My latest dumb efi question

[Quickstep192]

Lately my daydreams have turned once again to EFI. One of the barriers for me has been ITBs.

While searching for “Porsche EFI throttle body”, I stumbled on the attached.
Is there a reason that ITBs are preferred for earlier models like mine over a throttle body like the one in the picture?

Sorry if this is a dumb question, but sometimes you just don’t know what you don’t know.

[patkeefe]

I would imagine it is an airflow efficiency gain with ITB's over the single throttle body manifold. It is also likely optimization which is beyond the cost efficiencies of mass production.

[tirwin]

Good explanation here:

https://youtu.be/uS3yHPfT9I8

In a nutshell, you’re getting air into the pistons very efficiently like Pat said.

[scottrx7tt]

But, you have to wonder why almost every auto manufacturer in the last 30 years either use a single throttle body, or a 2 throttle body system, one for each bank. Wondering if if really makes a HUGE difference?

[pampadori]

ITBs: sound, looks, increased access to top of engine, throttle response.
Single throttle: Easier to include idle air cont valve, easier to tune part throttle (itbs require syncing which often isn't perfectly sync'd)

You can run a plenum on itbs too but most folks don't. Clay.O on here has one of the best looking ITB setups where he included GT3 plenums. Looks killer!

[tirwin]

Originally Posted by scottrx7tt

But, you have to wonder why almost every auto manufacturer in the last 30 years either use a single throttle body, or a 2 throttle body system, one for each bank. Wondering if if really makes a HUGE difference?

BMW has used ITBs on several cars.

It’s most likely because of cost and reliability concerns for cars that are mass-produced. ITBs have more parts versus a single throttle body so less points of failure and cheaper to produce.

It does make a huge difference in throttle response.

I’m guessing there are some fluid dynamics differences too. With ITBs each throttle body can be isolated. With a common throttle body you have valves opening at different cylinders that probably creates some turbulence in airflow as you have all cylinders under a partial vacuum at all times that changes as the common throttle body opens and closes. I guess the effect varies depending on design.

[Gretz]

As someone who would like my engine to start like a modern motor, I value the idle air control valve. I know some folks have options for ITBs, but I'm not against a single plenum as shown above. There's a reason a large number of Singers use a single plenum / throttle body.

[tirwin]

Originally Posted by Gretz

As someone who would like my engine to start like a modern motor, I value the idle air control valve. I know some folks have options for ITBs, but I'm not against a single plenum as shown above. There's a reason a large number of Singers use a single plenum / throttle body.

I don't think the Singers use a single throttle body, do they? Pretty sure all the ones I've seen have ITBs.

I was under the impression they use a common plenum in conjunction with ITBs. You can still have a plenum with 6 individual valves.

If you look at the pictures in the Rasant setup, I think this is more like what the Singers use.

https://rasantproducts.com/rasant-products-intake-system-is-6-individual-throttle-bodies-964/

[scarceller]

ITBs tend to be tuned for particular RPM range and it's usually very high up in the revs 6500-8000RPMs but the common plenum in the 3.2L was designed for 5800RPMs and helps create a nice flat torque curve in the 4800-6200 RPM range.

Then more modern common plenum like the 993 can achieve multiple tune points via flaps and moving chambers in the plenum. This helps tune torque for more than 1 RPM point.

Even the 964 plenum has 2 tune points 5800RPM and again at 6400RPMs, the resonance flap in this plenum allows the 964 Engine to take a 2nd breath of air in the 6400RPM point and it's worth 10-15 ft/lb of torque at 6400RPMs!

And then you have cost and simplicity of common plenum, only 1 throttle body and such.

Emissions are also a bit easier with common plenum.

And ITBs need significant valve overlap to really be useful but valve overlap fails emissions!

[Gretz]

Originally Posted by tirwin

I don't think the Singers use a single throttle body, do they? Pretty sure all the ones I've seen have ITBs.

I stand corrected. I thought I remembered seeing several that used a single plenum with single throttle body, but indeed upon review of a number of Singer photos it looks like they do use ITBs.

Any idea what they use for idle air control?

[tirwin]

Originally Posted by Gretz

Any idea what they use for idle air control?

Not sure. I have seen a few of them in person and I snapped a bunch of pics of the engines close up. Let me go back and look and see if I can tell. I know they use AEM Infinity ECUs.

['76 911S 3.0]

ITB's also need to be sync'd just as carbs need to be, the airflow into each one at various throttle inputs needs to be balanced across all cylinders. The common plenum, single TB eliminates the need for this and all the complexity involved, hence why many OE's opt for single TB common plenums.

If you have a common vacuum manifold, you can run a PWM idle valve into the manifold for your idle control. Mind you this will not be as effective as it would be on a common plenum, but for things like cold start, it will be sufficient. The trick with ITBs is to set the blade base position to be what gets you the target WARM idle RPM. Use the idle valve to bring the idle up on cold start and once warm, the idle valve more or less is removed from the picture.