Throttle placement advice

[Xnke]

SO the Z is getting supercharged; initially with an Eaton M62, and eventually with either a TVS1320 or an Eaton M90, depending on funds and availability.

 

Yes, I am going to use the M62 first. I have one in as-new condition, with the rotor timing revised and the blower case ported, and it is MUCH easier to fit up to the engine. The M90 is a little bulky for the passenger side of the engine bay; mounted above the alternator and along the valve cover. Some modifications will be needed...And I don't currently have one, even a poor condition one.

 

So far the plan is to mount the super to the passenger side of the engine bay, above the alternator, similar in position to the power steering pump on power-steering equipped cars. This does make for problematic spark plug and valve lash maintenance; but those are not frequent and removing the super will be setup to be a rather simple job, if a little more annoying than just not having it there.

 

Here's the dilemma: The super originally had the throttle plate on the inlet of the supercharger, and blew down through an intake manifold. I will be blowing through an air-to-air intercooler prior to entering the engine; so the setup will be much more similar to a centrifugal supercharger setup than a typical roots-style blower setup. I know that runner length is dependant on the position of the throttle in relation to the back of the intake valve, and I also know that positive displacement blowers have a reputation for bending throttle plates in blow through applications.

 

If a large blowoff valve is used, would it be preferred to keep the twin 50mm throttle plates where they are now, about 7.5" from the manifold mounting flange, or is relocating to a single 70MM throttle body ahead of the supercharger the only way to go? (roots blowers loose efficiency REALLY fast as the inlet to the supercharger is restricted, so a larger than normal throttle plate is acceptable here.) This blower does have a vacuum/boost operated bypass valve installed already, integral to the housing.

 

With the long intake path that would result from having the throttle plate ahead of the supercharger, I would expect silly levels of low-end torque, but having the intercooler in the way changes things, I'd think. Also, the total charge path would be 12"+32"+24"+18"or a total intake-valve-to-blower-outlet of 86 inches. Not sure how you'd figure the length from the throttle plate to the blower outlet; mean distance would be 13.5", but how do you figure in the rotor's disruption of the air?

 

Bueller? Bueller?

[ttodhunter]

I've got an M90 in the garage that, someday, I want on my Z. A while ago, I came across the Magnuson bypass and bookmarked the page, which has since been lost. I found the description out there still:

 

"The best kept secret in forced induction is the little known bypass valve. This small valve, when properly installed between the supercharger and the air throttle body, allows the supercharger to become extremely efficient in terms of economy and parasitic power loss. Our M90 supercharger uses less than 1/3 of 1 HP at 60 MPH cruising. The bypass is operated by a vacuum actuator control unit that is normally closed. When vacuum is high (idle-cruising) the actuator opens the bypass valve, equalizing the vacuum pressure throughout the system. When boost is required (accelerating) the vacuum is decreased and the bypass valve instantly closes, causing pressure to increase into the cylinders. This equalized vacuum condition virtually eliminates the normal parasitic power loss of a forced induction system."

"Magnuson Products, Inc. Standard Bypass was designed for those applications where space limitations or plenum/manifold shape or other considerations dictate a remote bypass. The four bolt flange should be mounted on the inlet tube or casting and the 1 1/2†tube should be plumbed to the discharge side of the supercharger."

 

This might be what you're looking for, maybe not... Either way, I'll be following the topic! BTW, what year is your Z?

[Xnke]

That's the best part about the supers used on the GM 3800...the bypass valve is integrated into the blower housing...I've already got that exact valve. (By the way Magnuson doesn't make any of the blower stuff...it's all OEM'd by Eaton. They just make the manifolds for the specific engines.)

 

The M90 you have...what is it from? generic application or pulled from a factory production car?

 

My Z is a '72, mostly.

[Six_Shooter]

You're thinking about that runner length a little too much in relation to the throttle body.

 

You're going to use a stock (style) EFI intake manifold, correct? If so, the runners that have an effect on the torque curve will still be the same length as stock, about 7.5" long each, including the port in the head. Look into the Helmholtz Resonance (and there's another property people sometimes cite, but is not coming to me right now). The air chamber of the plenum has an effect on how the resonance in the runners work, not the distance to the throttle body.

 

Another way to look at it, is ITBs, or triples that are so popular, the throttle plate is right near the head, but the runner length is measured out to the end of the "air horn." In theory, when the throttle is all the way open, the throttle plate should have no effect on the air flow. In reality, there is slight turbulence around the throttle plate and shaft.

 

Back to the problem at hand, it is recommended to keep the throttle body at the intake manifold plenum, when an intercooler is being used. There are a few reasons for this, throttle response will be increased, both when opening and closing the the throttle. There will be a large volume of air between the supercharger inlet, and the intake valve. If this is behind a throttle plate, it will be under vacuum when not at WOT, and will take more time to become pressurized, than if it was already at atmospheric pressure, it will also take longer for the engine to exhaust the volume of air, when the throttle is snapped shut, causing a slow rev down to happen.

 

There is also the issue that there are couplers between the super, the IC and the IC to the intake manifold. These can, blow off, or tear, or just simply fail, especially when they get a bit older. If this happens, you have ZERO throttle control, and the engine will rev as high as it can, it takes very little throttle opening to rev an engine when not under load. It is for this reason alone, I would place the throttle body at the intake manifold plenum inlet.

 

I've thought about using a pre-compressor throttle body, there are reasons to do this, one being that it eliminates the need for a blow off valve, but only when a conventional air to air intercooler is not used.

[Xnke]

No, I built myself a custom twin throttle body manifold based off the large-diameter SU carb manifolds grafted to an EFI flange. There is very little plenum volume with this setup, the runners are longer and have a pronounced taper from 50mm down to 38mm, and there is no "plenum" persay, behind the throttle plate. Very similar to an ITB style manifold, from a tuning standpoint.

 

All in all, I think I'll leave both throttle plates as they are and run a blow-through style setup.