I have a 390 in pieces all over my pool table out in the shop, and another one sitting in my utility cart waiting to go in another truck. But this truck came along that I liked so I picked it up.

Ratsmoker I have really enjoyed the torque and hp curves you provided for some of the guys. I am hoping you could post the curves for what I got in this truck. I know it is less than what I was planning to build, but I'm still currious.

(C8AE-H) heads standard valves no porting
Stock Pistons .030 over
Edelbrock Performer Cam
Edelbrock Performer 600 CFM Carb.
GT 390 Stock High Riser Aluminum Manifold (C7AE-9425-F)
Hooker Super Competition Headers

What else do you need for input?

 

I need to know the compression ratio. Different year 390's had far different ratio's. I'll be more than happy to post a graph tomorrow. Gotta go to work now.

 

Ratsmoker,

Since I don't know the pistons I can only guess at the theoretical compression ratio. I assume it has 69cc heads since most C8AE-H heads are that. Also it is a 3.78 stroke and I asume a .041 gasket. But I don't know the valve eye brows in the piston top.

I did however check the actual cold cranking compression with a pressure guage. They are all within specification, between 135 psi and 155 psi with 143 psi the average. Since atmospheric pressure is 14.7 psi then 143 psi divided by 14.7 psi gives the true actual ratio of 9.7 to 1 compression ratio. This should be better than the theoretical compression ratio anyway.

Ratsmoker thank you for doing this for me.

 

Here it is. Different size cams will give you much different cylinder pressures so the best way is to do the geometry but I used 9.7:1 for yours since the cam really has no overlap.
http://www.clubfte.com/users/ratsmoker/horsetruck.jpg

 

I don't think that compression ratio would be right. I have a rebuilt 360 with nothing special in it and the cranking pressure is 175~180. I also have a 428 w/ about 10.25~10.5 and it cranks at 210 with a Crower 16903 cam.

The actual eyebrow size is usually 5 to 10 cc's and that's not all that much of a difference. Knowing if it's zero decked is more important. Either way, the 143# sounds like it's close to 8.5 or so (not counting for cam changes)

 

Thanks a million Ratsmoker for your time. Now that my logic has been questioned, I realize I made a mistake by over simplifying how I figured my compression ration.

I started thinking about my chemistry and physics days and remembered compressed air does not simply double in pressure when the volume is cut in half. It would do that only if the temperature of the comprressed air stayed the same, but when gas is compressed, it heats up.

So, this supports what Karlijay was saying that 10:1 compression ratios should have gauge pressures not just 10 times the atmospheric pressure of 147 psi, but something much higher since the temperature is also increasing in the compressed air.

You guys are good, I had to pull out by old text books just to look up those old gas formulas from physics, and sure enough everything you guys said makes perfect sense now.

The bottom line is, I cannot simply determine the compression ratio by converting the gauge pressure. And since the pistons being 1/8 inch down would drop the ratio to 8:1, I really have no way of telling what I got unless I pull the heads, isn't that right??

 

I'll redo it with 8.5:1 compression. You should have a little more than 8:1. The graph posted above will change about 8:00am tomorrow.

 

I think 9.5:1 is pretty close. I have a compression ratio calculator that I built in Excel. I input info like stroke, bore, gasket compressed height, deck height, chamber volume, piston dish/relief, rod length, and piston compression height, and it determines static compression ratio.

I plugged in the standard 390 specs (10.17" deck, 3.78" stroke, 6.488" rods, 4.05" bore) with the additional information he provided (.030" overbore, 69 cc heads, .041" gasket). I also estimated piston reliefs to be good for 10 cc, and I used the compression height of standard 390 pistons, which is 1.680". Those figures yield a static compression ratio of 10.0 to 1. Deck clearance is 0.012". 74 cc heads yield a CR of 9.5:1. That makes sense, because C8AE-H heads were not truck heads. Their smaller chambers were used for car engines that had higher compression ratios than the truck motors. The references I have also show a 145 psi cold cranking pressure at sea level to be equivalent to a 10.0:1 static CR.

I'm using C8AE-H heads on the 410 I'm building, but I'm also using the shorter 360 truck pistons (1.660" compression height) to keep the compression ratio down to something manageable. Even so, I'm still going to be at 10.0:1 static CR. A relatively long duration cam should help keep cylinder pressures down and detonation to a minimum.

 

Karlsd,

So based on what you are saying, your estimate of 9.5:1 is not far off my original post estimating 9.7:1

The real question is how far down the pistons are. Were the piston clearance from the top of the piston to the top of the deck always the same in all 390's?

Or another way to ask it is, what is the normal gauge compression in psi for 390's are they always the same for stock engines?

Is mine (143 psi) fairly standard or low?

 

I'm not sure if all 390 pistons that Ford originally used had the same compression height. If so, all 390s would have the same deck clearance. I know that the aftermarket 390 pistons available vary somewhat in compression height. Still, a deck clearance of .012" is close enought to zero deck that you really don't want to get much, if any, taller. Compression ratio was usually adjusted not through piston height but by how much dish (or dome) the piston had, and how large the combustion chamber was. FE heads that I'm familiar with have combustion chamber volumes ranging from 67 cc to 76 cc. You can get another 15 cc or so of volume with dished pistons.

 

After talking to my dad about his experience with engines, I am pretty sure I am in the 8.0 or 8.5:1 compression. Ratsmoker, I would like to see that graph you were going to post when you get a chance. Thanks.

 

I changed the post above to 8.5:1 C.R. yesterday.