How much HP/Torque gain going from 8.2 to 9.5 compression?
 

Hi all,

Working on dissasembling my 390 for a rebuild. Bought a '73 F250 390 4spd two weeks ago that was running very poorly, a compression test showed one weak hole and one dead hole so I figured it was due (though truck only has 81,000 on it).

Upon taking it apart I found 1 push rod on the valley pan and one very bent rod still in operation. Since I was this far (engine already out) I've decided to go through it anyway since I plan on keeping this truck for a long time.

The rest of the engine "looks" great, can't even feel a ridge at the top of the cylinders and the cylinders themselves look great. I plan to have an engine shop check this stuff out, but I'm guessing the stock pistons might be re-useable, just need a hone job and new rings.

OK, so I was originally figuring I was going to need to bore it out and get new pistons so I was going to go with the keith black silvolites (1130 I think they were) and get it up to 9.5:1, but is it worth the extra money ($250) if I don't need them now? How much do I gain going from stock to 9.5?

I plan to use a Crane 901 cam, headman headers and a 4bbl setup to make a good tow horse.

Thanks,
Jordan

You will gain around 5%. So if your engine is making, say, 260hp at 8.2, you will be around 273hp with the 9.5. The rule of thumb is about 4% per point of compression until you get up around 11, then it drops to about 3%.

-Scouder

Do it, dude. I rebuilt my engine in Feb and didn't know one iota about a 390. Use those pistons (1131 I think) and that cam. I'll be doing the exact same thing in a couple of months, only trying to get enough compression to use the 941 cam. You'll love the responce once you get it all back together.

Check this out. Ebay 4508485852

Do get the pistons, the cam, the headers, and the 4-barrel. They all work together very well. The extra compression complements all the above parts. I recommend a Holley 600cfm vacuum secondary carb for this setup.

DD2k is showing 313hp @ 4500rpm, and 428ft-lbs at 2000 and 2500rpm when it's all said and done. It should tow very well.

Oh, and dont be tempted to re-use that rod. Get a new one.

Ok, thanks guys. That auction says the 1131's give 10:1, thats too much for 87 octane, I'd like to stick to 9-9.5 if I can. I think I need THESE

I think I'm going to spring for adjustable push rods. Since this already has bent some it would be worth it to blue print the valve train. Hopefully that can be accomplished with a rod length checker, some adjustable rods and lots of time and patience.

Does anyone have a specific part # recommendation on what valve springs to get? They seem to be size specific, not engine specific on the Jegs and Summit racing site.
OR better yet, anyone know if crane has a package with the 901 cam, lifter AND springs? That would be awesome and if so, what is the part #?

The cam and lifter set is 343902. If you're going adjustable there is a much better way to do it. Go down to Napa, and get 16 of their adjustable rockers. These would be for an early car 352. They cost me $14 per rocker at Napa. Then get a set of "427 Ford Adjustable Pushrods" at DSC for $40.

While you're down at Napa have 'em get new rocker shafts if yours are worn, which they probably are.

The Crane recommended springs are part #99837.

If you have adjustable rockers, is it still necessary to have adjustable push rods?

Thanks for the part #'s!

Yes. The pushrods themselves aren't adjustable, it's just a designation to say that they go with the adjusable rockers. The regular non-adjustable system uses a ball on both ends of the pushrods. The adjustable system has a ball on the adjuster screw on the rocker. Thus, the pushrods for this system have a ball on the lifter end and a cup on the rocker end. Thus the need for different pushrods.

aaaah, gotcha. So I would assume they are easier to setup, or is it just a more reliable way to go?

More reliable? Thought you said you found a pushrod in the pan, and one bent? But adjusted correctly, and maintained, they might be as reliable as the stock setup.

Actually I think that when adjusted correctly and quality components are used, meaning that the adjusting screws aren't loose, the adjustable system will be more reliable than the stock non-adjustable setup. Here's why. With the non-adjustable setup, you have no way to adjust lifter preload. So if one lifter can have almost no preload on it, and when wear occurs the valvetrain on this lifter will start to make noise.

You can "adjust" the non-adjustable system with various length pushrods, but this is a major PITA, and if things wear down the road and you lose your lifter preload, you're SOL. It's easier to go with the adjustable system from the beginning.

Thanks for all the help, I'll look into doing that. I want this engine to last me many years.

Just currious as to what makes you believe you have 8.2:1 stock compression? Take a look at the pistons you have now. Are there any overbore markings on them (like 030 stamped on top). If they are the pistons with more than "eyebrows" cut into them, you will definitely gain from the 1131's. If your current pistons have just the "eyebrows" you are already there and good to go. Have you checked the stamping number on the crank and verified it's a 390 crank? Check with your local machine shop for parts first, you may be able to get a fresh adjustable rocker set (rockers/shafts) for around $200. The 901 cam is a good choice and will work nice with a 4brl and headers (that's actually my set up..)

Just covering bases, --Mike

Nope, not sure what compression it really has, that was just a generic guess, figuring stock compression is likely around there.

I haven't cleaned the tops of the pistons off yet, but here are a couple pictures, maybe you can tell me?

Picture 1
Picture 2

Also I haven't checked the crank yet. I'm in the process of wiring my new 30x40 shop and only have 120v outlets. I'm putting in a couple 220's this weekend so I can get my air compressor going and remove the flywheel and crank pully. Assuming its the stock motor, it's a 390 according to the vin.

thanks,
Jordan