[enduro54]

I have a 355 with a XE-262 comp cams cam in it and am only pulling about 15" of vacuum at 1000Rpm. I need 17". Will going from 336x heads to 461's increase my vacuum 2"?

[F-BIRD'88]

No but increasing the spark advance at idle will.

[SDLuck]

What he said.

[maxpower_454]

I thought raising compression will increase vacuum?? no?

[cobalt327]

Originally Posted by maxpower_454

I thought raising compression will increase vacuum?? no?

I've never thought about this before, but it would seem to me that the change in CR of a point or two will not affect vacuum to any large extent.

The vacuum is influenced far more by cam and ignition timing, air/fuel ratio, engine tune as well as how good the engine is sealed, combustion-wise.

[lg1969]

I agree with you, Colbalt

[enduro54]

Ok so i played with the timing and got it to pull 17.5". I have a stock aluminum intake and a set of 461 heads that I'd like to put on. Will either cause me to lose and vacuum over my stock cast iron intake and 336x heads??

[cobalt327]

As long as both pair of heads are the same, condition-wise, and the intake gaskets seal correctly, and the timing is put back the same, it should be identical for all intents and purposes.

[enduro54]

By condition-wise, you basically mean how well the valves seal on the heads, correct? Both are in good shape. The size of the intake ports doesn't matter?

[cobalt327]

Originally Posted by enduro54

By condition-wise, you basically mean how well the valves seal on the heads, correct?

Yes, the sealing of the valves is what I meant.

The size of the intake ports doesn't matter?

Those two heads are within spitting distance of each other, so no, it will not matter. Same thing w/the intake. If they are both the same design and the "new" intake doesn't have 4" of open spacers stacked on it, it will be the same for all intents and purposes.

The ignition timing, cam timing then I'm thinking intake design and carb size are also players in how much vacuum an engine will make. The intake port volume will affect the vacuum too, but to a lesser degree unless the port volume is much larger than before. Even exhaust tuning can affect vacuum, although I doubt you'd see it at an idle.

[enduro54]

Thanks. that makes sense. So will the 461's give me much of a performance increase over the 336x's? They will definitely be raising my compression ratio but mainly I figured they breathe better than the 336x's so I figured I'd see some more power.

The only difference with the intake is that the aluminum one has a hole that connects the passenger and driver side where the cast iron one doesn't. I was thinking that this might affect my vacuum but then realized that it shouldn't since the two sides are connected by the carb spacer anyway.

[cobalt327]

Originally Posted by enduro54

Thanks. that makes sense. So will the 461's give me much of a performance increase over the 336x's? They will definitely be raising my compression ratio but mainly I figured they breathe better than the 336x's so I figured I'd see some more power.

You will be making more power most likely- I would go as far as to say most assuredly. How much more, I cannot say w/anything but a WAG. I'd say 15-20? The CR difference won't by itself add much HP, it will enable you to run more cam- which WILL add significant HP, as long as the ignition timing doesn't have to be compromised. It's a balancing act.

The only difference with the intake is that the aluminum one has a hole that connects the passenger and driver side where the cast iron one doesn't. I was thinking that this might affect my vacuum but then realized that it shouldn't since the two sides are connected by the carb spacer anyway.

Is there a notch in the plenum divider, like the RPM Air Gap for the SBC has, or is this the OEM Q-jet intake?

[enduro54]

OEM q-jet intake

[cobalt327]

Originally Posted by enduro54

OEM q-jet intake

Yeah. That hole was only there for the EGR system. Plugging it would be better than allowing the sides to "share" like that. The furnace cement shown HERE, post #36 would work to plug the two sides.

If you thing the hole in the Q-jet intake is bad (which it sort of IS), the OEM 2-barrel aluminum intake for the '80 229 V6 is a single plane (as well as other years of the 229 and the 200 V6, I'd bet). Has nothing to do w/the intended usage or performance- it simply was cheaper to manufacture that way.

[enduro54]

Yea I thought about plugging the hole but don't want to have to worry about the putty coming loose and making it's way into my engine. It's not worth it. Why should I plug it? Will it reduce my vacuum?