small block : trying to shift the power band.. 7 "open plenum vs 4-5 "open plenum depths . current depth about 7" .power is best in the upper rpms . I need to drop the rpms so I gain more use of the 4-6 k rpm range and use more of the torque. would this be that much help . don't need a ton . now is best in the 5-7500 range . don't want to change the gearing. opinions .??

If you have the room to mill it down that much, I don't feel you would hurt anything. Worst case, if it doesn't work, you could put spacers back to get your plenum depth in line. Maybe advancing the cam CL would help lower the power band.

Is there a turtle for your manifold?

It would be nice to know, with some punctuation, what it is you want to accomplish other than shift the torque curve down.

Do you have enough converter or gear, or ar you trying to bandaid that situation by modifying the intake manifold?

Is the cam incorrect for your application? Is the cam timed correctly?

I ask because the very LAST place I'm going to correct a power issue like you are claiming is to mill the plenum down. That seems to me to be an odd way to do what you think you want. I'm not saying you don't need to correct the power curve, I'm saying that's the LAST way I'd attempt what you want.


I'd be interested to hear what other guys who do heads and intakes think.

Lengthen the runner dividers into the plenum that should help some torque improvement mid range, Good Luck

I would not shorten the plenum depth based on my own experiences adding spacer under the carbs helping the bottom end on a nearly stock 1969 440 HP cop car motor Almost every NHRA SS single four barrel motor being raced have tall plenum, but they do have high stall converters also
Advancing the cam will really help the bottom end, I'm not sure how much on your deal as I don't know the cam specs
Every Mopar street and strip or race only motor I've advance the cams on went faster at the track and felt better down low on the street as well as picked up torque on the engine dyno

sometimes I try to give more info than I need to. why do they make 3", 4",5",6" and 7" plenum depth manifolds ? from the shortest to the tallest , what if any effect will it have on the rpm range for torque and ,or horsepower ? I am well aware of advancing the cam. I am curious about how great the effect of plenum depth is in general. all open plenum ......

I do have 1 weiand x - pert [old ] with an open plenum with 4" depth and a turtle !current is a flat bottom open 7 " deep.

Try them both at the track and make sure and flog the tuning, jetting and timing, for the best results on both on both of them and let us know your results
I don't know the answer your seeking, sorry
The M1 single plane BB intakes have a lot more depth in them than the Eddy, Weiand and Offy single plane intakes I've used in bracket cars, I didn't do testing on any of them comparing them to each others at the track.
I did get to test a Eddy Torker 2 intake against a Eddy performer RPM and then a Mopar M1 single plane 440 on the same day on the same motor on the dyno with the same carb and pump fuel
The RPM was better than the M1 below 5500 RPM at WOT and both the RPM and M1 where 20+ HP better than the Torker 2 was from 2500 RPM tp 6500 RPM IHTHs

Sometimes it rules, sometimes it's just being able to fit something under the hood that gets different plenum volumes.

In my experience, adding volume to the plenum gained bottom without a notice drop in upper RPM power, to the point of making it taller just made it taller.

It's been years since I tested spacers like that. Some of what I've found is most intakes like the bottom of the carb farther away from the floor of the manifold than what comes stock on them. Again, that's almost always a hood clearance issue.

The closer the carb is to the bottom of the intake, the harder it is for the air/fuel to make the turn into the port. At low speeds this is much worse. The air can make the turn, but the fuel can't and it ends up hitting the floor of the intake. Once the fuel falls out of the air, it's damn hard to get it back into it.

Many low speed fuel distribution issues can be helped with just a simple 1 inch open hole spacer.

Again, before you lower the carb, I'd look for other ways to get what you want.

I have access to a flow bench with a fixture that is adjustable so you can raise or lower the floor of the plenum on an intake manifold and test what happens. Interesting testing.

From my experience, a 1.5" change in plenum height on my hemi moved the power enough to change my 60' ET by over half a tenth. HUGE. Runners were also slightly larger, but not much. Both were single dominator cast manifolds. I test on the track, not the dyno.
Going from 7" to 5" should do what you want. If it isn't enough, look into a Brodix turtle or extend the runners.

manifold has the turtle ... its 2" shorter . I advanced the cam 4 deg also . worked on exhaust system to gain a little velocity too ! ! I think it will be huge myself !!

I I kinda asked a question like yours,dint get too much feed back,trying to decide which manifold to use here

ok... 3 " less plenum depth = dropped the power 800 rpms earlier at the top . as for overall power -- added none to and took none away.. it did pick up a little in the lower - mid rpms. prb. just down on compression , worn out...

What Super Stockers do with plenum size increases wouldn't apply here as they are stock carb CFM limited.

I disagree, if the carb remains the same size and model the results on making the plenum bigger or smaller should have the same results
I like to compare a inline dual quad intake to a tunnel ram when using the same carbs on both intakes on discussions like this
I think the more you move, the longer it is in motion, the better the mixture gets on atomization

true ... but , getting that mixture going in the first place would take longer .. right ?? so the ultra hi rpm's would keep it rolling ? right ? short plenum = faster air at less rpm 's ??

I don't think the air will flow faster in a small plenum intake compared to the bigger plenum if every other part remains the same Every motor I've put a carb spacer on like them, regardless on the street, track or on a engine dyno test

Post hi-jack...Actually you got a bunch of VERY GOOD feedback but, you didn't recognize it apparently. Sincerely, don't take that wrong. I am pointing it out so you may consider.
Back to the orig. post... There is so much more to an intake manifolds performance than solely 'heights' so when you said 'sometimes I give too much info' I feel for you to get an answer you should have posted A LOT more info. It sounds like you have an interesting combo, if it wouldn't hurt you to share it more. And to your credit you did make a change and posted your results.

Do you have any pics of both these manifolds? Is one a fabricated piece? are they dual carb intakes? Do you have anti-reversion plates? What if the amount you lowered the plenum made an improvement in the way the carb sat in the hoodscoop? So many things really part of that interplay.