Tunnel Ram Plenum

[mike fabish]

What effect does the volume of the plenum of a tunnel ram have on torque, horsepower, RPM of the engine? The intake runner length is a factor but how does the plenum figure into the performance, or does it. Some of the recent discussions on carbureator sizing and the distribution and atomization of fuel upstream from the valve began to "fester" in my brain. Just hoping to better understand the whole thing!. Last year we dynoed a large volume cast tunnel ram on our 598 Big Chief and to our surprise the motor made a lot of torque way down low 4000-5000,
however the motor didn't really make any more horsepower than the single carb
manifold that we tested first that day.The T/R seemed to accelerate faster on the dyno as well. We didn't spend a lot of time optimizing the carbs on the T/R , which we borrowed from a Top Sporstman guy. The carbs were on the fat side, but not that bad. Any thoughts or comments would be appreciated. Mike fabish

[Ron E]

All good questions. I hope some who have done specific plenum tests will jump in. The single 4bbl intake you were using, was that a Dart single plane? As far as the increased torque, how did the runner lengths of the T/R compare to an average of the 4bbl intake? All I can share on plenum sizing is, one that's too big can still look good on the dyno, and be a turd on the track. They don't recover well. You very well may be past that level of understanding, but, I'm trying to bait one of the smart guys.

[mike fabish]

Ron E : Thanks for responding, I wasn't trying to "bait" anyone , I was hoping to start some discussion and learn something about intakes/TR's.
The single carbed manifold was an Edlebrock that was fully ported by Wilson Manifolds last Winter. We had used the Single Dart manifold but wanted to try something different. We wanted to try one of Darin Morgans
USP but it didn't fit well, there was not enough China rail to cover the port roofline on the heads [CFE] . Because the Edlebrock matched up fairly well
we had it done. The tunnle ram's runners appear longer by 1.5"[roughguestimate] but it was the plenum and the volume of it, and its impact on performance that I was interested in. The tunnle ram is a Schmidt Motor Works and we dynoed it last year and saw a lot of torque down low which surprised everyone. Generally the engine made more power at a lower rpm with the T/R and I was trying to find out why.Peak torque was 896@5500 with
HP 1146 @ 7400. The Wilson @ 5500 838 ft. lbs. with peak @ 6200 of 872, the HP was 1152 @7500. At 7000rpm The T/R made 1126 and @7000 the
singleplane made 1145. I was under the "assumption" that the T/R would run upstairs in the rpm range but it appears that may not be true. Mike.

[Big Dave]

Plenum volume and shape can effect signal carb sees similarly to the way a muffler muffles noise in a chambered exhaust. The pulses traveling up the runner from the valve seat can reflect around in the plenum and cancel each other out.

A couple of engine simulators I have seen use and assume the tunnel ram manifolds act like individual runners, which it often does equate to in power gains and tuning. But just like adding a spacer effects performance and the plenum volume with a single four application, the same must be assumed for the volume and shape of the tunnel ram plenum.

My understanding was in the days before the advent of 4500 series carburetors a plenum allowed more than one barrel of a carb to feed a single intake runner if the flow of that runner exceeded the cfm available from that barrel. Today with multiple huge by large carbs I do not know if that theory still holds sway; but then as many a street fighter has learned, no matter how big you are somewhere else is someone bigger than you.

I would think some one like Wilson could answer this question, though from the number of racers who cover their induction systems with blankets and wraps, and use a top secret need to know mentality this question may not be answered any time soon.


Big Dave

[Erland Cox]

The plenum acts like a pulse damper between the ports and the carbs. If the plenum is to small it is possible for air already drawn through the carbs to be pushed back up through the carb again to draw fuel a second time and this makes the engine run rich. If bsfc increases at higher revs the plenum can be to small. On a V8 engine with a dual 4-barrel tunnelram the plenum volume usually is between 50-80% of the engines volume depending on rpm. Erland.

[SchmidtMotorWorks]

Mike,

Two questions come to mind:

What did the port match look like?

What top did you use?

I have made a few different top styles, some with a merged design and others with a tapered booster. I haven't been able to find a certain rule but a few people that switched from the booster style to the merged style said they found a gain in the upper range.

[mike fabish]

Jon, We used the manifold as cast. Your oval port finish is smaller than our rectangular port opening, and our port opening tranistions to an oval about 1.5" 's into the port. The 'OVALS' are about the same size, the heads oval may be slightly larger. I'm assuming we used the standard dominator top. Mike

[SchmidtMotorWorks]

My guess is that if the runners weren't port matched that it may have been too restricted. That manifold fits a variety of heads with ports with different sizes and positions so I had to leave about 0.100 stock per side at the opening. If you do decide to match it, the work isn't difficult because the added stock is mostly in the the area of 1" from the flange.

I made a couple of different Dominator tops that were "standard" in thier time. While most people bought the Booster Plate design (it looked the coolest), I think the simple plate with either shear plates or the flow plates I make are at least as good in most cases and considerably better in some.

[Troy Patterson]

My two cents;

Plenum volume / manifold design aside, I'd like to know more about the single carb and the tunnel ram carbs, how they were set up and to what degree the a/f calibration was optimized, there respective dyno and / or track numbers if available.

Again, plenum volume / manifold design aside, there is a possibiltiy the fuel curve delivered to the engine by the tunnel ram carbs could have been closer to the needs of the engine, whereas the single carb as built, may not have, or, probably would not allow such a fuel curve.

Curiously, people tend to see cross rams as low to mid range intakes, and tunnel rams as mid to high end. Realistically, a tunnel ram is a cross ram stood up, and vise versa.

I remember tuning a guys cammed 440 Chrysler running the long early '60's cross ram intake and a couple of Competition Series Carters. The engine was in a owner built 4 door crew cab pick up (before the factories built them) with an 8' bed. A real tank at 6000 lbs.

Without getting crazy with carburetor modification, the thing would burn rubber from a 10 mph roll without difficulty - at 1500-2000 rpm. I was surprised as it was my first experience with the manifold.

I am all for free flowing runners and plenums.

The rich running tunnel ram carbs may have compensated for something. I had a 400 Pontiac I ran (for experimental purposes only) a 108 lca cam, 244* @ .050". I was also experimenting wildly with carb circuitry and so forth at that time. I inadverdently ended up with the engine pulling the equivelent of a main jet of say 120. The thing smoke badly, but never ran stronger, responded stronger (at any rpm), nor turned the '67 Firebird sideways in 2nd gear faster.

I realized a criticle rule about overlap. Too much allows the working fluid to escape. I have observed this time and again. If the t/r carbs ran terribly rich and made you some power, it could mean something.

Give us some specs on the motor.

[N/A Power]

I have first hand experience with SBC plenum tuning, my 406" motor ran well with enlarged Holley Pro dominator T/R plenum. Next I ran this same setup including carbs, on a 331" with no initial changes. The motor would hesitate going to wide open(even carefully)than it would accellerate decent until the gear change, where it would stutter briefly until about 8k.
I added a stuffing block to plenum and cured all of its sluggishness.I dont have measured volumes for plenum and spacer but i would estimate about 20% reduction. The car matched and bested the 406" 60 ft. time with the small motor, so it was a nice tuning mod. Hope this helps.

[mike fabish]

I'll dig out the dyno sheets and post that info along with engine spec s later this evening. I'm really pressed for time this morning! Mike.

[67RS502]

Aside from the 1.5" longer runner length of the TR what was the CSA of the TR compared with the single carb intake? I'm guessin the single carb intake has a larger runner CSA (more tapor too?), and its the reason why it made better high rpm power.

[shawn]

I agree with what everyone else has said. You need to get runner length, cross section, taper, and plenum volume on both manifolds to be able to evaluate the difference and why. Another thing you can look at is your fuel flow in the higher rpm's. One phenomina that occurs is a "resonance" in some tunnel ram type intakes. It doesn't seem to happen on all of them, or all engines, but in higher rpms the engine will quit taking fuel and when you think it should just be starting to rip, it lays over. There is different durometer rubber inserts you can use between the intake and the top as well as spacers that allow the carbs to "float" on o-rings that all but eliminate this problem. It's some thing else to look at.
shawn

[SchmidtMotorWorks]

The dimensions of the runners on that manifold are:

Length is close to 6.000 on most combinations
Plenum opening 2.950 with 0.375 rad (csa 6.835)
Flange Opening matched to heads (1.660 x 2.405 as cast w 0.650 rad)
(csa 3.630)

The plenum volume is 400 cubic inches


The runner taper if both ends were round is 3.81 degrees per side and ports aren't matched (a little less if they are)

Flange end = 2.150 dia

[Strange Magic]

I truely think that your variables and your findings are inconclusive due to the fact that different carbs were used during testing. I wouldn't conclude your findings to be gospel. You can sit all day long on the dyno back to back testing (ABA) dual dominators and find significant power gains and significant power losses. If you really are looking for true results you will need at least 2 to 3 days of testing with manifolds and carbs and dyno correction must be done frequently as well during the daily process and day to day process.