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Thread: Pushrod vs OHC Reply to Thread
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  Topic Review (Newest First)
10-28-2005 11:56 AM
nedro
Re: Pushrod vs OHC

Quote:
Originally Posted by luvtolean
Update to my first post, the S has over 10k miles now, and is getting around 30MPG commuting. When driven hard it is still well above 25 MPG. It's broken in quite nicely.
I think this speaks volumes. You can't drive a vette hard and expect anything close to that.
10-28-2005 11:41 AM
luvtolean
Re: Pushrod vs OHC

Update to my first post, the S has over 10k miles now, and is getting around 30MPG commuting. When driven hard it is still well above 25 MPG. It's broken in quite nicely.



10-28-2005 11:34 AM
nedro
Re: Pushrod vs OHC

A real world comparison would be, how many MPG do you get per tankful. Sure you can squeeze 33 MPG on a freeway with a vette, But what was the MPG while you were accelerating to get there. I would suppose that the 4cyl OHC would never get close to the gas consumption of the big V-8 while accelerating. And unless you are on a long trip or drive like somebody's father, the 4cyl will end up with better fuel economy in the real world.
10-28-2005 11:14 AM
luvtolean
Re: Pushrod vs OHC

Dan, good luck with the wrenching.



10-27-2005 10:00 PM
dan henderson
Re: Pushrod vs OHC

Quote:
Originally Posted by luvtolean
Now, if you want to argue 4V allows better filling, that's fine. But that still does not demonstrate to me how a pushrod (negatively) effects an engine's volumetric efficiency.
did i say it negatively affected the VE? no, i did not......now while you may have educated many people on the board here, do you have some information that i already may not know? btw atmospheric pressure is usually around 14.7 psi at sea level, and naturally aspirated engines actually have a lower VE at a higher altitude, however, a forced induction type setup is not as much affected at a higher altitude, would you like to explain why that is? there is no debate on if a pushrod type setup is a "negative" effect, it is simply just another way of controlling the A/F mixture that enters the combustion chamber....again, IN THEORY, a multiple valve type setup(usually found on OHC engines), will allow for more valve surface area(4 smaller valves equalling more than 2 large ones), in turn will allow for more air/fuel to enter the C.C.....just two different ways of controlling the burn.......along with many other factors and designs.........now what exactly were you trying to prove?
10-27-2005 8:31 PM
Skully
Re: Pushrod vs OHC

Quote:
Originally Posted by luvtolean
Dan please, if you have some information, don't simplify.

But let's see if I can help you.

Volumetric efficiency is a number that gives the theoretical efficiency, vs actual efficiency of an engine's "breathing" as a percentage. The theoretical efficiency is based on the swept volume of the engine's cylinders and the volume of air they'd contain at 1 ATM. The old wive's tale, an otto cycle engine is just an "air pump".

An engine's peak volumetric efficiency is normally at or very close to it's torque peak. MOST naturally aspirated engines never get to 100%. If you look at the runners on one of your maligned FI V8's intake manifolds, you might notice there are long runners there. This is to tune the reversion pulse in the intake charge created when the intake valve snaps shut. The magic number is 3. You want the wave to travel up and down the runner and on the 3rd time, for the intake valve to open just when that wave gets there. Then you get the most efficient filling of a naturally aspirated engine. A ram effect using the energy of the reversion+ gravitational effects on the charge. Very cleverly designed engines, with highly tuned cam events/intake/exhaust combos can actually get over 100%. Believe it or not, this can be achieved with good old cam in the block, 2V heads.

The best and easiest way to improve VE is to add a turbo or supercharger, or nitrous. Then you get much more O2 in the cylinder than 1 ATM of pressure can provide. If you have someone tell you they are pushing 9 PSI with their turbo, what they really mean is 9 PSI over atmospheric pressure. Obviously VE is much over 100%. Ever seen a turbo/super/N2O pushrod motor?

Now, if you want to argue 4V allows better filling, that's fine. But that still does not demonstrate to me how a pushrod (negatively) effects an engine's volumetric efficiency.
Damn I didn't know we had Kevin Cameron on the board.
10-27-2005 1:33 PM
CBR929RE
Re: Pushrod vs OHC

Quote:
Originally Posted by SemperSquidly
Good post LTL. After reading that, I'm no engineer, but I do feel like I stayed at a Holiday Inn Express last night.
haha me too. maybe we stayed across the hall from each other.
10-27-2005 1:07 PM
SemperSquidly
Re: Pushrod vs OHC

Good post LTL. After reading that, I'm no engineer, but I do feel like I stayed at a Holiday Inn Express last night.
10-27-2005 2:00 AM
luvtolean
Re: Pushrod vs OHC

Dan please, if you have some information, don't simplify.

But let's see if I can help you.

Volumetric efficiency is a number that gives the theoretical efficiency, vs actual efficiency of an engine's "breathing" as a percentage. The theoretical efficiency is based on the swept volume of the engine's cylinders and the volume of air they'd contain at 1 ATM. The old wive's tale, an otto cycle engine is just an "air pump".

An engine's peak volumetric efficiency is normally at or very close to it's torque peak. MOST naturally aspirated engines never get to 100%. If you look at the runners on one of your maligned FI V8's intake manifolds, you might notice there are long runners there. This is to tune the reversion pulse in the intake charge created when the intake valve snaps shut. The magic number is 3. You want the wave to travel up and down the runner and on the 3rd time, for the intake valve to open just when that wave gets there. Then you get the most efficient filling of a naturally aspirated engine. A ram effect using the energy of the reversion+ gravitational effects on the charge. Very cleverly designed engines, with highly tuned cam events/intake/exhaust combos can actually get over 100%. Believe it or not, this can be achieved with good old cam in the block, 2V heads.

The best and easiest way to improve VE is to add a turbo or supercharger, or nitrous. Then you get much more O2 in the cylinder than 1 ATM of pressure can provide. If you have someone tell you they are pushing 9 PSI with their turbo, what they really mean is 9 PSI over atmospheric pressure. Obviously VE is much over 100%. Ever seen a turbo/super/N2O pushrod motor?

Now, if you want to argue 4V allows better filling, that's fine. But that still does not demonstrate to me how a pushrod (negatively) effects an engine's volumetric efficiency.



10-27-2005 12:25 AM
phobiaphobe
Re: Pushrod vs OHC

That's all well and good but there is such thing as a 4-valve pushrod motor, so the point is moot.
10-27-2005 12:21 AM
dan henderson
Re: Pushrod vs OHC

the theory that multiple valves yeild more surface area than single valves, ex. being a mitsubishi 4g63 2.0L vs any small block chevy....do the math on the valve sizes(dont have the figures on hand) but they will outflow any almost any small block even with the big valves 2.02/1.?? .....the mitsu being somewhere(like i said dont have my figures right on hand) in the area of 2.55" on the intake and 2.0?" on the exhaust..........if youd like to get into this more in depth id be glad to bring up some numbers im an ASE certified auto tech with an AA degree in automotive technology/chassis fabrication/high performance engines
10-26-2005 10:10 PM
luvtolean
Re: Pushrod vs OHC

How? By what theory?



10-26-2005 10:03 PM
dan henderson
Re: Pushrod vs OHC

Quote:
Originally Posted by luvtolean
What does VE have to do with what is actuating the valves?

the thread turned into a "which engine is better argument"...........and in theory, it is easier to achieve VE with an OHC multiple valve type setup........in theory......
10-25-2005 12:38 AM
luvtolean
Re: Pushrod vs OHC

Also variable lift/duration pushrod motors.

And talking about electric solenoid valvetrains doesn't count for my VE question.



10-25-2005 12:35 AM
phobiaphobe
Re: Pushrod vs OHC

BTW there are such things as 4 valve pushrod motors.
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