EPL

I had the carb off of my '64 327/365 and it looks like the someone machined the plenum divider down to the base of the intake. Car runs fine, just looked strange. Were they all this way? The manifold is the correct manifold for the car. If anyone has a picture, I'm sure that would be helpful. Thanks in advance.

ctjackster

no, they were not all that way as delivered, but it is not uncommon to see the divider "hogged out" - for every 5 SHP intakes on ebay, 1 or more has had this sort of work done. Kinda defeats the whole pupose of the 180 degree manifold though, but I think the boys "back in the day" were trying to achieve some sort of improvement, maybe someone like Clem can tell us the perceived benfit to be achieved by doing this.

as for your request for pics, let me run a search for the manifold on ebay, and I can post up pics of it both ways, I bet.

wesmigletz

I believe Wieand makes a plenum divider that could be added. Just have your machine shop notch the plenum and you could epoxy the divider at the base. My friend did tis to a BB intake a few years back. It did wonders for his throttle response.

66427-450

You can "tune" the Power/RPM curve of a dual plane intake by modifying the plenum divider, check out some of the current Edelbrock RPM intakes (as well as some of the current GM aftermarket intakes) you'll see notches in the plenum from the factory.

Guys that are into performance want more power up where it counts, and the resultant low RPM reduction is not an issue, heck, it's even desirable if you're trying to launch with a big powerful motor on skinny tires.

The only Vette intake I know that was so modified/tuned from the factory was the L-88/ZL-1 intake.... and it worked out great! In fact, I just went with one for my '66 (I went from a stock L-72 intake, to a 1"x1 5/8" notch, to the 2nd model L-88 intake).

But I agree, many misguided guys hacked away at the plenum dividers without any testing, but it's no big deal, it's very easy to weld a divider back in



[Modified by 66427-450, 3:53 PM 3/30/2004]

mrg

While on the subject of this particular manifold. ... Other than the weight difference between the aluminum SHP intake manifold and the 4-barrel cast iron version is there that much 'difference' between these two manifolds - performance wise .. ? .. Not sure if this topic has been broached before ..

John

ctjackster

interesting stuff, I am always learning - now JohnZ will post up some torque & rpm numbers for every milimeter of plenum removed :D

66427-450

the manifold alone (over the 300HP CI version)... it probably produces a bit more power, at high RPM flow rates. but during normal/conservative street driving? i'd say it's negligible. remember, all these power producing parts work as a set (cam, heads, intake and exh sys).

EPL

Thanks for the insight everyone! As noted above, there are always interesting things to learn here!

66427-450

I agree, this a great forum.

What most guys don't understand is that you reach a point (during plenum divider removal) there you start to significantly alter the design air/fuel distribution, or balance, between cylinders. Again, when the GM engineers drastically altered the L-72 intake for the L-88 then ended up stagger jetting the carb, and adding "air flow tabs" to keep all the cylinders balanced. How many guys removed the divider without checking cylinder exhaust temps for lean cylinders? Or modifying the carb to compensate? See my point.

When in doubt, stick with the factory configurations :thumbs:


[Modified by 66427-450, 4:18 PM 3/30/2004]

SWCDuke

As stated, cutting down the plenum divider makes the manifold act more like a single plane manifold than a dual plane manifold. A dual plane (180 degree) takes advantage of some favorable wave dynamics at low revs to enhance low end torque and provides a better fuel metering signal to the carb. The carb will usually require richer jetting to maintain desired fuel-air ratio if the divider is cut or a single plane manifold is substituted for a dual plane. Chevrolet recommends specific "stagger jetting" (different sizes at each corner) for big blocks with cut down center dividers.

A single plan manifold or cutting the divider down on a dual plane will increase top end power (assuming the carb calibration is modified to maintain proper fuel-air ratio), but at the expense of low end torque since the favorable low speed wave dynamics are reduced or completely lost.

A dual plane is preferable for a street high performance engine as improves low end torque, and particularly on a 327/365, grinding out the divider is not a good idea as the 30-30 cam is already a major low end torque killer.

In the universe of 180 degree manifold designs, there is not much difference between A and Z, because there is limited space to package all the runners. Nothwithstanding the above, the Z-28/LT-1 is probably the best design extant for a street engine.

If you look at the SHP versus the 300 HP manifolds the SHP manifold has sharp bends where the runners turn to point at the head ports. I think this effectively shortens runner length and increases plenum volume, which would bias the SHP manifold to a higher rev range and compliment the SHP cams. I've never seen comparitive performance numbers for the SHP versus 300 HP manifolds on a SHP engine, however, it is noteworthy that by 1968 the same CI manifold was used on both the base 327/300 and the L-79, which was rated at the same power as prior years.

This did not change with the new 350 CID small blocks in '69. Both the base 350/300 and L-46 350/350 used the same manifold and essentially the same Quadrajet carb. The big change between the L-46 and L-79, aside from displacement increase, was the L-46 cam. It has about three degrees more duration at .050" lifter rise and is effectively retarded four degrees on the same 114 degree lobe center angle.

Could be that the more aggressive cam was added to balance out the inlet manifold that was more biased to the low end.

Duke

EPL

Thanks, Duke and everyone else. I have a lead on a manifold but not sure the date matches up with my build date (late May '64). Is the date on a 3844461 visible when installed? I would check this myself but I am out of town.

ctjackster

no one will ever see the date, get the nicest '44461 manifold you come across. beware of those that have a plug "welded" to the manifold in the by-pass hole, or those that have had the plenum divider hogged out

ps, the date is underneath, and even if you have the thing off and in your hands, if the shield is rivetted in place then you still won't see the date - if you are buying a date-correct intake manifold, that might mean that the shield is missing, fyi

EPL

Thanks CT. Would have missed the point about the shield!

jimtreber

Also back "in the day" Holley had come out with a 950 cfm three barrel. The rear barrel was a single oval throttle plate that went all the way across thus connecting where would be the two rear throttle plates. Holley was advertising a 30hp gain with this carb on a stock 327 350hp and 365hp. But to use it the divider in the stock hp manifiold had to be notched out quite a bit almost to the point of removal.
Luckily this carb had a vacuum secondary to prevent bog.
I ran this carb in the late '60's and it seemed to work.