Technical Horsepower, what happens if...?

Ok, I don't know if I asked this here before. 468 BBC in a salt flats car, makes 725 HP and has a terminal velocity of 275 MPH. You turn it into a 509 and make 900 HP at the same RPM, but everything else remains the same. Most will tell me the car will not go any faster because gearing, tire hgt, drag coefficient are unchanged. I disagree. For the sake of discussion the RPMs peak at 7500. Now I don't think it will go 300 but 280, 285, there has to be an increase in speed because there's more 'work' being done within the operating parameters. An internal combustion engine operates in cycles, yes? As fun way of putting it, there's a giant behind the car kicking it every second and now it's a bigger giant kicking it harder every second. If it was an electric motor at a set speed then I could see no change, and it would take less amps to do the same work were that to be the case, because the mechanical output is the same and it wouldn't work as hard. And if I'm all wrong then a simple 400 HP SBC pulling the same RPM would achieve the same terminal velocity, wouldn't it? Since we often get into really "egghead"*****ysis here (that's a compliment) I thought it might be worth ruminating over it a little. Let the games begin

 

more hp will run through the rpm range faster and get up to the higher end quicker in the run. so if it was redlined before, then it will redline ealier but go no faster.

But if it wasn't redlined at the lower HP (drag limited), then it will climb higher into the RPM range and therefore go faster, earlier.

-rick

 

It will go faster, however it will need gears/tires size to match the max hp RPM. If both engines max out at the same RPM and the lower HP engine can get to and hold that RPM then adding HP will not increase top speed it should however accelerate quicker.
Dan

 

@69fury you type faster than me. Well said
Dan

 

depends on if you can hook the HP to the salt
what if the original set up wasn't getting everything to the salt?
then your new set-up would go slower with more HP and more slip
you say terminal velocity....do you mean exit speed?

 

I was told there would be no math today!

 

As others have said, if the vehicle is gear bound at 7500 rpm, it will run the same speed. More horsepower will not make it go faster at 7500 rpm. However, less horsepower may not be able to achieve 7500 rpm and therefore the vehicle will go slower. The biggest obstacle to go fast is moving air. Wind resistance goes up by the square of speed. It really takes a lot of HP to move air out of the way.

 

Also realize that an engine only puts out the amount of power that is required to do the job. So if it takes 725 hp to push the vehicle to 275 mph at 7500 rpm. Your 900 hp engine will be putting out 725 hp at 7500 rpm and 275 mph. In which case it is the gearing that is limiting the mph not the hp. Now if you put a higher gear in the vehicle, you may be able to raise the speed with the 900 hp engine.

 

if we were talking about driving on pavement, then it depends on how it's geared now. If it's geared to turn less than 7500 at 275 mph, then it will go faster with the 900 HP engine.

But since we're talking about salt, top speed in A engine cars is usually limited by air drag vs traction. No matter how much power you make, you can't get enough traction to push that wall of air.

 

The highest theoretical top speed is when the gearing puts the top speed at the peak hp rpm. If you had top speed & peak power at 7500rpm earlier it should be a little faster with 900hp instead of 725 because you still have more than 725hp at higher rpm, perhaps you can overrev it to 7800-8000. You would, however, have an even higher top speed with different gearing that puts you right at the peak hp rpm at top speed again -****uming the traction is good enough to let you use those extra horses.

 

Never thought about not getting enough traction on salt, it’s got get a little hairy with wheel spin at 250+ mph.
Dan

 

It's all about aerodynamics at those speeds. Downforce to get enough traction, and minimizing forward drag at the same time.

It takes someone with a lot more courage than I have to drive one of those cars.

 

Before rev limiters at the salt, and also dirt like El Mirage, engines would over rev and explode because of lack of traction. Sixty years ago I met Rollie Free and heard many stories of the lack of traction at the salt lakes. As others have stated gearing, traction, aerodynamics and down force all have to be taken into consideration. Has to be balanced for optional and safe speed.

 

Since we have no idea what is currently limiting the top speed, we have no idea if more HP will provide more speed. That said, adding more HP may be what is required, and it may be the easiest place to start.
Cheating the wind may be a very good starting point but you may have already reached the best configuration on that element as well.
I suspect that it the reason guys make these runs every year is because speculation, experimentation, trial and error, minor adjustments and just plane luck may be all it takes to move the speed limit towards the higher number every year.

 

Not said by Highlander but the 468” is an A engine and a 509” is a AA engine under SCTA engine rules at Bonneville. He may be looking for a class change AND a higher speed. Over the last 50 years of racing on the salt it’s been shown AA is one of the hardest classes to run due the HP and torque trying to maintain traction on the various salt conditions, weather and tires. Custom made rearend gears usually are made or an overdrive used depending on the gearing needed. OD’s are not user friendly unless there’s plenty of HP then they are fine.
His mentioning 900 hp from a 509” indicates a gasoline class which is attainable with today’s parts.

 

I realize this is a theoretical question, but putting that aside, can one build two engines that one engine makes 175 HP more than the other at the same RPM? Also****uming that’s the max RPM/peak output of each?

 

Depends which way the wind is blowing

 

This is simple to figure out. Take 2 identical cars, one with each engine, run them side by side and see what happens.

 

Let's****ume that the limiting factor is wind resistance and that the 725hp peak power at 7500rpm gets us to a terminal velocity of 275mph and can not push any more. Upping the engine to a peak output of 900hp at 7500rpm will allow the engine to push past the 7500rpm mark and attain a greater top speed.
Working out how much extra speed it will gain is above my pay-grade...

 

The power requirement to handle wind resistance (and at those speeds wind resistans is basically the only thing to worry about) is proportional to the speed cubed. If you need X hp to reach 100mph the power to reach 110 mph could be found easily.
100*100*100= 1000000
110*110*110= 1331000
1331000/1000000= 1.331
You need about 33% more power in that simple example.

But that's regarding the power you get to the wheels, at that specific rpm, it requires optimal gearing.

 

As others have said, you offer a terminal velocity of 275 MPH with 725 HP.
Then offer up 900 HP at the same RPM.
Then mention a RPM peak (?) of 7500 RPM.

So using your*****ogy, yes giant is kicking harder (pedaling a bike harder), BUT it is not kicking/pedaling faster (RPM).
So as others have said, it can get up to speed faster (limited by traction), but will not go faster without pedaling faster (RPM) or changing final gearing (OD, diff or taller tires). The engine can make more power, but it's operating at the same RPM, driving the rear wheels to the same speed.
If the 725 was able to reach peak RPM and hold it for the mile, the 900 would not require WOT to reach and hold the same RPM.

 

Assuming speed maxed out based on hp/rpm stated, one would then****ume bigger motor makes equivalent hp at lower rpm, so then with the delta in hp/rpm, vehicle would go faster.

 

So........ 900hp at wide open throttle would reach a higher rpm.
Higher rpm with the same gearing would yeild higher mph.

 

If your talking about running on the salt you would need to run both engines in the car at the same event do to the salt condition.

 

That is true, but the OP was very clear in saying that "everything else remains the same".
I am taking that to mean what it says. Literally everything other than power remains constant, including salt condition.

 

If traction and drag remain the same, and that's what's limiting top speed, then adding more power won't make it go faster.

spend some time looking at the records for some of the classes, see how the big motor speeds compare to smaller motors.

 

Does “salt condition” refer to the coefficient of friction at the wheel?

 

Mostly.

 

Yes, but that is the issue. Peak RPM could mean peak HP, peak torque (doubtful, due to normal torque curves) or upper limit of RPM.
I chose limit. simple and easy to answer.
If you choose peak HP, then this then gets into a horsepower curve with no RPM limit defined (but peaks at 7500), pitted against a curve of drag which is undefined since the vehicle and it's gearing is unknown. This question is not answerable.
What is the upper RPM limit of a NA BBC?
How much does the HP curve fall off above 7500?
Was the car fully refined and geared to handle 750 HP?
Will it handle the additional force and cooling requirements?

All of these variables make the question not defined enough to give an answer.
I'm a fan of the racing, but not a member, not deep into the nuances of it.
I can share with you an example of a couple cars.
I posted the Aardema/Braun car videos in the OT section.
https://www.jalopyjournal.com/forum/threads/new-aardema-braun-v12-for-lsr-is-alive.1334552/
@A****** also posted the latest from SuperFastMatt.
https://www.jalopyjournal.com/forum...before-bonneville-2025.1338229/#post-15642310

Watch them and let me know which is putting out more HP and the speeds they ran. Hint; it's not linear.

 

Which shows that Enzo never tried to run on the salt.
Love this, Jocko. This is one of the main reasons a lot of us signed up.
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