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'39 LZ Compression Test Results


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I finally had the courage to make a compression test as a follow up to my vacuum test. Here are the results:

#1 cylinder 100 psi

#3 cylinder 95 psi

#5 cylinder 105 psi

#7 cylinder 85 psi (ouch!!)

#9 cylinder 100 psi

#11cylinder 105 psi

#2 cylinder 100 psi

#4 cylinder 100 psi

#6 cylinder 105 psi

#8 cylinder 100 psi

#10 cylinder 95 psi

#12cylinder 96.5 psi

Since is am not sure if this engine is a '39 or later engine I compared the the test results against the specified compression for an engine with a 6.7 compression ratio ('39) and 7.0 for the later engines with the larger bores.

The compression (at sea level) for a 6.7 is 118.50 (I interpolated since the Ford Service Bulletin did not list a compression for an engine with a 6.7 compression ratio). for the larger bore post-'39 engine the compression is listed at 123.

6.7 compression ratio = 118.50 psi

#1 cylinder 100 psi = 18.50(16%) psi below normal

#3 cylinder 95 psi = 23.50(20%) psi below normal

#5 cylinder 105 psi = 13.50(11%) psi below normal

#7 cylinder 85 psi (ouch!!) 33.50(28%) psi below normal

#9 cylinder 100 psi = 18.50(16%) psi below normal

#11cylinder 105 psi = 13.50(11%)psi below normal

#2 cylinder 100 psi = 18.50(16%) psi below normal

#4 cylinder 100 psi = same as above

#6 cylinder 105 psi = 13.50(11%) psi below normal

#8 cylinder 100 psi = 18.50 psi(16%) below normal

#10 cylinder 95 psi = 23.50 psi(20%) below normal

#12cylinder 96.5 psi = 22.00(19%) psi below normal

7.0 compression ratio = 123 psi

#1 cylinder 100 psi = 23(19%) psi below normal

#3 cylinder 95 psi = 28(23%) psi below normal

#5 cylinder 105 psi = 18(15%) psi below normal

#7 cylinder 85 psi (ouch!!) 38(31%) psi below normal

#9 cylinder 100 psi = 23 (19%)psi below normal

#11cylinder 105 psi = 18(15%) psi below normal

#2 cylinder 100 psi = 23(19%) psi below normal

#4 cylinder 100 psi = same as above

#6 cylinder 105 psi = 18(15%) psi below normal

#8 cylinder 100 psi = 23(19%) psi below normal

#10 cylinder 95 psi = 28(23%) psi below normal

#12cylinder 96.5 psi = 26.50(22%) psi below normal

The mathematical average of actual compression for all 12 cylinders -s 98.88 which is about 16% below normal for a '39 engine with the 6.7 compression ratio and 20% below normal for a 7.0 compression ratio engine.

Overall I would have to say these are poor readings. This might explain why on road tests I can't get this LZ to go beyond 65 mph and experience some power loss going up a steep grade.

If the top speed is rated at 90 mph, 65 mph represents (at least mathematically) a lost of about 28% which is fairly close percentage wise to the compression loss for larger bore engine.

I am baffled why I got such a good vacuum test results 19" (rock steady).

I am especially concerned about some of the large PSI drops between certain cylinders.

I am thinking of replacing the head gasket to see if that helps bring up some of the compression between the cylinders.

If anyone has any advice on next steps such as additional testing, changing my oil(I am currently using Castrol straight 30 (or using oil additives) to help increase compression or maybe a complete or partial engine rebuild I would sure appreciate the advices.

Thanks again to Peecher and V12 Bill for your recent help.

Additional note: on #4 cylinder I added about one teaspoon of oil into the intake manifold and retested. The compression jumped from 100 to 155 psi.

On another note I tested cylinders 2-12 and then re-torqued the head bolt for this cylinder head and then retested the compression. After tightening the head bolts the compression on average to each cylinder jumped up 5 psi !!!

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Try squirting oil in each cylinder and then see how much the compression picks up. if it goes up significantly, you most likely have a ring, or valve problem. This test would take a bad head gasket out of the picture. Does this engine smoke under a load?? If not I'd go for the valves. If that is the issue, that is a somewhat easy task and can be fixed while the engine is in the car.... if you have the patience. Is your 65 mph on a flat stretch of road? Has the distributor

been checked? I have found that on some of the olde engines, anything at or over 95 psi for compression in most cases is acceptable. I would think your cylinders @ or around 100 psi shouldn't be an issue...I think Dee may be able to confirm this.

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Daddio65,

No real significant smoking under load. I can hit 65 on a flat stretch of road (I can go up a grade at about 50-55 mph). The distributor is rebuilt (purchased from Earl Brown about two years ago). Condensors and resistors are all new. The coil has not yet been rebuilt. I have a spare that I am going to send to Jake Fleming for a rebuild.

My average compression per cylinder is just under 99 psi which I think could be a lot worse. The only real cylinder that worries me is #7 at 85 psi.

I'll run another compression test using the squirting oil test and see how much compression comes up.

Thanks for the advice

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Actually, those compression readings sound pretty decent for an old engine. Oil will help seal the compression rings but adding any quantity of oil to a cylinder will also take up space in the combustion chamber and likely raise the compression. I would look for other reasons for your lowered power. What do the spark plugs look like after a 65 mph run?

Other than the checking the coil you might try using one step hotter spark plugs if you find significant fouling.

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Frisco,

If you are sending your coil in, have Jake check out your distributor as well.... You never know things have a tendancy to change over time. Jake will shoot you straight on it if is right on or not... This way you are taking out all of the other questionables from your problem. Just my opinion though.

Jeff

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Peecher,

After about 20 minutes of driving at 60-65 mph the plugs look like they are getting a dark gray (charcoal) colored carbon build up around and behind the base of the bent (c shaped)electrode. The threaded insulator area has is almost the original color and maybe has a very faint patina to it. The plugs are relatively new (less than 200 miles on them)

The electrode gap area itself has almost no carbon deposits on it and is more of a medium gray color. The plugs seem dry with no oil or heavy gasoline odor to them. The plugs I am using are Champion H12's that Jake Fleming recommended because they are a "hotter" plug and are self-cleaning if the engine is run at high speed for prolonged period of time.

Do you have any oil recommendations to help with compression. Right now I am using Castrol 30 weight. I understand that there are oils out there designed for "older" engines.

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Your spark plug's conditon sound normal, maybe a little rich but I think modern fuels may burn this way along with a little oil burning. The H12's are the one step hotter so they should work just fine. Many folks use 20-50 oil in these cars; Castrol CDX 20-50 would do and Penzoil used to make a "high mileage" 20-50 which I used until it became unavailable in my area. I doubt that any of these oils really do much for compression but they probably help prolong the bearing life and give a little boost to the oil pressure.

You might consider sending your spare coil to Skip's ( Fords-R-Us] for a rebuild. Jake does a fine job but Skip will rebuild them with a "hotter" output; It made a difference with my own car. Dee

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Peecher,

Thanks I checked their website. This looks perfect. I think a hotter coil would help. I think the coil I am using is original and never been rebuilt. A 70 year old coil could probably use a rebuilt.

I also think I will send my water pumps for a rebuild.The improved impellers really look impressive. It is getting hot here in California so the timing is perfect.

Thanks for the information.

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