Hi members,

I've read through the TKX 5- speed threads on the forum, and overall, the reviews are very positive and appear to be the best mod you can do to the C3.

My question is this. Tremec recommends that you make the rear cross member removable (like they are on the automatic-equipped cars). My '69 factory 4 speed car is basically a shell at this point in the paint shop getting ready for paint. I have no desire or want to cut into my brand new replacement chassis that I installed last year, and I will NOT do that. I assume that the recommendation to create a removable cross member is for easier removal for servicing/clutch replacement with the motor IN the car.

My theory is as follows - if the motor is out of the car right now, there should be no need to touch the cross member to install the TKX. The way I drive the car, I don't anticipate having to replace the clutch for many many years, so aside from some failure of the transmission itself forcing removal, I assume that if I do not make the cross member a removable piece, that I will have to remove the motor and TKX as a unit if it needs to come out for some reason.

Original motor going into a crate to be stored away and wiil be replaced with a 507HP/515TQ 396 BPE motor, so if I decide, I'll likely get the close ratio with the .68 5th gear (for HWY cruising) as this motor generates 95 percent of that torque at around 3000 RPM. Even at 1900-2000 RPM, it should have no issues accelerating, or at least maintaining speeds on inclines at that RPM.

 

Sounds like you answered your own question. But if it was my car, I' definitely would make the cross member removeable. I talked to a lawyer friend of mine named Murphy and he agrees.

 

HAHA!. I dunno. I really don't want to cut into that beautiful new powder coated frame. The car will be stored on a lift so the underside will be quite visible.

 

I'd be interested in hearing from members who have done this conversion and what clutch assembly you decided on.

Also would love to hear from members who did NOT make their cross member removable. I've read the other threads, but did not see much about the cross member...

 

If the only thing holding you back is cutting the powder coating, I’d get in there and do it. You can always have the removable part re-coated if it gets messed up and there are good epoxy coatings that you can use to paint where the frame has been cut to make it look pretty.

I’m going to agree with lowbuck72 - if you don’t cut it you are pretty much guaranteeing yourself that you will need to pull it shortly after installation.

My money is on a noisy or leaking throw out bearing.

 

Well the cost kind of is as well. LOL. I've put so much moolah into this restoration already, another 5K into the project is a drop in the bucket, but still it adds up. I've heard nothing but good things about the conversion, but $5K+ just in parts to do this upgrade is a bit crazy. I can't imagine what paying a mechanic to install it would cost, but probably at least $7K - those are reaching Porsche prices!

 

What gear ratio do you have?

For reference, my other car has 500hp/522 ft lbs with a TKO-600 and a 3.50 rear. It's a pleasure to drive, third gear is a hoot and fifth makes for easy high speed cruising > 70 mph. If i were to do it over, i'd go with a 3.70.

If your in the 3.08 - 3.23 range, i'd stick with an M-20

 

I'm running a 3:55, but I also have a 3:36 on hand.

 

I can't tell you what I did, since it's still in boxes, but here's what I bought for the 79 (3.55 gears). It's going my wife's car, and she wanted a light clutch. I bought the hydraulic setup from SST, along with the rest of the kit.

https://www.shiftsst.com/hydraulics-complete-kits/p40

Confirm the part number with them, as that shows it's for the Tremec Magnum.

 

I'd leave the 3.55 in with an M-20 if you don't want to cut it up. But if you do, definitely go with the hydraulic clutch mentioned below - makes the clutch feel like a modern car.

 

Thanks for the input, fellas. Not to belabor the point, but is the only reason Tremec recommends to make the cross member removable is for ease of service if/when the transmission or clutch needs servicing without having to remove the engine and transmission as a unit?

If that's the only reason, it's really not a terribly difficult process to remove them as a unit as my '69 is bare minimum - no PS, PW, PB, or A/C. It's a lean mean machine, which is how I like it, so it's a weekend or two job, absent of other distractions. I know the TKX unit is slightly longer which makes it trickier to remove without that removable cross member. I also have a lift, so working underneath the car is pretty easy.

I've also read that folks have been able to replace the clutch without having a removable cross member, albeit a very tight fit.

 

Fortunately, both of my cars came with removable crossmembers (all 80-82 cars had them).

I plan to remove my crossmember, remove my (old) engine and (old) auto transmission as a unit, then install my (new) engine and (new) TKX as a unit, then re-install the crossmember. Perhaps you can still do that with the crossmember in place, at least for the smaller manual transmission? GM never had to do it that way, though. The body was dropped onto the completed chassis, drivetrain already installed.

With only jackstands, I'd probably pull the engine and trans together for a clutch job, too.

There are lots of TKX videos on YouTube, but the one's I've seen are by folks who have a lift.

 

Just so you have a data point. I didn't cut my crossmember when I installed mine. I did mine in a dirt driveway and while it certainly wasn't fun, it's definitely do able. On a lift with a transmission jack, no problem. So whether you decide to cut it or not, the transmission will still be serviceable.

 

I have a four-speed and don’t see the great benefit(s) of the five speed conversion. For me, I don’t care about gas mileage, have a recently rebuilt engine, and don’t spend hours doing 70+ mph on highways. Sure, if I could wave a magic wand and have a no-cost Tremec instead I’d take it, but the cost/benefit analysis doesn’t work for me. To each his own…

 

I didn't install a TKX (5 speed) used a T56 Magnum 6 speed.

T56 Magnum Install 78 Corvette. - CorvetteForum - Chevrolet Corvette Forum Discussion

Since my car already had a removable cross member, I still needed a different cross member for the T56 6speed. Started using a single disc clutch (way to stiff) then went to a Twin disc setup ( almost feels like a C6 manual )

But since then I have replaced the motor (LS3) / clutch. Not possible for me to install engine / trans as one piece. Since the LS3 swap is replaced the clutch 1 more time. If I had to pull both I would never DIY the process.

With a quick jack 5000TL there is enough clearance to leave the transmission on a jack and slide under the car and complete the clutch swap.

As for diff gears, I still have the factory 3.08 Rear diff. With the ZZ383 or the LS3 / 480 was no problem street or track days. I do have a 3:36 diff which best matches the C6 corvette (3.42) but I do have 2 gears for overdrive.

With the 3.08 / LS3, highway gas mileage was 33.1 MPG

1978 LS3 Swap Completed. - CorvetteForum - Chevrolet Corvette Forum Discussion

If you are planing on doing this once, install a twin disc to save your legs.

 

Another option is running a 3.08 posi rear end from the 68-69 BB. I've read in other forum posts that folks were recommending that as an alternative to a 5 speed upgrade, but not sure how that would work in practice in my setup:

Big BPE stroker (507HP/515TQ) - flat torque curve all the way to 6000 RPM with approx 95% of that torque available at 3000 RPM
3.55 posi rear end
M20
255/60-15 tires (27" diameter)

How streetable would this setup be, and is it worth considering? Obviously, such a setup puts it well below the ideal 10 starting line ratio (it's darn near perfect with the M20 and 3.55), but wondering with the amount of torque the engine is putting out, would it be that much of a noticeable loss in acceleration - enough to negate the more ideal highway cruising RPM range?

I'm not planning to drag the car, but don't want to create a dog off the light either. It would be nice to bring that 60-70 MPH cruising RPM down a few hundred RPM. 3.08 should put me at 2700 RPM@70, as opposed to 3100 RPM for the 3.55.

It's really not the end of the world if I stick with the current setup, Most of my cruising has been on the twisty country backroads with speed limits in the 40-55 MPH range, where you can basically keep it in 4th with the ocassional downshift to 3rd on hills.

And if it is a worthwhile consideration, what components (besides the rear end) are needed to make the 3.55 posi to 3.08 posi swap?

 

Anyone done the conversion I mentioned in my last post?

 

well i'll say ive not heard the ideal ratio was 10, i thought it was 9. i think thats high for a light weight vette. in fact a lot of the factory combos are between 8-9.

m20 1st gear- 2.52 x 3.55 rear= 8.9

i have 2 vettes both running an M21 with a 3.70- 2.20 x 3.70= 8.14
one is a L79 the other is a 427. neither one feels sluggish, the L79 feels perfect, the big block feels like it could have less gear in it and be fine.

if you dropped to 3.08/M20 you would be at 7.76. not much lower than where i am.

i do have a truck with a tko-600 in it, with 3.70. i cant remember which first gear it has, but its 3.73 rear which puts its way over a 10 ratio. it feels like a granny gear. im not a fan.

 

Here are some helpful ratios. The 8.84:1 first gear in my 80 (top bolded line) seems plenty quick. I try to have my fun between 2nd and 3rd gear, though. The highlighted line is what is going in my 79.

 

1-4 gear ratios in the TKX are very similar to your BW, but with overdrive 5th. The best of both worlds. Though, 1000+ RPM increase from 5th to 4th is one heck of a difference when downnshifting, and only amplified if downshifting from a higher speed than 70, but it is overdrive, so I guess that's the price to pay. It's really just a hwy cruising gear.

In a highly modified 911SC I had a few years back, I'd modified the 5th gear in the 915 transaxle from .83 to .59. Going from 1.1 to .59 was about a 1400 RPM drop, or increase depending on whether downshifting or upshifting. It was a bit extreme.