For anyone that's interested in continuing this discussion about TB sizes, here's some info I've dug up.

To convert vacuum to PSI is
(vac) X 0.4912

So if you have 2.036" hg vacuum at WOT,
That equates to a 1 PSI difference in air density.
1 PSI density makes about 7% power difference.

So if you have a 300hp engine that pulls 2" vacuum at WOT, it's theoretically losing enough air density in the intake to
kill off 21 horses.

Just like Ed Mayer explains, I do not believe you need any pressure drop or velocity through the TB or plenum.... so any vacuum there would just mean wasted power.

I don't believe that there is a significant change in air volume (CFM) through the TB by switching to a larger TB(though there is some increase)... even my mono-blade. I think the same volume of air moves through the plenum, and into the intake ports.... the density of that air will be different though. If you raise the air density ahead of the intake valve by 1 PSI, then I believe the greater pressure differential at the intake valve will cause more CFM to go by the intake valve and into the cylinder..... same volume in the cylinder, but denser charge.

The idea is to get more oxygen molecules into the chambers by increasing the air density.... going from vacuum to atmospheric pressure (14.696 PSI). The slightly increased air flow into the intake manifold would show up as a higher MAF reading because there is more mass moving through it. I will not be running a MAF though because of the restriction.

The question is always asked if it will hurt power to go to a bigger TB? I say no it shouldn't, as long as you do a little tuning to make up for the increased air density.
Just bolting on my monoblade without retuning would cause some pretty bad lean spots & rough throttle response.
I had just mentioned that one could DIY enlarge a stock throttle body for cheap.

Does anybody have any good arguments against my beliefs to discuss here?

 

You are correct. But your monoblade on an engine that can only draw lets say 650cmf will feed the engine at probably less than half throttle. So when you step halfway down on the throttle you will be supplying the engine with all the air that the engine can pull. So you would in effect be at full throttle at half pedal. That would make driving the car a little choppy. You can get away with a little more overill on a dry intake verses a wet intake but there is still a limit to maintain good driveability.

Hell, try it out and report back with the results, you never know till you try right.

 

<embed src="http://members.aol.com/n644ever/jimcarrey/ace1/alrighty.wav" hidden=true></embed>True.

 

I have a 52mm t/b on my stock cammed,stock head,stock exhaust manifold (at the time of the run)'89 Formula 350.

Why allow more air into the opening of the plenum when the rest is the stock choked parts? The TPI will only flow as much as the smallest port.

Get a bigger manifold,bigger runners,ported plenum, and THEN add the 52mm. Any gains from adding it first will not be realized until the smallest restriction is removed.

See my results below:

 

I didn't say that I was allowing more air into the plenum... but more like increasing the pressure (density) of the air in the plenum.
If say the intake valve was the most restrictive point (usually it is on stock heads), then the increased pressure differential between my plenum and the chamber would in fact increase the CFM (more air) going into the cylinder.... just like cranking the pressure valve up on a flow bench increases the flow. Given that information, your statement isn't correct.
You do not have to fix the worst restriction in order to make power gains by opening up other restrictions.... of course you get get best gains by fixing all of the restrictions. Just remember that the smallest point isn't totally responsible for all of the restriction.... restriction is cumulative.

 

This is exactly the point I was trying to make without the scientific explanation. You will see the largest gains by opening the worst restriction(s) but that doesn't mean that is the only way to make more power (hence the faulty/oversimplified "straw theory"). An engine is much more technical than that.

I will do a before and after test with a larger TB though in the next few months. Whatever the track results are is what I'll believe, not what the flow numbers are on a flow bench.

 

There are some things that merit a bebate, but this ain't one of them.

See ya.

 

My $.02 to add to this again is that Ed Maher and Jim85Iroc are right...different thread...same right answers

As for Formul8!!...Dude...change out your stock exhaust manifolds...I still think they are more of a restriction than the stock TB...why do you have a 52mm TB on your engine then if you have other, larger restrictions?

For that matter, what about the rest of your exhaust system...I mean S h i t, after the stock intake base and runners, isn't the stock exhaust manifolds the biggest restriction?

 

I bought the AS&M runners,52mm t/b, and ported plenum for $300 used. Then sold the stock stuff a months later for $200.

That is why I stuck the 52mm on. I just installed the headers a month ago and need to have the dual cats and custom y-pipe welded up. Since it is 10 degrees here, not much if a hurry to finish it.

Check out below what the car did run with the stock manifolds.

 

YES and if this was with a carburetor you'd have BIG problems because you'd need some pressure drop in the plenum to keep fuel suspended in the air and in some in vapor form. At zero vacuum most fuel drops out of suspension. You get zero vacuum for an instant even with stock TPI if you go to WOT at very low RPM.... but thankfully there's an easy way to make this work with fuel injection... you simply increase the acceleration enrichment or pump-shot until the lean spot is fixed.

With a carb you can increase the the pump shot as well, but it still squirts fuel into the common plenum area.... so without vacuum very little turns to vapor. With port injection it doesn't matter because fuel is sprayed into the open intake valve. This means you can get away with MUCH less pressure drop in the manifold. You just have to adjust the fuel delivery to match the mass of air entering the chambers...tuning.

 

The cutting continues...