[Caezar75]

okay, a turbo charger compresses the intake air, and "boost" is equivilent to 1 CR per 6lbs of boost. on a turbo the static CR is like 7.4:1? right? (not sure) any way...being that the function of a turbo, wouldnt you get the same results from shaving the head and using flattops to get the 10:1 CR? no hassle of turbo?

thanks for the input

jon

 

[Mack]

You're absolutely right, Caezar. The thing is that some people think a boosted motor makes more power than N/A even when the pressure inside the cylinders are the same. That seems ridiculous to me. I think most people nowadays just like that "Psssshhhh" sound of the blow-off valve. Turbo is a status symbol. To it's credit though, it is easier to install a turbo than it is to disassemble your motor to raise the compression. But it's also more expensive. So basically N/A is the way to go. Pssssshhhhhh!!!

 

[Kevin 280/300TT]

Well, compressing the intake air and CR are really two different things. Yes, a turbo compresses the intake air, meaning there is more air (oxygen) in the intake charge, so any given fuel/air mix in the combustion chamber will burn with more energy released. The CR on the other hand, is the difference (ratio) in volume of the combustion chamber with the piston at bottom dead center (most volume) to the piston at top dead center (least volume). So, by shaving the head, you decrease the volume of the combustion chamber, increasing the compression ratio, which again results in more energy to move the piston and make power. Both methods increase energy, just depends on which road you want to follow. Hope this makes sense, it did in my head while typing it. Not sure its clear though.

Kevin

 

[Caezar75]

okay so then from what you said kevin it makes me think that a turbo would supply more power because of to CR being changed by additional air...leaving a larger volume of mixture at an equivilent CR...thus a slightly more powerfull engine


ideas?

 

[MarkZ]

All i know is i was watching EPSN last night.They had the tractor pulls on, some ol'e boy had a 1968 tractor with the original engine in it, made 3000 ponies.A couple of little add ons were 3 turbos the size of my tires.I think you could shave and compress all day and not make that kind of power!I really like the idea of turbocharing my 240 and think setup right it would fly.But as easy asthe l series motor is to work on i think i am going to go the bigger cam higher comp. route.My .02

Mark

 

[Jeff260z]

Well sorta kinda maybe, but not really.. long

You will not get the same results from raising the compression as turbo charging. Well you can get more power from both (turbo or increased compression ratio) but the reason this happens is different.
Why does increasing compression ratios increase HP? Not sure exactly. You aren't compressing more air you are just compressing the air you have in the cylinder into a smaller volume. This smaller volume may help produce a much more efficient combustion as the air and fuel are more tighly mixed.(spectulating)

A turbo on the other hand helps an engine breath better and increases the volumetric efficiency of the engine. Imagine a normally aspirated engine piston trying to pull enough air into the cylinder to fill it COMPLETELY. When the piston is moving slowly the cylinder fills up very easily. As the rpm increases the ability of the piston to fill up the cylinder goes down because the air must travel much faster and has much less time to enter the cylinder before the intake valve closes. Because of the resistance in the intake and the airs inertia the air will never fill the cylinder completely. This is where volumetric efficiency comes from.

Volumetric efficiency = actual volume of air in cylinder / volume of cylinder

Why turbos make power. They increase the volumetric efficiency. They allow more fuel to be added to the cylinder because more air can be added.

The piston in a turbo engine doesn't have to suck as hard to get air into the cylinder as the intake charge is pressurized and air is forced into the cylinder. Not only are you able to get that cylinder completely filled with air but you can fill it with air at a much higher pressure than a normally aspirated engine.

Atmospheric pressure is around 14.7 psi normally (sea level). When a normally aspirated engine sucks air in, the air pressure inside the cylinder at the end of the intake stroke will never equal atmospheric pressure. Why? Because the volumetric efficiency of the engine is not 100%. The cylinder will be filled with air because air is a gas but that air will not be at 14.7 psi but maybe 13 psi(speculating). So you end up with a cylinder with less air than it could have and the capacity to produce less power. New cars with 4 valves/cylinder will be able to get closer to the actual atmospheric pressure but they will never reach it without turbo charging.

Well it's way past my bed time. I didn't read this over so if it reads like crap and i mostly bull crap, well you get what you pay for and this was free.

Good night.
Jeffrey

 

[nismotony]

Kevin is right, to a point. CR and forced induction are two completely different issues.

The CR of an engine does not change no mater how much 'boost' you have. Example...
Non-turbo motor, the intake pressure is approx 14.5psia @ sea level and you compress it into a space that is 9 times smaller (9:1 CR)
Put 8 psi boost on the same engine and now you have 22.5psia and compress it into a space that is 9 times smaller (9:1 CR)

Cylinder pressures will increase, but the CR remains the same. What is boils down to is this...

Increasing the CR increases the efficency of an engine

Increasing the intake charge pressure increases the amount of Oxy in the cylinder so you can burn more fuel, hence more power. (Almost the same as increasing the displacement of the engine)

I hope this helps...

 

[Stingray280zx]

Right On

Well I should goto bed, but no. That actually made alot of sense, in fact that should be copied and pasted, in fact I will. That is the best way i ahve ever read to understand the turbo and NA. Wonderful absolutley wonderful. Hats off to you.

-Emir

PS- In fact I say In fact too much, facts are useless lol j/k

 

[New2Zs]

Re: Right On

This being true..for us N/A runners...would Ram Air help in any way? What if I took the intake to the air filter and moved it where air would be pushed in? Would this disrupt the sensors ?

 

[Tony D]

Tractor Pull Engines

They routinely run three-stage turbosystems. This is good, depending on the compressor maps, for 150 to 200+ PSI intake manifold pressure! They inject water interstage at several gallons per minute to remove the heat!

 

[Tony D]

Good, Nismotony!

Now you are getting to the crux of it. I will read further and see if anyone else picks up the INTEResting twist you introduce...
BTW, 14.5 psia is at the carb mouth, more likely, the engine is running several inches of mercury vacuum even wide open, leading to pumping losses....

 

[Tony D]

BINGO! Almost...

Jeff is absolutely correct, but everone so far has overlooked the most important aspect available with turbo setups.. Intercooling the charge allows packing even more air into the cylinder. With some systems better than "perfect" intercooling is possible, thereby allowing great efficency gains.
Turbos have been referred to as "Variable Compression Ratio" devices. But more correctly, 6 pounds of intake manifold boost is more correctly comparable to 13:1 N/A compression ratio.
Turbocharging gets the advantages by completely filling the cylinder before the second stage of compression begins. This allows drastically more HP to be produced without revving the engine to stratospheric levels. You can get 300HP from a turbo motor at 5500 rpm, but no matter HOW you increase the C.R., you can't get that on a 2.8 liter motor.

But I've been drinking and have begun to ramble...

 

[Tony D]

"Ram Air"

Is passive supercharging using aerodynamic principles to increase the pressure within the intake manifold, making a N/A engine have an increase in volumetric effencicy.
Ram Air is very hard to do and mostly useless at speeds below 100 mph.
Cowl Induction, on the other hand can help at speeds as low as 30mph if set up correctly. Search the archives on "Ram Air" And "Cowl Induction" and you will find recent discussions of this topic. Real Recent....

 

[jeffp]

One thing to bring to light also is what Corky Bell mentioned in his book regarding the way a turbo helps out engine longevity. With the turbo application you also are aiding the movement of the piston on the intake stroke by forcing air into the cylinder in the rod bolt area of the connecting rod. The air being forced into the cyllinder helps the stress applied to the rod bolts at the point of 0 piston movement. This is a significant consideration when you start building high compression applications.