Djet in a hotter than stock cam engine

[Dave Thompson]

I recently bought a 1.8 built to 2.0 liter engine from a PCA club member I know who raced his 914 in club racing. He had bought the engine from an owner who had it built for autocross and carbs.
As my 2 liter had the stock FI on it when it blew up, I decided to buy a pair of Webers and try them on this "new" engine. I finally manged to get it set up and running pretty strong, although somewhat choppy at low rpm, like a racecar.
I strongly suspect it has a somewhat hotter than stock cam.
Anyway, as I want to sell the car, I decided to take out the carbs and install my FI and I just finished. The car runs very strong and much smoother now, however I find the engine sort of surges as I approach redline in second gear ( I haven't had a chance to approach redline in any higher gears yet, as I don't relish traffic tickets). Anyway, I am curious whether this sounds like what I should expect running the stock FI with a hotter than stock cam. Where can I find help tweeking the DJet system for optimum performance under these conditions?

[Dave at Pelican Parts]

From what I hear, the most typical symptom of trying to run a really hot cam with D-jet is that the vacuum signal at idle is very very low. The D-jet "thinks" that the throttle is open because there isn't a whole lot of vacuum (due mostly to overlap on the cam lobes), so it runs very rich.

Kjell Nelin has a great "primer" on what D-jet is and how it works. http://www.914fan.net/djet.html He also has some comments on tuning the D-jet system for non-stock configurations, particularly for 1910cc displacement.

Brad Anders has set up a much more detailed web-page on D-jet, with astounding articles on the Manifold Pressure Sensor and the ECU. http://members.rennlist.com/pbanders He talks in some detail about tweaking the MPS for non-stock conditions.

--DD

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Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling

[Joe Bob]

Hot cams stutter...unless you have the specs on the cam, it is hard to give accurate advice. Talk to the previous owner and track down the specs, then talk to the manufacturer...

[pbanders]

What DD says makes sense, but I have no actual experience with a hot cam and D-Jet. My previous engine with a hot cam was carb'ed. Like others, I've only "heard" how it doesn't work. D-Jet is remarkably tuneable - it may be possible to do something with a hot cam engine to obtain decent performance. Don't forget - even with carbs a car with a hot cam usually has idle problems, anyway.

Why don't you be our experimental test case?

Brad Anders

[Dave Thompson]

Thanks for all the quick responses so far.

To clarify the symptoms, I need to say that I HAD a low rpm stutter problem with carbs (2 40DCNFs) installed. With the D Jet installed, the low speed running is very good. The pull is also very strong and smooth up through the entire power band, until it reaches nearly 5000 rpm. Then the power sort of surges or pulses. When I had the original stock 2.0 engine in the car with FI, I could not rev quite as high, and the power just sort of faded out as I approached the higher rpm. That just seemed reasonable to me considering the normal torque and power curve characteristics of the engine. Now it feels like the power is there at higher rpm, if I could tune to take advantage of it.

I have my timing set at approximately 27btdc@3500 rpm, stock 2.0 distributor, vacuum line to the advance side of the distributor. Should I put the vacuum line to the retard side, as DD suggested at one time? Should I just plug the vacuum line and leave it off the distributor altogether? Should I be looking for a different advance number?

I will take a look at those articles you cited regarding the D Jet, and come back here with any results I get from tuning.
I am quite surprised by the strong power feel this engine is giving me, even tuned as it is now. I guess my old engine may have been in worse condition than I realized. As this is my first 914, I have no relative baseline to judge performance by other than the fact that EVERYBODY always pulls away from me on the track! The way this motor feels, that might not be so obvious now.

Dave Thompson

[SteveStromberg]

Check the fuel pump pressure it should be near 29 lbs, you might have a lean mixture at high rpm. Steve

[Joe Bob]

By chance do you have hydraulic lifters? If you do, they tend to flaot at the higher rpms...