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Discussion Starter · #1 ·
The Bosch K-jetron

The Bosch K-jetronic fuel injection system is a brilliant piece of mechanical fluid dynamic engineering that first appeared on German cars (Porsche, MB, VW) in the 70s and was adopted by Ferrari beginning in 1980 to meet ever tightening emissions standards. The thing to remember about this elegant bit of kit is that it was designed to primarily meet emissions regulations - it is NOT a performance fuel injection system. It's main problem from a performance point of view is that while it delivers stunningly accurate air/fuel ratios (AFR), the air must WORK its wat through a tortuous ducting system through a single throttle body after WORKING to lift the airmass flow sensor plate which otherwise BLOCKS the flow of air into the engine. In short, there is a lot of resistance to airflow. To complicate things a bit, the Kjet and Marelli ignition systems are wholly separate and do not talk to each other.

Mods available
Short of replacing the Kjet with Webers or Electronic fuel injection that both allow more AIRFLOW into the engine, Kjet mods primarily focus on decreasing resistance of the Bosch system.

1. Tune the Kjet to factory specs. I know it's obvious, but the system is going to deliver it's best performance from a good tune-up from someone with an exhaust gas sniffer and setting the CO, which sets the mixture. When set correctly, the ability of the Kjet to deliver a constant AFR is amazing. While maybe not meeting emissions, the best AFR is about at 13.2 or so. Stock Kjet settings keep the AFR a round 14, or a bit more. We're not talking gobs of hp here between 13.2 and 14, but a couple - certainly not with the required mild specification cams required for Kjet to work well.

2. K&N air filter. Everybody's got a differnt opinion about these re-usable high flow filters. I personally have seen them flow more air to the tune of about 5+ hp at high rpm settings on a chassis dyno and have 'heard tell' of close to 10 hp on a 328. I've also heard they don't do squat. Well, I've seen it with my own eyes and think they do allow more flow in the 5hp+ range, but nothing dramatic.

3. Bigger throttle body. Ed Maszula (advertises in 'Excellence', the Porsche mag) will bore out your throttle body to allow less resistance at this point. Matt 'Kermit' Morgan who runs a Ferrari aftermarket shop ( specializes in this service for Ferraris and has flow benched a 10% increase in airflow boring out to 69 - 70mm and adding a new throttle plate. He will be dynoing to check results soon. While theoretical, there is a possibility with throttle body boring that a little low end responsiveness will be lost in favor of more top end, but will have to wait on some driving reports. I don't believe that drivability will take a big hit myself.

4. That big ol' ugly serrated black rubber air hose that goes from the fuel distributor to the throttle body can't be good for flow. Durable1 (above) makes a smooth carbon fiber tube replacement to help airflow.

To be complete, there is a whole Kjet system upgrade, the Lenz Motorentechnik(, a German system which adds an ECU that controls the Kjet through the frequency valve and replaces the Marelli ECU tying the two together with more aggresive curves. The system also comes with a new exhaust. They claim 300hp (DIN?) from a 3.2 Euro motor initially factory rated at 270. Well, I'm not sure I believe that, but I'm sure it helps some. Check it out and let us know. I think the whole thing is $3 - 4K.

Cams - besides flow, the cam profiles in Kjet systems are very mild to prevent reversion waves that would confuse the airmass sensor plate. I would bet that the Ferrari cams are as aggressive as possible, although I know the Porsche community has experimented with grinds. Experienced cam folks like Elgin ( and Web-Cam ( can speak to regrinding, but the stock overlap of 24 - 26 degrees is probably about it.

That's it for now. We haven't talked about exhausts, emissions systems or other ignition systems - will save that for separate posts. Please add on your experiences and new systems as you find them. The Ferrari V-8 is a great engine and a lot of fun and is even more impressive the more you get to know it thru little tweaks to help it do what it was designed to do in the first place!
Hope this helps get folks started
best to all

Discussion Starter · #2 ·
Very nice info Russ, and very

Very nice info Russ, and very accurate imo.

The one part of the CIS system that can be modded to slightly decrease the Air Resistance of the Flow Sensor Plate is Fuel Pressure (system pressure). The operating priciple behind the basic CIS system is Higher Fuel Pessure = More AFP Resistance = Leaner Fuel Mixture, Lower Fuel Pressure = Less AFP Resistance = Richer Mixture.. The System Pressure is controlled by the Aux. FP Regulator. On the regulator is a plug set in the housing applying the exact pressure required by the spring & fulcrum inside to regulate the FP at a constant (after warm up). By changing the relationship of this plug to the internals also changes warm running pressure. The APR cannot be adjusted without specic tools & gauges, but with the proper knowledge & tools enhancements are possible, especially if one is running the larger throttle body.

Regards, JRV

2 Posts

We own a 1984 Ferrari 400i GT (VIN ZFFEB07B 0000 49041) with apparent ignition problems.

Car has 35,000 miles from new and until now has run strong and smooth.

On a long run from Atlanta to home here in SW Virginia yesterday, car began to cut out after 2 hours running. We managed to pull off at a truck stop, "popped" the hood and let it cool. After 1/2 hour, we restarted engine and drove away. About 2 hours later, same symptoms, same solution. Our last leg was north and at increasing elevation (cooler ambient temperatures as evening fell). No further problems. (it is not fuel related...we checked operation during running.)

We would like to replace this unit with an aftermarked Ignition System. Do you offer such a system, or can you recommend same?

Thanks in advance for your kind consideration and reply.

Ed and Cheryl Trottier
Moneta, VA
[email protected]
[email protected]
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