Understand the 300 from 73 to 86 CR is 8.0:1 and 8.8:1 from 1987 to 1996 with EFI

Have a carburetor 1980, 300 with bulild date of Dec 1980 on the door sticker, recently acquired from a wrecking yard

Want to increase CR to 8.8:1

Noticed two types of aftermarket pistons
1. concentric round dished
2. D shaped dished

Original pistons have offset round dish

The head has the tapped holes for air injection.

Which pistons are recommended to increase CR?

 

Assuming you plan to have the block machined which 300 pistons doesn't really matter. The factory pistons are set ~.045" below deck. If you have the block machined to put the new pistons at .010" below deck it will pump up the compression to ~9:1.

There are some flat top 351W pistons that will work too as long as you get the rods bushed.

Hypereutectic pistons are stronger than the stock cast pistons.

 

The pistons for the EFI configuration, 1987-1996 appear to be closer to .045" deep concentric recess.

The stock piston eccentric 1980 recess appears to much deeper than .045".

Will have to measure with depth micrometer to get actual recess depth.

Believe the combustion chamber on the EFI head is much different than the carburetor head. There is a raised region in the EFI head which creates a swirl for more efficient burning.

Understand installing the carburetor 240 cid head on the carburetor 300 will raise the CR without any valve train geometry issues.

 

Using the EFI head or a 240 head generally gives about a .5 bump in CR.

 

What are your plans for the engine? If your only goal is to bump up compression then a 240 head will get you to ~8.8:1. If you're going to rebuild the engine, then like I said you can have the block milled to put the pistons .01" below deck and get ~9:1 with a 300 head.

This should help with piston selection: https://www.uempistons.com/catalogs/...te_catalog.pdf

 

The piston catalog helps a bunch.
Appears the 69-79 with 3.22" dia concentric round recess has the lowest CR = 7.8.
Looks like the 1980 300 could be D shaped recess or off set round recess.
D shaped recess has CR= 8.0 and offset 2.34" dia round has CR= 8.7.
Find the 8.0 , unacceptable, but 8.7 is acceptable.

Suppose could look with LED light through the spark plug hole with a scribe probe to determine the shape of the recess. If the piston recess is "D" shaped, then using the 240 head to increase CR would be the way to go with out having to replace the pistons.

Looks like CR = 8.7 for EFI era from 87-96 with "D" shaped recess pistons.

Does anyone have the combustion volume for the 240 , 300 carb head & 300 EFI head?

Easy to determine CR, if this number is available, based on the piston dome volume or dish volume. In the case of the 300 engine, looks like all the pistons are dished.

 

FE pistons are another option- I used flat top 360 pistons in my last build, nothing special extra needed although that block was so worn instead of the .050 over I planned it had to go .080 over to fit a .030 oversize 360 piston. It didn't even run hot in the high desert where I was living at the time.

 

[quote=raven3;14008005]

Does anyone have the combustion volume for the 240 , 300 carb head & 300 EFI head?

I'm not sure where I found these numbers (probably over on fordsix) but I have in my notes:

<table width="256" style="width: 192pt; border-collapse: collapse;" border="0" cellspacing="0" cellpadding="0"> <colgroup><col width="64" style="width: 48pt;" span="4"> <tbody><tr height="17" style="height: 12.75pt;"> <td width="192" height="17" class="xl63" style="border: 0px windowtext; width: 144pt; height: 12.75pt; background-color: transparent; mso-ignore: colspan;" colspan="3">240 chamber is 68 cc +/- 2.</td> <td width="64" style="border: 0px windowtext; width: 48pt; background-color: transparent;"></td> </tr> <tr height="17" style="height: 12.75pt;"> <td height="17" style="border: 0px windowtext; height: 12.75pt; background-color: transparent; mso-ignore: colspan;" colspan="4">300 carb is 76 cc +/- 2.
300 EFI is 65.9cc


</td> </tr></tbody></table>

 

Did some research, based on Autozone parts data base for 300 pistons

Autozone has no piston part number for the 300 from 65-72.

Autozone has same part number, P768-std(standard size), P768-30 (thirty over size) for 73-95
Description indicates D shaped recess with 22cc negative dome volume with CR= 8.5

However in 1996 the part is different, PN H674P30
Further reseach indicates 1996 piston is different than 87-95 becuase the dish recess was decreased from .310" to .290" while the compression height was decreased from 1.776" to 1.767" , decreasing the piston deck clearance

Appears the after market pistons have D shaped dish recess, similar to the OEM "D" shaped EFI pistons
compared to round off center recess used on 77-86 model years.

The pistons on my 1980 have 2.34" diameter off center round recess.

Performed a study of CR vs head gasket compressed volume for various configurations.
Looked at Mr Gasket, Edelbrock, Fel-Pro, Ford racing, Victor.
Mr gasket has the smallest gasket volume, while Edelbrock has the largest volume.

Looked at the following engine configurations.
1. OEM round recess piston w/ 240 head
2. OEM round recess piston w/ 300 carb head
3. "Sealed Power", D shaped recess piston w/ 300 carb head
4. "Sealed Power", D shaped recess piston w/ 300 EFI head
5. OEM round recess piston w/ 300 EFI head
6. "Sealed Power", D shaped recess piston w/ 240 carb head

Mr Gasket, zero decking
1. CR =9.2
2. CR= 8.6
3. CR= 8.5
4. CR= 9.3
5. CR= 9.4
6. CR= 9.1

Edelbrock, zero decking
1. CR= 8.7
2. CR= 8.2
3. CR= 8.1
4. CR= 8.8
5. CR= 8.9
6. CR= 8.7

Notice the OEM round recess pistons with 300 carb head resulted in ~0.1 greater CR compared to after market Seal Power D shaped recess pistons.

Also, note that for the OEM round recess pistons, there is a difference of ~0.4 increase of CR between Edelbrock and Mr Gasket.

Included in the matrix was CR sensivity to machine decking the block.
On the average for all the engine configurations, .01" decking , corresponded to ~ .15 -.20 CR increase.

Conclusion: Using Mr Gasket with .02" block decking with "Sealed Power" after market D shaped recess pistons will result in static CR = 8.8, which I believe is the upper limit on a carburetor engine with 87 pump gasoline.
i.e. , 8.5 + 2 x .15 = 8.8