Pontiac Firebird 1967-2002 - Partially Blocked Secondaries on Pontiac 400?

Regarding a mid 70s 455, I saw the following over at: http://www.carcraft.com/techarticles116_0306_pontiac_455_cubic_inch_engine/index.html

"Even the intake manifold suffered, showing partially blocked secondary bores that were originally fed by a "detuned" Quadrajet."

Is the above true for a mid 70s Pontiac 400? Does this mean the intake manifold partially blocks the secondaries? How can I tell if my 400 has this issue and how can I solve this?

My block code is 500557 which makes it a late 75 or early 76 manufacture. The code on my factory cast intake is F207.

Thanks,
BT

The below statement isn't 100% accurate; in 1969 the highest output engine in the 69 Trans Am had a tab on the carb which did not allow the secondaries to open 100% - as Pontiac wanted the highest output engine in the (slightly heavier) 69 GTO Judge to be stronger and therefore faster than the lighter 69 TA.

This "feature" on the 69 TA's carb was quickly fixed by most TA owners; so, now (40 years later) unmodded carbs specifically made for 69 TAs are quite expensive.

Regarding a mid 70s 455, I saw the following over at: http://www.carcraft.com/techarticles116_0306_pontiac_455_cubic_inch_engine/index.html

"Even the intake manifold suffered, showing partially blocked secondary bores that were originally fed by a "detuned" Quadrajet."

Is the above true for a mid 70s Pontiac 400? Does this mean the intake manifold partially blocks the secondaries? How can I tell if my 400 has this issue and how can I solve this?

My block code is 500557 which makes it a late 75 or early 76 manufacture. The code on my factory cast intake is F207.

Thanks,
BT

Yes, the mid-70's 400 intake manifolds featured a "D" port secondary intake port. You can bore the blockage out and also block the EGR for more performance. Also, the factory intake, when properly ported, will out perform most aftermarket intakes.

Yes, the mid-70's 400 intake manifolds featured a "D" port secondary intake port. You can bore the blockage out and also block the EGR for more performance. Also, the factory intake, when properly ported, will out perform most aftermarket intakes.

So I remove the intake manifold and look for a blockage in secondary ports? To remove the blockage is this something that can be done with a dremel tool and stone or should it be taken to a machine shop? I have a dremel with an assortment of stones and cutters. How do I block the EGR? Currently the manifold has the vacuum line disconnected from the EGR. The small vacuum tube on the EGR is bent over on it self. I believe this has disabled the EGR in the closed position? Does this sound right?

BT

If the rest of the engine is stock, don't bother. The mid 70's engines have very small cams and don't need more flow than the stock manifold provides. Once you start modding it's probably easier to get an earlier stock or aftermarket manifold.

This is the "blocked" intake.
http://image.highperformancepontiac.com/f/9027145+w750+st0/0608P_02_z+modify_intake_manifold+before.jpg

Compared with an early intake on the right and bored out "blocked" intake on the left.
http://image.highperformancepontiac.com/f/9027199+w750+st0/0608P_08_z+modify_intake_manifold+compare.jpg

http://www.highperformancepontiac.com/tech/hppp_0608_modifying_pontiac_intake_manifold/index.html

Notice even bored out, the secondary opening is still smaller since the early intake actually bulges out around the secondary butterflies.

Nice job posting those pictures above. The early 68-72 intakes are preferable, but as mentioned, the stock cam profiles won't show much change. Upgrade the cam to something more aggressive and have fun. Also, compression is the other issue with the later motors. Get your compression up to 9 to 9.5 to 1. A head swap is the easiest way to do that on a Pontiac motor. Find some nice early heads with compression chamber volume in the 72-80cc range. The factory heads to look for are #62, #48, #16, #12, or #13. The Ram Air IV cam is a nice, easy choice, but there are better options today. Lastly, picked up Cliff Ruggles "How to Rebuild and Modify Rochester Quadrajets" book. Best book for getting the quadrajet to run great. Check out my results:

http://www.youtube.com/watch?v=vLe8IVGxMio