When is too big to big

[David Doddek]

>A question for all of us interested in vagaries of injection.
>
>I put Injection on my race car and lost 40 horsepower.
>
>History
>Engine  ISUZU DOHC 2 valve per cylinder 4 cyl normally aspirated
>        2 litre
>        Fuel 100-103 octane leaded AvGAs
>        Exhaust 1.75 primary 1.875 secondary 4-2-1
>        Carburettors    2 x Weber 48DCOE with 2 inch long ram tubes
>                        and 3 inch spacer from port to carby
>        Ports size 39 mm valve 44mm
>        Approx horsepower at flywheel 200
>        Approx horsepower on rear wheels 145
>
>I changed nothing in engine internals , exhaust, fuel and then installed
>following injection
>
>        50mm individual throttle bodies, Injectors from Nissan FJ20
>Turbo rated to 240 bhp flow. Autronic sequential engine management (
>Very Similiar to Motec designed by the original motec designer).
>General motors bosch type fuel pump. 2.5 bar regulator mounted at end of
>fuel rail.
>        Effective inlet tract length is 7 mm longer then carby setup.
>        manifold tapered from buterfly linear to port opening. Dowelled
>manifold to head.
>
>After 12 hours on dyno maximum power at rear wheels 110 bhp ( down 35 )
>bolt carbies back on and horsepower returns.
>
>        CO sensor shows good mixtures on the injection 4.5% 4.8% from
>4000 to max 8000 rpm. and power loss is pretty much all through the rev
>range.
>
>        Can only suggest the following ideas
>        1.      Inlet increase at butterfly from 42 to 50 mm
>                and lack of venturi effect is stalling air
>                speed in inlet tract. 2 other cars I race against use
>                50mm throttles..an FJ20 16 valve 2 litre nissan ,
>                and an A15 1500 8 valve overhead valve motor .
>                both with no problems
>        2.      Some type of pulsing problem ( not sure how
>                i would work this out).
>        3.      Computer design is bad ( have tried a seperate
>                new one as well with same table in it
>                so not faulty)
>
>        I would be happy to provide as much info as is requested but not
>sure where to start.
>
>I would welcome any comments suggestions as i have spnt a fortune
>already on the dyno and rebuild an engine because of the time on dyno.
>
>Regards
>Doug Robson
>Club Car Racing Register of NSW
>Sydney Australia

I can most likely guess that the problem is in the dynamics of the intake,
not necessarly the size of the throttle boddies.  You might try connecting
all of the tubes from the cylinders to a common plenum and placing the
throttle body on the plenum.  Also try monitoring the exaust with an o2
sensor and guage instead of monitoring for co2 content.  Find a mixture of
about 12 to 1 for gas or 8 to 1 for alcie.  I have experienced similar
problems on a v8 before.  Also, the fuel curve in the computer may not be
right.  I know that it might sound crazy, but you might also try throttle
body injection.  Quite often it yields better mixture, especially at high
revs.  Hope it was some help.

David J. Doddek                                          |pantera at dris.com
Owner SGD Electronics & Development Engr for Caterpillar |h 309 685-7965
Formula SAE Team Sidewinder 94-95                        |w 309 578-2931
89 T-bird SC,  69 Fairlane w/SGD EFI                     |fx 217 428-4686
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