Will machinig more of the divider out of
the plennum end of the runner decrease runner
length and end cause that porton of the runner to act like part of the plennum?
Already portmatched to bigmouth gaskets
accel base and modified plennum for siamese
runners. Will these mods improve top end HP
and torque?

 

Theoretically, yes. I could tell you for sure, if my new tranny ever gets here. If you're using the stock cam and or heads, there is a chance you won't see gains, but seeing you have a 383, I'll assume you have better.

 

go one better and remove all the material in the slp runners there by decreasing the runner volume about 6 3/4 inches making the intake about the same length as a super ram setup maybe a little shorter giving you the extra rpms you need

 

Beware - The farther down the runner you go, with the wall, the less likely that portion of the siamesed runner will act like a common plenum, and the less likely you can rely on the engine power being a linear function of runner length and RPM. Here's an excerpt of a message I wrote on the subject last week:

---
Machining out the divider will indeed increase plenum volume. However, the
downside may be loss of off-idle throttle response. Low-RPM torque will decrease, and upper power may very well increase, but the ratio of low-RPM torque loss to upper RPM torque gain is something that you cannot predict. You cannot know except through direct experimentation because a fully siamesed runner is not the same thing as the common plenum: The assumtion works at the entrance of the runner, but the farther down the runner you move, the less accurate this assumption. The idea of a short versus
long runner is related to something called "Helmholtz resonance theory" in which
a reflected pressure wave caused by the opening of the valve, and the rapid
expansion of gas into the vacuum of the cylinder propogates upwards through the
runner like a spring, and rams air into the cylinder when the valve opening coincides with the resonance frequency of the air column. For Helmholtz theory to work, the assumtion is that all the tubes end in a common plenum. If we siamese a runner down to the cylinder head, then the runner doesn't really end in a common plenum. It ends in an air cavity which is shared by only one other
runner tube: The closest cylinder. Helmholtz theory, and the assumption that runner length is tuned to a given frequency, is lost to several other non-linear variables which make it nearly impossible to compute.

---

However, practical experience (from others) has shown that if you siamese the runner down to the intake, you'll generally see some increase in upper RPM power, with some loss of low-end torque. I'm sure a dyno graph would show some interesting dips and peaks in power production that are not there in a true common-plenum design.

Some folks on the third-gen f-body.org mailing list are currently taking this even farther by machining out the whole runner divider, then machining out a couple of inches of the divider in the INTAKE as well!

------------------
Daniel Burk
http://www.isthq.com/~dan/fcar.html
'84 Trans Am WS6/L69
KB SFC, Moser axles, Torsen Diff. PST suspension, Braided stainless brake lines, Koni struts, 11-inch rear disks,Spohn Adj. torque arm,
Ported 305 heads w/1.94"intake valves, Comp Cams XE262H, Griffen alum. radiator,
Turbine Technologies 2500 stall converter, underdrive pulleys, Crane Hi-6 & more.
1.05g skidpad verified.
New best E/T! 14.039 at 100.82 MPH in 41 degree air at Stanton, Michigan.

 

I took my intake siamesed half way and epoxied the egr bleed holes shut.

I was afraid of getting too close to the injectors and having mixture distribution problems at lower rpms. At that point I would have created the 'carbeurated' port fuel injection.

I'm not sure if the reverse pressure wave will carry fuel back up that far, but with a batch fire system firing half the charge on top of the valve, I didn't want to find out the hard way.

I havn't experienced any problems as of yet. These kinds of setups would have to be matched to a higher end cam and low rear gears, high stall converters to really be effective.

Just a guess, but this setup will net a sizable performance loss on a stock cam, 2.73 rear type car.