I have the opportunity to get a ported siamesed plenum for my '90 Vette. Can I use the stock tube runners? I would probably have to use the stock gaskets and modify them for the ported plenum, but I assume the gaskets should match the tube runners. Would the smaller tube runners cause any problems with the larger ports in the plenum? Any information or ideas would be appreciated. The plenum isn't ported yet. Would it be better to just gasket match the ports to the stock gaskets? Thanks.

 

well if the plenum is siamesed then you'd want to go into the runner walls just enough to smooth outh the transition for the airflow (and match your gaskets accordingly). So go ahead and get the ported plenum and then match port the runner walls to that. There are websites and pictures of this around (if not, email me and I can shoot you some), but I'm not sure just how BIG the difference will be for you. I think most people siamese the BASE and runners for more RPMs in tandem with this mod.

Couldn't hurt though. :D

 

I got most of the pictures off thirdgen I did not photograph my engine, but heres what I found out! doing several similar engine mods

(1)almost any mod that increases air flow into the cylinders helps some

(2)the stock TPI intake is a huge restriction on airflow into the cylinders

(3)siameseing the ports and blending the runners and plenum entrances on a set of SLP runners worked very well (like this)


about 3" into the upper runner area then installing them on an accel or EDELBROCK ported base intake
is about the best combo Ive found on a basically stock config. TPI intake
having done similar mods and having dynoed the results (HERES A SIMILAR DYNO RUN) I can tell you that the results will be a loss in low rpm torque, below 5000rpm where your engine spends 90% of its time and a loss in volumetric cylinder filling efficiency below 5000rpm and a slight gain in hp over about 5000rpm due to the shorter runner length lower restriction to the air flow, (stock the TPI flows about 190cfm)
http://www.eecis.udel.edu/~davis/z28...age010511.html

notice the loss of 60 ft lbs of torque and the gain of only 10 hp! look 1/2 way down the page at the chart

now what would help would be porting just the upper 3 inches of the siamesed SLP runners and the PLENUM and useing an EDLEBROCK base with its bigger port size and straiter runners.

look the stock TPI has runners 15" long with about 3 more inches of runner in the cylinder heads , for a total of 18" of runner length, with that length runner both the 3rd and 4th harmonic wave help fill the cylinder WITHIN the ENGINES RPM RANGE
http://www.bgsoflex.com/cgi-bin/intakeln?length=18

BUT shorten the runner length to about 6" like THIS

proved counter productive by in effect extending the plenum volume down into the base of the intake and bye making the runners for all intents just an extention of the plenum and you get this result
http://www.bgsoflex.com/cgi-bin/intakeln?length=6

all the benefits of intake harmonics WELL ABOVE THE RPM RANGE OF THE ENGINE

but look what just shortening the runners by 3" does
http://www.bgsoflex.com/cgi-bin/intakeln?length=15

now Im not saying you won,t get a gain in high rpms, but your low and mid range will cancel some if not most of the gains you could have gotten buy porting the base intake to the max and only porting the upper 3" of the runners and plenum on a basically stock lay-out TPI intake

while the combined work and costs involved exceed the HOLLEY STEALTH RAM and the RESULTS are far less BECAUSE the HOLLEY STEALTH RAM has MUCH BETTER air flow (about 300cfm) and a cross sectional area that greatly improves the air flow at higher rpms and RUNNERS that give a strait shot at the back of the intake valves that are effectively about 11-12 inchs long measured from plenum roof to the back of the intake valve
http://www.bgsoflex.com/cgi-bin/intakeln?length=11 http://www.bgsoflex.com/cgi-bin/intakeln?length=12


[Modified by grumpyvette, 8:08 PM 3/14/2003]

 

Grumpyvette, is there any dyno numbers for the siamesed base and runners/plenum combo available? I'm interested in what exactly was gained in 5000+rpm range, say up to 7000? on an engine built for upper rpm use, and not concerned with lower rpm/torque output. If you know of any, please post or give a location/direction to where they can be viewed. Thanks. :cheers:

 

I included those in the original post
look 1/2 way down the page
http://www.eecis.udel.edu/~davis/z28...age010511.html

[Modified by grumpyvette, 1:00 AM 3/15/2003]


[Modified by grumpyvette, 1:01 AM 3/15/2003]

 

I had seen that one, but was looking for test on something with more cam, more compression, and better cyl. heads. Where is a good place on the web to find info on modified TPI? Is there any other forum with good info? Thanks again for your help. :cheers:

 

Sinister, if you are looking for high rpm hp, I think there are better manifolds out there. Miniram,stealth ram, or converted carb manifold.

 

:withstupid:

I don't think anyone would intentionally build a 7000rpm TPI motor?

 

I know there are better set ups, but I was curious how far anyone has gotten with TPI. I think that with enough work, they should be capable of getting enough flow to support higher rpm use. I'm not concerned about the loss of torque, which should be minimal. I just like to be differant, and do things others say are impossible, or not worth it. If I do the work myself on the tpi unit, all I'm out is time, but going to any other efi setup would cost me money. If I do not get the results I want, I plan on going carburated then I know Ill get what I'm after. :cheers:

 

the guys are correct!

your engines max hp depends a great deal on the AMOUNT of air that can flow into the cylinders and the rpm the cylinders can EFFICIENTLY fill at.
now with cams matching the flow
the stock TPI flows about 190cfm-200cfm (siameseing the base will add only slightly to the high RPM flow but hurt low RPM tq)
(tq peak normally about 2900-3200rpm)
the highly modified aftermarket TPI flow about 235cfm-245cfm (a 22% increase in flow)
(tq peak normally about 3600-4000rpm)
the stealth ram with minor port cleanup flows about 300cfm(a 50% increase in airflow)
(tq peak about 4800-5200rpm)
now the formula for hp is (tq x rpm / 5252=hp)
if all the engines made the same peak tq (lets say 370ft lbs)look at the differance in results
TPI ....370 x 3200/5252=225hp
aftermarket TPI ....370 x 4000/5252=281hp
stealth ram ....370 x 5200/5252=366hp

the problem with that comparison is that the far greater air flow of the stealth ram drops low rpm tq and increases high rpm tq so 400-450hp with the correctly matched cam is really more likely





[Modified by grumpyvette, 2:27 AM 3/16/2003]

 

I siamesed the stock base about 1" and ported the plenum & upper runners; dyno pulls are on my site.

The gain was 20hp at 5,400rpm with an unknown loss (it's an A4) of low end torque, though 60' times only increased by .01sec or so indicating a small drop in torque.

I just opened up the cuts to about 1.5" deep but seem to have a coolant leak so it may be a while before I can get any results data.