Isabel’s getting another gear! Ordered a new TKO-600 over the weekend and now trying to decide on a new set of gears for the rear. Here are the current numbers and some calculations using http://www.tremec.com/calculadora.php My rear tires are 25.95" diameter and I bought the TKO-600 with .64 OD (.82 is for road race).
Currently, with wide ratio toploader and 3.25 gears:
50 MPH in 4th: 2100 RPM
80 MPH in 4th: 3375 RPM (which is the reason for the new trans)
With 3.25 gears and the TKO:
50 MPH in 4th: 2100 RPM
80 MPH in 5th: 2155 RPM
I will likely try the 3.25’s with the TKO just to see how it is. On the surface, this combo might look good, but there is no way OD would be remotely useable around town in that 50 MPH in OD yields about 1350 RPM.
With 3.73 gears (what defaults using the calculator for a TKO-600):
50 MPH in 4th: 2425 RPM
50 MPH in 5th: 1550 RPM
80 MPH in 5th: 2475 RPM
With 3.89 gears:
50 MPH in 4th: 2525 RPM
50 MPH in 5th: 1615 RPM
80 MPH in 5th: 2575 RPM
With 4.11 gears:
50 MPH in 4th: 2650 RPM
50 MPH in 5th: 1700 RPM
80 MPH in 5th: 2725 RPM
I have of course made my possible gear choices with an eye toward a better off the line launch so obviously the RPM at 50 in 4th is pushed upward. The problem I am having is playing that off against the cruise RPM at 80 MPH (can you say “Power Tour”?).
The other thing I am trying to get my head around is what would be satisfactory stop and go around town RPM range (when getting into 5th is not likely) vs. best mileage/lower RPM’s when cruising on the highway in 5th. I don’t want a gear that makes 4th around town too hard on the engine and cruise that is not low enough RPM but I don’t want the cruise RPM down around idle either in order to get 4th gear around town to be closer to the 3.25 with the 4 speed. Does that make sense?
Looking for advice or guidance from those who have traded situations like this off and came up with a good balance of highway cruise and street off the line/1/4 mile drag strip performance.
No clue Bill but I can say that I really do not have any perceptable detonation under any conditions since changing to the 750 CFM carb. Why do you ask? The direction I am going is going to raise the RPM in 4th in any case so lugging would be less likely and so detonation less likely as well. Unless you are driving at the cruise RPM in 5th at lower speeds? I think that 5th is going to be unusable below a certain speed, like it is in most cars, just trying to make sure that I do not create a weird combination with my choices! So tell me what you are thinking?
FWIW, I originally had 4.11:1s in my '69 M-Code with TKO. Although it was real snappy in 2nd through 4th, and highway RPMs were acceptable in OD, 1st gear was way to short. I’ve since changed to 3.60:1 rear gears, which seems to be a good compromise.
When I did this with my '69 351-4V Mustang convertible, I used 3.50:1 gears and liked that combination. CatVert had the T5 with 3.25 gears, but that was a 302. Both combinations had good top gear acceleration even from 50-60 MPH and excellent driveability, but neither one ever went drag racing.
Thanks for your post. I was worried about the same thing and this is why I chose the TKO-600 (2.87 first gear) over the TKO-500 (3.27 first gear). The 600 ratios are a lot closer to the wide ratio toploader ratios. This choice was a bit painful for me because the TKO has a 26 spline input shaft so I have to change out a perfectly good 10 spline 11" dual friction (Centerforce) clutch disc to a 26 spline unit in order to make the swap.
Since you did this on a '69 a couple of questions if I may. I plan on using the short shaft kit which will allow the TKO to direct bolt to the original Ford bellhousing. Looks like (with a FMX/C6 1310 yoke) I can use the same driveshaft as my toploader, any comment there?
Also, what can you tell me about beating up on the trans tunnel? Think I am going to need need to clearance the floor support to clear the top corners of the TKO, I know I have read some small tunnel “mods” are needed but have not seen specifics (but have looked for them!).
Thanks Bill. Good top gear accel from 50-60 meaning in 5th? My 3.25 calculations show 1350 RPM in 5th at 50 MPH, that sounds like a difficult hill to get up! My engine is built for low end but thinking any engine is going to lug at that low an RPM. Do you remember what your T5 OD ratio was?
Bob, I don’t remember the OD ratio. Yes, I was referring to 5th gear acceleration. When I sold CatVert, the buyer and I went for a test drive. I was on a freeway at cruising speed in 5th gear and punched it to show him how it performed. It was good enough to immediately feel the acceleration. That seat of the pants observation is really all I can offer.
Back when I bought my TKO, the 3.27 1st was the only choice available. Had the 2.87 been an option, I’d have definitely gone that route. I elected to go with the short shaft, rather than use a spacer. I’d previously converted my toploader to a 26-spline input, so the TKO dropped right in. Although clearance is tight, the trans tunnel didn’t require any mods to fit the TKO. The only sheet metal tweaks I made were a pair of dime-sized notches to the shifter opening, in order to access a couple of bolts. Technically, with enough patience they could have been tightened without the notches, but this made it much simpler/faster, is completely hidden, and doesn’t hurt a thing. I used the stock driveshaft with the larger front yoke. Works fine.
Finally, I highly recommend installation of an aftermarket scattershield in place of the stock bellhousing. Back when I still had the toploader, I once had a pressure plate let go at the strip. The cast-iron bellhousing completely disintegrated. The force was such that some of the shrapnel penetrated the floorpans. I was extremely lucky that neither the brake nor fuel lines were severed, and I escaped injury. Made a believer out of me though.
You had mentioned before that you were getting some pre ignition due to dynamic compression issues. Most of the time that happens at low RPM and is most noticeable in a higher gear. Just trying to evaluate how touchy it might be to being lugged at low RPM’s. Modern engine designs with variable valve timing are able to run low RPM’s in overdrive with no issues. For example the 5.4 3 valve is very happy at less than 1500 RPM hauling a heavy F150 with 3.23 rear gears. Maybe you have enough torque to make the 3.25’s work for you. (4th is the tallest ratio you have now so if you get pre ignition in 4th with 3.25 at a certain RPM, you can calculate backwards to see where the engine needs to be, RPM wise, to move the weight at speed. It is more about the torque curve than anything else.
"Match the trans to the rear gear your running.Shoot for a 10:1 first gear ratio.Too much or to little 1st gear makes for a lousy combination.The OD gear is another option to consider. "
Seems like your advice is well founded Andy. With the TKO-600, 2.87 first gear yields a rear gear of 3.50:1 using the 10:1 rule of thumb. I avoided the TKO-500 for the very reason cited in what the post quoted (not what I quoted), too low (high numerically) first gear.
So smart money says I should definitely try the TKO-600 with my current 3.25’s and make a new rear gearing decision based on RPM numbers in various gears/speeds.
Not sure I understand your monkeying with the trans gearing comment though Andy. There are just a few choices available and think I made the right choice on the box I chose based on it’s ratios.
I mean if you’re going to swap the 1st gear or whatever, do it before you install the box.
What I run into is that the final drive can make either 1st or 5th worthless in older cars. (and all the other sources linked here and Bill’s post, etc. ) So, it’s best to stick with a numerically lower final drive. Install the 1st gear you want, and then see if you like it.
In the mountainous areas around here, 5th is all but worthless except if you’re doing 2wice the limit. And then the fat front tires and poor aero of the 2nd gen cat start to float the front around 135 or so. So, it’s really better to engineer it to keep 1st useful for a nice jump away from Newton’s 1st law, and then have fifth as a “maybe” gear.
The true benefit of going to the 5 speed is stepping away from the delicate 4 speed box. The overdrive is a bonus, but the better engineering of the new box is worth every penny. A hot cleveland can gut an old Borg Warner as easily as an FE or Lima.
But not a toploader, that’s NO “delicate 4 speed box”!!! The WR TL will live on either in my basement or sold to recoup some of the TKO costs. I also recently went through it and put in a couple blocking rings and 1st gear synchro keys. It’s shifting like new up (and down) through all the gears. Been with me a long time though (since I was 17 or 18), might be hard to part with it.
My chosen ratio’s (TKO600 over TKO500) are done to match the WR TL, the .64 OD is for mileage and comfortable 80 MPH cruise. Just need to get that rear gear choice right. Driving it with the current 3.25’s, noting some RPM numbers and the engines “note” at those numbers should help me with the choice. The posts here and on MCnet on the topic should too!
Thanks,
Bob
P.S. The concept of gear changes in a manual trans generally does not work. You (again, generally) choose the box for its ratios, not what you want to put in. The concept of changing gears would involve not just the gear you want to change, but the cluster gear as well that it meshes with. This to my knowledge is not “how it works” with manual transmissions.
That’s what I meant. You can change the clusters, but you better make sure you have the right ones. So, focus on the 1st gear ratio and don’t sweat the 5th.
Also, I don’t hold the same opinion about Toploaders you do. But that’s why they make Fords and Chevies.
If your transmission swap calls for a different clutch spline, recommend you check with the Car Quest store downtown Royce City. They are closely connected to a rebuilding operation and may be able to switch out the spline portion of your disc. In years past they have been a big help to several members of our NETR AACA club in Greenville. Nice looking 69. Black Cherry? My 69 Vert is medium blue metallic. Surprised our paths haven’t already crossed. Ever show at the harbor district? Regards Mike
Think you melded Andy (who is in NC) with me (in NJ). Already bought the 26 spline clutch disc, $138 shipped. Changing out the center did cross my mind but I have no idea who does that around me (in NJ).
Thanks Re: my car. It is painted with 40th Anniversary Mustang Crimson Red which is a burgundy pearl color.
