Breathing choices - exhaust or manifold?

[Guest]

Hi again,

This is related to my earlier post on exhaust systems (BTW thanks for your advice - any more greatly appreciated). No real front-runners have emerged after looking into aftermarket kit as yet but I'll keep trying.

While talking to Hayward & Scott I got embroiled in a conversation about manifolds. They don't have a jig for 944 Luxes but would build one 'onto the car' for about £450+VAT.

I remember this used to make a decent power difference when I was into Minis. But BMC A-series engines weren't well designed from the start, whereas Porsche seem to get things (nearly) right first time. However, while browsing through lists of official Porsche safety and technical recalls/bulletins (one of my nerdier moments [8|]) I discovered that early cast iron manifolds were recalled/revised in March '87. Mine certainly looks pretty rusty, tired and VERY heavy.

So, here's the point: if you were mainly interested in hp rather than nice sounds and you had to choose between EITHER a new 'sports' exhaust or a new manifold (stainless versus cast iron) what would you go for?

Both is the obvious answer, but I'm on a budget (again).

Cheers
Rod

 

[Fen]

Based on the virtual lip flapping I've just done in your other thread regarding 4:1 vs 4:2:1 manifolds I would have thought that the manifold makes more difference to performance than the rest of the system in an NA car (assuming the system is not hideously badly designed).

What about trying to get the post-recall manifold?

Also, I posted a thread earlier this week with the contact details of a guy who produces chips, I think his Lux one is around £30 and I have read that it helps smoothness and driveability. I'm happy with the S2 one I got from him.

 

[ClubSport 911]

Leaving gas flow to one side (i.e. pumping losses) the first question that needs answering w.r.t to engine breathing etc is the inlet / exhaust overlap.

I don't know your car so well, but if you could discover this, then I might be able to guide you as to whether an exhuast makes massive or minor difference

Example:-
SSI's on a 911 SC (big overlap between inlet and exhaust)= +20hp.
SSI's on a 911 3.2 (small overlap) = 5hp at best, and wait for it...

"free flow back box" = -2hp. Of course, sound is another matter

 

[Guest]

What about trying to get the post-recall manifold?

I'm currently trying to find out more about the recall. At the moment I just know there was one, but I don't know what it was for. If it it was a proper redesign to increase flow then I'll get one and then see if Hayward & Scott can improve on it.

BTW I spoke to Richard Watson re: chips yesterday and one is in the post - will keep you posted...

Cheers
Rod

 

[Guest]

Leaving gass flow to one side (i.e. pumping losses) the first question that needs answering w.r.t to engine breathing etc is the inlet / exhaust overlap.

I don't know your car so well, but if you could discover this, then I might be able to guide you as to whether an exhuast makes massive or minor difference

I'll try and find out - I don't have a decent workshop manual I'm afraid, only the next-to-useless Haynes jobbie, and that has no detailed timing info in it. Will get back to you...

Thanks
Rod

 

[Guest]

Leaving gas flow to one side (i.e. pumping losses) the first question that needs answering w.r.t to engine breathing etc is the inlet / exhaust overlap.

I don't know your car so well, but if you could discover this, then I might be able to guide you as to whether an exhuast makes massive or minor difference

OK...

Intake opens 1deg after TDC
Intake closes 49deg after BDC
Exhaust opens 43deg before BDC
Exhaust closes 3deg before TDC

If I remember how do do it this makes the overlap - nothing?!.

Or am I just being stupid?

Anyway, how do the timing figures above compare to the SC and 3.2 examples you quoted?

Don't have numbers to hand but from what I remember from my mini days it seems pretty untunable!

Cheers
Rod

 

[ClubSport 911]

That is a lot more overlap than a 911 3.2. All this means, is that whatever you do on inlet, will / could affect exhaust. This includes the airbox, right through to the outlet itself.

 

[Guest]

Steve,

Sorry, my previous post in this thread (which I've since edited in a desperate attempt to save face []) was a load of old c&ap. My calculations were slightly confused, to say the least. Just shows what comes of trying to sneakily use the forum when I should have been doing something more useful at work...

Now I've got home and have had a chance to dig out an old tuning book (nitrous oxide Allegro anybody! - Jesus I used to read some mad rubbish) I can now see what I should have done.

There is actually no overlap. In fact the exhaust valve is closed 4deg BEFORE the inlet valve opens. So there's no chance of contaminating the charge in the cylinder with old exhaust gases from the previous stroke. There is also no chance of beneficial 'scavenging' (but I won't go into that). As I understand it there is therefore little or nothing to be gained from trying to increase the flow through the exhaust, whether via changing the exhaust itself or the manifold.

So, without changing the very 'safe' standard camshaft for something that produces a decent degree of valve overlap it seems to me that you can't really gain any power using aftermarket exhausts. Some of them claim 10-15hp gains on NA cars... I don't understand how these claims can be genuine. If someone knows better, please let me know.

Sorry for the confusion [&:]

To anyone else reading this: If you've been encouraged to order an expensive exhaust after reading Steve's last post, don't be blame him if it doesn't blow your socks of when you fit it - it's not his fault it's mine. Sorry.

All very embarrassing. I'll get my coat...