Intake manifold modification

[Dubs1]

Hello Jay  and the brain trust

Recently found at a swap meet   an early Holman and Moody intake manifold  for a 289/302.  Purchased it for my boys 289 Mustang coupe.  In the center plenium divider  the divider has been machined down about 1 and 1/4 inch.
What affect does this have on engine performance?  More top end ?  More bottom end?  I thought I read  on this site a while back that it improves top end  power.  If so does that lessen any midrange or low end power or torque?

Thanks much   

Larry  in Denver     

[gt350hr]

  Larry,
      The manifold ( as you see) was designed with four round holes beneath the carburetor. Testing "back in the day" found that a spacer (1/4") or milling under the carb (1/4") helped the fuel distribution in that manifold. Later versions had two "oval slots" on each side for similar reasons. It gave a small boost in power throughout the power range with no appreciable loss in bottom end power. Not a "top secret" modification , just an improvement.
   Randy

[Joe-JDC]

From my experience, lowering the divider loses torque significantly, especially 1 1/4"s in a 289 manifold.  On some manifolds, milling the divider down like that will cause a stumble below 3000 rpm, due to fuel coming out of suspension.  If it was milled level, front to rear, then I personally would suggest welding in a divider and restoring the separation of planes.  Use a spacer if needed afterwards.  Joe-JDC

[machoneman]

Agree 100% with Joe. But, another way would be to mill a  two square slots, one in the front and one in the back wall and easily slide in a fitted piece of aluminum level with the carb pad, the carb's base plate holding it down and avoiding the issues with welding aluminum.

[66FAIRLANE]

The other thing I have done with manifold on car is to add a phenolic 1/2" spacer. This then has the afformentioned slots milled in for a slip in/out divider.

[jayb]

I agree that cutting down that divider will cost you low and midrange torque, but if the engine wants a lot of air it could also pick up horsepower at the top end.  Cutting it down effective increases the plenum volume of the intake, allowing the cylinders to share more, so if one or more are getting starved for air, that modification will help.

[Tobbemek]

On some manifolds, milling the divider down like that will cause a stumble below 3000 rpm, due to fuel coming out of suspension

There is known cases with smf engines wear a "to far" milled down divider give such a bad fuel distribution that you actually can heart a cylinder running lean, bee careful.

[machoneman]

Yes, as Jay pointed out the top end hp increase potential, one may pickup some gains if the engine is operated in the high(er) rpm ranges, given one doesn't mind losing some bottom end. Simply, a engine used mainly as a streeter may not be happy to drive due to the loss.

I added a welded in divider long ago in a very hot, steep-geared, 4 speed 427BBC Camaro (sorry!) as the previous owner had milled out the factory L-88 divider in a hi-rise dual plane. Once reinstalled, the difference in low end when taking off in 1st gear was quite noticeable, all in the seat-of-the-pants feel. In a small CID engine, I'd guess the loss of not having the divider would be even more noticeable (heavy car relative to the CID, torque,hp and low engine speeds) with smaller engine/car combos. 

I agree that cutting down that divider will cost you low and midrange torque, but if the engine wants a lot of air it could also pick up horsepower at the top end.  Cutting it down effective increases the plenum volume of the intake, allowing the cylinders to share more, so if one or more are getting starved for air, that modification will help.

[mbrunson427]

I think I'd be even more concerned about this being from Denver. It might be pretty poor at the low end. There was a guy at my work that was a super good aluminum welder, but he moved out of state, so I don't really have a reference for you here in town to get it fixed up.

[My427stang]

Although it may lose a little grunt with the shared plenum, I wouldn't be too concerned.  Proper cam choice and keeping intake centerline early it'll pull hard enough on the intake

[Dubs1]

Thanks  everyone.  The 66 stang is not a racer, Its Isaiah's school car with factory  cam, heads et all.  Swap meet price was super cheap. 

I like the idea  of notching  the fore and aft inner walls and inserting  an aluminum piece.

All the best

Larry

[machoneman]

Dubs, some years ago, some spacer manufacturer did make a spacer with those milled slots and a few pieces of kinda' T-shaped, drop in dividers (some dropped down to allow say a 1" gap at the top, another was a full wall piece. Damned if I remember who it was but...one could potentially buy a spacer and with some heat-resistant sheet plastic replicate this idea without fooling with machining an aluminum intake......far easier to say the least.  These btw were racer parts for track tuning but it would work on any streeter too.   

Here's a homemade version that depicts what I'm talking about: https://www.ebay.com/i/351974502747?chn=ps&dispItem=1