Trying to figure out if I have a High Compression Head

[We1969knight]

Hi All,
I have a 1930 Coupe with a fairly fresh engine.
It was apparently "really" gone thru.
Ungortunatly I did not get the paperwork re. this work when I got the car last summer.
And it's not available - I checked.
My question is: How can I tell if it's got a High Compression Head - will a simple compression test tell me?
What should the "stock" compression be on a fresh engine w. a OEM head?
TIA ... again
Chris
Union, ME

[Y-Blockhead]

Originally Posted by We1969knight

My question is: How can I tell if it's got a High Compression Head - will a simple compression test tell me?
What should the "stock" compression be on a fresh engine w. a OEM head?
TIA ... again
Chris
Union, ME

Fresh, stock engine ~ 65psig. Hi Compression... Higher. Depends on the ratio.

[daveymc29]

I have a similar issue. I have no history on a great engine in a coupe i bought. The owner got it from his dad and can only remember years ago he helped put a "rebuilt" engine in the car. It does have a police head with the large "d" on it toward the front half of the head. 94 psi in all four cylinders. Wish i knew where it came from and if they still do engines. This one is great, but no idea what is inside it.

[Gary WA]

Originally Posted by We1969knight

Hi All,
I have a 1930 Coupe with a fairly fresh engine.
It was apparently "really" gone thru.
Ungortunatly I did not get the paperwork re. this work when I got the car last summer.
And it's not available - I checked.
My question is: How can I tell if it's got a High Compression Head - will a simple compression test tell me?
What should the "stock" compression be on a fresh engine w. a OEM head?
TIA ... again
Chris
Union, ME

Any markings numbers letters etc on the head?

[AL in NY]

Do a compression check. Take the average value you get and divide by 15. That will give you a close approximation of your compression ratio. Stock heads have a 4.2 compression ratio.

[Terry, NJ]

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At least you would think so, I did! 4.2 X 14.7= 58.34 Lbs (Check my math) But it's usually higher. Why? Because of compressive heat. Compress a gas and it creates heat.And the heat creates some pressure too. Look for about 65 lbs in a good Mod A engine with a stock CR . More than that and I think you may have a "Head", though God knows which one.
Terry

Originally Posted by AL in NY

Do a compression check. Take the average value you get and divide by 15. That will give you a close approximation of your compression ratio. Stock heads have a 4.2 compression ratio.

[denniskliesen]

Using Terry's formula I get 88.2 lbs. for my engine. Using Al in NY i get 6.7 compression ratio. Somethings wrong here... Snyder said that head is 6.0:1 I'm getting 100 lbs. in all 4 holes.

[mike657894]

i get 67 on my stock head. no idea how fresh. I think stock can be up into the 80s. over that high comp in various state of wear.

[Y-Blockhead]

Originally Posted by denniskliesen

Using Terry's formula I get 88.2 lbs. for my engine. Using Al in NY i get 6.7 compression ratio. Somethings wrong here... Snyder said that head is 6.0:1 I'm getting 100 lbs. in all 4 holes.

Other factors effect compression also, as cylinder bore size. My 5.5 Snyder with a .040 over bore has 85psig.

Dennis, where you hang your hat in SoCal?

[We1969knight]

Also, should compression be done on a cold, warm or hot engine?

[denniskliesen]

Originally Posted by Y-Blockhead

Other factors effect compression also, as cylinder bore size. My 5.5 Snyder with a .040 over bore has 85psig.

Dennis, where you hang your hat in SoCal?

South of you down by the 405 and 5.

[mike657894]

Originally Posted by We1969knight

Also, should compression be done on a cold, warm or hot engine?

I always do them cold because when your engine wont run you wont be able to warm it up. So at that time you will be comparing apples to apples. also I always hear people say do the test with throttle open but I see no difference on the gauge.

[Mike V. Florida]

How to do a compression test

[Chris Haynes]

Sadly my compression tester is too small for my head. I guess I'll have to break the porcelain out of an old spark plug to test it.

Originally Posted by denniskliesen

South of you down by the 405 and 5.

I'm off the 405 in the SF Valley.

[Y-Blockhead]

Originally Posted by Chris Haynes

Sadly my compression tester is too small for my head. I guess I'll have to break the porcelain out of an old spark plug to test it.

FYI, The vendors do have an adapter for a 14mm plug. Nice to have for future testing.
http://www.snydersantiqueauto.com/spark-plug-adapters

My Snap-On compression tester has the Model A size included.

[Chris Haynes]

Originally Posted by Y-Blockhead

FYI, The vendors do have an adapter for a 14mm plug. Nice to have for future testing.
http://www.snydersantiqueauto.com/spark-plug-adapters

My Snap-On compression tester has the Model A size included.

Breaking the porcelain out of an old plug is much cheaper, :-)

[Tom Wesenberg]

Originally Posted by mike657894

I always do them cold because when your engine wont run you wont be able to warm it up. So at that time you will be comparing apples to apples. also I always hear people say do the test with throttle open but I see no difference on the gauge.

Yep, it never made a difference for me either.

[Y-Blockhead]

Originally Posted by Chris Haynes

Breaking the porcelain out of an old plug is much cheaper, :-)

Haha, So true. You could also use a plug that come apart, like an old 3X.

But the one from Snyder's is threaded for a 14mm plug and my compression tester screws right in. Unless you have a el cheapo gauge with just a rubber tip, then that is a moot point.

[Chris Haynes]

Originally Posted by Y-Blockhead

Haha, So true. You could also use a plug that come apart, like an old 3X.

But the one from Snyder's is threaded for a 14mm plug and my compression tester screws right in. Unless you have a el cheapo gauge with just a rubber tip, then that is a moot point.

I have been using my rubber tipped El Cheapo for over 50 years.

[mike657894]

I used one of the o-ringed ends that comes with the cheap kits. had to hold it with two hand and have a helper run the starter.

[V4F]

with the formula above I got 8.4-1 on my 7-1 head . 126#/cylinder

[Terry, NJ]

Dennis, It makes my head hurt to try to figure out these numbers! So I asked the question, How much did the compression ration increase by boring out a block .040? Someone younger and more mathematically more adept than I am, came up with the figure of approximately (IIRC) .047. So a mod. A bore of 3 7/8, bored out .040 increases the CR by .047 or it goes from 4.2 to 4.247. It ain't much! But one thing to recall is it is not the same no matter what the diameter. As the dia. increases, the amount of stock removed increases.
Terry

Originally Posted by Y-Blockhead

Other factors effect compression also, as cylinder bore size. My 5.5 Snyder with a .040 over bore has 85psig.

Dennis, where you hang your hat in SoCal?