Model A compression

[Jeff/Illinois]

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[HoarseWhisperer]

When I'd would tell club members of having low 70's compression (checked with multiple gauges), they would insist I must have a high-compression head.

But it was a stock engine & head.

Specs in the DeAngelis book calls for 76 psi.

But, engine specs in Bratton's catalog says 64 psi on a NEW engine.

Which is correct?

[Logan]

You also have to think about altitude. In Texas the lowest you really want is about 50-55, but when we were in Colorado for the high country tour, they were saying about 35 pounds was pretty good for being up that high.

[Tom Endy]

We recently checked the compression on a young fellow's 1930 Tudor. I was expecting to see somewhere around 50-55 lbs. Instead each cylinder measured a healthy 75 lbs. There is some thought it may have a Snyder 5.5 head on it. How can you tell the difference between a Snyder and a stock Ford head.

Tom Endy

[HoarseWhisperer]

Originally Posted by Tom Endy

We recently checked the compression on a young fellow's 1930 Tudor. I was expecting to see somewhere around 50-55 lbs. Instead each cylinder measured a healthy 75 lbs. There is some thought it may have a Snyder 5.5 head on it. How can you tell the difference between a Snyder and a stock Ford head.

Tom Endy

Tom,

The head on mine had the raised "W1" casting mark; W representing Windsor, Ford of Canada.

Not sure about US heads.

[Kurt in NJ]

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It is only good to compare compression # between different cars when the same compression tester is used, i have seen as much as a 30% difference between testers of the same brand ---name brands, cheapos can be worse ---there are 5 tester where I work, all read different. --if the car runs there is enough compression, if the # are even it should be ok, a leakdown test tells more ---bubbles in radiator--problem, air out intake-valve problem, air out ex valve--- too much air out breather---piston /rings/cyl

Another thing that can cause a false low compression reading is carbon dislodged when removing spark plug--it can get caught under a valve ----to prevent this loosen plugs 1 turn, start engine, goose gas a few times, then do compression test

[Popeye31]

there is a guy in my club that has a coupe that will runna nd its got 25 psi its hard starting but it runs. we will be changing the rings as soon as his wife is feeling better.

[BILL WILLIAMSON]

All compresion gauges are not equal. Also, check it on warm engine, throttle wide open, good hot battery, and let it go at least 10 strokes or 'til gage no longer climbs. If one is low, keep checking it, as some carbon may be stuck under a valve from removing the plugs. Bill W. ( compression will read higher on a 12 volt equipped car also)

[BILL WILLIAMSON]

Originally Posted by Kurt in NJ

It is only good to compare compression # between different cars when the same compression tester is used, i have seen as much as a 30% difference between testers of the same brand ---name brands, cheapos can be worse ---there are 5 tester where I work, all read different. --if the car runs there is enough compression, if the # are even it should be ok, a leakdown test tells more ---bubbles in radiator--problem, air out intake-valve problem, air out ex valve--- too much air out breather---piston /rings/cyl

Another thing that can cause a false low compression reading is carbon dislodged when removing spark plug--it can get caught under a valve ----to prevent this loosen plugs 1 turn, start engine, goose gas a few times, then do compression test

Also, bubbles can be seen better on warm engine, idling slow and radiator filled to top of overflow
tube. Smell the rodiator also for a silghtly gassy, semi exhaust smell, better still, have somone check it with an ehaust analyzer for HIGH hydro carbon reading. Bill W.

[Frank Miller]

I am not sure how significant it is but the cam profile as well as the bore can change compression pressure. Boring the engine increases the swept area while the combustion chamber remains almost the same. If the block has been decked it can raise compression. Like I said this may not be significant but something to think about.

[Ron W]

"Specs in the DeAngelis book calls for 76 psi.

But, engine specs in Bratton's catalog says 64 psi on a NEW engine."


Todays pistons rise above the block .032 ". That would account for a slight increase in compression. My stock head showed 75# average. My high compression head shows much more. Ron W

[Fred K-OR]

Can a guy that knows nothing about compression testing jump in here and ask a stupid question. When checking compression, does the engine need to be running? Or can you just crank it over? Like I said, very "Junior" question from someone that has never done it and thinks he needs to do a check.

[Tom Wesenberg]

Warm up the engine. Remove all the spark plugs, leave the ignition OFF, open the hand throttle, and have someone crank the engine (with the electric starter) while you hold the compression tester in each spark plug hole. If you have a screw in tester then you can crank the engine and watch the tester. I usually go about 3 or 4 compression strokes for each cylinder. Be sure to release the tester pressure after each cylinder check.

[Patrick L.]

Again, all good info.. Watch the very first needle jump and take note of the reading as well as the ending reading.. All gauges are not created equal so they will give different readings and can vary considerably.. As long as all cylinders are within 10% everything is fine.. In my old 'A' that was last done in 1961 the compression on all 3 of my gauges are at 45#.. The engine doesn't leak or burn any oil,but, its getting tired, it starts easily and runs good..

[Marco Tahtaras]

Originally Posted by Ron W

"Specs in the DeAngelis book calls for 76 psi.

But, engine specs in Bratton's catalog says 64 psi on a NEW engine."


Todays pistons rise above the block .032 ". That would account for a slight increase in compression. My stock head showed 75# average. My high compression head shows much more. Ron W

That occurs when the block has never been decked and everything is dimensionally the same as what Ford produced.

[MikeK]

Too many variables with cams. The real health check is a cylinder leakdown test as well as a compression check. I don't know why everyone is so concerned about identifying a specific compression number. Calculating the % of leakage is more telling about valves and rings.

Here's a simple do-it -yourself leakdown tester LINK

[flatheadernie]

AMEN to the leak down test, actual comp # has so mwny variables almost meaning less in real world , I built a v8 flathead with 200 plus # it only ran fair ? ernie n tx.