Hi

When I overhauled the 435 HP tri-power carbs, I missed to check if the center carb vacuum source to the distributor is ported or manifold type.

When I adjusted the timing today, this vacuum feels like ported. I thought it should be manifold type ?

There is hardly any vacuum at idle, but it increases instantly as soon as the trottle plate is touched. Is that correct for a tri-power 435 HP setup ?
I hope I took the correct fitting, the one on the pax side. The one on the front of the carb I used for the carb pods.

This shows the port I used for the ignition vacuum.It is the port on the carb. The one on the manifold is for the crankcase vent.


Günther

 

I don't know Gunther, but either way, it will run better with manifold vacuum. T into the manifold line, and cap the ported vacuum ports.

 

Ported is correct, but the previous post brings up an common question. Which is better, ported or direct (manifold) vacuum? The answer is, it depends. Historically, older cars (like ours) lived on good gas, typically 98 octane or better, and could tolerate lots of timing right off the bat. Then came the low octane, unleaded, low compression '70's. US manufacturers went to direct vacuum to maximize mileage and reduce detonation on acceleration in their (mostly) big, heavy cars using existing engines and fuel systems. Once computers showed up in the mid '80's they took over timing functions slowly but surely.

Depending on how you drive, one type may be better than the other. Direct vacuum maximizes timing at light throttle or acceleration, when cylinder pressure is low, and helps overcome low octane issues. Plus, mileage is typically better, but not always. If you have a heavy foot, or performance engines that require quality fuel anyway, direct vacuum actually hurts mileage and performance because timing vacuum DROPS right when you need it most! In that case ported works well.

I'll give a practical example. In the late '70's, there were lots of folks buying the BB Pontiac Trans-Am Firebirds, screaming chicken decal and all. These came set up with direct vacuum and a quadrajet four barrel. I got tons of complaints of BAD gas mileage that they were a "dog" on acceleration. All I did was switch 'em to ported vacuum, fatten up the idle mixture a little, open up the air intake scoop, and give 'em a couple more degrees timing. What a difference! Better mileage (a little), more power, and it got rid of a common EGR related hesitation. I had some pretty happy customers!

Anyway, sometimes it pays to play around with timing to see what works best.

Hans

 

Hi

Thanks for the expalnations.
So, the tri-power is originally using ported vacuum !
There is no free manifold port on the center carb . The only one
would be the one for the small choke opening diafragm pod.
Don't know if the take off for the 2ndarie pots is ported or manifold.

Will try it out.

Günther

 

just to help clarify above-once the throttle blades open far enough for the ported source to see "vacuum", ported and manifold vacuum values will be the same from that point to WOT. at least that's how it works on my carb'ed vehicles . . .

also, do a search on tri-power; seems like there was a thread recently discussing vacuum port identification

 

Hi

Well understood.

CHILTON says to adjust the timing to 4 deg only.
I adjusted to 12 deg in order to get 36 at 3000 RPM ( 52 inlcuding vacuum ). So this should do for trial as is.
Later on, I will try manifold vacuum , which will add 15 deg at idle and bring it to 27 deg .

Will report how it works best for me .

Brgds. Günther