What top speed translates to maximum revs?

David in San Antonio

David in San Antonio

Well-known member
I’m running a rebuilt original block (not Burtz) with insert bearings, counterbalanced crank, Burtz head and cam, updated lifters.
Question: What is the maximum RPM (red line if it had a tachometer), and what speed in 3rd gear does this translate to?
I don’t want to over-rev it.
Thanks everyone.
 
It’s not really a problem in third gear. The torque curve drops off sharply above 2500, so you can mash WOT and it won’t go any faster than about 60. After that it can’t overcome drag unless you’re going down a hill. Also your self-preservation instinct will kick in.
 
2,500 rpm is a reasonable red line, as Alexiskai said.

With a rear end ratio of 3.74:1 and 30 inch OD tires, 2,500 will be 59.7 mph. Any differences will ratio. For instance with a rear end of 4.11:1 the speed would be 3.74/4.11 X 59.7 = 54.3 mph. Or in another example, if you have an overdrive with an overall ratio of 3.00, then the car speed would be 3.74/3.00 X 59.7 = 74.4 mph.

It is quite easy to set up a spread sheet to calculate the mph. The only thing you need to know is that a mile is 5,280 feet and some simple math.
 
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More importantly, are you running an overdrive or different rear end ratio? That will have an impact on the speed part of the equation.
 
alexiskai said:
It’s not really a problem in third gear. The torque curve drops off sharply above 2500, so you can mash WOT and it won’t go any faster than about 60. After that it can’t overcome drag unless you’re going down a hill. Also your self-preservation instinct will kick in.
Click to expand...

Perfect....
 
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I figured whatever the stock Zenith would allow— 2 days after I put it together aft I poured my own mains it rocked on compression at shutdown .
The next day I found it would do 67in a trip to Wisconsin on the way back it was at 65 the whole length of the PA turnpike and I am still trying to break it
I know the balance is off some ,the numbers on the speedometer end at 75 — that is my goal— stock compression,stock rear ratio but now I am working on the shocks and front end.
I bought the Stal 550 19 radials and had to change the speedometer gear to the one for 3:54 ratio to get the speedometer correct again.
I have driven the car over 60 a lot.
It is comfortable to drive 60-65 and on the long trip I averaged 23 mpg at that speed.
 
With a 3.74 rear end, 30 inch diameter tires, at 5,000 rpm the car would be going close to 120 mph. At 10,000 rpm the car would be going close to 240 mph. :D:D
 
As several have said it depends on the gear ratio, if you have an overdrive and what the tire size is. What I was told back in the 60s by several different old timers is "Kid, you can't hurt it if you follow one simple rule,
when your ears start to itch its time to shift gears or slow down."
 
I raised the question of maximum revs because above about 50 mph the engine becomes much louder and cacophonous. It doesn’t sound happy. But the consensus here seems to be it can sustain whatever high gear top speed it can muster. It has the original 19” wheels with Lucas tires, the original transmission and rear end, and no overdrive. It’s touched 70 mph on a wide, very smooth road. On typical Texas secondary roads I lose confidence much sooner. I never drive on interstates.
 
“Your self preservation instinct will kick in”, still laughing at that one. It’s a coool line to remember and use sometime. Made my day.
 
Given that it is a modified engine with unspecified inserts and unknown oiling the limits for a original stock engine may not apply—
 
With 2 Morton-Thiokol solid fuel boosters you should reach escape velocity in 3 minutes.Take out the motor and drive line to lighten it . Self preservation does not apply with this application.
 
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