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View Full Version : 3.8 to 4.2 stroker



Nkuk35
11-06-2016, 06:29 PM
Okay guys so I am wondering if a crank from a 4.2l out of a f150 will fit in my 1990 xr7 the connecting rods are the same but I don't know if their are any differences between main journal size or if they are balanced differently. Let me know what you guys know about this and if you have ever done this swap I would like to talk to you.

XR7 Dave
11-06-2016, 08:05 PM
The rods are not the same. If you swap in the crank/rods/pistons from the 4.2 they will fit fine, but the 4.2 has a balance shaft which won't work in the SC block so you'll need to rebalance. By the time you spend that money, you probably won't want to use the stock 4.2 rods/pistons because they are weak. Maybe if you keep Hp under 300 it would be ok, but above that the 4.2L parts get a bit sketchy.

There are lots of 4.2 motors in SC's, but they pretty much all use aftermarket rods and pistons.

bowez
11-06-2016, 10:07 PM
Would it be possible to keep a 4.2SC under 300hp? I know it wouldn't be practical.

XR7 Dave
11-06-2016, 10:21 PM
Would it be possible to keep a 4.2SC under 300hp? I know it wouldn't be practical.

Sure, there are plenty of people who have dropped a completely stock 4.2 shortblock into an SC. You'll net an instant torque increase and HP will go up as blower drive and efficiency go up. I don't think I'd try an MPX on it but a 94 supercharger would work great.

Nkuk35
11-07-2016, 07:18 AM
Sure, there are plenty of people who have dropped a completely stock 4.2 shortblock into an SC. You'll net an instant torque increase and HP will go up as blower drive and efficiency go up. I don't think I'd try an MPX on it but a 94 supercharger would work great.

It says that the 3.8 and 4.2 have the same rod length and bore the only difference is the stroke

XR7 Dave
11-07-2016, 07:41 AM
It says that the 3.8 and 4.2 have the same rod length and bore the only difference is the stroke

Starting in 1999 the two motors shared connecting rods, but nothing else (pistons are different), and that doesn't apply to the SC 3.8, only the Mustang 3.8, and only starting with the 1999 model year.

Regardless, rotating mass is different due to different piston dimensions.

TBirdJKC
11-07-2016, 01:10 PM
Starting in 1999 the two motors shared connecting rods, but nothing else (pistons are different), and that doesn't apply to the SC 3.8, only the Mustang 3.8, and only starting with the 1999 model year.

Regardless, rotating mass is different due to different piston dimensions.

Is this the reason 99+ 3.8's were labeled as a 3.9? I'm guessing due to a slightly longer stroke?

Jacob_Royer
11-07-2016, 04:12 PM
Sure, there are plenty of people who have dropped a completely stock 4.2 shortblock into an SC. You'll net an instant torque increase and HP will go up as blower drive and efficiency go up. I don't think I'd try an MPX on it but a 94 supercharger would work great.


Will an 00 3.8 stang shortblock hold up in a stock SC application?

bowez
11-07-2016, 06:44 PM
Will an 00 3.8 stang shortblock hold up in a stock SC application?

From what I've read the block is fine, rotating assembly is a no.

1FSTBRD
11-07-2016, 09:13 PM
Is this the reason 99+ 3.8's were labeled as a 3.9? I'm guessing due to a slightly longer stroke?

I believe that it was just on the 2003-2004's that the 3.9 happened.....as far as I know, Ford started using the cranks that they had in excess supply from one of the V6 trucks.

XR7 Dave
11-08-2016, 09:23 AM
I believe that it was just on the 2003-2004's that the 3.9 happened.....as far as I know, Ford started using the cranks that they had in excess supply from one of the V6 trucks.

Yes, 2004 actually and it's from the Freestar van, not the truck. I'm not sure why Ford changed the crank, but most likely it had to do with harmonics rather than any kind of performance or emissions reasons.

1FSTBRD
11-08-2016, 03:09 PM
Yes, 2004 actually and it's from the Freestar van, not the truck. I'm not sure why Ford changed the crank, but most likely it had to do with harmonics rather than any kind of performance or emissions reasons.

Ah, that sounds correct!

Not to threadjack this too much, but the cam you had spec'd for my Mustang was a great choice, Dave.....lumpy enough that it's uncivil, but still good for a street car.

XR7 Dave
11-08-2016, 10:17 PM
Ah, that sounds correct!

Not to threadjack this too much, but the cam you had spec'd for my Mustang was a great choice, Dave.....lumpy enough that it's uncivil, but still good for a street car.

Seems like it needs tuning. Shouldn't be that radical.