8V turbo compression test

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SteveNZ

8V turbo compression test

Post by SteveNZ »

What is a healthy compression test figure in psi for a 2L 8V turbo eng, Croma, Integrale etc? Is there a service limit?
Guy Croft
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Post by Guy Croft »

No, there isn't.

The thing about a comp test is that it is only a diagnostic tool that only tells you the cranking compression... when the engine is firing the ring pressure is far higher.

The test really only comes into its own for measuring variance between cylinders to ID valve or ring leakage. It is useless for determining how powerful the engine will be, especially because - as you know - n/a and boosted engines both rely for top-end power on overlap inlet ram and cylinder cross-scavenge. Things a compression gauge - can't measure.

In my experience a normally aspirated engine will be ready for a rebuild with less than 130psi hot cranking compression, and at that level will use about a pint of oil per 800 miles.
A 'mint' unit will give 190psi + depending on cams. I've measured GC StII units with comfortably over 230psi. The more overlap the lower the comp test will give, but don't count on it!

I take this low '130psi' figure from a 1608cc 124BC that I owned at university in 1977 - which did - 800 miles to the pint..! She still flew along, but you could tell the torque was well down..

A low comp turbo unit, well I'd set 130psi as the min on a worn unit and aim for 165-180 hot, wide open throttle. I mean at least 180.

To be honest, to make any sense of compression tests, you have to build the engine and measure it at build, after run-in, and archive the results for every engine you do to make any sense of it.

GC
Last edited by Guy Croft on November 27th, 2006, 6:43 am, edited 1 time in total.
1969race125
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Post by 1969race125 »

These figures are not from a turbo, but perhaps may be useful for before/after comparison.

My 2.0L high-compression NA 8V race engine had the following characteristics:

"Fresh" (after run-in):
Compressions (1/2/3/4) 204/198/201/201
Power (at rear wheels) 149 bhp @ 5700
Torque (at rear wheels) 154 lb/ft @ 4300

"Tired" (after two years and approx. 15 race meetings)
Compressions (1/2/3/4) 195/160/165/190
Power (at rear wheels) 134 bhp @ 5500
Torque (at rear wheels) 126 lb/ft @ 4000

This seems to translate (very roughly, given all the other unmeasurable factors involved) into about 2 seconds of lap time in a 1m25s lap.

Whether the compression loss is due to head gasket leak between 2/3, or just rings or valves, we will find out when we strip the engine down shortly.

Andrew
Last edited by 1969race125 on November 27th, 2006, 3:56 am, edited 1 time in total.
Fahrell
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Post by Fahrell »

Another important consideration besides the compression value alone, is the difference between the cilinders.... even if your engine have a good value, the difference must be none or very small.
Andr’‚© Farkatt
Chris H
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Post by Chris H »

Static tests are at best a gauge to see if the compresions are even or if its worn out.

Loads of factors vary the results.

Static tests are a good test of cylinder sealing and thats about it.

Cranking speed is slow and you get a high reading due to the fact theres more time for the air to get in.

You can do a dynamic/running test which is a better test or for diagnosing issues.

You need a decent tester with a bleeder valve in it. I have a Laser one which does the job lovely.

Basically leave all the plugs in apart from 1 where the tester will go in. If its efi unplug the injector on that cylinders plug, if its carbed or even efi with no cat just leave it it won't cause a plroblem unless you let it run on 3 or whatever for a long time, earth the HT lead for the cylinder being tested.

Start the engine. The gauge will go up reasonably quickly, when the idle stabilises press the bleeder valve to reset the tester, it will quickly go back up to a true reading.

Say the static test showed 200 PSI, with the engine at idle it will be around 100PSi or so.

Snap the throttle open, it should go to around 80% of the static cr. So in this case 160 PSi.

Check all cylinders like this.

This test is a test for cylinder breathing, you may find that the static test showed each cylinder to be within 1% of each other but on this running test there could be a difference of 20%!

Say you get 200 static, 100 running but only 110 snap. Then you have an issue, it could be cam lobe wear, clogged up valve from egr/heavy breathing engine or in instances with the rubber connector on carbs engines the things collapsed inside and sucking closed (they can look fine from the outside). On newer cars like BMW, Toyotas with the inlet butterflies to increase inlet charge speed these can be stuck shut. Same with dual plenums like on vx V6's etc.

Say its 200, 100 and 200 again. This is an exhaust issue, pumping losses are high on an engine like this, could be collapsed exh baffles, melted cat, or as I seem to see more and more squashed exhaust due to incident with speed bump! Thats only if they are all high. If its just 1 cyl then worn cam lobe, poor valve gapping, bad lifter etc.

I find its a good method to use, it can save a lot of time, people thinking injectors are defective when in fact the cam lobes severly worn (Ford CVH is well-known for it).

try it.
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Post by sumplug »

Standard 2.0 ltr 8 valve should register 205psi when fully healthy and all cylinders should be within 10% of each other. Once the best figure gets below 150psi or there is a big margin between cylinders, that means there is a problem with sealing and needs a rebuild.

Andy.
vandor
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US engines

Post by vandor »

US engines had low compression ratio (8:1), which I assume is similar to that of a turbo engine. Because of this their compression test figures are lower than those of higher compression engines. From my experience a new stock US-spec engine will have 150-155 psi compression and I've seen several well running used ones in the 130 psi range. I'd say anything below 120psi is ready for a rebuild.

Csaba
SteveNZ

Post by SteveNZ »

Thanks for the replies
Ptarmigan
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Re: 8V turbo compression test

Post by Ptarmigan »

I realize this is an old thread but I would like to say that it is very useful.

Thank you to all those who shared their knowledge so that I could learn something.
If it can help others in the future my static values for a hot engine were:

141
100
122
107

on a 2l dedra turbo integral with 90k km that was using 1/2 litre oil every 600 miles but otherwise driving seemingly quite well. It seems a good time to remove the head and investigate further.

Many thanks again,
Ptarmigan
Guy Croft
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Re: 8V turbo compression test

Post by Guy Croft »

Thanks - nice to see you here - and I am pleased the thread helped.

Maybe do the test again and see if the cyls with low comp take noticeably longer to achieve a peak pressure reading - it is usually valve leakage.

Before you pull head do a 'wet' comp test - hot engine, plugs out, wide-open throttle but before cranking squirt some oil (about 2 teaspoons) into each bore just prior to cranking and test again. The oil wets te rings and can improve the seal. If the comps shoot up it's worn rings/bores. If they don't its valve leakage or head gasket.


G
Ptarmigan
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Re: 8V turbo compression test

Post by Ptarmigan »

Guy,

Thank you very much for the additional input.

Putting a tea spoon of oil into cylinder 2 showed 150 then dropped to 119. Test were performed hot for 7 seconds each, with a pause of 60 seconds between each run. I'm not sure if the incompressability of the oil itself makes for a false high reading on the first run as I had to test by myself (not resetting the gauge part way through). Either way if I have a moderate budget Im hoping I can get the engine back working closer to spec. I'm ashamed to say I would rather have someone who knows much more about what they are doing do internal engine work than me. It took me a whole week last time I put a new piston in a very simple two-stroke engine.

If I understood correctly Guy you don't undertake work on these engines at the moment? I apologize if this forum is not the place to ask these things.

Best regards to all,
P
Guy Croft
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Re: 8V turbo compression test

Post by Guy Croft »

No problem prepping a block or head or providing parts such as we are able, we won't do full turbo engines though.

email me:

guy.croft@btconnect.com

Please be aware we don't do basic (standard) 'reconditioning' as such and frankly I'd challenge anyone to try on yours because you can't get half the parts now. All our crankcase assemblies are on race rod and pistons and have fully-prepped & balanced crank fw setups. None of the work is 'a five minute' job and none of it is er, 'cheap'.

G
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