Inline six carbs

[slomotion]

Maybe this should go in the "Engine" section, but it's related to the "Post Your Straight Six Pictures" thread, and I didn't want to hijack that thread with a question.
Having said all that, I noticed a lot of 4BBL carbs on the engines. Some of them had the primary's facing forward, and some were facing outboard (to the left). I mounted mine with the primary's facing left because my manifold is a dual-plane, and I reasoned the primary's would service one plane, and the secondaries would feed the other.
I'd like to read some idea's as to which way is better, or if it even matters, and why......

Thanx!

[PanelDeland]

I've seen posts claiming the sideways mounting "should" create a more equal distribution of Fuel/air.I've seen others claim it makes no difference.I would like to see a photo of your intake since I was under the impression that all inline 6 cyl intakes were basically a single plane design.

[kyull67]

I really don't think there would be much of a difference but I can see a slight variation on the spark plug colors in my engine. On my Holley and Offenhauser setup it would be difficult to run it sideways with the linkages and I imagine that's why most people do the four barrels that way. However looking at the carb, I can see mounted sideways will equally space the barrels side to side. Maybe I'll give it a try..

[silverstreak]

One thing to keep in mind when mounting the carb sideways is how making a sharp turn (u-turn) will affect it. In other words some times the engine can stall if the fuel were to slosh to one side and uncover the jets or go temporarily rich (high fuel level) if it were to slosh towards the jets. Some carbs are more prone to this than others.
I can remember the 76 Impala my Dad had, would stall durning a panic stop (sudden red light) as the Rochester 2 barrel had the float bowl on the front of the carb, and all the fuel would slosh forward away from the jets and kill the engine.
Holley 4 barrels used on the drag strip would also bog down pretty bad when they launched, as fuel sloshed towards the rear uncovering the secondary jets. You can get jet extensions from Holley to fix this though.

[Old Bluey]

I've got an intake from a 67 Pontiac sprint motor that cam stock with a Q-jet. The primaries are close to the head, with the secondaries on the outside.

I've got my Holley/Offenhauser in the same configuration. went with a Lokar cable throttle, and it works good.

[292farmer]

do u have pics of the pontiac intake installed? thanks!

[RichardJ]

Old Bluey you should take fresh look at that Pontiac OHC Six you said you have. Chokes and primaries are outboard away from the engine on all of them that I have ever seen or seen pictures of and it's pretty hard to turn a Q-jet around. I hope no one is confusing the secondary air valve plates with the chokes.

Before I mounted the Carter AVS and Clifford, I measured the differences between the longest and shortest runners with a tape measure run through the passages. By mounting the carb with primaries to the front, the distance from the primaries to cyl #6 was about 15" longer than right primary to #3 cyl. By putting the primaries inboard the distance between the longest and the shortest was reduced to about 13". By putting the primaries outboard, the differences in distances between the longest and the shorted runners is reduced to about 8".
I am a little fuzzy on the exact numbers because it was about five years ago that I did it.
I liked the idea of putting the primaries outboard and the fuel line, vacuum hose, PCV hose, mechanical choke cable worked best that way. I had already decided to use a cable throttle so that wasn't a problem.

[jayman68]

Originally Posted by slomotion

I mounted mine with the primary's facing left because my manifold is a dual-plane, and I reasoned the primary's would service one plane, and the secondaries would feed the other.

Be carefull, the primarys are just that the primary source of flow to the cylinders and the secondaries only function when you are accellerating hard. If infact it is separate planes, it could cause a lean condition on half your cylinders when driving normally. Just my $0.02

[Shaky]

I did it like this. It worked. Can't say that I thought about it a lot.

[Old Bluey]

RichardJ, My bad, You're of course correct. Hadn't looked at it in a while.

Originally Posted by 292farmer

do u have pics of the pontiac intake installed? thanks!

Although the ports on the Pontiac line up to the 250 head, the mounting holes & flanges do not. After a bunch of modification, i was still left with a lot of vacuum leaks. So I yanked it out, ran the stock Rochester for a while and replaced it with the Offenhauser.

Before:


After:

[292farmer]

sweet thanks for the pics yeah i found out they wont work, but never seen a good pic and was curious what they looked like, how was the q-jet performance?

[Old Bluey]

The look is a lot like the Clifford intake. The weight of the cast iron Pontiac on the other hand is impressively heavy. Especially compared to the aluminum Offy.

I never used the Q-jet, I used a Holley (same carb on both setups) with an adapter. The stock single barrel was a Rochester Monojet. It's new home is on the shop shelf.

[slomotion]

Originally Posted by PanelDeland

I've seen posts claiming the sideways mounting "should" create a more equal distribution of Fuel/air.I've seen others claim it makes no difference.I would like to see a photo of your intake since I was under the impression that all inline 6 cyl intakes were basically a single plane design.

Panel, you're right, my bad. I recently sold a '76 Ford stepper with a 300 inline six, and that was the one that had an Offy dual-plane. I have the Offy 5416 on my 292. I don't know why Offenhauser doesn't make a dual plane for GM. Maybe we should start a campaign..............

RichardJ, thanks for the numbers, it never occured to me to measure distances. In the stock configuration, and using a single bbl carb, the numbers would be scary!

I'm guessing that's why the guru's recommend a heated manifold thereby keeping the fuel mixture atomized while it's trying to decide which way it's going to go.