Are there any pistons out there for the 351M that are higher than the stock 8.0:1? (with no 400 conversion)
What are the hp/torque gains to be expected, as well as mileage?

The 400 was 8.0:1 as well wasn't it?

Thanks!
Matt
1978 F250 4x4 - 351M

 

[font size="1" color="#FF0000"]LAST EDITED ON 10-Apr-01 AT 05:40 PM (EST)[/font][p]Matt:

Federal Mogul makes a TRW forged piston (#L2466F) for the 351M. They claim it gives 8.6:1 CR with the stock bore. As far as I know, that is the only high(er) compression piston option for the 351M.

Changing the pistons alone will give you somewhat more power, but I couldn't quantify it. Changing both the pistons and the camshaft (if you're running a stock or "RV" type cam) would be more beneficial.

The 400 compression ratio varied during it's production life ('71-'82). Early 400s ('71-72) had approximately 9.2:1 CR. Late '70s car 400s were down to 7.33:1 CR. Truck 400s were 8.4:1 CR throughout their production ('77-'82).

BubbaF250

1980 F250 4x4 Custom, 351M/NP435/NP208/D44HD-TTB/D60-FF/3.55s 6750 GVW, Rust & White.

 

What specs on the cam would match up with these 8.6:1 pistons?
As well, what type is ideal for the stock rebuild? Is an RV cam or stock cam good enough? I've heard they were crappy from the factory.

Thanks!

1978 F250 4x4 - 351M
Build 'em, don't buy 'em!

 

Along with a cam, you need a timing set that puts cam timing a "0" rather than the 8 degree retard that Ford use to keep emissions down. It also kept HP down!

Nathan

 

[font size="1" color="#FF0000"]LAST EDITED ON 11-Apr-01 AT 01:58 PM (EST)[/font][p]>What specs on the cam would
>match up with these 8.6:1
>pistons?

Matt,

Something like the stock cam used in the 351C 4V engines (206/221 duration @ 0.050" and 0.481/0.490 gross lift) would probably work well with the higher compression pistons and a 4V carb. You could also go with something a little milder like the Comp Cams 255DEH (203/216 @ 0.050" and 0.469/0.495 gross lift).

>As well, what type is ideal
>for the stock rebuild? Is
>an RV cam or
>stock cam good enough? I've
>heard they were crappy from
>the factory.

If you put in the TRW pistons, it won't be a "stock rebuild." If you do a stock rebuild with 8.0:1 (or less) pistons, you should probably stick with nothing more than an "RV" cam (around 198-200/204-206 @ 0.050 and 0.450" gross lift) for a slight performance improvement.

BubbaF250

 

>Along with a cam, you need
>a timing set that puts
>cam timing a "0" rather
>than the 8 degree retard
>that Ford use to keep
>emissions down. It also kept
>HP down!

Nathan ,

The cam timing retard specs I've seen say it was about 6 degrees. Retarding cam timing effectively moves the torque curve up in the rpm range. The cam retard likely increased the hp number (more torque at higher rpm), at the expense of lower rpm torque and driveability.

BubbaF250

 

I do believe the 351C and the 351M/400M share the same piston/pin location. Only the rod is longer, because of taller deck height of M motor.) If thats the case I'd use a piston from a 71 351C CJ. It has a true flat top. My 71 with 4V heads yielded 11:5.1. That way you'd bump the compression up with only a piston change. Again, I say check piston/pin location and you're there. I don't have the formula in front of me but I'd say you would gain a .5 to a whole point in compression.

I did a similar build on a 84 bronco with a 351W. Instead of using the dished replacement 84 piston. I used a piston from a 69 Mustang with 351W 4V. It didn't have the dish and piston/pin location was the same. I raise compression just a bit.

Hope this helps

Xaviar
96 F-250HD 4x4 460/E4OD 4.10
Banks Power Pack, Banks Trans Command
MSD 8227 Coil, MSD 32199 8.5 Wires, MSD 6A

 

351C, M, and 400 pistons are not all the same. The 351C uses a smaller diameter wrist pin than the 351M and 400. The connecting rods are the same between the 351M and 400. The difference in stroke is accounted for in piston height. The 351M uses a piston with a much taller compression height than the 400. Now, 351M/400 rods can certainly be bushed to work with the smaller 351C piston pins. But I doubt you'll find 351C pistons tall enough to work in a 351M. With a 400 crank, on the other hand, you can probably find Cleveland pistons to give whatever compression ratio you want.