Question about gearing for Top Speed

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harrisonm

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I have a question about gearing your car for top speed. I mean like Bonneville Salt Flats top speed. Let’s say you have a 69 340 barracuda and at 6000 RPM you are at 120 miles an hour with 3.55 gears. And let’s say that same 6000 RPM would give you 140 miles an hour with 3.23 years, and 160 miles an hour with 2.76 gears (all theoretical-i know there are charts available online that give speed for given RPR/tire size and ratios). I know there is a point at which continuing to increase the overdrive gear ratio will actually start to slow you down. I assume the ratio will get to the point where the engine’s torque and HP will start lo lag. At some point, to go faster, you are going to need more horsepower not just more overdrive gear ratio. I just wonder if anybody knows where that is. I am guessing it would probably be somewhere around a 1:1 ratio rear axle
 
Let's see:
2.94s at 70 mph was running 2500 rpm, that equals 5000 rpm at 140 mph

Then 7000 rpm would be 170 mph.

Wind resistance and how much power under the hood are going to be the limiting factors.

Probably going to need at least 350 horsepower to get to 140 mph. From there on is your guess.

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This guy might know, 500 plus cubic inch.

☆☆☆
 
It's unclear to me what you are actually asking. Are you asking; "how fast can I go with a 340 that makes peak power at 6000rpm?" If so then you would need to know how much real power it makes up there. And you would need to know a few things about the chassis it is going into.
Power required increases with the square of the speed. So if your car takes 30hp to go 50 mph, then the formula predicts 640hp to go 200mph. To hit 200 at 6000 rpm with 28s, would take a final drive ratio of 2.50

here is the formula;
mph = (rpm x TC)/1056 x FD
where TC is tire circumference and FD is your Final Drive
Example;
200= (6000 x 88)/1056 x 2.5= 200
Below is a power curve for a hotrod 360 sorta like the one I built; with a 230/237/110 Hughes cam. You can see that at 6000 it is just beginning to slide down the backside of the curve. The point is that the 340, to peak at 6000, is gonna need more cam than a 230/237/110, lol.


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My experience with bonneville, when you think you have enough horsepower to achieve a desired mph at your selected gear ratio...........double it.
:lol:

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In my opinion, gear ratio is not a deciding factor, near as much as horsepower and drag, unless of course, the gears are too low.
Theoretically, a slant six with 5000 rpm capability, with 2.76 gears is capable of 150+ mph. Do you really think it will?
Theoretically, at advertised peak horsepower, (390 at 4900 rpm) at 4.10 geared A12 car wont break a hundred.
We all know thats not true.
My friend had a 305 camaro, with 2.29 gear in the back. That gear certainly limited top speed of that car, cause there was NO WAY that engine could pull that gear. Top speed would have been better with 3.55s.
Im guessing there is a rough guideline that land speed record racers are familiar with, but i dont know it. Even with the right gear, power and drag will determine how fast a car is.
 
It's unclear to me what you are actually asking. Are you asking; "how fast can I go with a 340 that makes peak power at 6000rpm?" If so then you would need to know how much real power it makes up there. And you would need to know a few things about the chassis it is going into.
Power required increases with the square of the speed. So if your car takes 30hp to go 50 mph, then the formula predicts 640hp to go 200mph. To hit 200 at 6000 rpm with 28s, would take a final drive ratio of 2.50

here is the formula;
mph = (rpm x TC)/1056 x FD
where TC is tire circumference and FD is your Final Drive
Example;
200= (6000 x 88)/1056 x 2.5= 200
Below is a power curve for a hotrod 360 sorta like the one I built; with a 230/237/110 Hughes cam. You can see that at 6000 it is just beginning to slide down the backside of the curve. The point is that the 340, to peak at 6000, is gonna need more cam than a 230/237/110, lol.


View attachment 1715727142
I understand the math. Maybe I wasn't clear. I totally understand that higher gear ratios (ie 2.76) will produce a higher speed at a given RPM compared to a lower ratio (ie 3.55). I was just assuming there was a certain point at which the engine would bog down as it tried to overcome an extremely high gear ratio. Kind of like trying to start out in 6th gear. Or maybe it becomes a matter of overcoming friction from wind drag. In which case, you need more HP. Just wondering.
 
Or maybe it becomes a matter of overcoming friction from wind drag. In which case, you need more HP
Ok , I wasn't sure, but
I have seen that with a 350 motorcycle, published in an FSM, back in the 60s or 70s. So you're on the right track. I just can't help with the numbers.
However; I got this;
A friend and I headed out to BC from Manitoba in July 1975; he on a 750 and me on a 350. The only way I could keep up to him at 75/80 mph was If I lay down on the saddle and hung my ankles over the rear signal lights. A few hours after we left Regina Saskatchewan I vowed to get rid of that lil 350 and a few more hours we came to Tabor Alberta and the first thing I saw was a big ol' Suzuki dealer.
A couple of hours later, I was sitting tall on a brand new 1975 WaterBuffalo! We were flying! and I was sitting tall.....
Now If I remember right, that 350 was rated 29hp, and the 750 was 70hp.
So yeah yur on to something lol.Wish I could help. There was an article in some muscle-rag back in the 90s, about a certain E-body Barracuda that was built to cruise at high speed. I cannot recall the numbers but I think it was brown, and had custom fender flares; does that count for anything,lol. That article was somewhat technical, from what I do remember.
 
I understand the math. Maybe I wasn't clear. I totally understand that higher gear ratios (ie 2.76) will produce a higher speed at a given RPM compared to a lower ratio (ie 3.55). I was just assuming there was a certain point at which the engine would bog down as it tried to overcome an extremely high gear ratio. Kind of like trying to start out in 6th gear. Or maybe it becomes a matter of overcoming friction from wind drag. In which case, you need more HP. Just wondering.

Gearing for top speed generally the car will dead out of the hole lot of speed record cars need a push car to get them going but as you get up in mph you get up into the engines powerband. Say you go a 500 hp engine at 6500 rpm and geared so peak power is at the top speed you want and that's enough power to do so it shouldn't be bogging down your at peak power and have 500 hp going to the ground minus drivetrain loss.

It depends on the top speed your looking for, a drag is geared to be around peak rpm at that top speed it can do in a quarter mile so like a 12 second car at 110 mph its not really gonna bog anywhere.

Do you have a goal or is this a general question ?
 
I have a question about gearing your car for top speed. I mean like Bonneville Salt Flats top speed. Let’s say you have a 69 340 barracuda and at 6000 RPM you are at 120 miles an hour with 3.55 gears. And let’s say that same 6000 RPM would give you 140 miles an hour with 3.23 years, and 160 miles an hour with 2.76 gears (all theoretical-i know there are charts available online that give speed for given RPR/tire size and ratios). I know there is a point at which continuing to increase the overdrive gear ratio will actually start to slow you down. I assume the ratio will get to the point where the engine’s torque and HP will start lo lag. At some point, to go faster, you are going to need more horsepower not just more overdrive gear ratio. I just wonder if anybody knows where that is. I am guessing it would probably be somewhere around a 1:1 ratio rear axle
As someone whom has foolishly driven his Mopar in excess of 160, -get a longer, wider, heavier car. I may get beat up on the A body forum, but for YOUR safety I'll risk my reputation. Besides HIGH-END Brakes, a driveshaft safety loop, internally balanced engine, rebuilt differential, new wheel bearings-overpacked with grease,, Z rated tires, new,, an aerodynamic vehicle, with low-body weight and a roll cage...

...at any rate high speed steering corrections with a short wheel base are too often followed by lethal loss of control. I feel a B-body would be better for top speed, but you need to spend more on safety than a plan of high horsepower engine, gears, and brakes.

In my humble opinion, though the physics promotes narrow wheels for speed, I recommend wide wheels for control as well.
 
And no, the engine won't bog down with high gears
I dont know about that
I grew up on dirtbikes, and I remember several of them where the gearing outran the engine and when you shifted from 4th into 5th, you'd actually start to slow down

Now I understand we're talking something more powerful then a 2 smoker, but I would imagine the principle carries over
 
I dont know about that
I grew up on dirtbikes, and I remember several of them where the gearing outran the engine and when you shifted from 4th into 5th, you'd actually start to slow down

Now I understand we're talking something more powerful then a 2 smoker, but I would imagine the principle carries over
I guarantee it. I have driven 60-90hp/200 ft-lbs torque engines 130.
CelloMom on Cars: How much horsepower do you need? (cellomomcars.com)
How Fast Can a Car Go? (itstillruns.com)
HP required to go 100mph? (pro-touring.com)
 
I have a question about gearing your car for top speed. I mean like Bonneville Salt Flats top speed. Let’s say you have a 69 340 barracuda and at 6000 RPM you are at 120 miles an hour with 3.55 gears. And let’s say that same 6000 RPM would give you 140 miles an hour with 3.23 years, and 160 miles an hour with 2.76 gears (all theoretical-i know there are charts available online that give speed for given RPR/tire size and ratios). I know there is a point at which continuing to increase the overdrive gear ratio will actually start to slow you down. I assume the ratio will get to the point where the engine’s torque and HP will start lo lag. At some point, to go faster, you are going to need more horsepower not just more overdrive gear ratio. I just wonder if anybody knows where that is. I am guessing it would probably be somewhere around a 1:1 ratio rear axle
I have a question about gearing your car for top speed. I mean like Bonneville Salt Flats top speed. Let’s say you have a 69 340 barracuda and at 6000 RPM you are at 120 miles an hour with 3.55 gears. And let’s say that same 6000 RPM would give you 140 miles an hour with 3.23 years, and 160 miles an hour with 2.76 gears (all theoretical-i know there are charts available online that give speed for given RPR/tire size and ratios). I know there is a point at which continuing to increase the overdrive gear ratio will actually start to slow you down. I assume the ratio will get to the point where the engine’s torque and HP will start lo lag. At some point, to go faster, you are going to need more horsepower not just more overdrive gear ratio. I just wonder if anybody knows where that is. I am guessing it would probably be somewhere around a 1:1 ratio rear axle
it is strictly a matter of horsepower needed to overcome aero resistance.
 
I guarantee it. I have driven 60-90hp/200 ft-lbs torque engines 130.


i had to look it up, i took a 59 HP, 76 Ft/lbs kadett up to 120...but that was pushing it

there have got to be other factors then gearing and HP that come into play though, like the rolling resistance, or air resistance

Like the powertrain combo that GM put into the early 2000s pick-ups that hunted up and down at ~60 mph into and out of lock-up.

exactly
but it wouldnt surprise me if that was an MPG driven ECU micromanaging things
 
it is strictly a matter of horsepower needed to overcome aero resistance.
By horsepower you mean torque and gearing. Acceleration does not have to do with reaching top end power for the sake being at top end power.
 
i had to look it up, i took a 59 HP, 76 Ft/lbs kadett up to 120...but that was pushing it

there have got to be other factors then gearing and HP that come into play though, like the rolling resistance, or air resistance


exactly
but it wouldnt surprise me if that was an MPG driven ECU micromanaging things
there are other factors, wind, traction, friction input vary at varying speeds, (pressure from) altitude, -but basically like you said, "59hp 76ft lbs"
gearing is like Archimedes? lifting a ship by himself with pulleys. It makes up for the torque to some extent. As a point of interest, the early steam engines didn't need much in transmission to accelerate-excessive torque.
 
I agree with TT5.9. The only math that matters at Bonneville is the time slip. All I know is that it took 790 HP to go 199.997 mph, and then $30,000 more to get 840 HP to reach 200.991 mph! Aero is about all that counts when you get up to the big end.
 
By horsepower you mean torque and gearing. Acceleration does not have to do with reaching top end power for the sake being at top end power.

By horsepower he must mean horsepower, torque is “basically”output of one revolution and hp is all the revolution added up over time “rpm” can’t have an either or situation an engine make power only which we use Hp to represent and hp is the sum of torque and rpm. You won’t find a formula telling how much torque you need to do x amount of speed but you will with hp.
 
By horsepower he must mean horsepower, torque is “basically”output of one revolution and hp is all the revolution added up over time “rpm” can’t have an either or situation an engine make power only which we use Hp to represent and hp is the sum of torque and rpm. You won’t find a formula telling how much torque you need to do x amount of speed but you will with hp.
I agree with your definition of Horsepower. But essentially a single piston briggs and straton engine making 75 hp and worn chevy 305 V8 making 75hp NEGATE your opinion;torque is what is going to move the vehicle (outside of gearing) and rpm will dictate the final speed of the OP's objective.
Torque AND at max rpm for th eOP's objective is what is needed. Horsepower is also a measure of acceleration and driving constantly at top speed isn't acceleration.
 
I will say that going over 130 and an a body is terrifying! Emphasis on aerodynamics. I was doing about 130 in my dart and went from the center of the road to one lane in a swooping motion by no means quick, air got underneath the front and well my life flash before my eyes I could feel my car leaving the runway I managed to getback into my lane and thank God I'm still on this planet.
 
I will say that going over 130 and an a body is terrifying! Emphasis on aerodynamics. I was doing about 130 in my dart and went from the center of the road to one lane in a swooping motion by no means quick, air got underneath the front and well my life flash before my eyes I could feel my car leaving the runway I managed to getback into my lane and thank God I'm still on this planet.
I looked down to change the radio at 160 and had shifted lanes in a blink.
new bushings and a tight front end was quintessential to safety, my friend with a 70's charger said his would lift up at 140mph-aerodynamics and that giant air scoop being all the physics stopping him at 140. Not the same with my plymouth though i was very excited
 
Ok I do think a cross wind was a major culprit because I have had my Swinger over 100 many times and it as vary true at that speed.I also think l had my torsion bars turned up at that time.
 
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