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I was curious what would come of this.
Firstly, I tried an '85 exhaust cam in my modded '85 (had 2.1L head and both 2.1L cams) and could not tell any difference.
One thing you don't have comparative data on is the lift of each of these cams. Another important characteristic is the leading and trailing ramp of the lobes, that's how fast it opens and closes the valve. It's possible to have two lobe profiles of the same quoted duration that will flow very very differently based on the slope of the lobe, the width of the lobe at full/nearly full lift, and the total lift.
I believe the peak lift of each of those different cams are a little different.
The link you posted is very generalized, and gives good baseline numbers for getting you thinking, BUT the design of profiles for an N/A engine versus a Turbo engine are very different.
Less overlap will provide better performance at low RPM, more overlap better at high RPM, but with turbo engines there's a tradeoff that has to do with the duration of the exhaust cam. Less duration will have greater pumping losses, BUT it may keep the velocity of the exhaust pulse higher, giving more kinetic energy to the turbine to help turbo spool. It's whether the balance results in enough additional power from moving more air through the turbo to overcome the additional pumping losses during the exhaust stroke and if the exhaust is designed well enough to efficiently scavenge the chamber.
Greater overlap cams don't idle as well, there is some mixing of the intake and exhaust charges during the time both valves are open, with dilutes the fuel/oxygen mixture. At high RPM the inertia of the intake charge flowing into the cylinder helps carry exhaust out the open exhaust valve during the last portion of the exhaust stroke and helps scavenging some, helping the fuel/oxygen ratio.
The belief with the '85 cam was that the decreased duration would help spool in the turbine enough to counteract the flow losses, but like you rightly said, this may be a case of fable becoming accepted, there's not hard data. I was originally planning to test a few different combinations back to back, but when I couldn't even feel a difference and so didn't bother.
If you do test it, see if you can log EGT during the runs, the shorter duration cams should raise is a little bit, which would also help spool.
One comment on your setup, I have almost the same setup as you, but have tube headers with a TD-05 16G and a 10 cm^2 turbine housing, and a full 3" exhaust, and do not have any of the low RPM hesitation is sounds like you might have, the T3/T4 with a 2.5" exhaust is way too much restriction.
You will want to at least consider finding a later style 10 row intercooler and the larger aluminum tubing intake pipes vs. the stock ones if you don't have it already, I would bet that breating issues are causing your odd running. It will open the high range up much better too, and is probably causing the abrupt boost delivery just after spool between 2500-3000 as you describe.
I think I have the extra '85 exhsaust cam hanging around somewhere since I'm not using it, if you want to use it for some dyno runs I'd be happy to send it to you - I have always been curious about what the numbers actually are.
Best,
Drew
posted by 165.124.118...
, Fri, 9 Jan 2009 18:50:33
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