[keith646671]

Trying to figure crux of biscuit in egr interchange issue. Is the 73 up egr passage the small rectangle directly above the square port in the center of the intake flange? My 6x-4's have it, 96 d-ports do not. Does it basically go like this ?? ....

1. Heat from exhaust enters intake thru square center port on head and intake flange. To warm pocket under carb for cold starting and to function choke on "stove-type" manifolds on both egr and non-egr manifolds. Nearly all factory manifolds at least. Called the "heat crossover".

2. An additional smaller rectangular port was added above the center square or round (early) for EGR in 1973up. This egr passage routed to the carb to be reingested into engine thru hole under primaries into air-fuel mixture. Not sure where egr head passage begins?? I see square passage goes into center exhaust port checking with pipe cleaner.

3. My Eddy Performer RPM, Torker 1 and 2 have the flange square passage and pocket under carb to heat carb though T2 has a straight smaller pocket. None have extra rectangle over the square .. likewise the Holley Street Dominator I just got (for old times sake). Victor does not have any of that. Saw on Eddy installation instruction there was both non-egr and egr Performer plain pn's listed but only 1 number for Pontiac Performer RPM intake. Looked like SBC RPM had both. So appears to me most aftermarket manifolds DO NOT have the rectangular exhaust passage to mate to egr head hole factory setup but DO have the square hole warmup passage.

4. Covering the rectangular portion of head with a simple gasket might be ineffective at sealing egr head to non egr manifold since its hot exhaust. Have an Edelbrock gasket that has huge tombstone-like hole allowing rectangle to add to square hole passage after rebounding off intake manifold rail. Not too much perimeter width, looks like would blow out easy. Maybe not much pressure there.



My conclusions ...

Disabling the EGR likely to have limited effect on cooling the intake charge ... ( as an "air-gap" might ) unless larger square passage blocked as well. It may have an effect on diluting it to a degree. My 4x/400 stocker enduro doesnt foul plugs and runs pretty strong though.

Sealing the 1(non egr) or 2(egr) head passages whatever the mix should be accomplished with the "tombstone" gasket .. either egr will function or it wont depending on manifold-head combo.

Dont really understand how indirect path egr appears to take accomplishes any reduction of emissions or reuse of unburnt fuel.



2 friends involved right now ...

- 1. Wants a non egr manifold from my parts stash .. for a 71 2bbl Bonneville street application .. have several egr 4 bbl intakes, likely non-egr on 3 engines I am hesitant to disassemble .. a 72 400, 71 455, 69 350. My thought is for a street car an egr 4bbl intake should be indistinguishable from, and swapping egr intake to non-egr head should be a matter of using a gasket like I mentioned that lets each flow to each other like I mentioned. But see below.

- 2. Has a whistling noise from the 69 GTO he just rebuilt a 0071 block/62 head butler parts 400 for. Performer rpm with 800 Edelbrock carb. Dont believe the RPM would have the extra egr rectangle but might. Weird whistle was thinking possible heat crossover-egr leak. Car runs good but whistles, not an apparent vacuum leak. Said he had to double-gasket intake.

Have messed with mostly 60's stuff and not much of that really, even fuzzier with 70's, any help understanding would be appreciated.