Winter wish list

[Porsche917]

I am sitting down compiling my winter rebuild wish list - Steve, you mentioned that you have had good luck with your exhaust system - who supplied it? Am going over the fuel system (will get the loan pump back to you next season Steve unless you need it sooner in which case I can post it?), wiring, gauges etc etc to try and get some dependability - can't learn how to drive these cars fast if I always standing watching you lot! Long range forecast for next year is hot and sunny!!!
Cheers
Geoff

[Steve b]

Hi Geoff

My exhausts is a bit of a bitsa but mainly from BTB exhausts!

The most important thing to remember is to allow for expansion in the mounting system! As obvious as it sounds as the exhaust heats up it expands and gets longer. 20 - 30mm. The mouning rubbers need to support the system, prevent it swaying from side to side but NOT restrict movement along the car.

Ive seen systems from some companies claiming to know what they are doing which are completely locked up with no allowance for expansion. The result is that when it tries to expand it stresses the system and it cracks (usually at the manifold or down pipe!)

I'm personally not a big fan of flexi joints but plenty use them! If you must, then make sure its supported at both ends so its not taking the weight of the system otherwise guess what.....yep it will crack!

Hope this helps

Steve

[martinhall]

Hi there,

Steve is pretty much spot on, with his post in that the system has to be able to move or will crack or fatigue fracture. Our system uses slide joints, held together with springs, pretty much like a rally car uses. The reasons being that it seals up pretty well to save power loss, and is very tollerant of being bashed when the car goes over kerbs, or when I have run out of tallent and taken to the scenery.

If you are going to have a system made, dont go too large on the pipe size, you will actually loose power. Also bear in mind that it has to pass the noise check. Whilst there is a lot of information on the web, most of it is pretty useless because we have to use the cast iron manifold.

Mart

[Porsche917]

Thanks for the tips guys - any suggestions on diameters etc? I'm convinced my exhaust though quiet and solid (not fallen of or cracked) is draining quite a bit of power. Did BTB make it up as they went along or do they have a template exhaust? Sorry to ask so many tedious questions - just don't want to send a couple of hundred quid only to find I end up with one that causes trouble. I really need a car that gets me from start to finish as many times as possible next year.
Cheers
Geoff

[martinhall]

HI there,

2 1/2" diameter would be a good starting point - however, have a look at an old book called Scientific Design of Exhaust & Intake Systems. Yes it talks about VX 4/90's and measurement before you could buy an electronic box of tricks but its still valid - not sure about a brooklands exhaust silencer though! Primary pipe length, length to joins, and stubs are all in there, however for whatever reason, the 924 engine seems to like a bit of backpressure i.e. a bit of restriction in the exhaust. Sounds mad, but I think it was Clive Trickey's book on mini tuning which showed that an open exhaust could loose power compared with a silenced exhaust.

Mart

[Porsche917]

Hi All
I have another question/request. Does anyone have a spare strut brace? I missed one on ebay recently .....or if there aren't any spares about how about a plan or one that is currently off the car that I can borrow and have copied? Got to have all the trick bits and seems like everyone but me has a brace - so to prevent me having an inferiority complex can someone help?
Cheers
Geoff