What mods to driveline and suspension are critical if adding 140 hp to a stock 71 coupe

[Hansjgode]

So I recently bought a 71 coupe ( 302 2bl / c4 / 2.69 8” rear end / front disc / power steer) that hasnt been run in years but has no bondo, surface rust in obvious areas, paint is a horrible color and shot, BUT it is completely numbers matching to the tag on the carb. I’m not a professional body/paint upholstery and would obviously keep every part i takeoff. My issue is that despite A numbers matching good shape car it isn’t worth that much. That being said I’d like to add HP and driveability without destroying the original value, i.e not drilling and hacking in case someone down the road would like to bring it back to original.
what are the forum pros advice on ways to do this. Can I put in a 370 hp blueprint 302 with a TKX and a new axle and upgrade a suspension package that doesn’t involve machining or welding to the car? I also don’t want to twist the car in half either, is there a middle ground?
please, any and all comments are appreciated.

 

[Sheriff41]

Howdy and welcome from Texas!

Your mods are very reasonable and will make for a fun car. There are plenty of suspension upgrades you can do that are pretty much bolt on -- sway bars, springs, shocks, etc. I would recommend subframe connectors, but they require some welding.

Driveline upgrades are covered in your list with the exception of the differential. With the 5-speed I'd recommend a 3.50:1 gear with traction lock or better.

One other thing to address is a brake upgrade. If you're running drums up front you will want to upgrade to disc brakes. Others will recommend discs all around, which is good, but the front disc/rear drum arrangement works well on the street.

Keep us posted on your progress!

 

[71coop]

Assuming the 8" rear end is in good shape, and at least a refreshed suspension, 370 HP at the flywheel is not at all excessive for this car. Too many people assume an 8" reaer end is weak - its not as strong as a 9", but its still good. Just make sure the brakes, suspension, any degraded bushings, etc are cared for.

Edit: with the TKX, I agree you'll want a little more gear. 3.5:1 is a good ratio. You can still stick with the 8" with that power level. With my old '79 vette and a TKO 600, I ran a 3:75:1 ratio and it was nice in 5th gear. That gearing can depend on the RPM range the 302 power band is in.

 

[tony-muscle]

With higher performance in this older chassis i think subframe connectors are really important. However, it will be hard to add them without it being a permanent mod.

 

[Cleveland Crush]

Happy to see you you have included an axle upgrade Hansjgode. When building North of 350HP on a driveline factory engineered for about 250HP your going the find the "Weak link" in the driveline, especially with a manual tranny. Here's my old factory 28 spline back when I started to exceed 400HP. I second the motion, if you currently have 4 wheel drum, change them out to at least front disc. Lastly, you can install "Bolt-in" sub-frame connectors since you don't want permanent mods. Welding them in is preferable, but a bolt-in is certainly better then no connector. Good luck with your project.

 

[Hansjgode]

You guys are awesome for responding, really appreciate it! All the comments make sense seems the consensus is going over 300 hp good practice to support the frame. More questions if that’s ok.
-any major objections to altering “original value” with welding frame connectors
-am I correct in assuming this car totally restored to factory original still won’t have near the value of a Mach 1 or Boss
-did the Mach1 / Boss cars have stronger chassis / shock towers than the coupe? Is that why these mods are necessary?

 

[71coop]

Just my opinion - many may not agree.

I think the frame connectors are needed if you believe the integrity of the body/frame has degraded or you are planning on tracking the car. And there is absolutely nothing wrong with them, they will stiffen the chassis up and improve the rigidity of the vehicle. I've thought about it, but for a street cruiser with around 350-375 HP, it doesn't make sense to me.

IF you car is in very good condition, (think low miles and no rust) and you aren't driving like a maniac, I don't see a problem with 400 HP with a stock body. If you are roasting the tires from stoplight to stoplight, or taking the car to the track, you might want to invest in some structural upgrades. As I recall, these cars were made to allow a big block like a 429 with lots of torque (think around 450 ft-lbs).

As far as value with the connectors - depends n the buyer.
Mach 1s mostly likely will draw more $ than a stock 71-73, but again, depends on the buyer.
A Boss most definitely is worth more than either.

 

[Bill Perkins]

Welcome from Opelika, Alabama. I like your 72 Coupe as I have one also that is a numbers matching car and it is a one owner.

 

[mrgmhale]

I find a lot of those Big Nose Mustangs have pretty high rear axle gear ratios, like 2.79:1 or 3.0:1. That is great for saving money on gasoline. But, if you are after some really snappy performance, unless you add a lot of horsepower and torque from the engine, those steep gear ratios are going to muffle how snappy the Mustang is. Just going to a lower gear ratio, like 3,91:1 or 4.11:1 will get you some really spirited performance, especially with an engine that has been built to produce more power than the stock engines used to deliver. But, you will be spinning some pretty high engine RPMs at 60 MPH also. What you need to do is find a suitable balance with your rear axle gear ratio.

The prior owner of our 73 Mach 1 replaced the original 302 2v engine with a 1994 F-150 351W (not C) engine that was built to produce 360 HP on a dyno. HE also replaced the original 2.79:1 rear axle gear set with a 3.5:1 TractionLok gear set. I found his choice of 3.5:1 ratio gears to be a nice compromise where we still get very spirited performance, but the engine was only turning just under 2,900 RPM at 60 MPH in 3rd gear (C4 three speed auto). Our 73 Mustang Convertible has a 302 2v with a C4 tranny, and 2.79:1 rear axle gears. I decided to not do anything to increase the power of the engine unless I was also willing to put in a lower ratio rear axle gear set. The 2.79:1 ratio gears work, but the vert is very pokey in acceleration.

Anyway, to your question re: what to beef up when building an engine for more horsepower... Luckily you already have power front disc brakes. Unless yu you plan to track and race the car there really is no reason to upgrade to a 4 wheel disc system. You have plenty of stopping power as long as the pads and shoes have enough frinction materiaal on then, and the drums and rotors are in good condition, as well as the master cylinder and power brake booster. The stock C-4 tranny may work well for you with a little mor epower being sent into it. 140 additional horsepower is not going to cause the C-4 to fail as long as it has good ATF in it, is not overheating, and you are not thrashing the car or doing a lot of racing, Keep the bands properly adjusted, especially intermediate band. If the C-4 needs to be rebuilt the Sun Gears and Pinions out to be upgraded to a high performance set of gears. The valve body can be improved with firmer shifts with any number of aftermarket valve bodies or valve body improvement kits. If you are going to be racing around an external automatic transmission cooler would be a good thing to get.

I would be looking at the suspension for worn bushings, weak springs, and even stiffer anti-sway bars.

For our 73 Mach 1, with its C-4, I began to get some slippage in Wide Open Throttle 1-2 upshifts. Rather than rebuild and beef up the C-4 I opted to get a well build AOD 4 speed auto tranny. I video recorded the entire process, including showing how to adjust the Throttle Valve Pressure. Now at 60 MPH in 4th gear I am only spinning the engine at just under 1,900 RPM. Yet, with the power of the built 351 I have plenty of HP and torque to overcome the air resistance at the higher speeds the OverDrive gear ratio provided. In fact, the AOD worked out so well that if I wanted to so turn the Mach 1 into a really mean street machine I would move to 4.11:1 or even 4.88:1 rear axle gears (with TractionLok), as the OverDrive gear ratio would help keep the engine RPM at a reasonable level as highway speeds. But, in my case I am not chasing more performance, so I will leave things as they are.

In cased you are interested in swapping in an AOD I have a series of links below, plus attached a few files, showing how we did the swap, and a list of parts and vendors we used.

Have fun!











The following link is for the Neutral Safety/Backup Light Switch splicing:















https://youtu.be/vdR-z3KV0UE



https://youtu.be/YCBBgd-olQ0



https://youtu.be/NcYr68atRjM



https://youtu.be/iFNXTGXNNcQ



https://youtu.be/2tTwnbcZZkY



https://youtu.be/8QncIKQTvNo



https://youtu.be/b4lnPReGWuM



https://youtu.be/zfYe--Gsjdk







Setting up Throttle Valve Pressure from the carburetor cabling, and how the original auto tranny floor shifter behaves with the AOD when shifting manually:



https://youtu.be/KYIAqqYUKNo



More below

This video shows there is a difference between TV pressures in Neutral vs Park:



https://youtu.be/6hZJEnGCPvY

 

[giantpune]

I find a lot of those Big Nose Mustangs have pretty high rear axle gear ratios, like 2.79:1 or 3.0:1. That is great for saving money on gasoline. But, if you are after some really snappy performance, unless you add a lot of horsepower and torque from the engine, those steep gear ratios are going to muffle how snappy the Mustang is. Just going to a lower gear ratio, like 3,91:1 or 4.11:1 will get you some really spirited performance, especially with an engine that has been built to produce more power than the stock engines used to deliver. But, you will be spinning some pretty high engine RPMs at 60 MPH also. What you need to do is find a suitable balance with your rear axle gear ratio.

My '73 has a 2.75:1 9" rear. It'll chirp and pin the tires without having to hold the brakes or dump the clutch. The car is more than fast enough around town, and it'll cruise at 60MPH with the tach still down around 2k with no overdrive.

My '69 has a 3.55 rear. This car, you have to wind the motor up a lot faster just in your every day driving. Its rowdier and louder. It definitely feels like you're doing faster, but I think just due to the seat-of-the-pants factor. All the noise and RPMs tricks your brain into thinking you're hauling more butt than you really are. With having to open the motor up like that 24/7, it is impossible to drive the thing without grinning ear to ear. This car does have an overdrive. And in 5th gear, at 60MPH, the engine is going about as fast as in the other car that doesn't have an overdrive.

 

[droptop73]

These cars can be made to handle really well with a few basic mods and a good alignment. Remember it's generally not power that breaks parts, it's from shock loads.

 

[mach71351c]

Times have changed on the all stock numbers matching deal. Especially on the run of the mill coupe. If it was a 4V 4 speed Cleveland or 429 car then that's a different story. So add that blue print 302 and TKX. Make the car fun safe and yours.