Compression test on 205 gti
Discussion
Following my recent dissapointment on the rolling road I figure there is no point getting my carbs re-jetted if my engine is a pile of pap.
I am going to do a compression test. I am going to warm up the car and test each cylinder,
Q's
1) Go I need to give it some gas while cranking
2) I have been told I may have to leave the plugs attached the leads on the ones I'm not using otherwise it will mess up my ignition (HOW?????)
3) What should I expect, anything above 10bar seems to be common opinion, any help appreciated.
I am going to do a compression test. I am going to warm up the car and test each cylinder,
Q's
1) Go I need to give it some gas while cranking
2) I have been told I may have to leave the plugs attached the leads on the ones I'm not using otherwise it will mess up my ignition (HOW?????)
3) What should I expect, anything above 10bar seems to be common opinion, any help appreciated.
Yep, throttle wide open throughout. Disconnect the low-tension side of the coil to disable the ignition safely.
You'll find a really useful page here...
http://www.pumaracing.co.uk/comp.htm
Good luck.
You'll find a really useful page here...
http://www.pumaracing.co.uk/comp.htm
Good luck.
V8ish said:
Thanks, that is a useful article.
But how do I disconnect the low tension side of the coil? What am I looking for?
Remove the cable from the LT (not the HT, although you could remove the king lead) of the coil so that when you crank it over it doesn't suddenly spark you won't want the engine to fire when compression testing.But how do I disconnect the low tension side of the coil? What am I looking for?
You'll have one of two types of coil on the 205, either a traditional can (mounted to the inner wing) or a more modern looking compact coil (mounted to the inlet manifold). To disable the former you need to disconnect the spade terminals either side of the HT, and to disable the latter just unplug the multi-plug near the HT. Job done.
It's differences between cylinders that you want to worry about. Anything more than 10psi difference will be a cause for concern. 140-150psi is a healthy figure IIRC (could well be higher though as I had a lowered compression turbo conversion engine). I think that you'll be lucky to see 140bhp at the wheels, let alone the fly (which is a meaningless made up figure anyway). Post up the results of your compression tests first.
will do. Regarding disabling the ignition, I have 4 spade connectors on my coil. Surely just removing the HT and at least the two towards the front of the car will suffice? I'm doing the test tomorrow. I will do a wet test if need be.
The car came with a spare head reputed to be from longmans though I have serious doubts. It is painted lime green but I reckon thats were the longmans connection ends. The ports look standard, as does the cam and its not polished. Any thoughts on how I check to see if the cam and other bits are standard or modified?
Cheers
The car came with a spare head reputed to be from longmans though I have serious doubts. It is painted lime green but I reckon thats were the longmans connection ends. The ports look standard, as does the cam and its not polished. Any thoughts on how I check to see if the cam and other bits are standard or modified?
Cheers
Yeah, just removing one set of the LT spades will be fine.
The standard head is pretty good in the grand scheme of things, so a decent head-job is most likely to be an optimisation of the standard design and may not look too different from OE. I don't think polishing is something that decent tuners bother with. I'd have also thought that cams will look much alike, and you'll only be able to tell the difference between standard and non-standard through measuring.
Having said that, chances are it isn't a Longmans unless there's supporting documentation!
The standard head is pretty good in the grand scheme of things, so a decent head-job is most likely to be an optimisation of the standard design and may not look too different from OE. I don't think polishing is something that decent tuners bother with. I'd have also thought that cams will look much alike, and you'll only be able to tell the difference between standard and non-standard through measuring.
Having said that, chances are it isn't a Longmans unless there's supporting documentation!
V8ish said:
right, just done the test. I have a massive 14.5 bar or 210psi!!!!
I assuming someone has taken a massive chunk off the head at some point. Whats the implications of such a high reading?
Cheers
210psi would be a light skim on a very healthy head. 180-190psi would be about right on a healthy XU9J.I assuming someone has taken a massive chunk off the head at some point. Whats the implications of such a high reading?
Cheers
Alternatively, it could be a HUGE skim on a knackered head....
I think you should also take any readings with a pinch of salt, unless you have a very expensive gauge that's been recently calibrated.
All you're looking for is decent consistent readings across the cylinders and vaguely healthy readings. If you've got those you're probably alright.
All you're looking for is decent consistent readings across the cylinders and vaguely healthy readings. If you've got those you're probably alright.
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