Have you ever used a fulcrum an lever to lift something b/c you didn't have enough strenght to lift it without one? It's more or less the same concept.

 

It's not just 500 lb-ft of static torque the engine is producing, but it's spinning those lug nuts at 5252 rpm (as a convenient example). That's also 500 brake hp. Try that with your torque wrench!

Actually how much hp do you think you could exert turning a stubborn lug nut (actually a mini dyno)? Assume you could apply about 176 lb-ft with a long breaker bar on the wheel mounted vertically (lug nuts up), and you could run around the thing once in 10 seconds. Calculate your hp output.

 

i see what your getting at here....lol. Thats what I was trying to get my stubborn brain around , i left out some important factors like rpm and brake horsepower. Thanks for clarifying that.

 

Way to go!!

Jon

 

LOL! Subscribing.

 

Yeah! I finally get to argue with Old SStroker. While your general theory does seem to make sense, there is little denying that at the same displacement, undersquare motors seem to generate higher torque at low rpm, while oversquare motors seem to deliver less torque, but carry it to higher rpm. Obviously this is affected by other factors such as high rpm friction with undersquare motors, and larger valves with oversquare motors. I am also aware that real-world testing HAS shown the opposite of my theory in a small handful of instances. I feel that the real pitfall would be in assuming that the BMEP for the engines would be the same. Since it really does boil down to how much fuel and air you can burn, the smaller bore at a given displacement would have a higher peak BMEP, offsetting the smaller surface area of the piston pushing on the longer crank throw. It seems to me that the BMEP of a larger bore, while perhaps averaging the same, would be less at peak. I'm also curious as to how thermal efficiency affects this whole concept. As usual, if I'm wrong, please point out why so I can learn somthing.

 

Good comments, game. As always it is all about the combination, not just one thing like bore/stroke (B/S) ratio.

While burn is critical, it's even more important how much (mass) air and fuel you trap in the cylinder. Oversquare (B>S or B/S>1) engines often breathe better as mentioned because they can have more valve curtain area per cubic inch. That could make it easier to make more lowend grunt that a B<S combination, which flies in the face of the "rule".

Frictional losses, especially at high rpm, favor shorter strokes (B>S). The way to make power is to make as much torque/cubic inch (BMEP) at as high an rpm as possible. F1, Cup, ProStock are all good examples. Ironically, F1 has a hard rpm rules-imposed limit, while NASCAR controls max rpm on some tracks, but not others, and PS guys just keep pushing it up. PS makes the highest BMEP (torque/cube) at power peak rpm (pprpm) of any NA engines I am aware of.

Just like the "rule" that says single plane V8 manifolds don't make low end torque like dual planes, the "rule" that says B<S engines make better low end torque than B>S engines, is far from a "rule". It's all about the combination. The engine only sees the combination, not the individual parts.

It's always fun to compare engnes by calculating BMEP both at torque peak and at power peak rpm. This is especially fun for magazine engine tests. When they get much over 205 psi @ power peak rpm I'm interested.

A recent magazine featured a 466 cube 687 fwhp @ 6700 (STD correction) engine.That's a BMEP of 174 psi @ pprpm. That same engine builder regularly builds Engine Masters Challenge engines which make over 205 psi BMEP @ 6500. Do the math for a 466 at that point.

174 psi @ 6700 for a 427 LS7 would be just shy of 630 fwhp. Shoot, there are 630 rwhp Z06 (NA) engine packages out there. 630 fw is around 550 at the wheels, or a tad less.

FWIW, what's the BMEP for 925 hp @ 9000 from a 440? If you could get that power @ 7500 instead, what would the BMEP be? Which engine would be more impressive to you?

Keep up the thinking, guys and keep an open mind.

 

BMEP is the name of the game.

Thermal effeciency takes a hit with the larger bore combinations as there is more surface area for heat to leave the cylinder. Mechanical effeciency also take a hit with large stroke combinations as pistion speed and accelerations go up. This may be why its seems long stroke combinations seem to get winded a bit at higher RPM.

 

I thought this was due to piston speed. If you have too high of a piston speed, the rings wear out significantly faster, and have a hard time sealing, generally speaking.

 

Do u think a magazine is a credible source of information???or just trying to sell a product???

 

3.4 camaro put a good bit in there. Piston speed becomes a big player and if you were ABLE to spin those combinations high, the cylinder head (for the most part) would run out of steam. Cylinder pressure is always the name of the game.


As far as your lever arm argument pat, the (Rod/Stroke Ratio) argument is a thing of the past. As Old SStroker said, it is about the controlled explosion of the fuel/air mixture at the top of the piston, not the said "rotating assembly and it's components". Now, I am not taking away from the machining and parts, but just speaking in generalities.

Hopefully my post makes some sense.

 

IF the mag prints the real numbers seen on the engine dyno and the correction, the data is useful. I look at who did the test and who wrote about it. In the referenced article, the magazine cover said 700 hp, but the dyno showed 687. That helped with their credibility. So did the reasonable 174 psi BMEP.

If you want to raise the BS flag on magazine dyno tests, check the powerpeak rpm BMEP vs. the engine build. There was a comparo a few months ago in some rag..er, mag of old muscle car engines. All but one had reasonable BMEPs for engines of that era, even with some modern parts. One, the "winner" had unusually high power and very questionable BMEP. Why they through in a ringer beats me. It blew a lot of their credibility.

 

Magazines are there for one thing.

You should NEVER believe anything you read or hear, and believe only half of what you see !

 

Evidently you have had some up close and personal dealings with some of the mags, or you have had none. I can't tell by your post. Tell us your horror story with a car mag article.

Like everything else, some things are better than others.

Never say never. Never say always. You will often fnd yourself stepping on your tie when you do.

My $ .02

 

As with everything man made.. there are compromises and when you look at the history you can generally tell how things ended up the way they are.

The name of the game is to release as much chemical energy from the fuel as possible..

With that said and focusing on basic piston engine designs, there are limitations on bore and stroke..

Here are a few basic limitations

Stroke (S>B) limitations
Piston velocity
Valve area/ Flow
Impact to squish/quench/clearance volume

Bore (B>S) limitations
flame speed
cylinder fluid motion and gas exhange process
Piston to valve clearance

Rpm (both) limitations
piston speed
Piston acceleration
Valvetrain dynamics
Cylinder head flow
Friction!!

This is a basic list but goes to show how when the engine application is begining the design process.. each of the factors are weighted and determined whats the best

As a rule of thumb, 12-13 Bar BMEP is a all a NA engine will achieve (which is about 174-188psi).. if you want higher than that Forced induction is the way to go. Dont mind the engine builders getting 205.. that requires a bit more effort than most are willing to put in an engine.

So speaking purely in BMEP terms, we already know we are limited NA.. to make more power from a given displacement you now have 2 options.. RPM or forced induction. Forced induction will increase BMEP and RPM will increase ultimate power.

So.. using formula 1 as an example.. they are limited to NA and limited in displacement.. (effectively 'Torque' limited).. so they had to increase RPM. To tackle that challenge they decreased stroke, increased bore, and worked on valvetrain dynamics. In the end, you can see they took the same tradeoffs that any engine design has and just tackled the issues that RPM limits..