Hi Folks,

I've been doing some reading on 302 compression ratios...actually a lot of reading...probably too much, but whatever. (I searched on here as well).

Engine is a 1984 flat tappet 302 with D8OE heads.
Stock compression I believe was 8.3:1
Pistons are flat tops with 4x valve reliefs.
The D8OE heads I believe are 69cc.

So, I bought a set of rebuilt GT40 Explorer heads, and according to my DIY CC job at home, the chambers are 65-66cc. I've read elsewhere that this is common.

According to an online compression ratio calculator, that small decrease in chamber size doesn't increase the compression ratio much. Maybe from 8.3:1 to 8.6:1 ~ish.

So, my first question is...what pistons were in the Cobras and Explorers to get the compression ratios for both in the area of 9.0:1 or 9.1:1? I've done some reading, and some folks say that the GT40 heads were milled more for the Cobras to get the compression ratio UP...but others say that this is not true, and that all the GT40 heads were 65cc +/- 1cc. Did either of them have domed pistons?

Second question is, where do I want my compression ratio to be? This is just going to be a weekend driver for screwin' around. It's just a little 2wd flareside that won't be hauling or towing anything.

If the Explorers actually had 9.0:1 compression or higher, how did Ford get it there?

Picture below shows my stock pistons.



Would love to hear from the experts on this.
Thanks folks,

Rem

 

The Explorers got a different piston at some point in production, I know, I've seen these in a couple of 97-98 motors I took apart. Ford played with the pin height over the years in the 302. The E7TE's had a nominal 64 cc chamber (+ or - 2ccs) The GT40 heads also had a nominal 64 cc chamber, with the Cobras getting an additional two milling passes to get them to a nominal 60cc chamber.As for those who say that wasn't true, how many of these guys have actually taken apart a virgin 5.0 GT40 headed Cobra engine ? I'll bet not many, and certainly fewer took the time to cc those heads after they came off the engine. I would shoot for a 9-10 to 1 ratio for that motor. Choose your pistons wisely as the pin heights vary from 1.585" to 1.620". That .035" difference makes a big difference in the static ratio, that's about 8 cc's difference in the swept volume, given the head gasket yields around 9 cc's (.043" thick). You need to measure the distance the piston tops are "down the hole" at TDC to figure in your calculations. The piston top volume with the valve reliefs will be around 4 ccs. I would stick to the lower end (9 to 1) as a limit with the block you're working with, it's an extremely thin casting all over (120 lbs total weight vs 136 for the pre 1980 blocks and 126 lbs for the 86 -up castings) Your decks are thin and don't handle more compression well.

 

As for domed pistons in the 302 ? Never happened in production aside from the Boss 302

 

I'd wager your shortblock has already been rebuilt as those pistons don't look stock to me, OEMs were dished with no valve reliefs. Can you see any numbers stamped into the underside of the pistons, that is where you would find something indicating an overbore size and if you see that you know its not stock.

 

Only stockers I know of without reliefs were the E6 headed motors and some mid 60's 390's. 1984 motor would have had reliefs. But like you, the absence of a slight dish might indicate that they're aftermarket pistons. Cleaning the tops would reveal an over size piston if that's what they are.

 

This motor is original and was never touched until I removed it from the truck. It was a one owner vehicle and only has 35,000 miles on it. Everything is bone stock original...even had the original plug wires, rotor button, etc.

I bought a set of E7 heads before the GT40's, and I saw the pistons in the engine they came off of, and it had dished pistons. That was a truck 302 engine...not sure what year, but it had a roller block with a non-roller cam in it.

 

No markings on the pistons up close...this is the cleanest of the bunch below. But you guys are saying the markings would be on the underside if there were any?



I'm taking the block and heads to an engine shop for assembly, and machining if required. The compression was good on all cylinders and thing wasn't burning oil or anything, so I'm hoping it just needs a cylinder hone and re-ring, etc. If it ends up that it needs a bore...then so be it.

I was really just trying to figure out what compression I should have, and if you guys suggest in the area of 9:1, then that is what I'll aim for and ask the shop to do for me. The thing that had me puzzled was...my searches were telling me that the Explorer had 9:1 compression...and if I had flat-top pistons, then why wouldn't a direct GT40 swap get me 9:1 compression? Maybe it was the GT40P headed Explorers that were 9:1, and the 1996 GT40 headed Explorer was less than 9:1. I dunno...I'm just rambling anyway.

I appreciate the help and comments guys, really. I know the shop will handle this stuff just fine, but I'd like to know what I need, and what I need to do to get there, that's all. Reading decades old info online gets tiring after a while when the info varies so much.

 

Good info to know, thanks. Something else I noticed with this block was that the lower head bolts go through into the coolant jacket, and I read that this was a change that only came along a little later with the roller blocks.

 

Are your piston crowns down in the hole any at TDC?

All of the EFI 5.0 motors are roughly 9:1 except perhaps the very first year with E6 heads, everything before that back to about 1976 were smog era motors with dished pistons and/or large chamber heads, the dish may not be very big but every little bit counts. The pic below shows what I am used to seeing in a stock motor.

 

I just checked it. Set the piston at TDC with a dial indicator, and it looks like the tops of the pistons are 0.014" below the deck. I did only check one.

I also checked the factory head gaskets I removed, and the non-compressed sections measure 0.065", and the compressed areas measure 0.045". They're going to be +/- a thou or two, but they seemed fairly consistent.

No idea if these numbers are normal or not.

The compression calculator I was using is linked below. The only reason I assumed that it was half way accurate was that when I enter 69cc for a chamber size, it comes back with 8.3:1 compression, which is what my truck was supposed to be originally AFAIK. When I punch in the 65cc for my "new" GT40 heads, I still only get 8.6:1 which is why I started asking the questions in the first place.

http://crt-performance.com/compression.htm

I could be way off on my assumptions...that's always the first possibility.

It's not a big deal, I just want to get this thing right for the rebuild.

 

I just looked it up and that is within spec for late model engine which should be 8.8:1 CR with a 64cc head.

 

Thanks Conanski, much appreciated. You guys have been a big help in getting my little brain wrapped around this stuff.

Shouldn't take much work then to get it to 9.0:1 compression or there about. I'll ask the engine shop to comment on the pistons, etc and if they see any evidence of previous rework, but it is supposed to be all original as it sits. As always, anything is possible.

I'm pretty sure I'm going to splurge on getting the shop to do the break-in and dyno tune for me. That way I can just drop it in the truck and turn the key (pretty much) when it's all done.

Do you still have your desktop dyno? Will this old thing make it within reach of 300HP?

Holley 600 cfm carb
Edelbrock Performer 289 Intake
Comp Cams XE250H full kit
GT40 heads and hopefully 9.0:1 CR.
Headers of some kind...not 100% sure which yet...

Any comments good or bad?

 

Yep. The closest Ford cam to that Chevy grind is the XR256H-10, select the 35-234-3 to get the HO firing order. With headers of some type the software says it'll be just shy at 294hp at 5000rpm but torque output is very healthy at 300+ across the board and a peak of 335lb/ft at 3500rpm. This assumes exactly 9:1 CR, if you can squeak out 9.3:1 that should put it oven the top.

 

Thanks for that Conanski, much appreciated! I had been reading quite a bit trying to determine where this might end up, and found guys that had done similar set-ups were anywhere between 275-325 HP...but even then, most of this info came from the Mustang forums, where they were usually talking EFI roller blocks, and often times RWHP, not crank HP, so I really wasn't sure.

I already have the XE250H cam (Comp # 31-230-3). Actually, purchased the whole kit p/n K31-230-3.

Would there have been any benefit in going with the HO firing order, and/or the 256 grind? I did look at the XE256H, and did consider that one for a while, but ended up with the XE250H. I don't mind buying another cam if there's a better option, but the XE250H seemed to be well liked in the searches I did. Any comments good or bad?

(I think I asked you about this previously, but didn't get back to you).

Thanks,
Cory

 

The only "stock replacement" pistons I've found are the Speed Pro L2482 which is an older 302 type piston (as far as I know) and the other is L2488 which is a replacement for the 5.0 HO motors. The L2482 has 2.7cc valve reliefs with a 1.605 compression height and the L2488 has 2.0cc valve reliefs with a 1.619 compression height.

There are several ways to get the compression you want. Milling the heads and decking the block are some things I would recommend on a rebuild which sounds as thorough as yours. The head gasket compressed thickness can also be reduced but I've seen that doesn't get as much compression as the other mentioned ideas.

What I'm getting on my calculator is if you can reduce the deck height down to about .007 and your combustion chamber to about 59cc you could get 9.5:1. And that's with a 4.00 bore and a .047 head gasket. That's with the 2.0cc valve relief too.