THE TRUTH ABOUT INLET SYSTEM DESIGN
HEMI INLET SYSTEMS AIR FLOW AND HP
There is a lot of confusing and misleading information on the various Hemi inlet systems (stock, aftermarket and Kenne Bell). Next to the supercharger, our number one tech call is about inlet systems. I guess it should be as it’s by far the most important part of any supercharger system- and the first upgrade made to any N/A or supercharged car or truck.
INLET SYSTEM PRESSURE / HP LOSS
Does anyone really believe that a relatively high restriction stock inlet system works as well on a supercharged application? We hope not. Limit the air supply to the supercharger and you choke or starve it for air. It becomes less efficient, runs hotter and uses more engine HP to drive it. That applies to all supercharger kits. But worst of all, an inlet system that incorporates ANY restriction / pressure loss to the supercharger results in a loss in HP just as your engine would see at a higher elevation. God gave us 14.7 psi of pressure / boost at sea level. That is the weight of our atmosphere at 0 elevation. That 14.7 psi pushes air (boost) into your engine. The higher the elevation the lower the atmospheric pressure (boost) and the greater the HP loss. It’s all about boost- God’s and/or the supercharger’s.
In Denver (5000 ft elevation), your engine sees about 2.5 psi less boost or 12.2 psi. 2.5 ÷ 14.7= 17% pressure / boost loss in HP or efficiency. Your 350 RWHP engine will lose 17% or up to 59HP. Now take a stock inlet that isn’t even adequate to support 350RWHP. Now let’s assume your 7 psi supercharger kit makes 475 or 35% more HP/CFM than Dodge engineers anticipated. There’s substantial pressure loss at that HP/CFM. Then the stock inlet is obviously too small and restrictive. After all, it was designed for 350HP and not 475 with a 2.3L TVS - or more with a 2.8, 3.6 or 4.2.
Say it now loses a mere 1 psi. No big deal, right? You saved some money on the “kit with the stock inlet.” Wrong. Add the 7 psi boost to 14.7 for 21.7 “absolute” pressure / boost for your engine. 1 ÷ 21.7 = 4.6% x 475HP = 21.8HP loss. Oops. And it gets worse.
Kick it up to 600 (25% more HP and air flow through the same restrictive stock inlet). Now the boost loss from inlet to restriction is 2 psi and 2 ÷ 21.7=9.2% x 600HP = 55HP loss.
Yes, the HP loss will be less at 550 and even more at 700. It’s all relative to air flow. The greater the air flow in any inlet tract, the higher the losses and the less “boost” the supercharger sees entering it. The supercharger has no choice but to suffer with less inlet pressure or speed up with a smaller pulley and consume more engine HP to drive it and run hotter! No one inlet system fits all unless it’s bigger than the maximum kit HP potential.
We have done thousands of tests on supercharger inlet system components. One thing you can bet your house on. As HP increases so does air flow (approx 1.5 CFM/HP) and restriction (HP loss).
The best solution is to select the BIGGEST HIGHEST CFM system for your supercharger. Keeping in mind at all times that HP / air flow to the supercharger will ALWAYS BE LIMITED TO THE SMALLEST COMPONENT IN THE INLET TRACT (filter, pipe, elbow, throttle body and inlet manifold). As the old saying goes “You are going to pay for a good inlet system whether you get it or not.”
Since our first Twin Screw kit in 1990, we knew the most important consideration for any positive displacement supercharger (Twin Screw or Roots type Eaton) was a low restriction inlet tract. They all love “0" restriction. The bigger the better at ANY HP or air flow. And the larger the inlet, the greater the HP per psi of boost. We used our own design - the first Ram Air Kit for a Mustang, a 4.5" hose to an under bumper air scoop.
THE HEMI INLET SYSTEM
Back in 2008 we bought a new Challenger and flow tested the complete inlet system piece by piece and collectively. You can see some of the test and data on our website at Hemi 5.7 Supercharger
The complete Dodge inlet system (canister, filter, hose and throttle body) flows a wimpy 744 CFM and a little more with a K&N which isn’t much help in the stock canister. By contrast, the Camaro stock inlet is much better, 964CFM.
The 81mm stock Dodge TB is 924CFM. See anything wrong here? Remember any inlet will only flow as much air as the smallest component.
Good ‘ol common sense tell us that this Dodge 744CFM inlet coupled to a 924CFM TB is NOT in the best interest of your Hemi, especially at higher HP levels as on a supercharger kit. The inlet tract starves the TB. Think you still want one of those cheaper “good enough” supercharger kits using the stock inlet system?
That’s probably why there’s a dozen aftermarket companies selling replacement CAK’s for the Hemis. The best one we tested was 1087CFM (see website) and it was 25HP at around the 450HP supercharged level. But now the 1087CFM CAK is limited by the 924 CFM TB. Oops.
One of the best written and most informative tech features ever done on throttle bodies is by Richard Holdener. Testing took 3 solid days. Tests were on a Ford but it’s all applicable. Check it out on the KB website Tech Articles under Throttle Bodies, “Air Apparent.” It is guaranteed to put it all into perspective.
HEMI THROTTLE BODY UPGRADES
The “ported” 81mm throttle body we tested didn’t flow more air because the throttle blade and exit size REMAINED AT 81MM. Same size hole as the manifold.
A larger throttle body? Let’s pause here a moment and briefly discuss one of the most ridiculous product applications we’ve ever seen- a larger 90mm throttle body that bolts to the 81mm stock manifold hole (the same one that matches the 81mm stock TB).
Careful with that one if using the stock 81mm Hemi manifold as with a centrifugal or N/A. It may be possible to bore out the stock Hemi manifolds to 90mm. Never tried it. Looks shaky to us. But a 90mm TB typically flows 1150CFM. A stock Camaro uses a 90mm.
A 1380CFM 102mm is good for up to 40HP over a 90mm on a Camaro or Hemi. So logic told us that higher HP / air flow would be most efficient with a larger throttle body and a CAK with 4.5" Kenne Bell design hose and filter, which flows 1850CFM as compared to the “smaller” 3.5-4" 1087CFM. If the throttle body, inlet manifold and intercooler is adequately sized to support a 4.5" 1850CFM inlet supply, it’s all there. No upgrading would ever be necessary.
KENNE BELL CONCEPT
Therein is why Kenne Bell chose to use the maximum size 4.5" on ALL our Mammoth kits 8-28 psi. It’s coupled to a rear inlet manifold that accepts a 148mm 1850CFM or 168mm 2350CFM throttle body. This 4.5" inlet system will support big HP (up to 1400HP) while providing max air flow and HP to our smaller standard 2.8L or larger 3.6 and 4.2 direct bolt on upgrades. All KB 2.8 entry level 8-10 psi 91-93 octane kits produce max HP per psi boost as they are over engineered for minimum inlet restriction.
Our 4.5" concept was sold on thousands of 86-12 Mustangs and Cobras. Also the infamous Buick Turbo 84-87 GN’s. So the concept is hardly new.
Basically, we started with a 4.2 max HP kit that also accepted our lower capacity 3.6 and 2.8 superchargers. That covers a range of 650-1400HP. The downside is our Mammoth concept on the 2.8 costs more. It is the only way a KB kit is offered. But it makes more top end HP, leaves nothing on the table, is easy to upgrade and offers tons of room to grow by merely replacing the throttle body and supercharger.
If you choose not to buy or upgrade to a more efficient inlet system and need an alternate HP source, there’s always headers. They aren’t cheap either. Considerably more than the cost of an efficient inlet. And remember that headers drop boost 1 psi requiring the supercharger be spun faster, thereby using up progressively more engine HP. Then the 35HP and the additional +7.3% air flow through the inlet (35 ÷ 475=7.3%) still results in even higher inlet boost loses. “Small” never gets bigger and more efficient by itself. Larger inlets are simply a no lose investment.
The lower initial cost of a supercharger kit with a HP robbing inlet system may appear attractive until one considers the real data and facts. The selling price of a Kenne Bell kit would be $600 less with a stock or under-hood “filter on a stick.”
HOT AIR KITS
When will we get this right and quit misleading folks by chassis dyno testing with the hood open instead o f CLOSED, like they drive their cars? We see 25-55HP loses vs a filter located outside the engine compartment. That’s equivalent to a set of headers. Anyone believe 200 degree under-hood air makes more HP than 70 degree outside the engine bay air. Yep, and the earth is flat. We don’t want to ever forget about our old Hemis or any drag cars with those hood scoops that directed 100% cool air to the carburetor(s). And the 500HP muscle cars of that era that used hoses. Oldsmobile, Ford, etc. ran two big 4" hoses not a single 3.5".
The stock Hemi canister sucks OUTSIDE COOL AIR from the fenderwell. Many of the “redesigned” CAK’s we’ve seen stick the filter behind the radiator and try to shroud it and seal it to the hood relying on those little openings around the radiator to supply cool air and a rubber seal to the hood. We’d like to see the temps/HP tests on these things. At best, the supercharger sees a mix of hot and cold air. The 2011 5.0 Mustang and 2010 Camaro use little snorkels in FRONT of the radiator to suck in 100% cool air. Again, the Dodge uses cool fenderwell air which is okay for an aftermarket kit. There is a lot happening under the hood. Hot air from the engine, exhaust manifolds or headers and radiator (that’s a bunch) all competing to get to that relocated Hemi filter.
Consider this. A 4x3x1 deep engine bay cavity is 12 cu ft. At 500HP, the filter is inhaling 750 cu ft./min or 12.5 cu ft./second! That’s all the air in the engine compartment every second. We’ve seen a fender “suck in” on a Cobra if unable to get sufficient outside air flow from the fenderwell. We have all the dyno (hood open vs closed), drag strip, idle and street temp and HP data. Again, same as the 60's. The more things change the more they’re the same.
We hope this explains why Kenne Bell goes through the expense of locating the filter and air source OUTSIDE the engine compartment. Fresh, cool unrestricted air flow has always been and always will be the key to max HP. They can dance, run and spin but they can’t hide from this one.
Also, if selecting a larger more efficient inlet system, be sure to buy it with a tune as the engine can run leaner. A 10% increase in HP will lean AF approx. 10%.