DIY Duraboost Mustang

Ive recently grown to dislike working on transverse engine cars, it makes even simple jobs 10x harder than they need to be. ive decided i want to get back into rallycross/stage rally with either a RWD or AWD car, but it has to be a Longitudinal engine car. This leaves a few obvious choices, Miata, Impreza etc. In an ideal world, an XR4TI is a perfect fit, but being quite rare, and difficult to get performance oriented parts for, plus the inherit limitations with the stock engine, have led me to look for an alternative, leading to this plan to be hatched. Take a 1994-1995 5.0 Mustang, remove the heavy v8, and insert a turbocharged Duratec 4 cylinder. This has a couple benefits, moves the CG of the car much closer to 50/50, makes heaps of room to service the car in the engine bay, and has commonly available parts for both the drivetrain and the chassis. The HP limitations of that motor are okay, seeing as the common consensus is that 250WHP is all that rally tires can make stick on a 2WD car. There are off the shelf parts to adapt the duratec to a T5/T56, and it appears that there may be bolt in motor mounts as well. being a 94/95, in my state, means that no OBD2 plug in emissions testing is required, allowing an aftermarket ECU to control the engine with minimal fuss. beefing up the suspension shouldn't be terribly hard, the stock rear end is a 3 link, and the fronts convert to coilovers with ease. Tell me, where is the catch im overlooking? should be a pretty straightforward swap, even if i had to make custom motor mounts. most of the stock weaknesses of the SN95 platform have been addressed by the aftermarket. even if it isnt the fastest RWD car, it should be a hell of a lot of fun, being a similar formula to the current Ecoboost mustang. 

All I can say is do it. 

I see no downsides. 

Is there a benefit to starting with the v8 Mustang over a v6?

Only advantage of the V8 car is the 8.8" rear axle over the 7.5", otherwise they are the same. Also, a 1994-95 Mustang is not a 3-link rear suspension, it is the awful quadra-bind 4- link setup, 2005 is the first year for the 3-link. Changing it to a 3-link or torque arm setup is pretty easy and helps a bunch.

Other transmission options that may or may not be strong enough for what you want to do include the Duratec Ranger M5OD-R1, NC Miata 5-speed, NC Miata 6-speed, and while I haven't seen it done,all indications are that the Duratec should be able to bolt up to an EB Mustang MT-82 6-speed transmission as well.  Photos of the bellhousing pattern appear to be the same.  The only real question is if the starter mounts, crank mounting, and crank protrusion, are all the same between the Duratec and EB.

The stock rearend is NOT a 3 link.  It's a 4 link (Hotchkiss Link, I think) and people cut all that out for a torque arm for ROAD RACING because of how craptacularly it binds up and the roll center moves around.

EvanB said:

Is there a benefit to starting with the v8 Mustang over a v6?

there are conversion mounts for the 5.0 to duratec, im not sure if the v6 uses the same mounting points on the chassis.

Knurled. said:

The stock rearend is NOT a 3 link.  It's a 4 link (Hotchkiss Link, I think) and people cut all that out for a torque arm for ROAD RACING because of how craptacularly it binds up and the roll center moves around.

right, i got that mixed up with the later cars. it appears, though, that using spherical bushings removes most of the bind, or ill just cut and weld a 3 link in it.

Yes, contrary to much of the information out there, spherical bearings on both ends of all four links will eliminate all binding. It's not the cheapest route, but especially for rally uses I'd look into making all of my links using Johnny Joints and swaged steel tubes. I believe Currie even makes JJ's specifically designed to replace the axle end of the uppers, which are pressed into the axle housing rather than being incorporated into the, as is the case at the other 6 locations. 

Driven5 said:

Yes, contrary to much of the information out there, spherical bearings on both ends of all four links will eliminate all binding. It's not the cheapest route, but especially for rally uses I'd look into making all of my links using Johnny Joints and swaged steel tubes. I believe Currie even makes JJ's specifically designed to replace the axle end of the uppers, which are pressed into the axle housing rather than being incorporated into the, as is the case at the other 6 locations. 

It appears the kit is for a fox body, but IIRC the 8.8 is shared by both, so should still be compatible. If not I'm sure I could call them and come up with something for it. And those Johnny joints aren't all that expensive 8n the grand scheme of things, it's about the same cost as a full set of urethane bushings.

What about seths v6 car? Seems lije all youd really need is a solid and rust free shell. Preferably with a clutch pedal.

Fors would have used the same stampings for v6 and v8. Wouldn't make sense otherwise. 

Dusterbd13 said:

What about seths v6 car? Seems lije all youd really need is a solid and rust free shell. Preferably with a clutch pedal.

Fors would have used the same stampings for v6 and v8. Wouldn't make sense otherwise. 

Yup. Solid shell with a clutch pedal, no sunroof would be great

For off road stuff I'd look at a torque arm, for on road I'd look at the Steeda 5 Link (really a proper 4 link plus panhard, designed by a Ford Engineer originally)

I am absolutely NOT a Mustang expert and have several expensive mistakes to prove it. However I HAVE stayed at a Holiday Inn and I'm pretty sure about a couple of subtle items you may want to consider:

V6 models of that era are all 7.5 rear axles and therefore did not come with LS/Posi options so if you buy a V6 model you will only have an open diff. V8's had 8.8 and some/most had posi as an option.

Both V6 and V8 models have cooling fans capable of 2 speed operation, but only the V8s are wired that way. The V6 harness and relays cannot operate the cooling fan in 2 speed mode even though it has the wires on the motor side of the connector.

PS I approve of your basic plan. The Mustang engine bay is roomy without a DOHC hogging up all the space.

Sparkydog said:

I am absolutely NOT a Mustang expert and have several expensive mistakes to prove it. However I HAVE stayed at a Holiday Inn and I'm pretty sure about a couple of subtle items you may want to consider:

V6 models of that era are all 7.5 rear axles and therefore did not come with LS/Posi options so if you buy a V6 model you will only have an open diff. V8's had 8.8 and some/most had posi as an option.

Both V6 and V8 models have cooling fans capable of 2 speed operation, but only the V8s are wired that way. The V6 harness and relays cannot operate the cooling fan in 2 speed mode even though it has the wires on the motor side of the connector.

PS I approve of your basic plan. The Mustang engine bay is roomy without a DOHC hogging up all the space.

The 8.8 axle is mainly why I want the v8 model, plus, once its removed it can be sold to recoup the extra expense of buying a v8 car. The two speed cooling fans are nice to know about, although I'm probably gonna end up stripping most of the things like that out of the harness, and eventually making my own custom one for the chassis. And that's what I'm thinking, should lighten the car up a bit as well as making service work much easier. 

Maybe I’m missing something here but couldn’t you significantly  reduce weight and shift CG by swapping the cast iron cylinder heads for aluminum on the 302?  Maybe some other weight loss changes-relocated battery, fiberglass hood, under hood accessory removal, etc.

It seems to me like your creating a lot of work to do the swap when you are already close to or at your 250 HP goal after the head swap. 

There are tracloc options for the 7.5, I run one in a Lemons car.  The advantages are they are lighter and if out of a newer car 1.5 inches wider.  As long as you don’t put slicks on it and go drag racing, 250 hp on dirt will not kill it.  Rangers and Aerostars cane with decent gears and the FX4 even could be ordered with a torsen. 

So don’t rule out a V6 Mustang, plus they are basically free in a V6 manual configuration.

Gaunt596 said:

Knurled. said:

The stock rearend is NOT a 3 link.  It's a 4 link (Hotchkiss Link, I think) and people cut all that out for a torque arm for ROAD RACING because of how craptacularly it binds up and the roll center moves around.

right, i got that mixed up with the later cars. it appears, though, that using spherical bushings removes most of the bind, or ill just cut and weld a 3 link in it.

Putting spherical bushings in makes the bind worse, not better.    It moves okay straight up and down but if you want it to articulate, it is not so good.

This is why the softly sprung early cars and Fairmonts/etc had very large, oval bushings in the lower arms.  Was needed so the axle wouldn't bind up as it gyrated.  When Ford made the suspension stiffer for the later high powered cars, they went with stiffer bushings to control axle hop, which they could do because the suspension wasn't going to articulate much because of the aforementioned stiffer springs and shocks.  (Interestingly the '93 Cobra R had significantly SOFTER rear springs.  I don't remember what they did bushing-wise)

Three points define a plane, four points define a space.  If you have four points locating the axle longitudinally (or laterally for that matter) the geometry had better be perfect or there will be bindage.  The Hotchkiss (Satchell?) Link system uses four links to locate both laterally AND longitudinally...

A 4 link (with or without a panhard) with spherical bearings will only bind if the geometry requires one or more links to change in length.  As long as that doesn't happen, the suspesion will move fine. 

rslifkin said:

A 4 link (with or without a panhard) with spherical bearings will only bind if the geometry requires one or more links to change in length.  As long as that doesn't happen, the suspesion will move fine. 

The Mustang suspension requires one or more links to change in length.

Welcome to to wide wonderful world of mass production.  Compromises for packaging and ease of manufacture are over on the left, production-line tolerances are over on the right.  

The assembly tolerances for a Fox-body Mustang add up to over an inch of length when measured bumper to bumper, for example.  And that rear suspension design is just plain flawed.   The arms are like three inches long and the geometry is Fig. 1 for the Webster's entry on "Sucks (adj)".   

Knurled. said:

The Mustang suspension requires one or more links to change in length.

The only public comments I have found from people who have actually themselves investigated the Fox/SN95 Mustang geometry with spherical joints at all 8 locations both (one actually being the Design Engineer at Maximum Motorsports) independently stated that it was completely free of bind. Conversely, all of the other widespread comments I have found stating that it will bind are from people who have never themselves tried it themselves. Regardless of the ratio, I tend to favor the opinion of people who have actually tried it.

And from a kinematics standpoint, there is no reason it should bind.  Any object (i.e. live axle) inherently has 6 degrees of freedom relative to another object (i.e. chassis).  A suspension must maintain 2 degrees of freedom, as controlled by the springs/shocks, in order to have bind-free travel. By using links with spherical joints at the mounting points on either end, each link can inherently only constrain one degree of freedom.  Thus it takes 4 links having 8 spherically located mounting points, with at least (but not limited to) one being non-parallel, to fully (neither over nor under) constrain the movement of an axle. This is why a triangulated 4-link, like a Satchell or Fox/SN95, or a 3-link plus Panhard inherently do not have any bind the way a spherically jointed 4-link plus Panhard would if all 10 mounting points are not very carefully positioned.

That's not to say that the bind-free movement of a spherical jointed Fox/SN95 is supposed to be particularly desirable either, but the desirability of the bind-free motion is not at issue here.

From my research, it appears that the most likely culprit for this misinformation about the Fox/SN95 suspension links needing to change length is that the idea of what happens with a 4-link plus Panhard suspension (at least one link needing to change lengths through the range of motion) was misapplied to the marketeering of Fox/SN95 suspension components, and was subsequently misinterpreted by the masses as applying directly to the Fox/SN95 standard geometry...And thus a lie told enough times has become the truth that many (most?) have come to believe.

In reply to Driven5 :

You're looking at a static vehicle.  The static vehicle is made out of sheetmetal, not granite.  Under side loads and torquing loads, the tub is going to be flexing everywhere, and the axle is going to be flexing as well, although to less of a degree.  (Since the OP seems married to the idea of an 8.8, that would primarily be where the axles are plug-welded into the iron center case, although that case is going to flex around a bit too since it's a stressed suspension member)

Even if the geometry were perfect, the rest of the vehicle is not, and a design that marries two functions together like that is not very tolerant of imperfection.

Hell, Ford's "Escort Bible" describes how to make a proper 4 link to locate the rear axle.  They use rod ends (err, rose joints) on the axle end and TCA bushes at the chassis end.  Those aren't there for NVH reasons.  The 4 link design a simple parallel 4 link with arms 24" long and 6" from each other, not something that should ever experience any kind of binding.  (A Watts link is mounted to the diff cover with the bellcrank mounted on-center, so the axle's center of articulation is in the center of the 4 link)

To completely go off the established track of this thread... do you really want an XR4Ti?  Because you know I have a couple of them.

I'm not a mechanical engineer but I did own an SN95 and read about this a lot. I remember hearing about Fox Mustangs in college that were basically stock with sticky tires that would tear up the area of the car's tub known as the 'torque box' during drag racing.  Something had to be wrong; how could basically straight-line forward acceleration cause this?  Binding is all that made sense to me at the time. 

Anyway, when I think about the Fox/SN95 rear suspension, I consider an above view where all I can see are the upper and lower control arms and the rear axle.  One wheels is going up and the other is going down, like in a turn.  All of those arms are swinging in an arc and none of them swing with each other.  The lowers are nearly parallel to each other so they kind of don't count as much although it's still happening with them, too.  The uppers are pulling away from each other / fighting each other's arc and it's more noticeable because they're so short so the arch they swing in is more acute.  If one of them could be made with a spring inside, so that it could grow or shrink depending on the demands of the other arms, a lot of that resistance to the other arm's arcs would go away.

Maybe I've got something wrong.  But ripping torque boxes out doesn't seem like it would be solved by an almost infinitely harder bushing in the form of a bearing in place of thick rubber bushings.  It's never seemed convincing.   

I don't know where the "Maximum Motorsports guy suggests spherical bearings everywhere" thing came from; I've not heard that before.  They've never sold such pieces to my knowledge but they do sell Ford parts that are basically the SN95 factory fit arms.  

Knurled. said:

Ford's "Escort Bible" describes how to make a proper 4 link to locate the rear axle. 

This, and it boils down to the longest 4 equal length parallel links you can fit- that's what I'd do if I were building a car with a live axle, regardless of size and original configuration.

That said, if you want a Mustang, I'd much rather be figuring out how to get weight out of a later S197 V6 than berkeleying around with the flimsy shell of a Foxbody (I rallycrossed one) or SN95.  If you're going to run something where it all needs to be reinforced, at that point I'd go older and lighter.