Acceptable compression test results

[steamy]

Advice or thoughts please.

 

Just done a compression test on my '67 TR4A with a warm engine using a Gunson tester.

 

Results as follows:

 

Cyl dry wet

 

1 150 155

2 160 170

3 160 165

4 160 160

 

I think the dry results are OK? but should I be concerned with the wet results?

 

I am not aware of any engine mods so assume standard compression ratio and size pistons but cannot be sure.

 

What compression would you expect from perfect engine?

 

Thanks for any input,

Ian

 

 

[Motorsport Mickey]

The good news is there ain't any perfect engine ! Or comparison compressions, however the figures you've given are pretty much in the range I'd expect which is good news.

Bare in mind that the motley collection of parts that make up an engine and the settings you've used ( tappers for example) which vary and give different figures.

 

Mick Richards

[steamy]

That's very reassuring Mick,

 

Thanks

Ian

[john.r.davies]

Wet effect, +5, +10, +5, +0

 

That's not a big effect, and was zero for No.4.

To me that says poor valve seal(s?).

 

Why were you doing a compression test?

Symptoms?

 

John

Edited March 18, 2017 by john.r.davies

[steamy]

John, The 'power or acceleration' did not seem to be as 'crisp' and as I hadn't checked the compression thought it might show something?

As you think the valve seals might be in question is this because of the dry results?---where would you expect the PSI to be?

 

Thanks

Ian

[Motorsport Mickey]

"where would you expect the PSI to be?"

 

Aha, back to post number 2, I'd expect them to be just about where you have them...maybe.

 

When was the engine built ? What components were new in the build ? How many miles have you done in it since it was built ? What oil are you using ? What precautions have you been taking to prevent bore glazing ? Have you deliberately tried any BMEP running yet ?

 

All this information has a bearing upon what ...maybe, wrong...if anything is, with the engine and it's compressions.

 

Mick Richards

[ChrisR-4A]

Hi Ian, have you checked to see if the vacuum advance is working.?

Chris

[steamy]

Mark, Their are many unknowns in your useful list of questions. Using Castrol Classic 20/50 and no I have not taken any 'precautions' re bore glazing and I had to google BMEP! It seems to me I will just have to work through everything systematically. This will include the vac advance that you suggested Chris...thanks.

 

At least I know what I will be doing tomorrow

 

Thanks all for your inputs

 

Ian

 

Lovely looking car Chris!!

[john.r.davies]

Ian,

The number that the gauge gives you isn't relevant - a compression gauge is not a precision instrument, and unlike even a rule, you can't compare yours with what someone gets on another engine with another gauge.

What's important is that the numbers should be similar across the block, to exclude a piston with damaged/worn rings. or low pressure in adjacent cylinders to show a blown gasket.

The wet test improves the ring seal temporarily, so that the pressure should go up, but a poor rise indicates that the valves - usually the exhaust - are not sealing. Where they seat in the head, I mean.

 

If none are damaged, then lapping them in may cure it, or at worst recutting the seats (followed by lapping) but even just lapping will remove the "lead-memory". You should watch for valve recession thereafter, or else get steel seats pressed into the head.

 

Just checking, how much oil did you put in? A teaspoonful, 5mls, is the usual amount, followed by a quick buzz of the starter to spread it about.

 

Poor performance could easily be a valve seal problem and no wet rise at all could mean a valve with a chip out of the rim.

 

Inspection means the head comes off.

John

[steamy]

John, Thanks for your explanation...I have learnt a lot. Two 'squirts' from oil can so possibly a bit more than tea spoon? but did not buzz starter before the compression test.

 

I will start with the other suggestions and then perhaps head off if nothing else shows up.

 

Ian

[Motorsport Mickey]

DON'T take the head off !

 

Unless there is something ACTUALLY wrong with a car just drive it. If you googled what BMEP is it should have given you the idea that how cars are driven and oils used count for a lot.

Your car doesn't make noises, it doesn't overheat, it's mpg is presumably ok (otherwise you'd have mentioned it) the only thing you picked up on is that "the power or acceleration" seemed to be not as you expect or remembered it.

 

Last time you filled up did you use the petrol used before ?

Being as it's only March is the petrol in the tank "old" having been put in before Christmas with the car laid up over winter ?

These days petrol degrades quickly and even my mower doesn't like a half tank of old petrol from last November and it's performance is diminished.

Have you tried another tank of petrol ? it's not unknown (although unusual these days) to receive a tank of petrol which may not be to the correct quality etc etc.

From your lack of response to the earlier questions I take it you have no prior knowledge of engine rebuild dates, etc how long have you owned the car ? have you driven it much previously to allow you to compare how you think the engine should be performing ?

Redo the compression checks if you must, start the engine first and run for maybe half a minute to clear the bores of any existing oil. Then carry out the test, dry first making sure you hold the throttle open whilst the engine turns over for about 5 revolutions per cylinder. Record the figures and then add about 5cc (it's not critical) of oil to each chamber one at a time and carry out the compression check with open throttle and for about 5 revolutions for each cylinder and then see what you have.

 

You are much to early into thinking ...suspecting even, that something may be wrong. Johns comments are feeding your paranoia and muddying your thoughts, unless you have a desire to involve yourself in weeks of engine work do the simple (and sensible things) first.

 

Mick Richards

Edited March 19, 2017 by Motorsport Mickey

[stuart]

Fresh fuel will make a lot of difference and to be blunt how do you drive it? if youve been pootling around and doing short runs locally with it hardly off choke then that will make a lot of difference to the performance especially with regards to you noticing it had "Dropped off a bit"

It may just need an "Italian service"

Stuart.

[Motorsport Mickey]

" if you've been pootling around and doing short runs locally with it hardly off choke then that will make a lot of difference to the performance"

 

Back to the prior BMEP connection, drive it on torque in top gear (just a couple of hundred revs below 3500 revs) and give it full throttle, down to the floor and hold it on, up motorway gradients are good to load the engine. When it gets to about 4200 revs drop off the throttle, brake the speed down to about 3300 revs and do it again, and again. Mine gets this treatment once a month to keep it in nick, easyfooting it in top gear doesn't do it any favours, the full throttle in top gear at BMEP expands the piston rings and helps them bed in to the cylinder bores without harming them, helps to develop better compression (this is where we came in) better power, and less oil usage.

 

Mick Richards

[graeme]

Ian,

 

Have you checked the points, the gap could have closed up.

 

Cheers

 

Graeme

[steamy]

Firstly many thanks for all your inputs...very grateful that the TR fraternity are so helpful to each other.

I an not rushing in any time soon to take head off Mick. To answer a few of your thoughts: I had not driven it since November and since then been doing a full retrim including seats (interesting).

Old fuel and pootling about might well be a factor so will be giving it some throttle as per your suggestions Stuart and Mick.

I tried checking the vac advance as per Chris's suggestion today and it may well be a contributing factor but I didn't have enough suck but can check later in week when I can borrow a vac pump from my railway club.

I have only had the car since August 2016 and getting back into classics.......had an MGB in the day and as they say my other car is a Austin seven..(6.5hp)!!

 

Electronic ignition Graeme.

 

Ian

[Guest]

My 3A was in hibernation from October until last Wednesday. The weather was fine etc... The car ran like sh*t! But I am used to the symptoms after a winter under a tarp. A trip to to the pump for fresh 102 RON fuel (yes, readily available 500m from my house) cured the problem. Within a few miles the car ran (runs) like a Swiss watch again.

 

Reading your ideas about the compression hadn't crossed my mind, knowing that my car's engine is in good nick. However, thinking about the symptoms you encounter, I can see what you think. But like the others wrote: try fresh fuel first and a good trip to blow off the cobwebs.

 

Equally important: after a few miles on the road, open the bonnet and check for small leaks of fuel under the SUs. Sometimes, the gaskets dry and start to leak the moment fresh fuel hits the carbs.

[john.r.davies]

DON'T take the head off !

Johns comments are feeding your paranoia and muddying your thoughts, unless you have a desire to involve yourself in weeks of engine work do the simple (and sensible things) first.

 

Mick Richards

 

http://mrclan.com/fastdl/tfc/sound/misc/owned.wav

 

 

 

 

 

 

Quite right, Mick, but then he did ask the question.

JOhn

Edited March 20, 2017 by john.r.davies

[ChrisR-4A]

Hi Ian, re vacuum advance, you don't need much suction to move the baseplate, pump not needed. Since 1996 I have replaced two vac adv units, current unit been on for about 10 years and is ok at moment. First sign when it's not working is lack of power (torque) at low revs going uphill. Over about 2000 revs engine should still run fine.

[steamy]

Thanks Menno I will be putting in new fuel and give it a thrashing as per other inputs.

Re vac advance Chris my 'suck would just produce the slightest movement on base plate but I couldn't hold it there with sliding my tongue over end of pipe....this may well be faulty then...the pump I can borrow will give 12-15hg continuous and will show if is leaking etc. however I think? my problem was across rev range but will check all when the sun comes out.

 

Ian

[Lebro]

Your pump, if continuous won't show up a small leak. The tongue is the best way in my opinion. It should hold the new position till you "disconnect"

The diaphrams very often get a small puncture. I had to replace mine due to this.

 

Bob.

[Motorsport Mickey]

Thanks Menno I will be putting in new fuel and give it a thrashing as per other inputs.

 

Ian

Just remember Ian, revs won't cut the mustard, it's the torque applied at a held full throttle throughout the rev range 3300-4200 revs (no more than 4200 revs) under a full engine loading which expands the rings and beds them into the cylinder bores.

 

Mick Richards

[steamy]

I probably misled you Bob with saying continuous....There is a limit switch which is set at say 15hg and pump will cut out when it reaches this limit but if it looses some vacuum the pump kicks in again which will show if there is a leak. I think there is a faulty diaphram which is why I cannot get enough 'suck'?

 

Point taken Mick--cheers

 

Ian

Edited March 20, 2017 by steamy

[MOOBUG11]

Anything over 100 is OK or 9kg..