Low Comp - what are my options

mkitbna6 · Postby **mkitbna6** » Sun Oct 12, 2014 4:30 pm

Hey guys. About 6 months back I picked up a silver NA6 with a pretty attractive mod list, for an equally attractive price.
Some of you may remember it from: viewtopic.php?f=57&t=41205

I used it as a daily for about 4 months, then after battling with cold start issues I decided to have it looked at, with a view to having it re-tuned. The cold starts were fixed however a compression test revealed bigger problems. The car is currently running between 90-115 across the four cylinders. I am fairly certain the issue is the piston rings, rather than anything else. I am not overheating or experiencing any specific negative side effects and its still making 120hp but this is obviously a problem.

I am wondering whether I should pursue a rebuild or simply sell the engine as is an drop in a replacement. I've not really had a chance to do much with the car because of my work and study, but I'm hoping to get cracking with something over the summer so I can fully enjoy the car. Any advice is welcome. Also if anyone is looking for a worked, albeit tired engine then we could talk swaps etc

Cheers, Matt.

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Magpie · Postby **Magpie** » Sun Oct 12, 2014 5:12 pm

Do a leak down test.

Can you please give an explanation as to how you did the test? How many cranks, throttle wide open etc... Were the plugs oily or built up with carbon?

Mr Morlock · Postby **Mr Morlock** » Sun Oct 12, 2014 6:12 pm

Magpie knows so do others but a mechanic should also be able to identify if its worn- rings etc. In all likelihood if you rebuild it the cost will be higher than buying another engine Older cars become problematic because they are often at the bottom end of the resale market. You will have to do the sums and decide. It also depends on how long you are likely to keep the car. If the car is not blowing smoke you can also just keep driving it. A professional it costs you something in the order of $70-100 per hour.

93_Clubman · Postby **93_Clubman** » Mon Oct 13, 2014 11:05 am

As Magpie alludes, it's critical how the comp test was done:
viewtopic.php?f=29&t=59832

manga_blue · Postby **manga_blue** » Mon Oct 13, 2014 12:35 pm

You need further diagnostics:wet/dry test, leak down, ...

Low compression can come from the bottom end (worn rings, pistons, bores) or the top end (worn valves, valve seats, valve guides) or both. If you have a good head and a stuffed bottom end then it might pay to just replace the bottom. If the bottom is good then you have reasonable options of reconditioning the head (which might be well worth it if it's already well worked) or replacing it. If it's both then an engine swap is probably best. Your own level of skill, access to good tools and workshop space are big factors.

sailaholic · Postby **sailaholic** » Mon Oct 13, 2014 1:48 pm

Given that the head and cams on that car came off a race escort (and i assume fairly highly built), it's entirely possible some of your loss of compression is from big cams and port volumes giving very poor efficency at the relatively slow speed your starter is cranking over at.
Manga is probablly better able to comment on how much of a drop you would expect.

manga_blue · Postby **manga_blue** » Mon Oct 13, 2014 2:08 pm

Sorry, I didn't realise it was that worked. Big cams can make a huge difference to cranking compression tests. Scavenging overlap reduces the effective length of the compression stroke. Depending on cam timing, 100psi could be fine. Do you know what the cam timing is, or at least the overlap? Do you know what the static compression ratio is? e.g. 10.5:1 pistons with 50 thou off the head

mkitbna6 · Postby **mkitbna6** » Mon Oct 13, 2014 2:52 pm

Thanks for all the advice guys. The compression test was done at hypderdrive when it was in for a tune, it simply came up in conversation when I picked the car up, Andrew recommended a leak test however he said in his opinion it was the piston rings. I had the car listed for sale recently, and had a friend who was interested come to have a look and he did a cranking compression test. There's also some evidence to suggest that the 3rd cylinder is only firing at high rpm, removing the spark plug lead on idle doesn't actually alter the idle.

In regard to any further information on the specs of the engine, I only have what is discussed in Les' earlier thread, the guy I bought it off hadn't owned it for that long and wasn't able to shed any light on the modifications. I have a mate who is really keen to do a rebuild, and is quite capable and I have access to the parts through work so I'm thinking that might be the best option at this point. I love the car, but it will probably have to go within the next 6 months as I'm heading overseas next year, so I wanna give it some more life and move it on to someone else, its too good to let it slip anymore.

mkitbna6 · Postby **mkitbna6** » Mon Oct 13, 2014 2:58 pm

Also, no smoke, no other issues, bit smelly and a bit thirsty but otherwise it runs and drives fine..

sailaholic · Postby **sailaholic** » Mon Oct 13, 2014 3:54 pm

Big cams, head work and itbs can equal smelly / thirsty.

From what I remember of les saying the exact specs on the head are a bit hush hush.

I'd be doing the tests before diving in. There may be very little wrong with it.

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mkitbna6 · Postby **mkitbna6** » Mon Oct 13, 2014 4:28 pm

Thanks heaps. Obviously, without being able to provide hypderdrive with specs, the advice was purely based on what Andrew could see and the info he was working with on the wolf ecu and likewise with my mate who was interested in purchasing the car. I did try to reach out to Les through facebook (bit stalkerish I know) a while ago, and I haven't been able to find a listing for "yager performance" so I'm not quite sure what to make of everything... At the end of the day it drives fine. Might just throw up an ad and not worry about it? I don't wanna be a dog and sell this on with problems, but I don't know how else I can be better informed. Pulling apart the engine would certainly give answers, but I'm not prepared to do that on a whim because its a rather substantial undertaking that could end up worse in the long run.