This thread is dedicated to how an engine builds power in general. Not specifically Corvette or even GM.

I have heard that a long narrow intake runner makes high torque and a short fat runner makes power. (Lets assume for this that the volume is the same) Why is that??? Is the same true for the exhaust runners? Now what about if the volumes are different?

What about camshaft timing, why does closing the intake valve before BDC increase cylinder pressure and thus torque (or so I have read)? Wouldn't that decrease the intake charge and produce a low pressure just before BDC? If I am wrong (probably am, ) on anything here please don't hesitate to let me know and correct me.

I used to be an engineering student, so I am more interested in how and why rather than just a Laymans terms explanation.

Thank you!

 

I think that's more of a topic for books than a forum thread.

 

Titles? Internet articles? I am having difficulty finding that detailed info. All I have found is what I just posted. Just the result and not the why or how.

 

This should be interesting.

 

Plenty of books on this subject. Most David Vizard books and articles cover related subjects.

More than the human mind can absorb without deleting vital stored data.

 

 

Short answer: an engine is an air pump. The more air (and fuel) in and out, the more power the engine will make.

Long answer: David Vizard.

 

Thanks guys, I will be off to the library with his name in mind!.......might need some coffee.

 

Personally, I find beer works better Getting beer into the library is difficult, but worth it

Seriously, the reason DV gets so much attention is that he makes difficult concepts pretty straightforward, and he backs up what he says with dyno testing - like our resident tuning wizard, Lars. The books are a must read.

My only negative to DV is nobody - no. body. - can get the kind of power he does out of ported stock heads. He's got a flow bench in his brain, connected to his eyes and hands.

 

Beat me to it.

/thread

 

(Horse) power is a function of torque. You make it sound like two different things.

 

As noted, the engine is just a big air pump and the fuel let's it run itself. The more air you can pump, the more power you can make.

Think of the intake track as a garden hose. On a short, fat hose, you can quickly move a high volume of fluid through the line. A longer skinnier hose cannot move as large a volume of air as fast, but what air does move though it picks up velocity.

With the added velocity some things are improved, such as scavanging (Vernouli Principal) and these lead to a higher torque rating at a lower RPM.

 

Lets say Hp is the tooth fairy, and torque is your mom putting the money under your pillow. Hp is a made up number based on a calculation. Torque is a measured force over a perpindicular distance....yes they are the same but are also different. Your avg horse can do 1 HP which = lifting 550 lbs of coal up a mine shaft 1 foot in 1 second. The term 'horse'power became popular from James Watt as an advertising/marketing tool to tell people the approx equivalant, in horses, his new fangled stream engines would produce....or so I have been told by expensive engineering textbooks for years and countless hours of homework dealing with that exact formula

Hence the asking of why and how....I am more interested in why the velocity and density and pressure each affect the engines ability to produce a measured force....'torque'

Even reading some David Vizard articles just tells you the result of the change, not why it happened.

A 800 HP 10,000 rpm engine is of no use to me....450 ft/lbs at 2500 rpm, now that is

Looks like I need to do more research. I was once told that stealing from one person is plagarism, but stealing from many is research...

 

The truth is that how an engine makes power is actually pretty complicated. The basic principles involved are pretty simple, but the number of variables, and how they effect each other, quickly gets overwhelming as you dig deeper into the subject. Any attempt to describe such a complicated system in one or two sentences is bound to be a gross oversimplification. Search for "David Vizard" on you tube, and you will quickly see what I mean. I swear, if you listen to that guy for too long, you'll start to wonder if the color you paint the engine might affect it's power output.

That being said, the small block chevy has been around for almost 60 years now, and some pretty good "rules of thumb" have been developed by engine builders. So, you don't really have to know all of that nerdy stuff like connecting rod to stroke length ratio, and other esoteric stuff like that.


Keep the shiny side up!
Scott

 

ill make a small stab at this...

HP= TQ*RPM/5252

now that being said... to get high HP readings you must be able to flow well at high... very high RPM's if your heads, cam, intake ports, exhaust ports, valve train(float)or whatever restrict in any way, your rpm and HP will be limited as your TQ will start to fall off. TQ comes more from cubic inches.... displacement, and compression. keep the airways open, the TQ flat and HP will go up up up....

there is also a formula for port size and flow... your max flow will determine your hp...also good to find carb size...to determine your carb size use CI*Max RPM/3456

and so much more... but this may get the conversation started you want.

 

What it takes to make power - and the real question of where in the RPM range to make power - gets more complex as the power level increases. At levels below about 1.4 HP/CID...which is where 99% of street engines are going to be...it's not really all that esoteric.

Studying cam events at CompCams or in any number of books probably is the best overall bang for the buck...and Vizard and others lay these concepts out quite clearly and simply.

 

What are you now?

Two very general things to consider. A long thin runner will cause higher air velocity. This long column of air (due to it having mass) will have inertia. This inertia helps the air keep moving into the cylinder despite the intake valve starting to close. Higher air (mass) content means higher volumetric efficiency, which means more combustion pressure, which means more torque. This column inertia thing works well if you keep the intake valve open long enough for the long skinny column of air to get in the cylinder. This "long enough time" is only possible at low RPM. (Look up Helmholtz too, for amusement.)
At hi RPM the time that the valve is open is extremely short. For maximum VE/HP/Tq when the intake is open very briefly you need to minimize the intake restriction, hence big ports and valves.

That's a short sweet incomplete explanation.

 

69427: I didn't like sitting behind a desk all day doing computer simulations. So I am going into construction. I need to work with my hands and keep busy, just the type of person I am. I also teach skiing in the winter.

"you'll start to wonder if the color you paint the engine might affect it's power output." Different colours emit and absorb heat at different rates. So if temp is a factor in power, it could very well help...how much? Not a clue...lol

Ok new question: Why when you look at various dyno charts, sometimes you see a quick drop off or increase in torque. You know that smooth curve then a random mountain peak look? It seems to be only for a short rpm range.

 

What I would like to know is, where did you read that? Even a very mild stock camshaft will not close the intake valve before BDC. I'm not an engine expert, I only know that from trying to figure out how Dynamic Compression Ratio works. But I can tell you this: whatever you're reading, stop reading it! And go back to engineering school, we need engineers in this country. Did you know that every year, we graduate more lawyers than we do engineers? That's pretty messed up!!

What you may be thinking of is the fact that the sooner the intake valve closes after BDC, the more cylinder pressure the engine will have, because the piston can't really start squeezing the mixture until the intake valve is closed. This is why long duration cams typically need more mechanical compression ratio to generate cylinder pressure; the late intake valve closing allows the piston to be somewhat higher in the bore when the compression starts taking place. At higher rpm, you get a "ram" effect, which boosts cylinder pressure due to better cylinder filling, but at lower rpm, the port velocity is too low to get that "ram" effect, and you end up with poor low rpm performance due to low cylinder pressure. Sorry for the layman's terms, but, like I said, I'm not an engine expert.


Keep the shiny side up!
Scott

 

Thanks for the correction Scott. I knew it sounded wrong!!