Hemco Machine - Turbo Plenum
 
 
 
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HEMCO UPPER PLENUMS: PICK YOUR POISON

 

GS X-tra
Richard Lasseter

   
 
Poison to the competition, that is in our opinion. Hemco plenums do work and they are an easy bolt-on. We'll delve into this later, but first I'd like to repeat what many of you have discovered for yourself: there is a tot of "stuff' out there that simply doesn't work for many applications including yours. Some "stuff' doesn't seem to work in any application, and other "stuff" will actually slow you down. We've seen all of this just as you have.

We've also seen news-stand magazine T-R build-ups where they used thousands of dollars worth of parts just to go mid 12's when we all know it can be done for half that.That's called "keeping the advertisers  happy." What these magazines need to realize is that their subscribers should come first because without subscribers there would be no advertisers.

With all this being said, it's always a pleasure to cover a product that works in nearly every case we've heard of. This is how we regard the Hemco Upper Plenums. Matter of fact, we've never heard from a club member who said that they were dissatisfied with a Hemco plenum

Both units shown here are for the stock S8mm throttle body. Plenums are also available for larger throttle bodies. As you can see, one plenum is stock appearing while the other is not. Internally and performance wise, they are both the same.

Now, what will the Hemco plenum do for you? Well first of all, our test T-R is a typical "11-sec. recipe car" that runs 11.70's at over114 mph with leisurely 1.7 60 ft. times. Boost is always limited to 21 psi max, and only then thru the traps. Basic mods include T A 49 turbo. H210-205 / 111 cam, ATR 2.5" exhaust, "Blue Top" injectors and Champion iron heads. The original unrebuilt shortblock has 60,800 miles on it. This '86 T-type is a consistent bracket winner launching at 2800 , RPM with a "Red Stripe" converter sending 374 rear wheel horsepower back to MT 10.Sx26xlS slicks.  

We used Sunoco GT unleaded fuel which is rated at 100 octane. Our experience has shown that you can sneak up to 19 psi with  4-6 knock retard. This is with our test car on a 70 day. We made two runs at 19 psi max boost on this 78 afternoon with our existing set-up on the Sunoco GT 100 gasoline and got an average of 11.78/114.11 mph with several 6-8 knock retard "spikes:" On went the "early" Hemco p1enum (not stock appearing) and two runs were made. First of all, the Hemco plenum will reduce your boost due to more restriction in redirecting and equalizing air flow in all the cylinders. Most detonation and blown head gaskets occur on cylinder #3 with cylinder #6 a close second. EGT probes will show that these two cylinders will run richer (and cooler) with the Hemco plenum.

With the Hemco plenum on our engine, we made two runs after the wastegate rod was shortened a full turn to give our previous 19 psi. The average of the next two runs was an 11.71/114.73 mph with 2 knock retard at the 1-2 shift and only 1 on the 2-3 shift. Now, that's a definite improvement. On the next pair of runs, we moved Our maximum boost to 20 psi (from our 19 psi baseline) and were rewarded with an average of ll.68/115.08 with several knock retard spikes of 3-4 at both the 1-2 and 2-3 shift points. At 20 psi boost, the stock upper plenum would've leaned several cylinders to give 10-11 knock retard as past experience has shown.

We don't have to interpret these results of all this to you. All you have to do is study the above raw data and you can see that the Hemco upper plenum will help correct cylinder-to-cylinder air flow distribution and provide some performance benefits in doing so.