351C to 460 Swap - Opinions

[mwallace]

Hello all,

I have a 72 Mach 1 with a 351C-4VCC backed with a Tremec 3550 5 speed trans. The 351 was recently built will very little miles. I started the project years ago and various life issues got in the way. It has 70 model 4V heads and a pretty aggressive roller cam (sourced from Brent Lykins) with compression in the high 9s. To make a long story short, my goals for the car are changing to more of a cruiser. About a year ago I backed into a cheap (essentially free) 460/C6 from a mid-80s motor home. From what I can tell the 460 is in great shape and I was told mileage was less than 70k.

The car has a 3.25 open rear which don't really work well with the current setup. Plans were to change to something lower but not so sure now. I also have a 3.50 9" on hand in good shape from a truck.

I understand the added weight of the big block and the changes needed on exhaust. Car has a large aluminum rad with Taurus fan setup so I believe that would work.

I would like to hear from folks who have been down this road - actually did this swap, did you love it or regret it? If I were to dive into this, I would probably sell the engine/trans to fund the project. The car was originally a 351C-2V FMX and I still have the original drivetrain in storage.

Thanks in advance

 

[TommyK]

I have done it and I don't regret it but my Cleveland was shot and in need of a rebuild. I too got a good running 460 for basically nothing so I did the swap. For me it was the fastest and cheapest way to get a running driving car again. You on the other hand, are probably just a cam change away from getting the driving experience you want which is certainly much easier and less expensive than doing a BBF swap.

 

[mjlan]

Are you planning on going through the 460 before using it?

Off the top of my head swapping to the 460 would be:

Engine mounts
Trans crossmember (what does the RV trans tailshaft look like?)
Shifter
Trans cooler
Exhaust

I would have to agree with tommyk- a cam and rear end ratio/ diff change and you are back on the road. I more street friendly cam and a 3.50 limited slip would be a good street combo. With the OD of your current trans you could probably also go lower (higher numerically) on the rear gear and still have the OD for highway if needed.

 

[giantpune]

Is the 460 complete with all the accessory brackets? And which version is it? Not all 460s were created equal.

If it were me, i would stick with one of your existing 351C engines and just make it less wild. Swap the cam shaft to something more streetable and the rear end to something in the 2.75 or 2.89:1 neighborhood. Definitely cheaper and easier than swapping the whole drivetrain and having to figure out how to connect a couple dozen things to the car.

I have a 69 tbird with the 429/C6 and 2.89:1 rear end, all factory. In 69, they still had the higher compression and straight up timing. Your later 460 would would have more displacement, but lower compression and ******** cam timing compared to mine. IMO, that combination is good for just driving around. It lacks anything that could possibly be confused with being peppy, sporty, or nimble. It does easily 65-70 in the highway without having the motor screaming. But it is not fast off the line. I would be disappointed and downright ashamed to own a classic mustang with that sort of performance.

 

[Hemikiller]

I'd talk to Lykins and have him recommend you a cam that meets your needs. With a roller, you can just pop the old cam out and drop the new one in. I'd run with the milder cam before doing the rear gear swap.

 

[c9zx]

As others have said, change the cam and, if needed, the gear. The extra 200 pounds on the wrong end of the car will not be helpful. That is just my opinion. It is your car so do what you want. Either way, good luck. Chuck

 

[mwallace]

Thanks for the feedback. I really appreciate it. I believe I'm going to just put some 4.11 gears in it before I go any further. With the OD I should be able to go that low without extreme revs on the road. That was my plan all along but its nice to hear from others who have been there.

The 460 I have is complete but far from a performance engine and it would not be a cheap option to make the move.

 

[rcadd1ct]

Thanks for the feedback. I really appreciate it. I believe I'm going to just put some 4.11 gears in it before I go any further. With the OD I should be able to go that low without extreme revs on the road. That was my plan all along but its nice to hear from others who have been there.

The 460 I have is complete but far from a performance engine and it would not be a cheap option to make the move.

TREMEC has a phone app that will help you bench race gear options to figure out what ratio will allow cruise and get the car off the line.

With my 4 Spd and 3:1 gears 80mph is 3200 rpm. I definitely need overdrive

 

[mwallace]

Thanks for the tip! That will help...

 

[giantpune]

TREMEC has a phone app that will help you bench race gear options to figure out what ratio will allow cruise and get the car off the line.

With my 4 Spd and 3:1 gears 80mph is 3200 rpm. I definitely need overdrive

Their phone app doesn't have the 3550 in it. Looks like there's a few versions of it, all with the same first 4 gear ratios starting with 3.27 1st gear. And either a 0.68 or 0.82 OD. Thats a big difference.

The app does have a TKO-500 that has the same ratios as the 3550 with 0.68 overdrive. I didn't see any transmissions in the app that matched all the ratios in all the gears for the 0.82 overdrive version.

You would want to match your rear end ratio with the OD ratio. Otherwise you could be cruising along, and the car would either not have enough gear or it would fall flat on its face when you hit 5th. I have a tremec in mine with the same ratios, but a 0.72 overdrive. It does well with the 2.75:1 rear end. 3.25:1 rear end would be about as wild as I would take it.

With that 3.27 first gear, if you were to go to a 4.11 rear end, you'd need to hit 4000 rpm to even get to 20mph. Thats a bit much for a cruiser.

 

[basstrix]

I did the swap from a 351c/FMX to a 429/C6 back in the 80's. The major components for that swap were:

Engine Mounts
Radiator, fan, & Shroud (hoses were larger diameter, pilot on water pump larger, and fan shifted farther over compared to 351)
All accessories (the car I swapped from was available, so this made the job much easier)
Front Coil Springs
Transmission, of course (was able to use the same cross member) Had to get the correct shift lever for the mustang's linkage.
Shorter driveshaft
Exhaust system....those hooker super comps fit nice, but they are super-low to the ground. Took away a lot of the fun when speed bumps were involved.

I put a fairly stout hydraulic cam in the 429 and ported the heads, so it definitely was not stock. I recommend sub-frame connectors to anyone doing this swap...the torque from these engines is pretty impressive. It would light the tires in 3rd at 35mph with street tires....definitely could have used some suspension work! Slicks were fine, so that's what I ran at the track.

All that being said, it looks like you're staying with the cleveland......personally, I think that's the better route, anyway. I would love to have that 5 speed in one of mine.

 

[Don C]

Thanks for the tip! That will help...

There's a spreadsheet in this thread that will let you play with ratios, tire sizes, RPM and speeds
https://7173mustangs.com/threads/tire-sizes.11266/

 

[tony-muscle]

As others have said, change the cam and, if needed, the gear. The extra 200 pounds on the wrong end of the car will not be helpful. That is just my opinion. It is your car so do what you want. Either way, good luck. Chuck

100% agree, specially the comment about weight added to the wrong end of the car.

 

[raydav]

A 460 in a 71-73 Mustang is a bolt-in. It was designed for it.

My wife drove her 71 Mach 1, 460 eight years, till it got rearended the second time. Then we got her the 73 Mach 1, and the neighbors 76 T bird for it's 460/C6. She now has over two decades on the 73.

As to the motor mounts to put a 460 in a 71-73, you remove the part that bolts to the frame, swap them side -to-side, and elongate a couple holes, and use 460 insulators.

For a smog motor 460 cam, get a 7 slot timing set, and advance twelve degrees.

My 73 Mach 1, 351 C4V/C6 has been on the road since the eighties. I have a 532 in a box for it.

Both our 73 Mach 1s have the stock 2.75 9 inch. Mine has 275-60-15 tires. Mine has been to North Carolina to open the 55th museum.

 

[rcadd1ct]

If you have Apple I use this, too.

Let’s you input any ratios for each gear and tire size

https://apps.apple.com/us/app/gear-speed-and-tire-size/id1048868556

 

[mwallace]

Thanks to all for taking the time to reply, really great feedback. I have the tall od gear with a 26" tall tire. In the app I'm using the TKO 3.27 0.67 setting as that is generally the same as I have. With the 3.25 the tall OD gear its only turning around 1600 rpm @ 60mph (not the app, real measurement). I would like to get this up above 2000 rpm which is why I was leaning toward the 4.11. Good point on the first gear rpms and that is a concern. Could go to a 3.89 so got some decisions to make. Got to just make a compromise somewhere. Its not a daily driver right now and probably won't be so I can live with the higher rpm it may be turning. The 9" in the car needs some other repairs (leaking seals and a bit noisy) so I got to get into it anyway so figured I would just start there. I am planning to take it on myself as I've done some of this before.

I have a 77 F250 4x4 with a 460 and 4.10 gears. Totally different machine but runs down the highway 2900 rpm @ 65 mph so I know how that is. I will probably just save the 460 for that truck one day as my motor has 200k miles on the rebuild but she's still going strong

 

[t.kohlmeyer]

Sounds like a fun car. I have a '72 Mach 1 I've had since 1977. It's an H code FMX and had 2.75 gears in it. My first change was to a 3.25 rear, then headers and a 4V 600 Holley. Then the FMX died so I landed a SB C6 and ran that for a while. This was about 1979. Next I picked up an M code '70 351C, went through it and built it super loose, top of the clearance specs, got a '71 CJ cam, and 4V Hooker SC headers, and a 700DP Holley. Put a B&M street/strip shift kit in with a CJ converter. Next, I put in 4.57 gears with a Nodular 3rd member, 31 spline axles and a Detroit Locker. It was in second before you could cross an intersection and at 70 MPH on the interstate was singing along at 4500+ RPM. A bit much, so I switched to a set of 3.89 gears. I love it.

So being a firm believer in the old axiom of "the only substitute for cubic inches is more cubic inches", I've got a 550+ inch BB under construction. '70 SCJ block, Kaase P51d heads, not sure which intake yet, and either an 850DP or a 950DP Holley. Hooker SC headers are hanging on my wall. I'm also considering a 6R80 trans. I don't do much interstate driving, I'm more of a stoplight and local strip blaster. Anyway, if you don't need to make it a daily driver, have fun with her!

 

[mwallace]

Thanks for the replay. Car looks awesome. Just as an update, I decided to get a set of 4.11 gears and try it out. Just got them installed alone with new bearings. I have only driven the car a few times for break in but so far I really like it. Fifth gear is actually usable now. Its running at around 2200 rpm at 65 or so which is just fine with me. Motor just wasn't very happy below that rpm.

 

[brooce]

A 460 in a 71-73 Mustang is a bolt-in. It was designed for it.

My wife drove her 71 Mach 1, 460 eight years, till it got rearended the second time. Then we got her the 73 Mach 1, and the neighbors 76 T bird for it's 460/C6. She now has over two decades on the 73.

As to the motor mounts to put a 460 in a 71-73, you remove the part that bolts to the frame, swap them side -to-side, and elongate a couple holes, and use 460 insulators.

For a smog motor 460 cam, get a 7 slot timing set, and advance twelve degrees.

My 73 Mach 1, 351 C4V/C6 has been on the road since the eighties. I have a 532 in a box for it.

Both our 73 Mach 1s have the stock 2.75 9 inch. Mine has 275-60-15 tires. Mine has been to North Carolina to open the 55th museum.

Could you walk me through making the motor mounts fit?
I've got a 460 going into a 72 convertible and am struggling to find good info and parts to mount it correctly.