Compression test questions

[Michael Oritt]

I did a compression test on the MGA engine in my Elva and got the following results (reading from cylinders 1-4):

Engine cold : 156-148-137-158

Engine hot: 137-140-125-145

W/squirt of oil: 147-150-133-160

It seems that cylinder #3 is low by about 10-15% across the board. But does the fact that the oil squirt raised all readings up proportionally point toward a bad exhaust valve? Would a leakdown test be any more significant?

I've also posted this over on the MG forum.

 

[Twosheds]

I always thought that the rings are the culprit when the oil squirt raises the readings. A leakdown testwould tell for sure.

I wonder why the readings went down with the engine hot. Woulda thought they would rise. But I'm used to airplane engines.

 

[Bugeye58]

I wonder why the readings went down with the engine hot. Woulda thought they would rise. But I'm used to airplane engines.

Maybe an ovality issue, that gets worse as the temp comes up?
I roundly second the leakdown test. I rarely even <u>do</u> a compression test any more.
Jeff

 

[Michael Oritt]

Thanks for the input guys--I ordered a leak-down tester from JEG's and should have it Friday.

 

[blkcorvair]

May go down when hot cause valve lash set too tigh I suppose.

 

[racingenglishcars]

Either way, the low #3 indicates a valve problem, possibly due to tight valve clearances.

 

[Bob Buxbaum]

A couple questions. Hopefully I won't get you mad.

1) Were the carbs WFO, and if they were CV style, were the slides propped open? I was taught to always have the intake fully open.
2) Has the engine seen a lot of hours of use? That could cause an "ovality" problem. Also, the rings could be tired which could cause the lower compression when hot.
3) Were the valves set before the test? That's a must.

Bugeye58 is really on the mark. A compression test is fairly inconclusive as it will only show gross leaks. A leak down test REALLY shows problems that are developing well before a compression test would show anything. Many guys I have known that raced 4 stroke 'anythings' (cars, motorcycles, etc.) would routinely do a leak down to see how the engines were wearing. Also, a very good test if the engine has been even mildly over revved.

When you do the leak down test you'll me able to check the condition of the bores by testing at TDC, BTC, and mid stroke. You'll just need to make sure the valves are closed. I've done this by simply taking the rocker assembly off of my GT6. If a cylinder is oval or tapered you'll find it. Don't forget to "lock" the engine when you do this as the incoming air can rotate the engine.

(Of course, all of the above assumes you haven't done a leak down test before.)

Good "hunting".

 

[Michael Oritt]

1) Were the carbs WFO, and if they were CV style, were the slides propped open? I was taught to always have the intake fully open.
2) Has the engine seen a lot of hours of use? That could cause an "ovality" problem. Also, the rings could be tired which could cause the lower compression when hot.
3) Were the valves set before the test? That's a must.

Bob--

I held throttles open when doing the test--I did not have the pistons held up. The engine has about 70-80 racing hours on it since new last year--I've generally kept revs at or below 6500 but no doubt there have been some higher revs. I did not think to check the valve settings prior to the test.

I expect to receive the leak down tester in tomorrow's deliveries and will work with it over the weekend and report back.

Thanks for the input.

 

[JerryB]

"""""""The engine has about 70-80 racing hours on it since new last year"""""""

A that point in time the numbers are academic.

If I were you I'd be thinking about a freshen up ...crack check the crank and rods also.

 

[Bugeye58]

70 or 80 hours?
If it were mine, it'd be due for its fourth or fifth teardown! I do mine about every 20 hours or so, and have an hourmeter installed to keep track.
Jeff

 

[Michael Oritt]

I did the leak down test today and the results were pretty bad with audible leaks through the exhaust on 1, 2 and 3. Four was good and held at 97%.

I pulled the head and will remove and evaluate the valves on Monday but clearly they aren't sealing very well. I also found one slightly bent pushrod on #2 exhaust.

With significant valve leaks it was impossible to evaluate the condition of the rings and bores in cylinders 1-3.

 

[Hap Waldrop]

The neat thing about doing leak down test, you know exactly which valve to address, no need to address ones that are sealing properly, I would suggest you just lap them back in and normally that take care of the problem, this is something I'm fully prepared to do after every race if needed, a post race leak down is part of my normal racing routine. You got to watch some of these race engine builders mentality, they like pretty skiiny seats on the outer edge of the valve, but that doesn't always play into the weekend warrior's game plan, better to go for a thicker seating area up the valve a little more that will last a little longer, you may sacrafice .5 CFM per port but you not lapping your valve after every weekend, there has to be a blance between what a weekend racer will do as part of their normal mantainence and all out performance, some times you can go further than all but the most serious racer is willing to do, pro racers may be willing to build a engine after race, we on the other hand maybe once a season.

 

[Bugeye58]

Hap is, as usual, right on the money. Another advantage to regular leakdown checks is that it allows you to spot potential problems before they turn into very expensive failures.
Jeff

 

[Deleted member 3577]

Better to go for a thicker seating area up the valve a little more that will last a little longer

So how many thou wide do you go on your 45?...(The shiny part)
(Wider on exhaust?)

I assume you go 30 on the back cut (or 32?)