<VV> was Conversion to EFI, now intake velocity etc.

[Bruce Schug]

On Jun 16, 2004, at 2:58 PM, NicolCS@aol.com wrote:

> Your comments are interesting too.  I think both the valves and the
> dual exhaust promote better re-filling of the cylinder (=more power)
> and do provide higher hp even at these lower rpms.  Of course gains
> are proportionately less since the target rpm is lower, but still
> useful.  Why would bigger valves reduce the overall velocity?
> I believe they would promote more airmass entering the cylinder and
> thus higher runner velocity. 
>  
> Your comments about the secondaries and decreased responsiveness with
> the 95 hp cam are exactly what I expected too.  Oddly, this 140 has
> much more noticeable response to the secondaries than any 140 I can
> remember (I've had 20+ over the years).  With my totally stock and
> happy 65 Monza/Corsa 140/3.55, when the secondaries come on you can
> tell but not nearly as much as with the '67 140/3.08. (Go figure.)  My
> theory is that the secondaries in the '65 come in well below the cam's
> efficient range where the '67's secondaries come in right around where
> the retarded 889 cam is really happy.  (This is one reason why many
> 140s cough when the secondaries open) I don't claim to fully
> understand why, but all who have experienced the car really notice the
> feeling of power.  I think that because it makes more power at
> lower/normal driving rpms, it just "feels" more powerful. There's no
> question that the secondaries impact is more noticeable than a regular
> 140. 
>   
> I'm having fun!  This is making me think. (ooh, pain!)
> Craig Nicol

Obviously 140 valves (and ports) and dual exhausts fill the cylinders
better but not until you have enough rpm to need them. The question is,
where will you reach this point, that is, at what rpm? I think it's
around 2,500-2,800 rpm. That's where 140's make more hp than 110's. Of
course, the extra carbs have something to do with this too. At economy
run speeds, (probably not exceeding 3,000 rpm with most of the time
spent closer to 1,500-2,500) I'll take a 110 port and valve.

The big valves, accompanied by bigger ports, reduces incoming charge
velocity because there is simply more area in the port - and the hole
(port opening which means valve diameter) is bigger. Think of it this
way: If you had a 1" pipe and hooked it up to an air pump (your engine)
which was providing a given volume of air or a given pressure of air
(like atmospheric pressure) the velocity of air coming in the pipe
would be X.

Now, let's say we hook up a larger 1 1/4" pipe (bigger port and valve).
What will happen to the velocity of the air coming in the pipe? It will
go down at any given rpm to less than X. This is why smaller-valved
engines with smaller ports are better at lower rpm. They maintain
higher charge velocity. This is why big valved engines with big ports
are better at high rpm's, they will flow more and make more horsepower,
but not as good at low rpm because the velocity of the incoming charge
is slower. In extreme cases, the incoming charge will not atomize the
air-fuel mixture well. You can get fuel collecting in the manifold
(puddling). Exactly where these conditions of poor atomization and poor
charge-speed occur will vary with engine design.

Remember the old Carter AFB? It has small primaries and large
secondaries. Why? The small primaries are better for lower-speed
driving; the big secondaries make more horsepower at higher rpm's.

I don't know the diameter of a 140 intake valve, maybe 1.6" or so. What
would it be like if you put a set of 2" valves in a Corvair engine?
Remember, many small block Chevy's use 2" valves. The SBC has maybe 350
cubic inches, the Corvair 164. Think of how this would affect incoming
charge velocity. How do you think a Corvair would run with these big
valves? Worse at low rpm's, better at high rpm's. But the Corvair
probably would never rev enough to NEED a 2" valve.

Regarding where secondaries "come in", remember, 140 secondaries are
mechanical. They don't open with an air-valve which opens some 4-barrel
secondaries when incoming velocity warrants it (like an AVS carb). 140
secondaries open when you open the throttle enough to open them. If you
floor the throttle, they open. Now there is an issue with when the
POWER comes in. This is determined by many factors, principally cam
design. With a 310 racing cam, you'll get "on the curve" at higher rpm,
maybe 4,000 rpm. With a mild cam, more like 2,500-3,000. If the engine
bogs when floored, it means there's too much carburetor opening - the
charge velocity is too slow. As soon as the rpm's pick up a bit it
TAKES off as it gets "on the curve". You've just experienced the fun of
owning a 140!

Bruce

Bruce W, Schug
CORSA South Carolina
Greenville, SC
bwschug@charter.net

CORSA member since 1981

'67 Monza. "67AC140"