Compression Ratio Help

[sparky72]

Well fellas, I have to unfurl the banner again:

I recently had my 390 worked over by a local machine shop. 60 over (was 30 over when I bought it), decked a bit, align honed, crankshaft honed, new pistons, all balanced, bearings, timing set, lifters, and oil pump (purchased through the shop; good parts, not parts store junk). Got a good price on having the heads worked on a bit too, with new valves, springs etc, machining on the bottom and exhaust side, and hardened seats. Less than a grand for the head work, which I figured was pretty good since this is going into an F250 and I don’t plan on moving THAT much air.

But I digress. My troubles began when I got home and realized that the pistons he bought are flat tops with eyebrows. I was expecting dished pistons, since we had agreed that between 9:1 and 9.5:1 was the ideal goal. When I picked everything up, he said he didn’t CC the heads, but figured it would be right around 9.25:1. Cool, that’s exactly what I wanted.

But! The H395P pistons, when sitting about 5 thousandths in the hole (which is about what he said they would be), give over 10:1 CR with 72cc chambers!! That’s way above where I wanted to be. I could run a fairly radical cam to help bleed off some cylinder pressure, but at the same time, this isn’t a Mustang...

I still have to measure exactly how far down the pistons sit, but I haven’t assembled anything yet because I’ve been slowly acquiring analysis paralysis researching the rest of the parts for the build. I’m really kind of miffed that I have a whole balanced rotating assembly now that will likely not work for my application.

Was this my fault for not being extremely specific about the desired CR, and not writing it down for him? I don’t know.

What are my options here? It seems like if I get dished pistons, my quench will suffer terribly and I might run into detonation. But if I stick with my current combo I’ll likely have to find a fairly thick head gasket, which lends itself to the same problem.

How does anyone get a 390 truck motor with a low 9s CR and good quench?? I’m kind of at a loss right now and could really use some help and guidance. I appreciate any assistance you can offer me guys.

[roboto65]

You could look around and find a thicker head gasket that should bring that number down.

[sparky72]

You could look around and find a thicker head gasket that should bring that number down.

From what I can see, thickest around is about 47 thou compressed. That would only bring me to about 10:1

[roboto65]

https://www.scegaskets.com/store/ford/f ... /p342072l/

Not cheap by any means but they are out there but ouch!

[roboto65]

Oh missed that they need Oring.

[sparky72]

https://www.scegaskets.com/store/ford/f ... /p342072l/

Not cheap by any means but they are out there but ouch!

Wow that’s thick! It would get me in the 9.5:1 ballpark but I would be worried about such a large quench height. Definitely something to keep in mind. Good find, thank you!!

Edit: Nuts I just realized that too. Wonder why you would need to go through all the extra trouble of adding an o-ring to the block.

[roboto65]

Looks like these guys have what you need. https://www.cometic.com/c-1280258-autom ... skets.html

[sparky72]

I guess I’m just wondering how guys get 9.5:1 (or thereabouts) while retaining a quench distance of .040 or so. It doesn’t really seem possible without using a really large dish piston (20cc or so).

My machinist seemed confident that he could pick a piston and deck height to get me between 9 and 9.5. Maybe that’s true with a 4.05 or 4.08 bore, but...ugh.

[roboto65]

Yeah only a few things to do, new pistons,machine the pistons you have a few thou not ideal and cam choice. The biggest thing that would get me is paying extra for the balance and probably having to redo that because of the new pistons or even shaved pistons. Another option is to run what ya got but you would most likely be stuck with premium fuel to keep detonation down god forbid race fuel . Maybe a adjustable cam gear set adjust the timing accordingly might help. One other thing but this is a high cost option is new heads Speedmaster As-Cast Cylinder Heads PCE281-2154 are 76cc but again this adds a lot of cost to a build that probably has cost enough without the headache.

I will say this is where checking and rechecking as you have done saves extra work down the road so job well done..

[sparky72]

Would 10:1 or so be okay in a truck motor? I’ve always thought it would be better to be in the low 9s for a big heavy vehicle like this, especially when running a big cam to compensate for compression will rob low end torque. This is my first motor build so I’m all ears for opinions!

[My427stang]

OK, so first, let's dig through a few things.

1 - That piston has a pin height of 1.759, which gives the crank, rod, piston stack a total height of 10.139
2 - The stock block deck is 10.170. No real way of me knowing how much yours has been cut, but it would take a .025 cut to be at .005 in the hole. Did you measure? That seems like a lot unless you were correcting for some damage. Every one I have done in the past few years cleans up at 10.155 for reference
3 - If you are at .005 in the hole, D2 and C8 heads tend to run large, so even with a clean up cut, and valves sunk a little, you are at 10:1 with a Felpro 1020 gasket and .046 quench. That could be real good if the cam is right. If you do the same calcs with a Felpro 8554, you are 9.74:1 with .058 quench.

Here's the deal though, I bet you are deeper in the hole. Let's say he cut you to 10.155, you'd be .015 in the hole and 9.79:1 and .056 quench with a Felpro 1020. That'd live depending on the cam

So, what cam you you need? A Comp 270H would be plenty if installed at 106 and be fine on gas as long as you didn't have too aggressive of a timing curve. A Edelbrock Performer Plus, 272/282 would also work super in a truck, likely even better.

Let me know the cam, and I can tell you more, but my hunch is, NOT using the thick blue 8554 and buying a Felpro 1020 .041 thick gasket is the way to go on this one. If the cam is 270 adv intake duration or higher, you are good to go

Two comments though

- A dish piston doesn't take away quench. There is a flat portion of all pistons and heads, those are the quench pads, there is always some, even on a dish piston.
- You could go with an 8554 Felpro even if you are really .015 in the hole. It would work, a million of them out there, but you'd be at .068 quench which is getting up there, and 9.5:1 compression. That's not the end of the world, nor is it like the truck 390s or 360s that were WAY down, but if you are running enough cam, I'd rather see the quench tightened up.

Get back to us with cam and how you are calculating .005 below deck. I don't think you have an emergency

[My427stang]

One last comment for now

If you do have the option to buy another set of pistons and return what you have, an L2291F60 is a decent deal.

Assuming a deck cut of 10 thou (big assumption, don't buy until you know) the 2291 is .004 below deck due to a better pin height, and with the cheaper 8554 head gasket, is 9.57:1 with a 72 cc chamber, it's also a forged piston

Now, if you wanted to get fancy, the next step is custom, but they start running double the price, but you can get whatever you want.

In the end though, current deck height, planned camshaft, and if possible, having a chamber poured (cc'd) is good stuff to know what will happen

[sparky72]

One last comment for now

If you do have the option to buy another set of pistons and return what you have, an L2291F60 is a decent deal.

Assuming a deck cut of 10 thou (big assumption, don't buy until you know) the 2291 is .004 below deck due to a better pin height, and with the cheaper 8554 head gasket, is 9.57:1 with a 72 cc chamber, it's also a forged piston

Now, if you wanted to get fancy, the next step is custom, but they start running double the price, but you can get whatever you want.

In the end though, current deck height, planned camshaft, and if possible, having a chamber poured (cc'd) is good stuff to know what will happen

Thanks so much for the replies! I plan on going in this week to have the head's CC'd, and I will likely take the block with me and have them measure the deck height. Once I get these measurements, I'll get back to you.

I haven't bought a camshaft yet, actually. I've been checking and rechecking what I have now before I order that, same with the intake and carburetor. I'm feeling pretty glad that I have been checking things!

Again, I really appreciate your help

[My427stang]

Any time! When someone is willing to do the homework and thinking it's fun

As far as deck height, you can have him measure the actual piston in the hole but here is what I would ask him

1 - Did you square deck it to the mains?

if the answer is yes

2 - Can you measure deck height off the mains and tell me the number?

You are done......we can figure everything out accurately, then just cc the heads

If he did NOT square deck and just cut the block flat, then you will really need to assemble a piston and move it to all 4 corners and measure, then we use the measurement closest to the head for the calculation.

I think you'll likely be fine. Likely the compression won't be an issue, but if quench is, we can talk about cost/benefit of going with a set of L2291s and pick a cam based on use, but it all depends on what you find out

[sparky72]

Any time! When someone is willing to do the homework and thinking it's fun

As far as deck height, you can have him measure the actual piston in the hole but here is what I would ask him

1 - Did you square deck it to the mains?

if the answer is yes

2 - Can you measure deck height off the mains and tell me the number?

You are done......we can figure everything out accurately, then just cc the heads

If he did NOT square deck and just cut the block flat, then you will really need to assemble a piston and move it to all 4 corners and measure, then we use the measurement closest to the head for the calculation.

I think you'll likely be fine. Likely the compression won't be an issue, but if quench is, we can talk about cost/benefit of going with a set of L2291s and pick a cam based on use, but it all depends on what you find out

He did square deck it. When I haul it all in I’ll ask him specifically to measure the main bore to deck distance. It does seem like a much more useful dimension to know than the distance from the piston to the deck, which is specific to the crank, rod, and piston combination.