[SVXRide]

Originally Posted by shotgunslade

Thinking about pressure drop on the intake system. The primary cause of pressure drop at high RPM is the pressure required to accelerate the air to a velocity of approximately 7000 fpm. This velocity is necessitated by the 3” diameter of the MAF and by what appears to be the 2.125” diameter of the two throats of the throttle body (don’t know the exact measurement), and by the 350 cfm of positive displacement pumping accomplishing by the engine. The velocity pressure (pressure required to accelerate air to a specific velocity) of 7000 fpm is 1.3 psi. The static pressure or friction losses along any reasonably aerodynamic pathway that is only about 2.5 ft. long are going to be small compared with this. So the strategy would be to avoid any unreasonable pressure drops.

Looking closely at the SVX intake system, I see 4 sources of unreasonable pressure drop. The first is the abrupt transition from the filter box to the throttle body. The second is the small radius of the two turns in the intake tube. The third is the corrugation of the intake tube. The fourth is the potentially unaerodynamic transition from the single intake tube to the two throats of the throttle body. I’ve never looked up inside that little plenum, but I’m guessing that there are no internal vanes.

The secret to good high velocity air flow is to get the air up to velocity smoothly, not abruptly and then to keep that velocity up and working for you. So, the transition from the filter box to the MAF should be a longer smoother transition from a rectangular filter to the round 3” diameter tube. Think long rectangular to round funnel. Then, the intake tube should be smoothed out and the radius of the bends should be made as long as possible. There’s no reason to increase the diameter of the tube as the air is already going 7000fpm and you want to keep it at that velocity, because the cross sectional area of the throttle body throats is approximately the same as the MAF. The intake tube should split, however with a good aerodynamic splitter and individual 2+” diameter tubes should run to each throat of the throttle body. These tubes could be very short, only an inch or so, but what is important is that the transition from a single 3” diameter tube to the two 2+” throats should be as smooth as possible.

I really don’t think we are getting any resonance effects upstream of the throttle body. I think the resonant benefits of velocity stacks and ram manifolds are pretty much limited to individual cylinder air pathways. In the combined intake pathway, we are looking at a pressure pulse frequency of 18,000 hz.

All this said, I would doubt that the static pressure drop across the current intake system is more than 1 psi, so we are talking about maybe dropping 0.5 psi of pressure drop. This decrease in pressure drop would increase the mass flow into the engine by about 3-4%. Not much, but maybe worth 6-8 hp.

'Slade,
So, you have your CFD model up and running yet? Here's what the intake manifold looks like where the TB bolts up (It's not this smooth in stock form )



Note: the EGR system feeds up both halves of the intake flow just inside the TB mounting flange (look at the lower part of the picture to see how the EGR porting runs)


I agree with you regarding the flow. I think everything you've noted is a reflection of engineering compromises that resulted from packaging the EG33 in a fixed engine bay volume. Move the MAF too far from the air box and you'll end up with a "trampoline" dynamic motion effect unless you add a support strut up to the MAF from the frame rail.

I've got air box pressure port measurements taken from a port above the air filter (engine side of the flow) at different speeds. Would this info help you in your modeling efforts? The only caveat is that I've got a true 4" diameter ram air system feeding the stock airbox with a "Green" filter in the air box.

-Bill