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100psi after engine rebuild?


mike91

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Hello again,
I finally got my 1500 engine running after having a total engine rebuild, it seems to run fine although I haven't been able to actually drive it yet...
However, I did a compression test on it yesterday, only to find that all four cylinders were around 100psi?! which seems crazy giving me a compression ratio of about 6.6?
I only had 0.005" off the head at the engineering shop and they supplied the pistons (flat tops) so they must be okay.
So, what could be causing such a low reading? I tried putting light oil down the bores which didn't really help...
Any suggestions welcome!
Cheers, Mike.

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Did you get the engine up to running temp before doing the test? A cold engine will give significantly lower results.

Otherwise, what work did you have done to the engine? At the end of the day, there are only a limited number of issues that can cause low comp, namely something wrong with rings/bore etc, something wrong with the valves, or something wrong with the headgasket.

In fact, that is a thought, what gasket did you use?? If you have recessed bores, did you use the correct gasket, or vice versa??

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Compression tests are best done warm, with all plugs removed and the throttle should be wide open when cranking.

The accuracy of some of these gauges is also suspect, so the comparison between cylinders may be more valuable than the absolute number.

It will take a few miles for the rings to seat, but really not very many if the bores were properly honed.   You can't directly relate cranking compression to compression ratio. Camshaft choice and even valve clearances can have a big effect on compression at cranking speed - the wilder the cam the lower the cranking compression.

Cheers

Nick

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What made you decide to do a compression test ? did you suspect something was wrong ?  Did you do the compression test with the throttle wide open ? are you sure of the accuracy of the compression tester ? It may be just a case of the piston rings needing to bed in. As all comp's are similar I would not think any thing serious. ie broken rings . You say it was a total rebuild, I assume you mean that the head was rebuilt as well, so should rule out valve seats not sealing. Are you sure valve timing is correct ?

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hello guys!
okay, where to begin... the engine was definitely up to temperature when i did the test.
it has new, +0.02 pistons in it. when i did the test the plugs were all out, although the carbs were closed? it's a brand new draper gauge and i had a second opinion from my dad's compression tester too, definitely 100psi! :(

the head was rebuilt with the engine, with unleaded valve seats and i seated them in, so i know they should be fine... it has a fast road cam in it. as for the cylinder head gasket, i bought a payen one from the rimmer bros website and just assumed it would be right.

the information in that website is certainly interesting, although not all that consoling!

i am beginning the lose the will to live with this engine, i seem to have had nothing but problems the whole way through :/ but thank you so much for all your help! don;t know where i would be without it.

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the cam itself is from canley classics... it is based on a spitfire 1500 cam and then reground to the TR5 pattern, so i timed the cam using the same timing marks as the old cam that came out in the first place?

http://www.canleyclassics.com/?xhtml=xhtml/product/camshafts.html&xsl=product.xsl

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i jsut spoke to the chap at canley classics and he said that my timing is correct and that would have no bearing on such low compression.
the trouble is that the car is not drive-able now, so i need a leak tester to come to me!

i don't see how it could be bores, given they are brand new. and it can't be valves, because it is so equal with 100psi on every pot.
so it brings me back to maybe a poorly fitting or incorrect head gasket?

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Quote:
so i timed the cam using the same timing marks as the old cam that came out in the first place?

The timing marks might not be matching between the cams.
Given that the actual cam timing is uncertain and that all cylinders are close to 100psi I would look at the cam timing first.
If it would be the bores or the valves I don't think all would be that equal.

You'll need to buy or borrow a DTI(dial test indicator) and magnetic stand, but the timing disc is easy to make yourself, just print and stick on cardboard.
http://3.bp.blogspot.com/_gbvTDtUDS-Y/S-B11DC8z_I/AAAAAAAAAkY/yNd4jJrRZN8/s1600/timing_degree_wheel.jpg


http://www.google.nl/url?sa=t&source=web&cd=4&ved=0CDUQFjAD&url=http%3A%2F%2Fwww.cranecams.com%2Fuploads%2Finstructions%2F803.pdf&ei=H8nHTdrZDMr1sgbXpZiXDw&usg=AFQjCNHtKFItCNIgwS96B9-WDgU5_iCHDQ

http://www.youtube.com/watch?v=la6-EgnCOl4

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i suppose a way to test if it is the cam or not, would be to take off the rockers(given that the head has got to come off anyway) and then test compression, if it rises then we know it's cam timing, right?

then if it doesn't rise, take the head off and have a gander.
now that you mention it, Richard, I do vaguely remember the engineering company mentioning something about the recessed bores and flatting them out or something? it's all just a colourful bur now though...

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438 wrote:
i suppose a way to test if it is the cam or not, would be to take off the rockers(given that the head has got to come off anyway) and then test compression, if it rises then we know it's cam timing, right?
...


Mike

I don't think this will work mate.
No rockers = no inlet valves opening = no air going in to compress.

Glen.

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438 wrote:

it has new, +0.02 pistons in it. when i did the test the plugs were all out, although the carbs were closed ?


You need to have your foot fully on the throttle when you do a compression test, because otherwise the butterflys are blocking the airflow. You need to remove as many inhibiting factors as possible, hence the following need to be done:

1)Warm engine, to allow any parts etc to fully expand and seal as they should. I normally go out for a quick 20min spirited drive, then do the test as soon as I dare put my hand near the engine!! A pair of gloves helps!!

2)Pull the plugs. This allows the starter motor to spin the engine over as fast as possible, to get a decent airflow moving.

3)Foot fully on the accelerator, so the butterflys do not block flow (remember they are held closed by several springs, so it takes a fair bit of force to open these. Ignore the pistons, these should move easily by themselves).

4)Crank the engine for a good 30 -45 secs, let the pressure reading stabilise out. It takes a few turns of the engine before the highest pressure is registered.

5)Ensure your battery is fully charged, if needed stick some jumps leads on it from another car to boost the power. You want that starter motor turning as fast as it can!

Good luck!!

Phil

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438 wrote:
take the head off and have a gander.
now that you mention it, Richard, I do vaguely remember the engineering company mentioning something about the recessed bores and flatting them out or something?


Has the ghead gasket got a rectangular tag at the rear? If so it's for a recessed block. If your shop removed the rings from the block you need an earlier gasket.

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good point about the valves not being open! hadn't even considered that...

i will head over there after work today to try again with the butterflies open, my local triumph suppliers have both recessed and non-recessed head gaskets in stock so all is not lost!

to be honest, i have been at work all day but been so worried about my engine i have spent most of it on the phone and internet trying to find a solution! it will be nice when i know it is definitely one thing or another.

cheers, mike.

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OK, this is (I think  ::) ) a picture of a recessed bore block. (At least  I hope it is, seeing as I have had three blocks with recessed bores and they all look like this!!)



I have always assumed that a non-recessed bore is straight flat across??

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I think you have been diverted by the compression issue. Yes they are low readings but they are also very even which is an important guide. Even at 100psi it should still run. Assuming the head has been built correctly then just put it back together and find out why it won't run - it will either be valve timing, ignition timing (don't forget firing order) or fuel. If those compression results really are accurate (which I doubt) then it won't be great but it will run.

If your carbs are clean, full of petrol, and everything is timed correctly with a spark then it will run. Keep these simple principles in mind and you will find the problem, and odds are it will be something simple.

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thebrookster wrote:
OK, this is (I think  ::) ) a picture of a recessed bore block. (At least  I hope it is, seeing as I have had three blocks with recessed bores and they all look like this!!)

I have always assumed that a non-recessed bore is straight flat across??


Correct. The non-recessed block bores have no step and are flat all across the block surface.

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438 wrote:

the engine was definitely up to temperature when i did the test.
it has new, +0.02 pistons in it. when i did the test the plugs were all out, although the carbs were closed ? it's a brand new draper gauge and i had a second opinion from my dad's compression tester too, definitely 100psi! :(


So, it does run fine and you did the compression test with the throttle closed.  Also you have a TR5 profile cam.

I think you are worrying about nothing.  The low figure will be a combination of the closed throttle and the camshaft.

Unlikely the gasket is wrong - if you use a recessed gasket on non-recessed block it seals the cylinders but water pees out the joint, non-recessed gasket with recessed block and it's unlikely you'll get any compression at all.

Nick

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Nick_Jones wrote:
if you use a recessed gasket on non-recessed block it seals the cylinders but water pees out the joint, non-recessed gasket with recessed block and it's unlikely you'll get any compression at all.


Ok Nick how did you find this out? ;)

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hello!
i have some good news!
i popped over to the car after work, and tried the compression tester again and sure enough 100psi all over.

so i put my foot to the floor and tried again, and it rocketed up, right up to 225psi!
so now i have gone from one extreme to another, although i am at a more pleasant extreme now :)

thank you so much for your help again!

cheers, mike.

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