View Single Post
beepbeep beepbeep is offline
Registered User
 
beepbeep's Avatar
 
Join Date: Jun 2001
Location: Sweden
Posts: 5,565
Hi!

I had a pic of 935 factory headers and they weren't equal-length design but quite short and stubby pieces. I'll try to find it on my old computer.

I believe OEM 993 HE's with turbo piping will be nice aftermarket add-on for 930's. As long as ovelap is non-existent and revs moderately low (which they will be as long as cams are mild), all lengthy equal-length stuff won't be utilized fully. Thus short, light and well made 933 HE's are probably better idea as long as heating needs to be utilized as well.

We had lot's of discussions in this thread. There is lot's of good info there.
EVIL ENGINE goes Twin Turbo - Your Chance to Design the 930tt Exhaust System


I also posted a dyno from car equipped with OEM short-stubby cast-iron headers and then nicely made equal-length headers. It's a 2L machine making well over 500hp. As you can see, the very restrictive OEM "headers" make more torque off-boost and at middle revs.



As turbo lives off gas flow, and that flow is partially generated by pressure difference fore/after turbine, and that pressure difference will be proportional to temperature difference and exhaust backpressure: short (but still well-flowing) headers and low backpressure is what is probably the most efficient design for turbocharged engine.

So short design with carefully crafted 3-in-1 mergers and minimal amount of bends is probably the best. Which is what factory also choose for 935. Equal length is probably the very best but given packaging constraints I would rather chose stubby short design and keep heat than long headers and gain weight and loose heat.

There is also a piece which is often forgotten: the more heat is retained in headers the more of it will be used to run the turbo, the more efficient engine will be while on-boost. This is probably of no importance for us, as nobody drives for longer time on-boost but old F1 cars were very efficient designs.

If 993 HE's are to be dynoed against equal-length "snake nest", one should pay attention to keeping the boost the same. Different amounts of backpressure might offset crude 930 boost device (a spring), and resulting 0.05 bar of difference might abtain false results.

CIS should be fine, it only measures air that has been sucked into engine and dispatches fuel accordingly. Thus, AFR's cannot change unless there is air-leak or some major EGR-reversion due to hot cams.

I agree with previous posters regarding 993 HE's being a nice trade-off regarding their reliability, light weight, price and availability. My guess regarding dyno results on otherwise stock 930 (as long as boost is kept the same) is: 993 HE's will give more torque off-boost and slighly more in middle-range on-boost. Equal-length will have a slight advantage on very top. 5-7HP or so. It's just a speculation of course.

Regards,
__________________
Thank you for your time,

Last edited by beepbeep; 01-04-2009 at 03:14 PM..
Old 01-04-2009, 02:50 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #67 (permalink)