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FDP Forum / Moe's Tavern (_8^(I) / Engine horsepower question for gear heads

RKSTRAT
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"Clapton is Good"
Mar 3rd, 2018 09:49 AM   Edit   Profile   Print Topic   Search Topic

This is kind of a beer-bet question.

So same car with different engine options:

specs are 250hp and 300hp

(Actually the engines are the same, just tuned differently by manufacturer. This is a common marketing thing these days.)

On the dyno (measure privately) the difference is only 28 wheel horsepower (whp) versus the manufacturer spec 50hp difference measured as brake horsepower (bhp).

I am guessing that the entire scale of whp is compressed, as whp is always lower than bhp.

Does the claim that the two engines truly differ by only 28hp have any validity as measured by whp on the dyno?

(This message was last edited by RKSTRAT at 11:52 AM, Mar 3rd, 2018)

Jake from PA
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Mar 3rd, 2018 10:35 AM   Edit   Profile   Print Topic   Search Topic

Correct that whp is less than flywheel measurements. Drive train friction must be taken into account.

Leftee
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Mar 3rd, 2018 10:41 AM   Edit   Profile   Print Topic   Search Topic

That is power to the ground. So, yes. It has validity.

When I had my Mustang I went the aftermarket tuner route. Ford is quite open in that area. All measurements were HP at the rear wheels. IIRC the “race” tune pushed that up 30hp. This along with a more aggressive power band across the tach made quite a difference from the drivers seat. It was something like 30hp increase at peak, but at lower rpm it was more like 70-80hp increase.

BlondeStrat
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Mar 3rd, 2018 10:48 AM   Edit   Profile   Print Topic   Search Topic

I'm not sure if you are talking about "same car" as in exact same vehicle ...

or same car as in same vehicle model from the manufacture with different factory engine options.

It would strike me that WHP on the dyno is impacted by gearing as well.

If a vehicle is has a different engine option it's possible it also could have different gearing options in the transmission or differential that could produce different rear wheel dyno results.

Just a thought.

Engines though can certainly be built and tuned to create vastly different HP using identical base engine platforms.

(This message was last edited by BlondeStrat at 01:02 PM, Mar 3rd, 2018)

RKSTRAT
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"Clapton is Good"
Mar 3rd, 2018 11:00 AM   Edit   Profile   Print Topic   Search Topic

Blonde-

Yes, same transmission as well.
Same car, exactly, just different tune.



K4
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Mar 3rd, 2018 11:48 AM   Edit   Profile   Print Topic   Search Topic

If it was measured on the same dyno on the same day, then yes, the power relationship is valid.

It has 28 more usable HP.

RKSTRAT
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"Clapton is Good"
Mar 3rd, 2018 12:36 PM   Edit   Profile   Print Topic   Search Topic

Not to get too detailed/obsessive here :-).....but asking in a different way, if you compared any two vehicles by bhp versus whp, will the difference always be greater with bhp? Or will hp difference match up by either bhp or whp? Since whp is always less than bhp, I thought the whp scale might be more compressed.

Alternatively, the differences could be truly unique to the vehicles.


De ville
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Mar 3rd, 2018 01:32 PM   Edit   Profile   Print Topic   Search Topic

I'm not sure I'm understanding your original question perfectly, but here's what I know about torque and gearing.

Really what your talking about regardless of terms like BHP or WHP, is torque. Rotational torque.

Ultimately if any two engines produce the same amount of torque, then the torque produced at the wheel comes down to the gearing. Calculating torque to the wheel is the sum of the engine torque at a decided RMP, plus the gear ratios, and wheel size.

The gearing decides everything after you set the engine torque into an equation. The gearing can add tremendous mechanical advantage to differing degrees depending on the gear ratios, IE tall gearing or low gearing. A side note: Tire size is also a factor. The larger the tire, the less mechanical advantage there is.

Example, lets say you have two 1/2HP electric motors. They're both hooked to gearboxes. One has a gear box with a 200:1 gear ratio, and the other is hooked to a gearbox with a 1000:1 gear ratio.

Both motors are 1/2hp and produce equal input torque to their respective gearboxes, but the output shaft on the gearbox with the 1000:1 gear ratio will have 5 times the torque as the 200:1 gearbox. Conversely the output shaft of the 1000:1 ratio gear box will turn 1/5th the speed/RMP at equal RMP input by the motors.

De ville
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Mar 3rd, 2018 01:51 PM   Edit   Profile   Print Topic   Search Topic

After reading your question a few more times:

"Does the claim that the two engines truly differ by only 28hp have any validity as measured by whp on the dyno?"

I would say yes only if the gearing/transmissions are exactly the same, and tire size is exactly the same.

If you want to get technical there are other factors like friction, traction, weight, and axle length, but at that point I think your cutting hairs.





6L6
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Mar 3rd, 2018 03:37 PM   Edit   Profile   Print Topic   Search Topic

Performance is more about torque.

saturn
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Mar 3rd, 2018 04:44 PM   Edit   Profile   Print Topic   Search Topic

The amount of power the engine can deliver, versus how much is sent to the wheels, not used up before it can push the car along, is down to lots of variables. More powerful engines tend to be heavier, though of course with turbos and such that is not always the case.

I tend to like heavy cars that hold their speed well and are relaxing to drive, so I like bigger engines generally, and am not so worried about the output as long as they have decent torque to point and squirt along!



K4
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Mar 3rd, 2018 05:31 PM   Edit   Profile   Print Topic   Search Topic

wheel horsepower is always lower than brake horsepower. It takes power to run all the items on the motor, like alternator, water pump air conditioning ect. It also takes power to turn the transmission and differential. Most manufacturers will publish power numbers lower than what is actually produced so even the bad engines make the number.

Published specs are on an engine dyno with only the water pump and alternator attached. The numbers you get from a "chassis" dyno are the actual amount of power that combination is capable of making on that day on that dyno.

Dynos are not exact, if you took that car to 10 dynos you would get 10 different numbers, but the relationship between the two cars should stay constant. the percentage change in total power should match the percentage change in power difference.

RKSTRAT
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"Clapton is Good"
Mar 3rd, 2018 06:09 PM   Edit   Profile   Print Topic   Search Topic

Great info! Thanks you guys.

Yes, torque is where it's at.

I was curious about whp.

jhawkr
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Mar 5th, 2018 06:37 AM   Edit   Profile   Print Topic   Search Topic

" Yes, torque is where it's at. "

To a point. But, torque is also affected by gearing. Two identical cars with identical engines but different transmissions or differential ratios will net different torque output. It is easily demonstrated by a bicycle derailleur. When the chain is on the small gear up front and the large gear in the rear, you have the greatest torque. Great for starting off uphill but you won't get much speed out of it.


FlyonNylon
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Mar 5th, 2018 07:03 AM   Edit   Profile   Print Topic   Search Topic

Both my vehicles are known for being pretty balanced in terms of torque/hp but I have to say if I needed power I would want more horsepower.

The Suzuki is a v twin that is perfectly fine lumbering around off road at 3k rpm, but is much happier at higher revs. The gearbox, fuel injection, power delivery is smoother from 5k to redline at 10k.

My car has a straight six that certainly has usable torque at 2-3k, however like the bike its much easier to rev match downshifts and carry speed when you keep it between 4-7k rpms.

One of the best feelings driving or riding is perfectly matching a downshift while braking into a corner, rolling on the throttle before the apex of a corner in the middle of the powerband and feeling the acceleration to redline and up shifting out of the corner. This is mostly horsepower.

And yes you can still have fun doing this at very safe speeds. Its more about technique than absolute velocity.

FDP Forum / Moe's Tavern (_8^(I) / Engine horsepower question for gear heads




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