 'blow by'. 2. Why is No 3 cyl showing such a high pressure when 'oiled'?

I know I will have to pull the engine and sort the bottom end out but I am puzzled as to what is actually happening.

Any ideas?

Regards

JohnT

John Turner (Midget & MGB)

You have to love Prop don't you?

Prop a 649 is not as you describe as "My guess is this cam is somewhere between race and hot street""

As usual there is no guessing involved.

The 649 is a well known factory special tuning cam
for full race and has a duration of 300 degrees. Nothing below 4000 RPM

John I could talk all day about CR but you are correct it is your problem that needs resolving and re inventing the wheel is not what you want to hear.

You have yet to mention what CR you have actually got? but with a 649 you will need a very high figure to make best use of it!

Regarding the pressures I have 2 observations.

1 A 649 is a high overlap cam and is therefore notoriously difficult to measure cylinder pressures accurately, for many of the reasons stated above!

This leads me nicely into the second point?

2 Which is why did you want to measure them in the first place? have you an issue you want to resolve?


I think you are getting roped into something here that is not being done correctly and for example the amount of oil put into the cylinder will vary the figure you get quite alarmingly. As you have noted.

As stated 250 is a high figure and on a normal engine would suggest that detonation would be happening everytime the throttle was floored, however what that would be like with a 649 cam is not something I have ever had to calculate or experience so who knows??

I think you should not worry too much unless you have some problem you are trying to sort out.

If you are unsure of the piston height or shape then simply remove the cylinder head and then you will be absolutely sure. Removing the head is a very simple task and is far better that trying to interpret potentially flawed readings.
Bob Turbo Midget England

if it ain't broke don't fix it
David Smith

John

Presumably you've been using the car during the intervening 16 months...has it given any cause for concern?

My compression figures (dry) for a 649 with 1.5 rockers and 11.5:1 compression were always around 180-190, so I don't think the cam is causing the huge variation.
Dave O'Neill 2

John,
The idea of the oil is to create a better seal at the piston ring. Then, if the compression remains low you know that there is leakage at the valve. If the compression increases, then the pressure loss is at the ring. That's the theory.

You only need a small amount of oil, and you really need to be consistent. I would add maybe 1cc of oil, using a syringe with a short bit of washer tubing attached so it can be directed to the edge of the piston and run down the sides of the bore to the piston ring. If you put much too much oil in then it reduces the swept volume and you will get a very high compression reading, which doesn't really help at all!

When taking your compression readings you are doing it with the throttle wide open each time aren't you? The throttle opening will also affect the readings. You also need to do it with all the plugs removed so that the engine spins reasonably fast and is consistent.

Guy
Guy Oneandahalf Sprites

Thanks to all - in order:

Bob - I can see there is more to cylinder pressures than I thought. I measured them in the 1st instance because I sensed a loss of power from early in the season to later on so naievely thought cyl pressures might give a quick indication of performance across cylinders

David - I am tending to agree!

Dave - The car has been running OK. Regarding pressures, even without oiling 2 of my cyls are indicating much higher pressures (250) than you are quoting.

Guy - I am afraid I was not at all consistent, again not really knowing what I was doing! I just poured a bit of oil down a funnel - very unscientific! All plugs were removed and with the throttles(DCOE)wide open


I am now minded to leave well alone and see how things go with the car in March.

Thanks again for all your inputs

JohnT
John Turner (Midget & MGB)

John

Individual gauges can give quite different figures, so comparing your engine's readings with mine isn't that meaningful unless the same gauge has been used. The important factor is the difference between individual cylinders when checked with the same gauge.
Dave O'Neill 2

John,
Sorry, I wasn't meaning to tell you the obvious. But one never quite knows on the BBS where to pitch things. Sometimes it doesn't do any harm to go back to the logical basics!
I can think of lots of reasons for low compression readings, but high compression readings are rather more of a mystery! I think that a healthy 1275 (or 1310) should be of the order of 190, and probably in the range 170 to 210, depending on the specific tune of the engine. 250 does seem to be very high though. In theory you can get high figures resulting from carbon build up on the piston crown, but that would be a lot of carbon!

I will be interested to hear about what you eventually discover!
Guy Oneandahalf Sprites

Is it possible that the testing gauge is inaccurate and reading too high and the suspect high pressures are in fact normal correct pressures, and what looks normal may be in fact be low.

Personally I would try with another CR tester before I made any further decisions.

Roly
Roly Alcock

Agree with Roly.

Otherwise it sounds like you have an infeasible coupling between 2 cylinders that are conspiring to make a 2-stage compressor... somewhere the MSA rules would make this illegal!

A
Anthony Cutler

This thread was discussed between 27/07/2009 and 22/11/2010

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