Back to the Basics... Please Help

[SSTibet]

Folks,

Some time ago I had read somewhere that "Hp output of any engine is primarily based on how much air can be put through that engine". Simply stated, More air through the engine = More Horses... Well, what is then the limiting factor in an engine to flowing more air??? For a given cubic inch displacement, why not just increase the rpm and get more air sucked in & out of the engine per unit of time and therefore get more power???

What is the reason for an 5.7Lt LT-1's limitation to 5,500 RPM instead of 8,000 RPM (just picking numbers for the sake of discussion) and limiting its power at 300Hp?

I know this is a pretty basic question, but I am of the type who needs to understand the basics first...

 

[Ken]

Nobody else bit yet? :L I'll be back later to help you understand, if someone hasn't already done so by then.

 

[rrubel]

A simple response, and I'm sure someone will correct/elaborate on it...

1. Your average engine can't spin 8k RPM without tearing itself apart. You need a compromise between the strength needed to handle the stress, versus the weight of the reciprocating mass, vs the cost the customer is willing to pay.

2. There will always be a limit to the amount of air that can easily be drawn into and pushed out of the engine... again, cost of engineering vs simplicity of design and ease of maintenance vs emissions.

Small lightweight engines like that in the Honda S2000 can rev higher much more easily than a large heavy engine like the LT1. Ever notice that while F1 cars use V10's, they're all tiny? Like 3 liters and under. Small, strong, lightweight pistons that can change direction quickly; lightweight springs that can still force a valve shut quickly but won't PREVENT the valve from opening; multiple tiny valves that can also change direction quickly.

About 10 years ago, a co-worker of mine who was a very knowledeable car guy ( designed and built himself a tube-frame/fiberglass/V8 car to go along with a digital dash he'd made for grad school) was trying to convince me that pairs of larger valves would always be better than 4 or 5 smaller ones because they were cheaper and easier to manage. At the time, I didn't know enough to argue, but the obvious rebuttal is that the smaller valves weigh a LOT less and can open/close quicker, thus allowing the engine to rev higher.

And if this seems a bit spacey, I'm somewhat buzzed at the moment...

[RICHR]

 

[Edmond]

What do the ZR-1's redline at? But they have a DOHC setup so that's an advantage at higher RPM.

However, the LS motors are making more than 1hp per CID with a pushrod setup.

Some of the things that those engineers can do is simply amazing.

 

[Ken]

Okay Tibet, rather than give a man a fish, I'll try to teach him...

My favorite Internet site (right after the CAC of course :L): How Stuff Works: Engines.

I think you'll like that site, and if you do, here's one that has definitions for automotive terminology: Automotive Terminology.

 

[SSTibet]

Hey thanks all, so as I understand it best "it is the physical constraints of an engine that limits its horse power".

So, would it be fair to say that the LT-1 engine is a fairly inefficient engine since it produced only 300Hp for a 350cid capacity??? I mean so much volume but low output???

And another question, I hear a turbo / super chargers can be added to the LT-1 to make it produce more Hp. If that is true, why is not the engine designed that way from the beginning; i.e. no chargers but a design that allows higher RPMs ??? What max RPM does the LT-1 hit with a turbo / super charger?

Any comments?

 

[Ken]

... "it is the physical constraints of an engine that limits its horse power".

There's more to it than just that. To begin with, an engine (as an air pump) can only be as efficient as the intake and exhaust tracts allow. An engine producing all the horsepower in the world isn't going do very well if it's restricted by a small carburetor (or throttle body), or a small restrictive exhaust system.

But there's still more to it...

Volumetric Efficiency

In a four-stroke naturally aspirated engine, the theoretical maximum amount of air that each cylinder can ingest during the intake cycle is equal to the swept volume (displacement) of that cylinder (0.7854 x bore x bore x stroke). Since each cylinder has one intake stroke every two revolutions of the crankshaft, then the theoretical maximum volume of air it can ingest during each rotation of the crankshaft is equal to one-half its displacement. The actual amount of air the engine ingests compared to the theoretical maximum is called volumetric efficiency (VE).​

There are many factors which determine the BMEP an engine can produce, but the fundamental determinant is the mass of air it can ingest into the cylinders, and there is a nearly-linear relationship between VE and BMEP. For contemporary naturally-aspirated, two-valve-per-cylinder, pushrod engine technology, a VE over 95% is excellent, and 100% is achievable, but quite difficult. Only the best of the best can exceed 110%, and that is by means of extremely specialized development of the complex system comprised of the intake passages, combustion chambers, exhaust passages and valve system components.​

Generally, the RPM at peak VE coincides with the RPM at the torque peak. And generally, automotive engines rarely exceed 90% VE. There is a variety of good reasons for that performance, including the design requirements for automotive engines (good low-end torque, good throttle response, high mileage, low emissions, low noise, inexpensive production costs, restrictive form factors, etc.), as well as the allowable tolerances for components in high-volume production.​

Knowing a claimed power and fuel-flow for a given engine, you can calculate an estimate of the VE required and use all these yardsticks for estimation of reasonableness.​

Engine airflow at 100% VE, in sea-level-standard-day cubic feet per minute (scfm) is:​

AIRFLOW (scfm) = DISPLACEMENT (ci) x RPM ÷ 3456​

The air-to-fuel mass ratio at best power mixture is typically around 12.5 to 1. If you work through the arithmetic, you find that the airflow required at that mixture (12.5 to 1) is:​

REQUIRED AIRFLOW (scfm) = FUEL FLOW (gph) x 2.72​

So, if a 420 cubic inch engine produces 480 HP at 4400 RPM on a BSFC of .46, then the required airflow is 601 scfm and the VE is 107%. You already know that is a VERY big number.​

​

Oh, I keep forgetting to address the power-to-weight issue also comes into play here, same as when dealing with the whole car. A big block weighs a whole lot more than a small block, but that issue's been addressed more recently with the increased availabilty independent manufacturer's aftermarket small block castings, not to mention the fact that head gaskets are now available to enable one to build a 454cid small block. I built a 417cid from a 400cid small block. ​

​

 

[SSTibet]

Ken, thanks for the information. It is now time for me to digest it. But I think I understand the basic reasons much better now.

So, what is your comment for what I said for the LT-1 that it is rather inefficient since it produces to little HP for so much cid capacity???

 

[JHL]

As Ken said an engine is not much more than a big pump, air and fuel in and gasses out. I read recently that 4 valve per cylinders can cover as much as 20% more area than 2 valves, The overhead cams act more directly on the valves allowing a higher engine speed and hence more reliable power, add in seperate cams for intake and exhaust and it is easy to see how this is a much more efficient set up, almost all new motors big and small are running this set up now, there are even some four valve per cylinder diesel engines availible in daily driver European Fords & GM cars. Some manufactures tried 5 valves such as Ferrari cars and Yamaha motorbkes.

In reality pushrod motors are dinasour technology these days, the main limiting factor being the valvetrain, although it is a testement to GM engineers that they have manged to extend the life of the venerable small block it now takes 6 liters to get a reliable 400hp which is a poor ratio when for years now the norm for performace engined cars is more likely to be around 100hp/liter. Same power is available from a smaller engine which in turn is lighter and more fuel efficent when cruising but has power for speed when needed. Add in electronic variable valve and cam timing and cam profiles that would have been previously unusable before can be electrically controlled to provide both high rpm HP with out sacrificing low end drivability torque. You also have electronically controlled exhaust devices wich can further aid both low end torque and high end rpm.

There is no doubt the new generation small blocks give bang for the buck but the technology is very outdated.

 

[SSTibet]

What exactly is meant by "low end torque"??? And why is it that important in most anything I read here about engines?

 

[JEFNLSA]

Hey Ken....
I discovered the "How Stuff Works" site a couple of months ago and, like you, it is one of my faves as well. Now I'm a plethora of useless as well as useful information.
SSTibet....you gotta check it out

 

[Ken]

What exactly is meant by "low end torque"???

That means an engine is making most of its torque down low in the rpm band, also known as the "bottom end" of the band. Torque is what gets you moving, horsepower keeps you moving.

 

[SSTibet]

Ken,

Thx for the information (as usual). Friends often ask me how fast I have driven this car, and I tell them it is not the speed that I like most about this car but the way it accellarates - especially from a stop. This, I know for sure is due to the torque.

Also when cruising at 4th / 5th gear and when open the throttle a bit it responds quite nicely is the speed is right for that gear. If the speed is somewhat low for that gear it does not respond as nicely and I need to donwshift and it gets wiiiiiiild....

 

[SSTibet]

Hey Ken....
I discovered the "How Stuff Works" site a couple of months ago and, like you, it is one of my faves as well. Now I'm a plethora of useless as well as useful information.
SSTibet....you gotta check it out

JEFNLSA, I have known "How Stuff Works" for many many years. There is lots and lots of "basic" information in that site, it has been very helpfull in understanding the "101" type lessons.

 

[Ken]

Have you been to KEVIN SULLIVAN'S AUTOSHOP 101 - Automotive Training and Resource Site for Automotive Electronics yet?

How about ENG-TIPS FORUMS - Technical Work Forums For Engineering Professionals?

Or maybe N2 PERFORMANCE - Home of the "Performance Professor: Jim McFarland"?

All great sites!

 

[SSTibet]

Ken,

Thx for the site information. Hey watch out, you are raising yourself a competitor here!

I am wondering if it would be smart to start a muscle car service shop in Ankara / Turkey. But then I am not a pro and there are not many US cars around here. I would love to run a business like that...

 

[Ken]

Well, gearheads being what they are ... You take what you've got to work with and go from there. Just about anything can be hot-rodded, or customized.

 

[WhalePirot]

Have you been to KEVIN SULLIVAN'S AUTOSHOP 101 - Automotive Training and Resource Site for Automotive Electronics yet?

How about ENG-TIPS FORUMS - Technical Work Forums For Engineering Professionals?

Or maybe N2 PERFORMANCE - Home of the "Performance Professor: Jim McFarland"?

All great sites!

TY for the new sites/links. Marked; soon to be perused; always willing to learn.

 

[JEFNLSA]

TY for the new sites/links. Marked; soon to be perused; always willing to learn.

Ditto

 

[SSTibet]

Have you been to KEVIN SULLIVAN'S AUTOSHOP 101 - Automotive Training and Resource Site for Automotive Electronics yet?

How about ENG-TIPS FORUMS - Technical Work Forums For Engineering Professionals?

Or maybe N2 PERFORMANCE - Home of the "Performance Professor: Jim McFarland"?

All great sites!

Ken, thx much again. Printed all of the articles from Jim Mcfarland (The Performance Professor) and I am making a folder to read at home. I also enrolled to the eng-tips forums.
You are great help Ken. Thanks again buddy