View Single Post
beepbeep beepbeep is offline
Registered User
beepbeep's Avatar
Join Date: Jun 2001
Location: Sweden
Posts: 5,565
Originally Posted by copbait73 View Post
Goran, there is no claim to HP on this chart however the only thing that is odd is the engine you just mentioned. Is the race fuel methanol? Please get more details, street?

My comment was maybe a bit hasted, I wrote it after few beers. Sorry.

Engine was a turbo-converted 964 N/A 3.6 with hot cams, twin Garrett GT35R's, twin huge intercoolers, cams hotter than GT2-evo and Rochester 1000cc-ish injectors. Pistons were 993tt stock pistons and it had H-beam steel rods. It was dynoed at 2 bar using standard (?) race fuel...I forgot which sort. On ordinary 98 octane RON fuel, boost could be upped to around 1.6 bar w/o knock. It had twin plugs as well (having N/A 3.6 heads)

I belive the graph is accurate but I suspect graph assumes 100% VE and low air temp. What I'm trying to say is that typical working installation is going to have different dynamic C/R at different revs, despite constant boost pressure.

As turbochargers go trough their different efficiency ratios, so does discharge temperature. The more heat in the air, the less oxygen is introduced into combustion chambers. Also, VE is going to vary wildly depending on cams, runner length etc.

Of course, Porsche aircooled heads aren't as good when it comes to knock as modern pent-roofed 4v designs with plug in the middle, but there are numbers of tricks one can use to prevent knock despite soaring dynamic C/R ratio. Ignition retard, twin-plugging, high octance juice and efficient intercoolers are some of them.

P.S. It's quite amazing what kind of boost you can run on modern engines. I have an old Audi UrS4 (93' model) that boosts 1.5 bar of boost on the top of it's 9.3:1 C/R.
Thank you for your time,

Last edited by beepbeep; 03-09-2008 at 01:28 PM..
Old 03-09-2008, 01:25 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #24 (permalink)