moregrip

iTrader: (8)

Originally Posted by Jim Bell

KENNE BELL SUPERCHARGER QUESTIONS ANSWERED

Outside of our test cars, Jeff was the very first customer to install our new 2.6 on a �99-�04 Corvette. I don�t think the instructions were even finished. Unfortunately, he and Rick also inherited some R & D issues because we had never tested a 402" engine with an 8 rib system. This resulted in a lot of speculation that put Jeff in a difficult position on a couple of issues. We had run hundreds of tests at up to 10 psi and 600 hp on the street and dyno with our �standard� 6 rib system and it does not slip. That was all the info we could give him except for some 8 rib pulleys to get started with.
Jeff was determined and agreed to do it at his own expense. We liked the idea because the 402 would be ��over the top�, fun and exciting for Corvette owners � I knew from my past experience with him at Dynojet that he would be honest and straightforward with the tests and feedback and � he was setting the car up for the street with pump gas.
So let�s get to it and hopefully answer some questions.

SUPERCHARGER PRINCIPLE OF OPERATION

There appears to be confusion about Kenne Bell Twin Screw supercharger efficiencies and how it functions. There�s a lot of information about this on our website under �Supercharger Efficiency Explained� that may be of interest.
Here�s a simplified explanation that may help. First of all, the TS is a positive displacement type high efficiency supercharger at any rpm and boost. Jeff�s car has our latest 2.6L which is capable of up to 1000 EHP WITHOUT ANY BOOST DROP OFF.
This type of supercharger will continue to produce more air(CFM) as engine speed increases. The airflow does not �go away� at high rpm, so boost drop off is not a function of the supercharger itself. For example our 8% smaller 2.4L on Jeff�s engine will develop the exact same air flow and boost IF it is spun 8% faster (higher ratio via a smaller pulley). HP will also be nearly identical IF the power consumption of the supercharger (engine HP to drive it) is the same. The compression of air by any supercharger requires engine HP. That is unavoidable but always a factor. Unfortunately, power consumption is a seldom discussed much misunderstood subject in the engine HP equation and is worthy of it�s own discussion. Briefly, a 2.4L, depending on application, may use only another 5 engine HP to drive but it has 8% less HP potential(920hp vs. 1000hp). The Eaton/Roots style supercharger will ALWAYS consume more engine HP to drive whereas the centrifugal and twin screws are very close in parasitic loss. Just something to consider when comparing supercharger dyno runs with the same boost. Also, if the VE(volumetric efficiency-airflow in vs. airflow out) is lower on a supercharger at a higher rpm, there will be the dreaded boost drop off.

So what else besides supercharger efficiency can cause �boost drop off� in a supercharger or naturally aspirated engine? Obviously, belt slippage but more important � restrictions in the inlet tract(filter, throttle body, tubing, mass air meter). That�s why there are larger throttle bodies, meters and filters for supercharged and naturally aspirated engines. Inlet restrictions reduce the air supply to the engine/supercharger. This is apparent on a supercharged engine via the boost gauge and HP output, but you would need a vacuum gauge for a NA engine. Consider this: 2" HG inlet loss is 1 psi of God�s boost(atmosphere weighs 14.7 psi at sea level) a mere 1 psi inlet restriction can cause a 6.8% engine power loss(14.7-13.7=1 psi. 1 psi�14.7 psi =6.8%) in an NA engine or 3.5% at 14 psi supercharged. No supercharger, particularly the positive displacement TS and Eatons, like to be starved for air. At Kenne Bell, we spend considerable time on the flow bench and dynos analyzing and testing various components. We can state once and for all that �boost drop off� is not caused by the Kenne Bell supercharger.

Example: This is the modified 4.6 4V Cobra motor out of the Earl�s Automotive/Kenne Bell test car- the first and only manual trans street legal non nitrous Cobra in the 9's. It made 960 hp at 21 psi with our 2.4l and cams on an engine dyno. There was no boost drop off with the 4.6 or 5.4 liter engines, 960 HP equates to around 1400cfm. Sorry for the Ford but we are currently comparing these engines to a 5.4l Ford GT at high HP levels. The supercharger doesn�t know or care if it�s a Ford or Chevy engine. It only cares if it can pump enough air to make the HP and boost desired.





ENGINE SIZE VS. BOOST

Next how does engine displacement affect boost? Here too, the actual formulas are on our site for those wishing to do their own calculations. If the engine displacement is larger, we merely increase the TS supercharger cfm via higher rpm(pulley change) or select a larger supercharger. The limits? The Kenne Bell 2.6L is rated at 950 HP and 25 psi max. That was all we could fit under the hood of the Corvettes. Our goal is for the supercharger to always pump more air into an engine regardless of size, cam, heads, etc., than the engine is capable of flowing. Thus, building up the desired back pressure(boost).

INLET RESTRICTIONS

If the Twin Screw supercharger size is adequate and the inlet system is NOT RESTRICTIVE, it will produce essentially the same boost from 2500 up. And, yes we dyno and flow bench tested a variety of Corvette inlet systems before deciding on the final configuration. Our combined inlet tube and filter flow of 1127 cfm, is more than enough to feed the stock 790 cfm throttle body. By comparison, the stock filter assembly flowed 751 cfm. There has also been a few question regarding the flat area of our inlet tube. This was necessary for hood clearance. The cross sectional area of the �flat� was increased and shaped to eliminate any restrictions, again it flows a whopping 1127 cfm. Someone asked if a 4" (101mm) tube would help. No it will not - by itself. Kenne Bell views an inlet system as an integral combination of components that must work together. A 4" flows 1327 cfm - but it can�t and won�t help with an undersized 3"(76mm) throttle body - or a 3.35" (85mm) mass air meter that restricts the flow of even a 90mm throttle body upgrade. However, the 4" may help if the 3.35" (85mm) mass air meter is eliminated and the TB is upped to 101mm. Get the point? It�s all about matching components on the flow bench and dyno to a particular HP level. Rick will be testing meters for Jeff to determine the actual HP gain from eliminating the meter in the 700 HP range. We concluded our production kit testing at 600 HP when the meter and injectors maxed out. We will be conducting more 5.7 test at considerably higher HP levels in the near future and comparing the results to Jeff�s 402".

CAMS AND BOOST

Finally, cams. We personally like the smooth idle of the stock Z06 cam. A good higher duration street cam will improve engine breathing and, therefore, may drop the boost around 1 psi. A 20-30 HP cam upgrade will not make the supercharger gasp for air. Don�t be concerned, no problem. Just take 4 minutes, screw on an 1/8" smaller pulley and the boost is back. Remember, it�s all about air flow - our pump making your pump(engine) more powerful with back pressure. If the supercharger has the potential HP rating, boost can be increased up to the manufacturer�s recommendations for boost and rpm. And if the boost drops off with out Twin Screw and the belt isn�t slipping, go work on upgrading the inlet system. You�re making 700 hp and want more? Just increase the boost and add more octane - that is if there is adequate fuel and engine reliability.
A good rule of thumb is it requires 1.5 octane to support 1 psi of boost which can range from 13-20 hp per psi depending on inlet restriction. Air flow is everything - the more air the more HP.
We love the Corvette market and we live superchargers and turbos. It�s all we�ve done since 1976. Hopefully, we�ve answered some of your questions.

Danville Performance

Thanks for the great explanation about the chargers and there efficiency ranges. It is always about flow rates and restrictions. The old saying about you are only as strong as your weakest link works for motors as well. Your biggest restriction is your HP limiter as well. Well Done!

AintNoHo

I love these articles on air flow theory. He mentions several ideas that caught my attention. The first is MAF restriction. In the Radix J-tube/90mm tb upgrade what size MAF is used or is it removed in favor of speed density? He also mentions going to a 101mm tb which supports my thought that 90mm tb is still too small on the Radix for a max effort.

moregrip

iTrader: (8)

now your thinking from what I remember of a conversation I had with someone,.... is that a 105mm TB would be optimal for a max effort radix application

Scott Mills

Awesome, any of you all want to unload that useless 90mm J-tube cheap?

moregrip

iTrader: (8)

couple of things to note:

Current Radix J-Tube won't support 105mm that I can see, so a a more customized setup would be needed.

No parabolic designed 105mm TB available

no readily available MAF to support it

F8L Z71

Funny, he keeps mentioning Rick. That's Rick@Synergy

The 90mm is about all youre gonna get for the stock Radix J-tube. You would have to make a custom J-Tube to utilize the extra flow of the bigger than 90mm TB. Can the Radix use that much CFM anyway? As for the MAF. Just go SD

moregrip

iTrader: (8)

I would think so as well, maybe farther down the tube you could get away with it

F8L Z71

So you are saying it might get wider the deeper down the tube you do? I've found that to be true as well

Whippled 496

iTrader: (4)

We're still talking about superchargers right?