[68 Firebird]

I tried a different factory iron intake today that has had the exhaust crossover removed and used
blockoff plates for exhaust at the heads. As expected the intake was cool at carb area and 130 degrees on intake runners after about 1/2 hour run. Running 180 t-stat & temp outside was 80 degrees with low humidity.

The idle and primary side of Q-jet is good and crisp. There is a slight bog/ hesitation when going full throttle or heavy from a stop that I would like to tune out. Would I be right to slightly increase air door tension to eliminate hesitation. There was no hesitation before just wanted to try an intake without crossover to keep carb cooler.

Gerry

[prostreet64]

The secondary air flap may be opening to fast as you suggest, and needs to be slowed down a bit for the fuel to catch up. However, it may be just as likely that it needs to open sooner because the bog is caused by a dump of fuel when you go wot. Only way to know is to try it one way and see what happens; easy to do, so no big deal if you get it wrong the first time. Also, the choke pull-off may be a factor as this needs to work in conjunction with the air flap to give a smooth transition from part to full throttle. If Cliff R. sees this post I am sure he can shed some light.

[68 Firebird]

I took it for a second drive later today and still am not fully in love with the modified
iron intake. Never went heavy throttle just cruised a few roads that I knew well to
compare how she ran. All in all it ran well but having the exhaust blocked at the heads
really changed the exhaust sound an made her noticeably louder and seemed to drone
a bit. This is not a waste as I had to intake to seal an oil leak from Valley pan and just
wanted an excuse to try it. If I don't use it makes for nice garage art!!!!!
Carb is dialed in sweet and don't really want to monkey with it.

And still have the 68 factory iron one to reinstall if I choose.

Gerry

[67cruiser]

I just switched over to a stock intake from rpm and picked up 3 mph. also check your choke pull-off, Cliff checked mine and it was coming in to fast, he sqeezed it down or made hole smaller, bog went away & felt like a new car.

[steve25]

You first need to lock out the secondarys and confirm that the off idle part of the primarys is not the cause of the bog and you need to confirm that the choke is not closing up under low vacume full throttle.

[Cliff R]

On the modified intake, is the divider still in place, or cut down/notched?

Not running heat to the intake has nothing to do with the "bog" or hesitation, at least not once the engine is warmed up and fully heat soaked.......Cliff

[68 Firebird]

The flange area for carb was not modified, factory 70 iron intake. No spacer on carb either.Only had exhaust crossover removed and water crossover separated. Running 180 t-stat and run for 1/2 hour. Tested on 3 runs and all similar. Just felt like she hesitated on a heavy take off and when going full throttle. Normal driving was fine . sprayed WD-40 and found no leaks
on intake. Had to address a minor valley pan leak so I just wanted to try this intake.

This is the carb you prepped and has been great on the factory intake for a few years, and
don't feel its the cause since I let car get very completely heat soaked. I did not that the
intake area just below the carb was down to 68 degrees while idling for 20 minutes and outside
air temp was 86!!! the runners were all 130 or so as well. Guess the venture effect was cooling
down the area of the intake. I wonder how cold the intake area under the carb would be on a cold
fall morning when air temps are just in the 50's.

Also on a side note the exhaust note is not as sweet as before.

Gerry

[Cliff R]

I experience similar issues with my own engine not having a crossover in the intake, but it disappears once fully heat soaked and engine is up to full operating temperature.

I've tested quite a few intakes, dyno, street, and at the track. ALL of them will induce stumble/hesitation/bog if they are cool/cold.

Fully open spacers on dual plane intakes can cause the same issue.

Modifications to the divider, cutting it down, or completely out will do the same thing.

Air cleaners can also induce stumble, filter element too short putting the lid too close to the carb, or even the X-Treme lid worked very poorly on my set-up.

I even had severe stumble trying to use the Performer intake on my last engine, it absolutely HATED it. Installing the intake killed that engine. It surged, stumbled, and puked all over itself on full throttle runs. Re-installed the stock intake and it was perfect again.

Even with all that said, never once had any troubles directly related to blocking off, filling, or not using a heat crossover in the intake, provided they were warmed up some before attempting any full throttle runs......Cliff

[68 Firebird]

Cliff, as always thanks for the insight. It is greatly appreciated.
I see the 68 factory intake going back on soon as it cures the
hesitation and exhaust note will be back to normal. This is hobby
and drive anytime the weather allows and the crossover really
seems to help in cooler temps like Sunday morning when
It was down to 58 and took longer to get happy.


Gerry

[steve25]

What is the cars water temp on the highway, if its less than 190 thats a problem as your oil temp needs to get to 212 degree`s to boil off condensation tha takes place in very motor when shut down.
Water in a motor with the added normal combustion byproducts forms acid which eats away at bearings and other aluminum parts quite fast.

[68 Firebird]

Steve I changed to a 180 t- stat to keep engine and oil at a more consistent temp.
That was done a few years ago and found better idle and got oil up to temp.
Water temp will hit 190 or more after long highway run and hang around 185 on
Back roads. Change oil every year and usually under 1500 miles for the year.
Oil change done at end of season to keep clean oil in during winter.
Thanks
Gerry

[68 Firebird]

Put 68 factory iron intake back on and all is normal again.
Yes it was nice to try but for all around driving with temp swing
the stock worked best for me. On a side note I have some lovely
garage art!!!!


Gerry

[leeklm]

68... regarding the change in exhaust/drone with the crossover blocked, I just went through this... When changing cams I decided to block the exhaust crossover. I ended up with a bad drone that drove me nuts! (RAM AIR manifolds and flowmaster exh) I then drilled a 5/16" hole in each block off plate, which took out most (but not all) of the drone, while still limiting the exhaust gas under the carb. Since drilling the holes, my engine paint has not burned off the intake like it previously did with wide open exhaust crossover.

The drone is not bad now, but still a little noticeable.