MFI space cam

r lane · 12-10-2022, 08:33 AM

I have a number of MFI engines. Enough to where I decided to cobble together a test bench. I am able to set up and test delivery. Make delivery adjustments as needed. What I would like to do is modify a cam to give me desired delivery rates. To do that you have to know where on the cam a change has to be made. The throttle angle lets you know where the cam follower is stationed radially so that is easily determined. The RPMs control the longitudinal position of the cam but how does one determine that location at a certain rpm. If the cam moves linearly from 0 RPM to say 7500 RPM, one could plot it based on that info, but I doubt if it is a lineral progression. I will appreciate any input. Bob

callard · 12-10-2022, 10:29 AM

I raced a '72 911E in SCCA's IT-S class and wanted to improve the power by enrichening the mixture across the racing rev range at WOT. I plotted the E space cam as you pointed out and found that the range I needed required that I modify the space cam (read grinder). I did some grinding and polishing on it and put it back in the pump, back on the engine, back in the car and ran on the track (I had a wideband Inovate system that captured CO and RPM). Two more iterations and I was able to achieve 12.8 from 3500 to 7200 RPM. A PITA way to do what I needed but think of the engineers that designed the space cams for the various cars back in the late '60s and early '70s. All analog work and their requirements were quite a bit more complicated than what I wanted. Hats off to them for the T, E, S, and 906 versions.

Showdown · Showdown's Garage

Try reaching out to [mention]Jonny042 [/mention] he did a lot of testing and modification of his space cam and recorded results to tweak delivery. His Project Heavy Metal thread has details and pictures of it all.

Rusty911 · 12-10-2022, 12:14 PM

There is another detailed thread here called open heart surgery III - reworking the space cam. Jeff Higgins started that and Mark Jung also had a post showing how he did iit. I believe he marked cam into small rpm increments of 250 rpm to make adjustment. Key seems to be to take very little off on lean spots since you can never reverse what you did.

Jonny H · 12-10-2022, 01:36 PM

There's a guy over here (UK) Mike Lucas who has a mappable computer controlled MFI pump (it has no space cam - instead it has stepper motors that control the MFI actuators).

I was at a dyno session with him once - he gets the engine dialled in with the computer controlling the pump axes. Once it is correct, he has some software that generates a computer model of the space cam to match the tune. He then gets the space cam machined and delivers the pump exactly per the dyno session tune.

It's not exactly what you want but he may be able to help you as he knows these things backwards. Website below which hasn't changed for years!

https://lucasdevelopment.co.uk