Discussion Forum - Northstar Chevelle Club

intake manifold reversion

From link below.

Two-plane intake manifolds tend to provide less plenum "damping" of reversion pulses than single-plane versions. In fact, when an engine is fitted with a fuel injection system for which there is no union of runner stacks through a common air chamber that helps dampen such pulses (effectively a plenum function), the problem can be even more acute. If nothing else, the condition further verifies the fact intake flow is bi-directional, under certain circumstances.


www.circletrack.com/enginetech/ctrp_1008_intake_reversion/

So....I'm right then..There just aren't really any dual plane intakes used on EFI? I want to run my single plane and EZ EFI on the 215.

it's all good..I gave myself a mister misty headache yesterday with Dynamic Compression Ratio calculators and cam profiles.

From how I understand it, Tuned port and LS-era engines are all about air volume which is measured and combined with a determined/matching fuel spray directly into the end of the intake runner-slash-top of the chamber. EFI specific manifolds account for this better and are designed without concern for any fuel suspended in that airflow. They measure the airflow and spray the directed amount of fuel at the directed moment. My manifold, for example, is basically a big open plenum up top, but a dual quad base underneath which gives me a lot of volume to draw from. The Ram Jet version (more noticeable on the BBC or LS engine) uses a smaller main plenum but longer runners like a tunnel ram.

For a Throttle Body type unit, you're essentially replacing the carb's metering ability with a more refined and hopefully more accurate system. Once combined it still needs to follow the best path in it's combined form. This is where manifold choice is more specific. You still want to choose to match your needs. I'd stick with the dual plane if that's the plan with a carb'd engine.

Another reason you see mostly single plane is that people think EFI will add gobs of horsepower, so they do the single plane manifold to match it. I don't think that replacing a correctly sized and tuned carb with EFI will gain you more than about 5%, EFI is more about not having to constantly find that optimum tune for different situations.

Remember, this is all based on naturally aspirated engines where the engine is what draws the airflow in.

or maybe I'm wrong......?

 

These things are happening freaky quick.

First with cam overlap, the exhaust and intake valve are both open for a brief instant but the exhaust is higher pressure and creates a pulse that flows into the intake (BAD). (that's the first spike in the graph)

Second is exhaust valve is closed and intake valve in opening, drawing in the air / fuel (that the blue line) (good). But now you have a mix of exhaust (not combustible) taking up part of the volume that you want to be all air / fuel.

Third is when the intake valve slams shut, the flow into the cylinder stops and stalls in movement. (smaller spike on end of graph)

All these pulses create a pressure waves that bounces around inside the intake runners and common plenum (Imagine reversion as water ripples bouncing around in a shaped pool.)

If the length and design is optimized, all these pulses bounces back at a resonator frequency in certain range of rpms (creates harmonics) that helps limit the exhaust reversion and push the intake charge into a cylinder (does not have to be the same one the valve created).

Dual plane intake manifolds also have longer runners which help minimize influence of these pressure waves, which screw up your fuel metering signal (vacuum carb). Large open single plane, not so much.

EFI is less sensitive as fuel metering is not as affected by the pulses.

The reason for a dual-plane is to carry both fuel and air. It's a whole different deal than carrying air alone. That's why dedicated EFI intake manifolds look so different than traditional carb intakes. It's also why throttle body injection intakes looked a whole lot like carb intakes! LOL! They were pretty much doing the same thing with the fuel introduced so far away from the intake valve. Curves and turns are different- lengths are still important, but not as much. Think how long the runners were on the early TPI intakes, and compare that with the later-model LS intakes. Similar displacement engines (305/350 for TPI, 4.8/5.3/5.7 for LS).

Spraying the fuel directly at the intake valve takes a lot of the work out of it for the intake. But it still has to deliver air at the best-possible velocity and it prefers a straight shot. For low rpm that still means a relatively long run in line with the port (think LS again). But at higher rpm the air has time to straighten out since it's moving faster, so a shorter port does fine. There are actually some intakes out there where the runner length changes with rpm to maximize this effect- some of them since the '80s, and most of them on imports.

It's all about maximizing the airflow into the port to give that great swirl motion that results in high efficiency. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased. This all changes with rpm, so the intake is always a compromise, like the camshaft. This is also why variable valve technology is also becoming a standard feature in modern engines.

It will all change again (however slightly) with the advent of direct injection. Without the valve to fight past, and by entering the combustion chamber in an absolutely consistent manner, direct injection promises even greater efficiencies if it's maximized. I don't know if it'll ever be retroactively engineered to older engines like ours, though.

Every paycheck puts me a little bit closer.. Its painfully slow though with no vacation till November..

Tony that intake will work just fine- you'd probably see some benefit from a spacer under the TB,and a turtle on the floor under it. I would bet a four hole spacer would wok best, but you'd be wise to experiment with all three types (open, four hole, and tapered four hole to open) to see which works best on the Dyno. Having distance between the TB and the valve will help the bottom end, and power should go up throughout the rpm range with a spacer in place. I'd run as tall a spacer as your hood will allow.

-- Edited by Scott Parkhurst on Sunday 27th of July 2014 06:55:06 PM