Bosch Motronic Alpha-N question

[Dammit]

Hi all, a question which may be very silly - I don't know enough to know, so please forgive me if this is the case.

Context: I'm having my 1998 Porsche 911 engine rebuilt, it's going from 3.4 litres to 3.7 and will be having some cams ground for the new capacity.

Lacking any sense of proportion I'm considering the Jenvey ITB kit for these engines - but that would mean working out how to manage air-metering.

I'll likely shelve this aspect if the only answer is Emerald/DTA/similar, as the time spent working out how to run the dashboard etc is probably extensive.

Hence my question - can the stock DME be modified/mapped to run happily with TPS/IAT instead of a MAF?

[Dammit]

I know that the DME can be mapped, and of course it will be mapped to suit the capacity increase, cams etc.

What I don't know is if the way in which it manages the engine can be changed - I suspect the answer is no, but I have found some reference to people successfully modifying Motronic ECU's to run Alpha-N in the BMW world, so thought I'd ask.

Being able to map the ECU ≠ being able to change from MAF to Alpha-N

[Dammit]

Or maybe it does, I don't know - hence asking.

[Dammit]

Could you give a bit of detail on why that's the case?

This may be obvious to you, but it's not obvious to me.

For e.g. I am uncertain why the physical properties of a device would change depending on who is using it.

[Dammit]

Would this lead to the highly fashionable "have cake and eat it" outcome of sharper throttle response with no loss of torque/changes to management+great noise?

[Dammit]

On a different note: vacuum.

This is required to run the brake booster, air-oil-separator, fuel pressure regulator and a bunch of smaller stuff.

If, for the sake of argument, one were to move from a single throttle body at the entrance to the plenum, and moved to individual throttle bodies at the end of each plenum runner this would (I am guessing) significantly reduce the level of vacuum in the intake/plenum upon which the various systems rely- potentially to a point that is too low.

?

[Dammit]

This is all bench racing at this point.

Background:
- I have a 1998 Porsche 996
- this is the cable throttle model year (more on this shortly)
- It's on 64,000 miles and in rude health
- I have no sense of proportion
- I'd originally planned on having it rebuilt at around 80,000 miles in order to fix the issues that Porsche didn't - bearing shells, IMS bearing, bore-ovality etc
- But then succumbed to a massive bought of "whilst we're in there" planning

Actual plan:
- I bought a couple of 3.4 litre heads from a breaker, they're both with a race-engine builder and head-porting specialist
- He's also got a complete intake, FVD-homage equal length exhaust manifolds, and all the gaskets that are needed to put everything together
- Once my spot in his queue rolls round he's going to put the heads on the flow bench and work out a) how they can be optimised and b) what the cam should look like bearing in mind what he has found, the new capacity of 3.7 litres, how many angels can fit on the head of a pin etc

Thing that caused me to start this thread:
- The engine designer mentioned that the current throttle may be too small
- This is usually resolved by simply banging a GT3 throttle body on
- But this won't work with mine - the cable version won't fit, the eGas versions won't work
- So I had a look on the Jenvey site and found their ITB setup for the 996 whilst looking for a larger single throttle body
- They do sound good

All of that said, the goal for the engine is to be a tractable, reliable, torquey unit that also gets nicely angry at higher revs - rather than a diesel style lump of torque then chokes, or a race engine that has nothing low down then it's armageddon from 5,000rpm upwards.

[Dammit]

These are the ones: http://www.jenvey.co.uk/products2/throttle-body-ki...



Interesting to hear of your experience with them.

A bit more context - I have a Volvo 850R that has a (much) larger MAF housing in order to give more resolution and therefore better control at higher revs. It's putting out around 360 bhp, and if fun - in a wait for it, wait for it, boom! type way.

But throttle response is not it's strongest suite - the 911 is a totally different story.

I in no way want to lose that crisp throttle response - if it could be improved then that would be awesome, but it can't lose the edge it already has.

I'm worried that a single large throttle body will take that edge off slightly - genuine concern, or not an issue?

[Dammit]

My maths is dreadful - forgive me if I make a laughable mistake with what follows.

The engine will be 3.7 litres, so will be ingesting 1.85 litres per crank revolution, which means that we have a rev ceiling of 4,800 rpm before we start to run into issues?

A throttle with a diameter of 80mm would have flow 200 litres/sec giving 6,500 rpm, so we need to go a bit further.

An 86mm throttle would stall (correct term?) at 7,400rpm, which sounds about right.

[Dammit]

The GT3 appears to have an 82mm throttle- that's the 3.6l I assume.

I'll see if I can find the size for the 3.8l.

[Dammit]

Good point, I had remembered that wrong.

[Dammit]

Ok, for now I think we'll shove the ITB genie back into its bottle.

Exhaust- the options are conventional 200 cell cats which run from the manifold to the silencer individually, and are therefore two separate units (one per bank) or an X-pipe arrangement, same cats.

Which of the two is better suited to a road car?

[Dammit]

Back to the throttle question as no one seems keen to proffer an opinion on the exhaust:

There are two throttles available for the gen 1 GT3:

Engine code M96.76, £1,600
Engine code M96.79, £483

Obviously that's quite a difference - and I'm struggling to find any data on which is which, would anyone know?

[Dammit]

Answering my own question it's the sixteen hundred quid one because of course it is.

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I should probably order a stock throttle body/plenum connector from a breakers and measure it- find out what other throttle bodies fit.

Anyone got some insight on that X pipe vs twin pipes q?

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Based on absolutely no technical understanding, but the ability to use Google on my phone whilst waiting for video-conferences to start it would appear that individual pipes are better from a breathing perspective, leading to more power, but that an X-pipe makes a nicer noise.

As I said, there's no technical knowledge behind this, just dyno-charts from a vendor on Rennlist.

[Dammit]

We now have some flow-bench data on the intake, head and exhaust manifold.

At 0.5" of lift with the 100mm bore (3.7) we get the following (what I'm going to describe as, knowing no better) path:

Intake* 247 CFM
Head (intake side) 277 CFM
Head (exhaust side) 197 CFM
Tubular manifold 168 CFM

So! Restrictions in both the intake and the exhaust, for the intake the only way forward that occurs (other than the ITB route) is the X51 alloy intake, but last time I tried to order that from Porsche they confirmed that it was available, but not when. On the exhaust side this was a huge surprise - it's an equal length, tubular manifold - similar to the FVD units. Does it simply need to have a larger diameter of primary? Or am I missing something obvious? (This is entirely possible).

• ie, all air entered through the 77.5mm plastic throttle body aperture complete with entry ring

[Dammit]

Ok, bear with me - this is a new area. Are you saying that comparing the figures that I posted (which are all at the same pressure drop) isn't helpful unless the PD is known? And that this is (assumption) because the PD is relevant/represents (in some ways) temperature and without knowing it we can't extrapolate?