1600 X-Flow compression

[snettleship]

I have a 1600 X-Flow (711M) in my 1970 Lotus Seven S4.  I race this at Vintage events.  It's got some upgrades like a performance Cam from Dave Bean, Carillo rods and 13:1 compression (which I would like to verify sometime).

 

I recently did a compression test, plugs out & Carbs wide open with the following results  (PSI):

 

Cyl 1     Cyl2     Cyl3     Cyl4

160        135      145      155

 

Cylinders 1 & 4 are certainly close enough.  3 is a little out  (10% off of cyl1) and 2 is more (15% off of cyl1).  I have heard that the likely culprits are:

 

- valves (bent?)

- rings

 

I can attest to an overrev even with the rev limiter (bad downshift).  Any advice on proceeding?  The engine has about 20 race hours on it.

 

Also, do the readings in general (say if it was 160 across all) indicate the "strength" of the engine?

 

Thanks

[xflow7]

Disclaimer:  I am not an expert

 

As far as I'm aware, the general consensus is that absolute values from a compression test don't indicate much of anything due to variation between gauges and the fact that the compression test is done at cranking speed and so doesn't account for the fact that different spec engines are tuned for different rev ranges, etc.  For example, a really highly tuned engine might have a silly amount of valve overlap leading to low-ish absolute numbers at cranking speed.

 

Having said that, for the same engine, at the same spec, with the same gauge, at the same atmospheric conditions it probably stands to reason that if all the numbers are equal, but 20 psi lower than they were the last time it might be an indication of the engine going off a bit.

 

As for diagnosing the problem, it might behoove you to do a leak-down test with which you can much better pinpoint whether a loss of compression is a valve problem or a ring problem.

 

Dave

[locostv8]

FWIW a friend many years ago rebuilt his Alfa due to a faulty compression gauge.

 

 

 

[slomove]

A cylinder leakage tester is a small investment (provided you have a compressor), I just picked up one from Harborfreight Tools for $34. That should give you some better idea what is going on.

 

Your compression numbers sound a little on the low side for a 13:1 engine but I don't know anything about Crossflows. My 10.6:1 Zetec usually does 240-250 psi. Was the engine warmed up?

 

Gert

 

 

 

 

[snettleship]

I warmed the car up to change the oil but I plan on changing plugs this weekend for a race at Watkins Glen.  So, I'll warm it up forst and do another check.

 

I will invest in a leakage checker since I live about 10 miles from a Harbor Frieght store (a dangerous thing to have nearby).

 

Thanks for the ideas.

[slngsht]

a dangerous thing to have nearby).

 

 

 

http://www.usa7s.com/aspnetforum/images/emoticons/iagree.gif

 

 

 

I always leave that place with more than I wanted going in.

[locostv8]

I'm kinda safe.  The closest HF is 100 mi away, but is only 10

from the VA hospital that I have to go to at least twice  month.