[erlindsey]

Hi guys,

I have a 400 bored 0.40 over. The pistons are the 8 eyebrow reliefs. It has an ex 262 comp cam in it. I had put a set of 670 heads on it. I was told that the compression would probably still be somewhat low. Was wondering, I have a set of 093 heads. Would these heads help any at all?

[geeteeohguy]

You have been misinformed. The compression with the 670 heads and 093 heads would be about the same: about 10.5 to 1. The 670 heads are about 72cc and the 093's around 65-69cc. Both are "closed chamber" design, require a lot of spark lead, and are very prone to detonation. Neither head will work well with today's pump fuel. You MIGHT skate by with the pistons you have, due to the relief volume of the eybrows, but those pistons have a rounded crown, sit well down in the bore, and invite detonation. You need to change to dished pistons or get some heads with a bigger chamber...around 85cc's.

[Ragtop Man]

Unless you need them for resto purposes or a judged class, stick with the later heads for a 400. Valve angle is better, sizes are bigger, parts are more plentiful. Maybe park the 670s for some later heads that are a little more friendly to pump gas... airflow makes power.

[erlindsey]

I have a set of 7k3's. Would they be a better choice?

[steve25]

Other than the factory Super Duty 389 and 421 motors, or the 400 RAV, Pontiac never sent out a production 389 or 400 that had over 10 to 1.
The over 10 to 1 compression specs that have been seen in tune up books and factory spec books are always high by .5 compression or more.
Your .040" over 400 with a 670 head and the added 4 CCs or so of the 8 notch pistions will come right in at 8.9 to 9 to 1 in regards to the true compression ratio.
Airflow wise you would be tossing out 20 HP with a .450" lift cam by going over to the 093 heads and your compression woild go up by .2, so its not a good trade off.

[cutlas]

I cc'd my .020 over 400 with the 8 eyebrow pistons and 670's.
worked out to 9.2 to 1.
I have not fired the engine so I don't know how the combination of those pistons and heads will work but it is what I have to work with wright now.

Joe.

[steve25]

Joe, factor in another 1.5 CCs for the top two compression rings back land volume and your at a true 9 to 1.

[chiphead]

I'm gonna have to disagree with the low compression crowd.
ClassicalPontiac's compression cacluator says this:

400 engine 40 over
stock stroke
73 cc chamber
10cc dish
020 deck height
.041 gasket thickness
9.65:1 compression.

On my engine, my 670s CC'd at 70CCs. With zero deck, .030 bore and .039" gasket, I needed a 21CC dish to get 9.3 compression.

[geeteeohguy]

I have to agree with Chiphead. Pontiac made thousands of 389's and 400's that had higher than 10:1 compression. Not just "super dutys" and "ram airs", but garden-variety engines...............

[cutlas]

I don't know about stock compression with stock pistons but I assembled the bottom end and used a light grease to seal the rings.
With my heads and those 8 eyebrow pistons and .039 head gasket compression worked out to 9.2 to 1.
I was told the 8 eyebrow pistons sit down in the hole a little more than stock.
Could be why compression was lower than some would have thought.
If I had the money to do it over again I would buy the Keith Black dished forgings.
My cam is a XE 272.

Joe.

[steve25]

Well I ran the numbers last night and I guess I fibbed a bit.
I forgot about the 1969 400s in the A and F bodys with a manule trans that where fitted with the remaining #48 heads form the 350HO option.
These heads with there 66 CC or so chamber made for a true 10.5 compreesion, other than this motor there was no production 400 with over 10 to 1 as even the RA4 with its 71 CC chamber comes in at 9.88.
To figure this out I use these numbers.

72 CC chamber.
6 CCs for valve notches.
1 CC for ring land back volume, this is a total of 79 CCs.
,020" deck clearance.
,042" gasket .

Many sources list a 65 CC chamber for the 389 with some heads, my CCing of some 20 of these head casting over the last 30+ years show they where 68 CCs at best.
The only street production motor to break the 10 to 1 mark with these 68 CC heads would be the 421 motor.


Here are some compression numbers for 389s and 400s using the above info.

389 68 CCs = 10.03

389 72 CCs = 9.64

389 75 CCs = 9.51

389 85 CCs =8.9
A 65 CC head with a .030" overbore will do 10.37 to 1.

Each .030" over on a 389 will bring up the compression ratio by .14.

400 65 CCs = 10.6

400 72 CCs = 9.88

400 75 CCs = 9.6

400 82 CCs = 9 to 1

400 85 CCs = 8.8

400 65 CCs + 030" over = 10.75
Each ,030" over will bring up the compression ratio by .15

I hope this helps out some!

[erlindsey]

I went with the 670 heads. Adjusted the valves. Tweaked on the carb a little. I was suprised in the torque. She'll fry the tires now.