RichardPON

Opinions wanted on this - found on another forum:



This is a basic mechanical explanation of what you can achieve with your engine.

It's all about volumetric efficiency as this is what defines how much power an engine is capable of producing. It's a measure of the efficiency of the power cycle - that is induction, ignition, and the exhaust process. Typical modern engines (in particular the latest german ones) will have a volumetric efficency rating of about 85-90+, with older engines like the mk1 & mk2 maybe at something like 80-85, whilst my old mgb falls at around 65-70 This means the tuning potential is a lot greater in my mgb with bigger gains to be achieved quite cheaply. The Japs managed to tune the mr2 engines to quite a high state of efficiency so if your mk1 is running at a healthy say 82% efficiency @ 122bhp, you could probably hit 150bhp @ 100% efficiency, whilst a mk2 rev 3 running at 85% efficiency @ 178bhp could theoretically attain 210bhp @ 100% efficiency.

Whilst this sounds tempting, please be aware that it is theoretically impossible to attain 100% volumetric efficiency, and getting up to the high 90's is gonna cost megabucks. So that's it i hear you cry? All is lost?

Nope. To increase the volumetric efficiency quite cheaply (well at least on my 'B) you can simply increase the volume of the engine. A bigger engine allows for wilder cams without losing grunt. The other way is to increase the amount of air that flows through the engine - in otherwords forced induction such as turbo or supercharging.

So throughout the whole cycle, a less restrictive air filter will flow air more freely into the engine. An induction pipe that holds more air than the volume of the engine will ensure you are not spending power from the engine having to draw air in throughout it's cycle. A port and polish will flow more air more freely into the cylinders. Better ignition such as more powerful coils, electronic timing, better HT leads and a spot on dizzy will provide peak ignition performance. Wilder cams will allow the valve timing to achieve optimium flow. A large bore exhaust system will then ensure the air gets out the back the quickest and a little louder

Finally, stroking the engine will allow more air to flow, and so will forced induction, making the effects of each of the above improvements more pronounced.

Just depends on how much cash you have!!!!

Paul J

This sounds wrong to me, forced induction allows in excess of 100% efficiency doesn't it???

Matt

richardpon, know your limits

Rick

Rich - that is complete bollocks

Rick

actually, its the biggest load of turd ive read in a long time!

Carlsworth

agreed....bollox.....


the cheapest way to make your engine more efficeint is by having a bigger engine.....

It's Czech Mate

Total horse shit

VE can only be quoted for a fixed Rpm

It varies at different rpm and peak VE usually happens at the same RPM as peak torque





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 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). An engine operating at 100% VE is ingesting its' total displacement every two crankshaft revolutions.







Forced induction can give way way over 100% VE

Anonymous

Sort of loosely accurate in a few places, but essentially the guy has really lost the fucking plot, well assuming he ever knew the plot to begin with which is unlikely!

foreigneRS

coincidence or magic? you decide

peak torque occurs at that rpm precisely because it is peak VE

Anonymous

Depends on how its mapped, if there is too much ignition retard or its too rich at the point of max VE then its NOT exactly the same as peak torque.

It's Czech Mate

coincidence or magic? you decide

[/quote]

Mapping - which I suppose can be described as magic

RichardPON

I didn't write it for god's sake Matt, you pleb!
I just knew something was wrong wtih what was written, but just wanted confirmation from far more knowledgeable people than me!

It's Czech Mate

I know you didnt write it you pleb

I can read

foreigneRS

wrong - physics

the highest VE will always give you the highest possible torque available (as it's when the most air and fuel are in the cylinders). whether the mapping takes advantage of it is another thing

It's Czech Mate

Cap in my ass, i clearly dont understand anything

You the man

GARETH T

who the hell wrote that tripe

Anonymous

Careful bit of rewording to now say "available"

Its still not actually 100% accurate though.

As if you have 0.1% higher VE at 5000rpm than at 2000rpm, or vice versa, where do you think will make the most torque in each case, would it change with the change in VE? Or would it be at the same point in both despite the different peak VE points?
(not a trick, question, think about things like heat relative to rpm, and piston speed and its effect on resistance, and also think about the flame front speed versus the speed of the engine and the effect that has on peak cylinder pressure and hence Det, then you need to consider the different amount of time for air to leak past the valves and rings.


When you have done all that answer these questions:

5000rpm 78.1% / 2000rpm 78% = which is going to give better torque
5000rpm 78% / 2000rpm 78.1% = which is going to give better torque


If you dont answer:
5000
2000

Then you have just given yourself a

foreigneRS

haven't fully read all that yet chip, but how can 78% be peak VE when you have 78.1% somewhere else?

gotta go and do a test now. will come back to it later

Anonymous

I havent said it would? (or not intentionally, LOL)


Read fully when you get more time

RichardPON

Except that was directed at the other "Matt" and not you, so learn to read Matt, you pleb

Matt

the other matt knew you didnt write it also

i was just making reference to your sig pic that i saw for the first time, thats all









you pleb

RichardPON

Oh yeah!

Several of these for me!

Matt

It's Czech Mate

afpmsl

foreigneRS

that's how i meant it in the first place, even if i didn't say it. we are talking theoretical things here after all

nothing is in the real world

in my experience of reciprocal things that pump (A/C Compressors), you tend to have a VE curve that has a very definite peak to it. i.e. it is rising with rpm until a certain rpm when it begins to tail off like the shape shown below (ignore the text on it, i haven't read that article at all, i just google searched for a VE curve to get the general shape).



i can imagine the same being true of an engine. how are you going to have 2 places so far apart in the rev range that have a similar peak VE?

you can have 2 places that have similar VE either side of peak, in which case we can start looking at all the other points that you have made below that I still haven't properly read.

foreigneRS

have read it now.

the truth is, i don't know the answer to that one. i think that there's too many unkowns.

Anonymous

Same answer i would give to that question mate, anyone giving a different answer to that either knows a LOT more or a LOT less about engines

foreigneRS

do you think tha there are people who know a LOT more?