I'm currently rebuilding a 351M for my 78 2wd F150 this is my first rebuild and I don't want to screw anything up in the process. The engine had new pistons new RV cam and a new timeing gear and chain put in it. I want a semi aggressive came with a lope to it but nothing too radical. this engine would hopefully be used for every day driving and mild racing and crusing on the weekends. how does a RV cam compare to other cams? what are it's uses? My ideal Rpm range would be about 2000-4000 Also what does the deck height have to do with a cam and how do I find it? also If I get a more aggressive cam I know I have to replace the push rods and valve springs but How do I know what kind of valve springs to get? do I need to upgrade the rocker arms too? Finnaly were the stock cams in the 351M's mechanical and can they be upgraded to hydrolic or roller? I know it's alot of questions but upgradeing an engines valvetrain seems a bit tricky to me. Thanks for any info

 

You asked a lot of questions, and most require long answers. First it sounds like the motor has already been rebuilt, and with a reasonable set of goodies. You don't mention what type of RV cam that you have, manufacturer, PN.

An RV cam, is a cam with a lot of lift and short duration. It's purpose is to improve the torque for pulling a heavy load at low RPM. The best RV cams are Hydraulic Roller cams, but they are expensive and require a lot of modification of the valve train. Hydraulic Lifter RV cams are cheaper and can do a good job. A racing cam will have more duration and lift and will produce a lope. They produce power at higher RPM than an RV cam, usually at more than 5000 RPM, at the expense of power at low RPM. racing cams require some modification of the valve train, usually just heavier springs to close the valves faster. The valve springs can be purchased with the cam. The racing cams can be Hydraulic, Hydraulic Roller, Solid or Solid Roller Lifter. The last three being for very high performance use. The 351M comes with a hydraulic lifter cam. The 351M is a low compression engine and it is well suited to a Hydraulic or Hydraulic Roller RV cam. It would be a mistake to put an agressive cam into a 351M with stock compression. To increase the compression you need to change the heads, or pistons, or mill the heads and deck the block etc.

The way that your engine is built now sounds fine to me, maybe it is not what you are looking for, but you will need to start over to get more out of it.

Read the post here for a while, and the tech articles, before you start your project.

 

Thanks for the info I've been shopping for cams lately and have ann idea of what I want. I was wondering if any one knew how much I would need to shave off the heads to acheave 8:7 cpr or 9:1cpr? The pistons and valves are stock AlsoI want to have as much torq and power through the whole powerband any cam suggestions? thanks.

 

First, you should find out what cam is in it now. You may not be much better off with your replacement.

The next thing you need to know is what was done when it was rebuilt. Were the heads milled, the block decked, or are the pistons stock compression height. A lot of replacement pistons have lower compression height that stock, also was it bored?

I assumed that everything was standard specs.

I boosted the CR to 9.1:1 by milling the heads 0.068"(?). I am not at all sure that you can do that. I assumed that the compression chamber is a 4 inch cylinder.

The most cam that I would use would be a CompCams 260H or similar. Dyno 2000 simulation gives Torque = 386@3000 RPM and HP = 297@4500 RPM.

I substituted an Aussie 58cc head and got 9.58:1 CR. I ran Dyno 2000 with a CompCams 268H. The results are Torque = 390@3500 and HP = 323@5000.

There may be some error in these figures, because I used 351C-4V head flow data in both simulations.

The Dynamic Compression Ratio is 7.7:1 with the milled heads and 260H cam, and 7.9:1 with the Aussie heads and 268H cam. These figures are good, but they could be higher. With more cam they will just get lower.

 

The previous owner Said that a new cam was put in 2 new pistons and a timing gear and chain and the heads were cleaned up a bit and that's it. So the piston bore should be stock. and this block is vbasically stock except for the cam. I"ve heard good things about the 268H I think that I will use it because I can run stock pistons (according to Jegs and Summit) but I need a cpr of at least 8.7:1 the heads are stock 4bbls and I want to bump up the cpr without getting new pistons. so that's why I want to get the heads milled. Thanks for the help and info!

 

If you are willing to mill the block to get the compression you want, have the machine shop to check the cylinders for wear, you may need pistons anyway and then you could use flat-tops to raise your compression. Just a thought.

 

Yes, if the original pistons are still in 6 of the holes, the cylinders are probably worn. I would have them checked before putting money into it. Of course worn cylinders can be fixed by boring and replacing the rings and pistons. TRW makes a piston for the 351M with more compression than the stock 351M piston.

The Comp Cams 268H is a good choice. I simulated it at Torque = 377@3500 PRM and HP = 308@5000 RPM. The Dynamic Compression with this cam and static CR of 8.7 is only 7.16:1. It should be >7.5:1.

Comp Cam claims these cams work with 8.1 to 9.1 CR, but I would not want to use the 268H cam with less than 9.1 CR.