I'm going to start this response with "I don't have many defendable facts" and "feel free to correct me as necessary"...

But I believe the weight difference between the I6 and the V8 was about 200 lbs. This basically isn't much more than some passengers weigh. In fact, this only increases the total weight of the vehicle by about 6%.

I'm guessing that the beefier brakes on the V8 were engineered to stop from higher speeds, not necessarliry the higher weight.

Moving on to a scientific approach, Newton proved that Force = Mass X Acceleration (or in our conversation, Deceralation - just a change in velocity). So algabraicly we can manipulate it to read: Deceleration = Force/Mass.

If we believe my assumption above about the Mass not being greatly increased by the larger engine, and 68Midnight has agreed to keep his speeds under 60 mph (presumably the I6 was designed to go that fast)- then I'm going to make the assumption that his vehicle should be able to stop safely in an adequate distance.

Now, this assumes that the brakes are maintained and function just as well today as they did when they were designed 40 years ago. It also doesn't address the comments regarding the ability of the small-er rearend being able to take the power of the V8. And I'm still curious about the VIN discrepancies...

Again, I'm an open-minded person. Please let me know where I have mistepped above. Thanks!

 

yay someone knows their physics

 

Crunchskippy, ya lost me

 

It's not just the smaller brakes, it's the entire front suspension that is weaker than its counterpart V8 equipped car. Take a picture of that rear end. Odds are it is an 8". If it is a 7.5 incher, then yes, it will break with the torque of a V8 if you get on it. And, maybe I missed it, but does it have a C4 tranny, or a 3 speed manual?

 

There were 3 issues raised about the V8-4 Lug combination:

[1] Safety of the Brakes
[2] Ability of the 7 1/4" Rear End taking the Torque of a V8
[3] VIN not matching engine & lug setup

I was merely trying to justify the safety of the brakes. (Mostly to myself, since I'm in a similar situation.) Sorry for all the babble...

But Soaring brings up a point I've never got my arms around: the front suspension differences. How can 68Midnight or I figure out what front suspensions we are running?

 

Go to any Mustang parts place, and order any suspension part, and they will ask you if it is for a 6 banger or a V8. The Six was lighter in weight and subsequently used lighter-rated suspension and steering components. Because of this, rear springs typically wore out and exhibited sagging far more quickly than the heavy-duty components found with the V-8s. This is one of many reasons why it is expensive to change from the 6 banger to the V8. Not to mention the tranny and rear have to be upgraded.

 

Diameter of the rearend has nothing to do with it torque capacity. My '85 came witha 7.5", it was fine until I broke the 300rwhp mark. Another example, my friend and I put an 86 GT drivetrain into his '83 ranger, which had a 6 3/4" rearend, the motor had a B303, E7 heads, headers and 2.5" exhaust, the rearend held up just fine.
As far as your brakes are concerned, Skippy is right, but what you do not take into consideration is dive. In an emergency stop situation, the I-6 suspension will actually travel further than the V-8 stuff, therefore it'll radically change alignment angles during a hard stop and reduce your contact patch with the road, in essence reducing your overall braking capacity.
Id make sure your suspension is adequate for the the weight. As for brake diamter I wouldnt be too worried about it, Ford thought 9" rotors were adequate for my '85 which has a GVWR (groos vehicle weight rating) of 3600lbs, thats as much as a '68 Chevelle [].
-P.

 

With the 60's model 6 cylinder Mustangs, the 7 1/4" rear was used. With the 70's model Mustang II's with 4 bangers, the 7 1/2 rear was used. They were called integral carrier rear ends because the differential and axle housing was one unit, and Ford used smaller and weaker components to build these rear ends as compared to the 8" and 9" rears. I don't know about 80's model stuff, but like the country and western song goes with these early Mustangs......"Size Matters."

 

Here is some fun reading on this subject.
http://www.nzmustang.com/Newintoold/newintoold.htm

 

'79-86 had 9" front rotors, 87 and newer had 12".
-P.