Bosch Motronic Alpha-N question

[Mud_]

227bhp said:

This doesn't make sense, unless you've piped off the signal from in between the butterflies and the head to somewhere else?
Is it forced induction?

I made a vacuum plenum and put the MAP sensor on it (behind a very small choke to help stabilise the signal) - point being there are tables other than VE that reference the MAP, like DFCO and adaptive spark. It's just NA.

[Mud_]

Dammit said:

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?

Those look a lot less subject to wonk than these: https://www.jenvey.co.uk/products2/throttle-body-k...

I didn't get very far with mine as I had to throw the old intake back on for an event, but the change in throttle response even just blipping the throttle or driving around my garden was orders of magnitude apart...and it sounded so much better even at low revs, I think in part because the metal intake dampens down the valve train noise a bit, and the IAC passage on the old TB roared horribly.

[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.

[anonymous-user]

Dammit said:

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.

Whilst your on the right track, you are over-emphasising the losses!

In my example above we used a 1kPa pressure loss as our design point. That is, roughly equivalent to a 1% loss in peak power, Ie a 300bhp engine is turned into a 297 bhp one. So, two things should be noted:

1) a loss of 3bhp is irrelevant for a road car, and even a lot of track cars (unless you're actually racing to win)

2) our design point did not include the pressure recover of the plenum. I mentioned it, but didn't include it in the final calcs. In reality, that 1kPa loss is more like just 0.3kPa for most plenum designs, where the throttle entry shape is reasonably decent, and hence most of the dynamic pressure can be recovered to static pressure. A 0.3kPa loss is 1bhp on a 300bhp engine!

I'm sure a bit of googling would turn up the throttle plate size on some later Porka motors that will give you an idea of the power supported by any given sized throttle!

For a road car, throttle are often using 80m/s as the design point in order to balance peak power against driveability at low throttle openings (especially if you're sticking to a basic cable operated arrangement)

[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.

[Mr2Mike]

Dammit said:

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.

A larger MAF housing won't give you any more resolution, it just increases the maximum airflow before the sensor saturates.

[Dammit]

Good point, I had remembered that wrong.

[anonymous-user]

Mr2Mike said:

Dammit said:

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.

A larger MAF housing won't give you any more resolution, it just increases the maximum airflow before the sensor saturates.

Indeed, it gives you LESS resolution....... (V/g goes down)

[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.

[Krikkit]

Could you not get an enlarged single throttle body from another source? Jenvey do some, but so do many manufacturers.

[Dammit]

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?

[dom9]

Dammit said:

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?

Aren't the Flat-6 engines flat plane/ 180° by their very nature thus don't need to interconnect the exhaust between banks?

My suspicion is that you'd be better off with twin pipes (which is what I ran on my trackday 996) rather than an X-Pipe.

I assume, however, the X-Pipe might make a nice noise, which is why the exist?

Interesting thread I've been following quietly.

[Dammit]

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

[227bhp]

Dammit said:

Or am I missing something obvious? (This is entirely possible).

Yes

You haven't quoted a test pressure, common is 10" and 28".

Hot air travels much faster than cool air and the pressure differential between a cylinder full of hot gas and the outside ambient temp is also huge so it will flow through a smaller orifice much quicker. One of the reasons ex valves are smaller than inlet valves.

A flow bench is a multi tool which you really know how to use to get the best of it, the figures it gives you are most definitely not the be all and end all of the whole picture, just pointers which need interpreting properly.

Edited by 227bhp on Tuesday 26th September 14:32

[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?

[227bhp]

Dammit said:

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?

It's called test pressure and is usually at 10 or 28" (but can be at anything you like) depending on how powerful the bench is. It's just like a unit of measurement, it's like you've given me 155 of something but not told me of what.
If whoever it was that did it knew what they were doing they will know what it was. As you can imagine it isn't hugely relevant until you come to compare something, say test A against test B or compare Petes figures with Freds - they all have to be tested at the same test pressure or there would be no point in comparing.