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Compression question

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Tabcon

Tabcon

Jedi Warrior
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I've been doing all the pre-tuning I can do without actually being able to run the car down the road, and after a compression test, all the cylinders are close to being within 10% of each other with the exception of the number 3 cylinder which is higher then the rest.

The readings are:

No. 1: 135 PSI

No. 2: 125 PSI

No. 3: 160 PSI

No. 4: 145 PSI

I re-adjusted the valves and did the compression test with the engine warm.

I installed a mild cam with a duration at .50 of 230 degrees and I was wondering if perhaps the valves should be adjusted differently than the standard 0.010 clearance. Should I use a different clearance for the exhaust valves and the intake valves because of the cam?

I was also thinking that maybe the compression chamber for the number 3 cylinder may be a bit different than the rest for some reason. The guy that did my head supposedly CC'd the chambers so it would have to be off a little to account for the higher compression.

Any ideas?

Should I do a leakdown test as well?
 
The proper clearance is part of the cam design, so you need to find out what the setting should be for your particular cam. It doesn't just depend on duration (or lift). The chart on Larry's web site might be a good place to start:
https://www.tildentechnologies.com/Technical/TriumphCams.html

I've heard a lot about mis-cut cams, so it might not be a bad idea to check that the valves on #3 act the same as #2 (in terms of lift vs timing).

But otherwise, I'd break it in first and then re-check. It might be just that the rings aren't seated in the other cylinders yet.
 
I have a non-std performance high-er lift camshaft in my TR-6 and the valve clearance specs are wider at 14thou intake, 18thou exhaust rather than the std 10thou.

If you shaved the head (I did by 100thou) then provided he had the milling machine set parallel with the face when he started you shouldn't have changed the relative cc volume of the head by much and I doubt that would explain the fairly wide variance you've got.

A leak down test would let you isolate if the valves or the rings are leaking.

Did you change the rings or valves?
 
I located the cam card and the correct clearance is .013. Not a huge amount of difference, but I readjusted the valve and checked the compression again. This time it was:

1: 130
2: 115
3: 160
4: 135

I'm not concerned about 1, 2 and 4, it's number 3 that has me puzzled. After the lash readjustment, the other cylinders changed a bit but not number 3.

The head was shaved, but I cannot recall by how much. Compression is supposed to be in the neighborhood of 10.5:1.

Everything is new in the engine, which is essentially built as a moderate race engine with the exception of the cam which is more of a fast road cam. I have a brand new cam I did not install because I thought it may have been a bit to much for street use. At this stage, I really don't want to tear it down and install the other camshaft, but I may have to if I cannot figure out what is up with the number 3 cylinder. My guess is that the number 3 combustion chamber may be a few cc's smaller (or larger?) than the others. If this is the case, pulling the head would not be a huge deal.

The engine runs great with the exception of a slight lag coming off idle to about 2,000 RPM's. I'm going to buy a Gunson Colortune and see if I can get the Weber's straightened out. Hopefully this will cure the lag. If not, I may need new idle jets I guess. I wish I could drive the chassis down the street but the local cops would have a s#$t fit!

I think you are most likely correct about breaking it in first before I do much more. It's just hard to adjust the carbs with the variance in the compression...but it can wait.
 
Tab,
To my mind 160 is what you should expect on all 4. I agree with Randall that the most likely discrepancy is from lack of running in (excellent link also, Randall). While not at all conclusive, a wet compression test that brings the others close to #3 might set your mind at ease. Tom
 
Tab,I think I might be wondering why dont the other cylinder's valves and rings seal as good as #3?
Did the head guy just grind and install,or lap afterwards for pattern check and
perfect seal.
I have done more MC heads(cylinders on my flatheads)than cars and never assemble without hand lap finish after grinding.Not sure if this is common practice on car heads with twice as many cylinders,and expensive shop time.
Have Fun
Tom
 
I agree, I think the compression should be higher also. The head was done by a Jerry Barker in California. I think Jerry is a member here. He was recommended to me by Kas Kastner. Jerry runs, or ran a Spitfire in F-Production and was SCCA national champion at some point. He came highly recommended by Kas and the head looked awesome when I got it back from him. There were some issues with clearance between the combustion chambers and he had to have a custom solid copper head gasket made for it by Gasket Works. It has brand new 89 mm liners from Cambridge Motorsports with J&E short deck pistons and Pauter rods installed. My compression gauge is a cheapo from Autozone, but I doubt this is the problem.

I really do not want to tear this thing down again, but I do think the compression should be higher in the other cylinders. I'm going to call Jerry on Monday and see what he thinks.
 
Tab,
Sounds like you have way better than average local auto machine shop.Curious what issues with clearance between combustion chambers could mean,and how a custom gasket solved it.
Tom
 
Tab,just looked at your website after my post,and realized I saw your rolling chassis in another post.

Wow,what beautiful workmanship ,and great of you to document and make avaiable on your site.
Thanks
Tom
 
TFB said:
Tab,
Sounds like you have way better than average local auto machine shop.Curious what issues with clearance between combustion chambers could mean,and how a custom gasket solved it.
Tom
Click to expand...

That's really my point, I don't know if it is solved. I'm thinking there could be some leakage between the chambers.
 
I'd suggest that running her, as suggested by Randall, is a good step here. She has sat for a bit if memory serves, and that doesn't help.
 
I agree. It did sit for quite a while before starting it even though I would turn it over with a wrench every now and then. I think the rings need more running in to seat properly.

It's now time to start on the body!
 
Last time I had one cylinder way out of line in compression it was a broken piston ring.
Compression test with oil actually did confirmed a ring problem.
 
I adjusted the valve per the cam specs which is 0.013 and ran the engine a bit more. The compression now reads:

1. 130

2. 130

3. 180

4. 175

I'm fairly certain that the rings have had time to seat and since the lower compression is only in number 1 and 2, my guess is that it's that the head gasket is leaking between the 1 & 2 compression chambers.

I'm going to to a leak down test tomorrow and will most likely have to remove the head to verify exactly what is going on. Worst case scenario I'll need a new head. I had my doubts about the head to begin with, but I thought I'd give it a shot in the event it was going to be alright. I may try super gluing some stainless wire around the chamber openings to see if this may work. If not, I may go with an aluminum head, but no more used ones at this point. I have over 2 grand invested in this one as it is. I love old cars!
 
have you retorqued the head? Not sure if it will change anything but maybe try that, if you can drive the car, drive it for a couple of hours and then check it again, if no 4 came up maybe the rest will come up too. then if all else fails pull the head.

Hondo
 
When you unshroud the intake valves, clearances between the cylinders (for sealing the head gasket) get pretty small. Same sit on my 4A. I have a GasketWorks solid copper head gasket that I modified (aka "custom") so that the gasket doesn't hang inside the combustion chamber and cause detonation. I think that it's the only option other than changing back to a unmodified head.

Here's what mine looked like. Notice the Sharpie mark for the jugs, it's rather close making a good seal harder to achieve. GasketWorks now sells copper sealing rings that might help the sitch:
 

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I agree. Something sounds off. You should be seeing 180+ on all cylinders.


For fun, I threw some of your specs into Pipemax. I did a lot of guessing, so don't put too much stock into these numbers.

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