I can't drive 55

[MatthewMcD]

I moved this to a separate thread to apply some focus.
My original question is, why is my top speed in 4th gear ~45-48 mph?

Background:
5/75 chassis with an estimated 68-70 F (PO said .5F but others have doubted that).
4 Speed transmission
32" tires

Vacuum advance on the distributor appears to work fine. At idle if I pull additional vacuum, it advances the timing noticeably. I pulled the plugs and they look amazing.

It has a mini-truck conversion to disk brakes in front. When raised the wheels turn easily. I recently rebuilt the knuckles and installed new calipers. (It drove this way before all that.)

Jacked up the rear driver's tire and rotated it. The drive shaft turned 2 1/8 revolutions for each turn of the wheel.

There are no vacuum lines associated with the Transfer case and the front drive shaft turns freely in 2H.

I ordered a tach so I can get some real RPM numbers to help the investigation.

The vacuum advance on my distributor.

 

[fjc-man]

If you’re running the original 4.11 thirds, that’s mostly why.

 

[MatthewMcD]

If you’re running the original 4.11 thirds, that’s mostly why.

Can you translate "4.11 thirds"?

 

[Skreddy]

A couple things that jump out: (great job spelling out everything you’ve checked btw!).

Vacuum leaks and timing.
In the other thread you said you had a Rochester carb on it. Is there a special spacer or anything that could have a leak? Does the Rochester have a secondary that’s not opening? Do you know your vacuum numbers? Do you know timing?

Does it start and idle fine or take some time to get fired up?

In my 40 or 60, I’m barely in 4th gear at 45. I usually shift either of them between 3000-3500 rpm depending on the upcoming road.

 

[Skreddy]

Can you translate "4.11 thirds"?

Your third members or front and rear end axle ratios are 4.11 to 1. So for every 4.11 turns of the pinion yoke (driveshaft), the wheels will spin 1 time.
If you’re getting 2.xx spins of the driveshaft per 1 turn of tire, if I remember right it’s because of the spider gears in the differential and turning only 1 tire.

 

[MatthewMcD]

A couple things that jump out: (great job spelling out everything you’ve checked btw!).

Vacuum leaks and timing.
In the other thread you said you had a Rochester carb on it. Is there a special spacer or anything that could have a leak? Does the Rochester have a secondary that’s not opening? Do you know your vacuum numbers? Do you know timing?

Does it start and idle fine or take some time to get fired up?

In my 40 or 60, I’m barely in 4th gear at 45. I usually shift either of them between 3000-3500 rpm depending on the upcoming road.

There is a spacer but the fit is very tight. I'll try spraying some fluid around the base to see if there is a leak.

As far as I can tell the carb is performing correctly. It starts up easily and runs smoothly. (I rebuilt it a while ago.)

I can recheck the vacuum numbers but last time I checked it (I believe) it was in the 18-20s at idle. There is a noticeable difference in the brake pedal when you start the engine. It really grabs the booster.

Once I get the tach I'll have a better idea of the RPMs and post that here.

 

[MatthewMcD]

If you’re running the original 4.11 thirds, that’s mostly why.

Looking at what @pb4ugo wrote: "If you enter these numbers into a calculator you will find your RPM's are going to be closer to 2600 using 4.11 diff gears, 32" tires, and going 60 mph" it appears that folks with 4.11 can get past 55 mph.

 

[Skreddy]

If your top is around 45, then you’re somewhere around 2000 rpm in 4th at that speed.
If you were shifting at 3000 rpm from 3rd to 4th, you’d be going almost 50 mph. Are you shifting at too low of an rpm?

 

[MatthewMcD]

If your top is around 45, then you’re somewhere around 2000 rpm in 4th at that speed.
If you were shifting at 3000 rpm from 3rd to 4th, you’d be going almost 50 mph. Are you shifting at too low of an rpm?
View attachment 3741211View attachment 3741212

These are the same numbers I came up with. Thanks for the confirmation.

My ear says I am shifting at the right time. Once I have the tach in (probably Saturday) I'll find out if my ear is calibrated correctly.

 

[clrussell]

Straight fan still? (Only ask for noise reference)

I bet you are turning less rpm than you think.

Or there is something thing holding your back.

I have 4.11 (confirmed when I changed the diff), 4 speed, original f engine and have zero issues cruising at 60mph with 33” tires. 1” taller tire won’t make 15mph difference.

For reference my timing is set at 14* base with hei and a Weber carburetor.

 

[Skreddy]

Straight fan still? (Only ask for noise reference)

I bet you are turning less rpm than you think.

Or there is something thing holding your back.

I have 4.11 (confirmed when I changed the diff), 4 speed, original f engine and have zero issues cruising at 60mph with 33” tires. 1” taller tire won’t make 15mph difference.

For reference my timing is set at 14* base with hei and a Weber carburetor.

That’s a great point. The direct drive fan on these is LOUD! Even when the clutch fan locks up on my 60 it is noticeably louder and sounds like it’s a lot higher rpm than it is.

 

[pb4ugo]

Looking at what @pb4ugo wrote: "If you enter these numbers into a calculator you will find your RPM's are going to be closer to 2600 using 4.11 diff gears, 32" tires, and going 60 mph" it appears that folks with 4.11 can get past 55 mph.

The numbers I mentioned would be the same regardless of the engine.

Have you checked your accelerator cable?
1st make sure the pedal goes to the floor. Then make sure when the pedal is on the floor that the carb is wide open. Of course check this with the engine off. Check your timing and also make sure the mechanical advance is working. Remove the cap, lightly turn the rotor one or the other. It should turn a bit with slight resistance and snap back once released. I'm curious, how are you getting your speed #'s? stk speedo or GPS?

 

[MatthewMcD]

The numbers I mentioned would be the same regardless of the engine.

Have you checked your accelerator cable?
1st make sure the pedal goes to the floor. Then make sure when the pedal is on the floor that the carb is wide open. Of coarse check this with the engine off. Check your timing and also make sure the mechanical advance is working. Remove the cap, lightly turn the rotor one or the other. It should turn a bit with slight resistance and snap back once released. I'm curious, how are you getting your speed #'s? stk speedo or GPS?

Speed numbers are from the stock speedo, speed indicators in our neighborhood confirm the accuracy... I'll check it with a GPS after I install the tach.

I'll check the accelerator cable as you indicate as well.

 

[pb4ugo]

The stk speedo has to be slightly off on the slow side, if your running 32" tires. Maybe 4 or 5 mph.

 

[MatthewMcD]

The stk speedo has to be slightly off on the slow side, if your running 32" tires. Maybe 4 or 5 mph.

The douche bag in the Lexus that honked at me seems to agree that I was going too slow...

 

[pb4ugo]

You should be going faster than the speedo shows.

 

[charliemeyer007]

Golden Earring - Radar Love (1973) - https://www.youtube.com/watch?v=Sm1q1XoN_Qo&t=10s

 

[MatthewMcD]

Straight fan still? (Only ask for noise reference)

I bet you are turning less rpm than you think.

Or there is something thing holding your back.

I have 4.11 (confirmed when I changed the diff), 4 speed, original f engine and have zero issues cruising at 60mph with 33” tires. 1” taller tire won’t make 15mph difference.

For reference my timing is set at 14* base with hei and a Weber carburetor.

Yes, straight fan still.

I'll recheck the timing and get back to you.

 

[clrussell]

Yes, straight fan still.

I'll recheck the timing and get back to you.

I bet your ear is playing tricks on your due to the fan. Tach will be a good telltale. I look forward to your results.

 

[Jdc1]

Is the High/Low gate there for 2wd?