When is too big to big

[Doug Robson]

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