What to do.

Here you can find info on what to do when you have L-jet installed.

The throttle plate

The size of the throttle plate governs the amount of air that can pass during a set period of time. Thus it also control available power to the engine as well as any other component in the intake tract. I've found the following to be a decent approximation:

So to find the appropriate bore you need approximate the power you have and substitute to the following:

Bore=SQRT(Power/130)*53 [mm].

This approximation gives for a 200hp engine:


This should be somewhere near the truth (For naturally aspirated engines). For turbos without intercooler you should use power=enginepower/SQRT(absolute mainfold pressure [bar]). With intercooler use a figure about 10% smaller. Not really experienced with turbos, try Mr.Kalalahti.

The Air-flow meter.

Couple of things you can do with this one. First, loosening the gear inside the meter tends to richen the mixture. 3 Notches give about 2% change in lambda. About 5 notches should be enough to bring cars with cats the 3-5% power increase advertised by some 'tuning' facilities. Use a lambda sensor for accurate mixture calibration.

The flap-type sensor itself is quite a big restriction in the air-flow. The only way to help this is to buy a bigger meter. To be sure whether the restriction really lies within the meter or somewhere else, measure the pressure drop in the meter. It should remain somewhat constant in whole range of WOT revs. If the pressure drop starts to climb fast when approaching high revs, it is likely that the meter is a restriction. My experiments showed a steady drop of .7InHg rising to 1InHg when the restriction started (At 140hp in a 4-cyl 2.0 Alfa with total of 1.5InHg at inlet valve with WOT and 50X50mm meter bore).

The intake manifold.

Making your own manifold is possible but very hard. If you find the restriction lies within the manifold, refer to acoustical engineering books to find the data you need to tune the plenum&runners. If you go this way, drop me a mail. I made my own manifold with 36mm runners.

To check if your runners are too small measure the bore and refer to any book on webers or such to find the _minimum_ bore you need for enough airflow at given displacement/revs. Also check that the ports in the head are big enough. The easy cure for too small runners is to fit multiple throttles that go into a manifold intended for sidedraught carbs. You still need to check the ports.

The Air filter

If you don't have K&N or something resembling K&N, buy it. Standard airboxes are crap, even if the filter element is good. I've fought enough with filters and suggest you pay the price for K&N. I'm somewhat poor so I made my imitation from an element from a Nissan 1800 Diesel :) Worked very well and gave no restriction. Still, I would have bought a K&N if I had retained the injection.

Fuel pressure regulator

This is thought to have the magic touch, to give power beoynd limits. Crap. The fuel delivery rate rises as SQRT(deltaP). Rising rate regulators are said to improve throttle response. Granted, if you have 100000m of intake plumming with million kinks in it and with .1inch diameter you could have some lag and these could be worth buying. Or if you have a turbo engine that needs rich mixtures with boost to reduce knocking. In my system I had 70cm intake tract with 63mm pipes and had no lag whatsoever. If you want to get richer mixture an adjustable regulator is one solution. Or if you've run out of delivery on the injectors it could help. Just remember to calibrate the air-flow afterwards.

This info is based on my own experiments. If you don't believe it, try out yourself and mail me the results.

A program called Revtest is available for Commodore Amiga. It is used for approximation of engine power&torque and is used in all of the power measurements I've represented on the other pages.

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