OK. This is ancient history, but I'm almost ready to start a motor with Jakes 86a based valve train kit, a heavily modified CB dry sump pump, etc... Point being I found a post from 2004 that suggest leaving the fan shroud open to the atmosphere if you are using a header (or at least aren't running heat):

http://www.shoptalkforums.com/viewtopic.php?f=1&t=72058

I'm curious if this is still the recommended procedure or if there is some new data out there? This car will predominantly a weekend warrior/autocross/track with sometime spent goofing around on the street.

Thanks.

-Aaron

Leave them open. The stock system (heater) allows air out those passages pretty much full time, so leaving them open is leaving them in their 'normal' position.

I was thinking of making some slugs to block of the heater passages, but make a smooth curve that would replicate a late model bus fan shroud. Is that a sane thing to test (I do have an MD1000 CHT) or is the bus fan shroud "more different" internally than just a smooth transition where the heater outlets once were?

Actually, I don't really think itll make a significant difference, so do whatever. Strikes me as wasted effort thought.

Yeah. Looking at the shroud it would seem I would need to remove it and split it to accomplish my goal. Last motor I ran with them taped closed - I'll try them open this time.