I have seen a set of Jahn Racing pistions with a domed top to them for the 4.9l I have a carb 1982 300 and have been entertaing the thought about turbo charging. Now i've heard if i install these pistions they will raise my compression to 11.1. Now would this be a good idea if i plan on forced induction, also how easy would it be to intall these without pulling the engine out?

 

You don't want that high with any kind of forced induction. In the old days they recommended no higher than 8.0 or 8.5 to one on a turbo setup. That figure has climbed since the introduction of EFI/computers. Electronic management allows precise control of fuel & timing so you can run it higher, but not that high. 11:1 is really probably about as high as you'd want N/A, especially with the carbed motor's chamber design. This is, of course, static compression and cam profile would have a great bearing on the effective ratio. One other thing I'd wonder about, since you are talking turbo here: Are those forged pistons? I've seen some Jahns for the six that were high compression, but still cast. I've been researching forged pistons for the six, and about the best thing I've seen so far are TRW forged dishes for a 351W. They've got 'em at Summit for about $33 each, and the wrist pin diameter should be right for a carbed motor's rods.

 

11:1 is too high for pump gas in a 300. There are lots of engines out there that will do it. A 300 won't, especially if it's a domed piston. If you used a 240 or EFI head it might do it. If you are going to do a turbo you have to decide if power output or driveability is more important to you. You need some compression just to make the engine driveable on the street when not under boost. 8:1 is as low as I would go. If you want decent mileage and driveability I would go about 9:1 and run less boost.

The small block pistons will only work on the very early 300 rods. I'm not sure what year the little ends grew, but there are very few years that share pin diameters with the small block. I've said it before and I'll say it again. If you are building and engine worthy of forged pistons it is worthy of a custom forged piston. If you are spending $300 on a set of pistons anyway, why not spend $400 and get exactly what you need. It never ceases to amaze me that people have no problem spending thousands on a new engine and then severely compromise its performance or durability using an off the shelf piston that isn't really what they need because it saves them $100-200.

 

Hey Silver Streak, who do you recommend for custom pistons? I'd really like some with stock compression (dished) for a late EFI motor, but forged. I'm not opposed to the idea of getting custom slugs, but would like recommendations on who to get 'em from.

 

My nephew just got some Ross custom ordered pistons. Them things sure are pretty, hate to hide 'em inside the engine . It's exactly 9.5:1, nice increase and not too much. Still able to add turbo later if desired.

 

There are several manufacturers that do excellent work. Many are listed in the pistons section of flatlanderracing.com. They should be able to start with a 351 blank and machine it for a 300 pin, but there might not be enough material in the pin area. Probe mainly does Ford parts so they might be a better choice for an oddball engine like the 300. I would call Flatlander and talk to their tech department. The guys name is Phil and he won't BS you about anything. They specialize in stroker engines, maybe they would do a kit for the 300 if there was enough interest.

 

I am running a built up 300 I did several years ago. After researching piston specs for other engines the perfect match turned out to be stock pistons for the 360 FE V8. The wrist pin diameter is a match for the later rod, compression distance was right and they had the flat top design that yielded me with a 10:1 ratio. The standard size for the 360 is .050 over the 300 bore. I see on another thread that someone is running 390 pistons which probably work too but I'm not sure of the difference in compression distance.

 

Ahh, thank you. This little bit of info will come in quite handy building the 300 on the engine stand in my garage...nice not to have to enlarge and maybe bush the small end, though I may go with custom pistons anyway to give 'er a little more overbore-ability. The Crane 272 cam is just the ticket with the higher compression, or a Comp 268 grind...

That compression ratio is with the larger chamber head, I s'pose?

 

I'm not sure what size my chambers are other than the head was off a '79 motor. The block I used was from a 60's heavy duty engine since the '79 block was cracked. The cylinder walls are about twice as thick in these so I think I easily have another overbore in there. Oh and I'm sure someone will ask now, I do have one of those coveted steel cranks tucked away in case I decide to try something radical one of these days.
The rods I used were the '79 rods since the early rods had a smaller wrist pin.

 

The 360 and 390 pistons are OK, but 0.050 over is really pushing it for an overbore. If you have intentions of running it hard I'd be concerned about cylinder wall stiffness. You should at least have the block sonic checked before going that far over.

 

Good advice (as usual!). The earlier blocks have a little more meat on 'em (isn't that usually the case, it seems...). I dunno about this spare EFI block I have---whether or not punching it out .050 is just asking for more heat and bore distortion. I read somewhere (Ford factory literature, IIRC) that the sixes were cast out of more quality stuff (higher nodularity, purer iron), so I would expect they're pretty solid. Nonetheless, it's always a good idea to get the block sonic-checked when going over .030. Some cheesy castings you can't even get that much out of (the 350 Chevy Mexican block comes to mind...).