Is there a correlation?

[EPorsche]

I was wondering, as I offend do, about my engine and the correlation between bore and stroke. Is there a set and hard fast rule that if you increase one or the other, it would correspond to an increase in either hp or torque.
This thinking came about when I was told the hp of the latest 991 (400hp/300tq). On my air cooled engine the hp/tq numbers were more or less even with the other, maybe a little bit more torque than hp. Today all the rave seems to be high hp numbers with significantly less torque, why is that?

[Quicksilver]

Horsepower is basically a "torque times RPM" measurement. In other words if you have a fixed torque and double the RPM you will have double the horsepower. The newer cars have higher redlines so they could have lower torque numbers but still end up higher HP numbers.

Be clear about it: If you have 2 engines with identical torque outputs at a specific RPM then they also have identical horsepower at that RPM.

As far a relationships between bore, stroke, torque, and horsepower...
The more air/fuel mixture you pump through an engine the more power it will produce. You increase the bore or stroke and it will make more power.
An increased stroke will tend to beef up low end torque for a few reasons:
- Longer stroke means more mechanical advantage pushing on the crank.
- A longer stroke means it is harder to spin the engine faster.
- A larger intake "gulp" can make head flow become maxed out at a lower RPM.
- There is a factor called "Rod Ratio" (that is a little complicated for this discussion) which is the ratio between the stroke and the length of the rod. A larger stroke compared to the rod length favors lower RPMs.
I'll kind of cut it off there because you could write a book on this stuff...


All of this stuff gets insanely complex and interrelated, and really requires some book reading to make sense. The best introductory book I've found for this is "Power Secrets" by Smokey Yunick. He explains things in very plain, easy to understand English. Some people might be put off by the fact that he mostly worked on American V8s but the physics are the same. To quote him, "An engine doesn't know what name is on the valve covers."

[porschenut]

The Jeep Wrangler has incredible torque down low, and runs out of steam at about 3k RPMs. It has a very long stroke, small bore. A perfect engine for pulling stumps, or driving over boulders.

One example of the opposite I can think of is the 3.2 short stroke motor that a lot of guys lust after. It makes its power up high, revs very high, but doesn't have much pull down low. Would be loads of fun on the track, with the right gearing.

[EPorsche]

Thanks for the info QuickSilver. I notice between the the SC and Carrera the only real difference is the stroke and Motronic with the same bore (piston pin be relocated).
What engine do you guys prefer for track/DE applications: high hp/low torque, mid hp/mid torque, or low hp/high torque?

[Dantilla]

Originally Posted by EPorsche

What engine do you guys prefer for track/DE applications: high hp/low torque, mid hp/mid torque, or low hp/high torque?

On my 944 race car, I uncorked the exhaust with 3" tubing all the way back to flow better at high rpms.

Much better top end power, very noticable decrease on power down low. For track duty, the engine spends most of the time at full throttle/high rpm, so that's the most important place to make max power.

To way oversimplify: Bore makes horsepower, stroke makes torque.

So oversimplified that all the engineers are pulling their hair out and screaming naughty words at their computer monitors about now.

[HawgRyder]

If you look at some of the older and perhaps current Formula 1 engines, they are over-square...or bore is much bigger than stroke.
Some of these things turn at 20,000 + RPM.
Also...they have a very small power band (narrow).
I remember seeing an old 50cc motorcycle in the Ilse of Mann races....a Scott...50cc...22 gears (11speed with 2 speed transfer case) and a power range of 22,500 to 24,000 RPM.
It sounded wild....and of all things...water cooled.
Bob

[EPorsche]

Seems like a Honda S2000. High hp but low torque and the only way to get the hp is to rev it to 9000rpm

[hcoles]

At the risk of over simplifying or even being wrong - in motor sports it is common to limit displacement.
For track use you want to maximize hp even if lowering torque at lower engine speeds because the gearing can be optimized for the high rpm use.
High rpm makes hp but is somewhat limited by piston speed, ft per minute. Short strokes reduce piston speed.
This is why race engines with displacement limits tend to have over square cylinder dimensions.

[afterburn 549]

Originally Posted by Quicksilver

Horsepower is basically a "torque times RPM" measurement. In other words if you have a fixed torque and double the RPM you will have double the horsepower. The newer cars have higher redlines so they could have lower torque numbers but still end up higher HP numbers.

Be clear about it: If you have 2 engines with identical torque outputs at a specific RPM then they also have identical horsepower at that RPM.

As far a relationships between bore, stroke, torque, and horsepower...
The more air/fuel mixture you pump through an engine the more power it will produce. You increase the bore or stroke and it will make more power.
An increased stroke will tend to beef up low end torque for a few reasons:
- Longer stroke means more mechanical advantage pushing on the crank.
- A longer stroke means it is harder to spin the engine faster.
- A larger intake "gulp" can make head flow become maxed out at a lower RPM.
- There is a factor called "Rod Ratio" (that is a little complicated for this discussion) which is the ratio between the stroke and the length of the rod. A larger stroke compared to the rod length favors lower RPMs.
I'll kind of cut it off there because you could write a book on this stuff...


All of this stuff gets insanely complex and interrelated, and really requires some book reading to make sense. The best introductory book I've found for this is "Power Secrets" by Smokey Yunick. He explains things in very plain, easy to understand English. Some people might be put off by the fact that he mostly worked on American V8s but the physics are the same. To quote him, "An engine doesn't know what name is on the valve covers."

I have to disagree with you about the Torq times RPM formula.
A engineer once told me the most purrfect engine in the world is the steam engine. As it develops megatons of touq at 0 RPM.
Needs NO transmission, can go backwords or forwards.
I guess one could argue for the electric too..but too be correct it is a "motor" rather then a engine.
(Nothing said here in a condescending matter)

[Tippy]

Originally Posted by afterburn 549

I have to disagree with you about the Torq times RPM formula.

That is exactly how the dyno calculates hp. Horsepower is a simple formula based on torque times RPM.

HP and torque always intersect at 5252RPM because that is how the formula is set up.

[Tippy]

You can also look at hp like this and is the best example I can think of.

Torque falls off at higher RPMS while the hp curve increases. Why?

Torque indeed is falling off but since RPM is increasing, there is more torque moments in a given time adding up to increase overall power output which is displayed as hp.

[gregwils]

Originally Posted by Tippy

That is exactly how the dyno calculates hp. Horsepower is a simple formula based on torque times RPM.

HP and torque always intersect at 5252RPM because that is how the formula is set up.

+1 You can find this in any formal explanation of how horsepower is calculated.

How do you convert engine torque to horsepower?

[afterburn 549]

Then...there must be a fallacy here in the formula?.....a steam engine has no torq>?
I am guessing this formula is the best way (torq x RPM at achieving a unified answeron a Dyno)?
In real life torq is the measure of twist, pounds of force..even with no rotation

[hcoles]

I think we have a basic technical misunderstanding, or perhaps this story will help the conversation, related to torque and how it is measured and translated to hp.

When an engine is run on a dyno a resistance to rotation is applied in pounds force at a given distance e.g. feet. The resulting "torque" is the product of these two measurements e.g. pounds-ft. Measuring torque, think of putting the tip of your finger on a spinning shaft and pressing down, there is a force on your finger parallel to the surface of the shaft and the distance is from the shaft center to the surface where you finger is touching the shaft.

Now the next step is to convert this torque measurement (force at a distance) to energy rate or power (hp).

What is a measure of energy? Let's refer to physics 101 - and we see it can be described as a force over a distance. How does that help? Let's go back to the finger and shaft. The shaft doesn't know if your finger slipped to apply the force or was attached and applied the same force as the shaft spun around and at the same time your finger spun around with it. So we assume the case was that our finger was attached and applied force as the shaft spun around. Looking at it this way your finger, for one revolution, moved a distance of Pi x dia. and the force was the constant force applied at the interface between your finger and the shaft in the direction tangential to the shaft. OK, so now we have energy for one revolution of the shaft in pounds force-feet.

Power = (energy x how much energy per unit time) / (desired metric e.g. HP)

To finish the story let's use an example.

1 - tangential force on the finger is 100 pounds force
2 - shaft diameter is 2 feet (thick shaft for this example but could be a large brake drum, this is sort of where the term "brake" HP comes from)
3 - shaft is spinning at 6000 rpm or 100rps
4 - your finger covers Pi x 2 x 100 feet in one second = 628.318 feet
5 - the force x distance is 100 x 628.318 = 62,831 pounds force
6 - the energy rate is 62,831 pounds force - feet per second
7 - we want a result in HP, 1 HP = 550 pounds force feet per second
8 - dividing we get 114.23 hp. (your basic 1971 BMW 2002)

There you have it, where "brake" HP came from and how to derive HP from torque if you forget the conversion number and hopefully a better understanding. The only thing you need to know is Pi = 3.14159.... the ratio of circumference to diameter of a circle.

This may be totally unneeded - disregard if that is the case for you. If you understood all this before and now are confused, I guess that could be good or bad.

[afterburn 549]

All is good.......I was musing over a small technicality..LOL

[combatic]

Originally Posted by EPorsche

Seems like a Honda S2000. High hp but low torque and the only way to get the hp is to rev it to 9000rpm

With an S2000, you have to keep the revs above 5k to get any excitement...it is a mental shift when driving a high revving car. Below 5k it drives like a fuel sipping civic.

I have driven a short stroke 2.8 in anger and the similarities in character are VERY similar and pleasing =)

Torque gets you out the hole faster though...even on the race track...after you didn't carry your speed correctly and now you have all the smaller engine...yet mysteriously faster cars breathing down your neck all of a sudden

I think most people like torque b/c it gives you real and a butt dyno feeling of power in most daily street driving applications. True race cars are always up near redline....and are built to live there as others have mentioned here and big $$$ are poured into developing/building such engines.

for DE stuff...run what ya brung....seriously. All of our cars are designed to be driven fast. The performance (rpms etc) mean nothing unless you are door to door racing in a class specifying displacement etc.

[Bob Kontak]

Originally Posted by Quicksilver

A larger intake "gulp" can make head flow become maxed out at a lower RPM.

Given the intake hole is the same size at low and high rpm, help a halfwit understand how it can work ok at higher rpm's.

Is it because once the higher rpms are achieved there are other pistons helping pull the air in?

[afterburn 549]

Yes ( VE)along with cam overlap

[Bill Verburg]

Originally Posted by Bob Kontak

Given the intake hole is the same size at low and high rpm, help a halfwit understand how it can work ok at higher rpm's.

Is it because once the higher rpms are achieved there are other pistons helping pull the air in?

modern cars vary the hole size w/ variable cam timing and variable intake geometry

at low rpm the geometry is smog/fuel economy oriented, the same engine at high rpm will have a geometry that compares favorably w/ the older RSR fixed geometry engines, hence the big hp #s


torque is what you want and torque at high rpm is better to have than torque at low rpm.

[j911brick]

Maybe somebody can explain this?

I was at a dyno one day and a Honda had a flat torque curve. Pretty much straight all the way across. But the HP curve was as one would expect. Is this even possible with a mathematical model?

Also: I have always wondered: wouldn't changing tire diameter effect the dyno results? Why or why/not?