I'm doing a rebuild on my 75 F150 with a 390. I had to purchase a replacement block which is an older block. I want to rebuild this engine for basically towing street use only. I don't want to have the 8.2cr that comes stock and I want a little better RV cam so my question is, for the cheapest way out, what pistons and cam do I need to purchase? I'm looking for a compression ratio of about 9.0-9.5. I want to do this with as little money spent as possible, so spending money or decking the block or heads is out. Just purchasing a cheap set of the right pistons and a nice RV cam is where I'm going with this. Keep in mind the heads are the original heads from my 75 engine and I'm keeping stock springs on the heads. I know this has been ask many times but with the searches I really wasn't clear as to what you guys recommend so hopefully this will be a straight forward parts number answer.
thanks in advance.
George

 

The cheapest way to get that compression ratio is to use 360 pistons. They will work wit the 390 crank and rods and put you around 9.5 to 1. Unless the block is in super shape you will have to bore it as the piston and rings will go higher in the cylinder. If you are keeping the stock springs on the valves then there isn't much you can do on the cam. It's a vicious cycle....higher lift/duration cam means you need higher spring pressure (change the springs)...more ability to generate more power means you need to get more air and fuel into the engine (4 barrel intake and appropriate sized carb)....more in means you have to get more out (headers). There is no cheap way out on this...lol. You can sorta do budget...get a stock 4 barrel intake and carb from a junkyard and put cheap headers on it....you still have to get springs to match the cam you install but the springs aren't that much money. I realize this isn't that clear of an answer either. There are tons of ways to go on these things. All the parts you choose must ultimately be a good match for it to be a good combination...if you put in a cam that peaks at 6000 rpm and you've got the engine choked with a 2 barrel intake and stock exhaust then you'll never see the power that's supposedly available. A good way to experiment with different combos is to run proposed builds through a software simulator (such as desktop dyno).

Good Luck!

Tracy

 

My truck did come with a factory 4 bbl intake and carb so I have that. I guess I can do the springs to help with the cam as the cam I have is shot. So with that in mind, any idea on the cam selection? I will be staying with the factory intake for sure, could change the carb in the future if need be. Headers I can do at a later date. I'm looking for an all around good RV towing cam. Nothing for high rpms at all. I saw these two piston selections in another thread:
United Engine and Machine Co.
Fast Engine Parts
Are either of these what I need?
But I'm still lost on the cam choice too.

 

Speedpro STL-H395P is a good piston. I used to use a Federal Mogul cast piston that I don't think is available anymore. The Silvolite 1131 is good. the second link looks to be a low compression 390 piston to me. The top of piston to pin height is different on the 360 and 390 plus the truck 390 pistons are dished which lowers the compression even more. You want a flattop with just the valve reliefs cutout. You can also select a head gasket to fine tune the compression ratio. Be careful on the CR...if you hit 10 to 1 with today's gas you will have a pinger for sure. For a cam, my last build I used a compcam 33-238-4. I like comp products...a lot of people prefer crane...I think something like a 343941 would be good. Both Comp and Crane have good websites that you can browse and help choose a cam....look at the rpm range, idle quality, hp and torque curves to help select. Honestly most people won't notice the differences in the seat of pants meter if you just select one based on max rpm and idle quality. You can get help from a vendor on this. Survival motorsports is very knowledable of FE's and can probably get you a good price on parts and help to make sure you've got a good combination.

Tracy

 

Thank You Tracy. I will check out the comp cam and the speedpro pistons.

 

Yep, 360 pistons will sit higher in the hole. With that in mind, the rings will sit higher also. Is there a ring ridge? That's a disaster waiting to happen.

I'm sure it's old-hat to you, but here's a good read: United Engine and Machine Co.

I did a search here with "quench" as the keyword. Looks like having the pistons .010" to .015" below deck will give the squish you want.

Good luck!

 

I just ordered pistons through summit and the a holes shipped me USA/INDIA made combo for a total of 8 so be careful with the piston supplier. The india product looked questionable at best, the metallurgy skills of that place scare me. You want a matched set and the pin height of 1.76. I got mine through EGGE, Inc. made in usa, $149.34 shipped next day air (part# L2291F .030)

 

Silvolite makes a good, inexpensive cast piston, which is what all 390's came with OEM. #1131 will give approx 10:1 compression, #1130 will give approx 9.5:1, and piston #1116 approx 9.7:1. IF your at sea level, might not want 10:1 without decking block to set quench clearance correct-would want ZERO deck meaning piston top is flush with head gasket surface of block. Flepro makes a good head gasket, blue in color, and is 0.041 inch thick. Typically your piston will be about 0.025 +/_ 0.010 below deck without decking the block. This means your quench clearance will most surely be over 0.050 which is getting a little too much to help stop detonation.
So maybe the silvolite 1130 is the best choice for a moderate 9.5 compression ratio. I would suggest blocking off the intake manifold heat crossover with stainless steel sheet about 0.015 thick, cut to about 1/4 inch in all 4 directions beyond heat riser passage on cylinder head, then glue the metal to cyl head with silicone or super weatherstrip adhesive, then lay intake gasket on head surface over the sheet metal to sandwich the metal between head and intake gasket.
Reducing heat of intake mix will help reduce onset of detonation. I have no problem in winter with heat crossover blocked off. The 390 intake design soaks up plenty of heat.
I would add dual exhaust, 2.5 inch diameter pipe minimum, headers if you can afford it, be sure to get flow rated mufflers such as dynomax super turbo or their ultra flow mufflers all rated at cfm flow so they actually muffle, but don't overly restrict the flow.
I use Edel 650 cfm carb, but had to get the Edel O2 sensor kit to tune fuel mix properly, was way way too rich even using the jetting charts that come with the carb.
Both comp and crane make decent cams. My only concern with crane is their 343901 cam has 0.533 lift on the exhaust, which really works the valve springs. Duration is what you need on the exhaust, not lift. So be sure to get a cam with split duration where the exhaust has longer duration than the intake. Ford cyl head has real poor exhaust port shape so needs dual pattern cam. The crane cam I mentioned does work real well. We put one in a friends ranch truck, he loves it.
From comp I would suggest cam # 33-206-3. Its an RV cam, works with stock exhaust system, has dual pattern I mentioned, says good torque and mileage. Valve lift is only 0.495 maximum which would work with stock valve springs, and be easier on the rest of your valve train.
Most folks on this forum love the crane cams, but compare them on desktop dyno for a mostly stock truck and your splitting hairs on performance. Hope this helps.

 

Saleen, I have had some bad luck with Comp Cams, more than once. Crane cams have not given me trouble. Of the '941 and '901 cams, use whichever one is bigger. But I would seriously consider using whatever new springs Crane recommends for that cam. If you can't afford new springs, you might want to wait on the new cam. On the pistons, the 381P is a copy of the low compression you have now. The 1131 might be more than you want. The 1130 might be okay. You could also consider the KB 150. But that idea that the FE has poor exhaust ports is not true at all. It is the exhaust manifolds that are awful. But if you are going to do headers when you can, then you will be fine. DinosaurFan, on work's old 'puter