Custom Inlet Manifold Plenum Volume???

[Duk]

I’m looking at building a custom inlet manifold for my Alfa V6 and the 1 big question I have is in regard to plenum chamber volume.

Over the years I’ve read all sorts of theories about ‘the best’ plenum chamber volume, but the one that is probably the most realistic is a volume approximately the same as engine capacity (3 liters). Smaller than this, is very unlikely.

What effect does the volume have on the performance and torque characteristic of an engine?

Thoughts, opinions and experience please.

And before you tell me to bugger off to the Alfa forums, I ask here because there are heaps more thinking heads than there are BS artists. “Well because it’s an Alfa engine, the laws of physics don’t apply……” if you get my drift?

[abently]

Give Beninca a call. ITB's with their custom airbox is all you can hope for.

Your problem isn't airbox dimensions, its actually engine space constraints.

Only thing I would do different to Beninca, is have the Injectors side/top fed close to the actual Head ports entrance not the bellmouths entrance..................

[The Real Roadrunner]

Only thing I would do different to Beninca, is have the Injectors side/top fed close to the actual Head ports entrance not the bellmouths entrance..................

well that just goes to show you know sfa about tough engines.

the size of the chamber doesn't have as much effect as runner lengths and if you go too big with chamber size it can cause dramas when it returns to idle also (too much vacuum in a big space causing stalling issues)

if you gave a better description of what your trying to achieve i would go into more detail.

rpm? na or fi? street or race? space restrictions?

cheers
linden

[Duk]

Basically the standard manifold has really short runners, so I want to build a manifold with much longer runners to improve low rpm torque.

I'm trying to achieve runner length of about 45-50cm (from the valve). Standard runners are less than 20cm.

Engine is internally standard, with a low standard redline of 5800rpm, a short crank stroke. It's quite a contradictory engine, short stroke, short inlet runner length, big standard throttle body (70mm), pissy little airflow meter, low redline and not a great amount of top end.

1 day, it may have a nice long runner set of extractors, ported heads and cam upgrade and a programable computer (prolly Adaptronic).

Not trying to build anything racey and high revving, just more torquey and flexable. Gearing is also quite tall and closely spaced.

I take it linden, that you are familiar with Alfas? It's a 75 and space isn't great, but I reckon there is enough to place the plenum chamber above the drivers side head and achieve my desired runner length.

[abently]

The problem with trying to get the correct runner length, is that your cam timing alters the exact length you need from the theoretical value.

The best advice you will get, is just fit a late model (2002 onwards) Alfa engine in it and call it a day. If you still want a little more power, then just fit higher comp pistons and add a cam with more lift. I have a feeling the cost would end up the same except this is 3.2L of torque (not to mention more HP) and an all alloy block.

[Duk]

The problem with trying to get the correct runner length, is that your cam timing alters the exact length you need from the theoretical value.

The best advice you will get, is just fit a late model (2002 onwards) Alfa engine in it and call it a day. If you still want a little more power, then just fit higher comp pistons and add a cam with more lift. I have a feeling the cost would end up the same except this is 3.2L of torque (not to mention more HP) and an all alloy block.

Reality dictates otherwise (read: no cash for that sought of thing).
Have read that later model Alfa engines are quite poor when it comes to low rpm torque anyway.
They persisted with their short runner inlet manifold and maybe cams that were a bit too radical for street use. Either way, they were pretty ordinary just driving around town.

http://www.autospeed.com/cms/A_0774/article.html Alfa owners world wide hated this article, but looking at the dyno curve and it's real world performance is more realistic than looking at your car with rose coloured glass's on.

[SillyCarS]

The problem with trying to get the correct runner length, is that your cam timing alters the exact length you need from the theoretical value.

i realise this changes the duration of the time that the intake valve is open for, but ultimatley isn't the major concern getting the length of the runners so that their volume corresponds to the volume of the bore when the stroke is at BDC? I could be wrong.

Also, i think the general rule of thumb for plenum volume was 3 time the voulume of the engine? or is this in forced induction applications only linden?

take a look at what grimboy did for his old 1g that is now is inocents ra23, im pretty sure it was a 6L plenum, no idea abou the runners though

[Draven]

Also, i think the general rule of thumb for plenum volume was 3 time the voulume of the engine? or is this in forced induction applications only linden?

Jeebus, I can't think of many supra plenums that have a 9L capacity...

Before I go any further, I know 3/5ths of fuck all about alfas, but...

The engine you describe sounds like a bit of a POS, and probably a transplant would be money better spent?

You say you don't have the cash for that, but a custom plenum with runners (and probably ITBs) is not exactly a cheap bit of kit, for what is (relatively) minor gains. I could be completely wrong here, not knowing the car - just offering an alternate perspective

[abently]

That's correct not to mention he has said 'porting the head' & custom extractors.......

Remember the 2002 onwards Alfa V6 Engines are the same which power the 147 GTA (1400Kg) 0-100 in 6.3secs, not sure where you got the idea that it somehow has no torque..... as that time is comparable with other performance orientated factory v6 (note; V6 not straight 6) vehicles.

[Billzilla]

Just go for the same volume as the engine - Bigger is better, but only for outright power as you assmptotically approach the situation where the inlets start to behave like inlet trumpets out in free air.
But it's not a huge affect, just got for something about the volume of the engine and make it so that you keep the inlet pulses under control and the airflow goes to the ports evenly and it'll be fine.

[Duk]

Jeebus, I can't think of many supra plenums that have a 9L capacity...

Before I go any further, I know 3/5ths of fuck all about alfas, but...

The engine you describe sounds like a bit of a POS, and probably a transplant would be money better spent?

You say you don't have the cash for that, but a custom plenum with runners (and probably ITBs) is not exactly a cheap bit of kit, for what is (relatively) minor gains. I could be completely wrong here, not knowing the car - just offering an alternate perspective

LOL
No individual throttle bodies, like I said, I'm after low/mid range NA torque and it's all DIY. As for being a POS, compared to a 1/2jzgte or even a 1uzfe it could be called that, but it has it's charm
A conversion into an Alfa 75 sure as hell aint easy either. Even using later model Alfa engines is a fair wack of work. If I was gunna do a conversion, I'd look at a 350Z donk. Prolly too wide tho

[Duk]

That's correct not to mention he has said 'porting the head' & custom extractors.......

Remember the 2002 onwards Alfa V6 Engines are the same which power the 147 GTA (1400Kg) 0-100 in 6.3secs, not sure where you got the idea that it somehow has no torque..... as that time is comparable with other performance orientated factory v6 (note; V6 not straight 6) vehicles.

I never said it had no torque, every engine has torque otherwise it wouldn't even turn under it's own steam. I said that I read that they had poor low rpm torque

[The Real Roadrunner]

what bill says is correct just go for something close to engine capacity in NA form and not much bigger than 2x displacement for FI.

400mm long runners is a shitload of runner length and ive personally never had any engine that likes more than 300mm but hey as you say the alfa engine is ummm "different"

could always try a twin turbo maserati engine check the classifieds

cheers
linden