More MFI queries - space cams

obrut · 09-21-2003, 05:39 PM

OK - I promise this is the last one...

The MFI set up I'm looking at was rebuilt by Supertec Performance. The throttle bodies were redone to 2.7RS spec - ie. 36mm & 38mm. The pump did not need a full rebuild and instead had all seals replaced etc.

Supertec state that a T space cam has a max flow of about 63cc/min whilst a RS space cam has a max flow of about 64cc/min. They also state that 40cc/min is all that is needed for 240hp or so. In their opnion the differences are neglible and they only replace the space cam if it is worn out (ie. not for performance). The main performance difference seems to come from the throttle bodies (air flow) side of things.

Any MFI guru's care to comment??

Cheers - Ryan

Tyson Schmidt · 09-21-2003, 05:53 PM

Ahhh, MFI, my favorite subject as of late.

Here's the skinny.

Flow rate is not the issue here. There is absolutely no question that a T pump is perfectly capable of supplying enough fuel. You can simply adjust the main rack to provide the proper amount of fuel under full throttle.

The issue here when using a T pump in an RS or S is the horsepower curve. The T space came is shaped to match a T HP curve which is quite a bit different than an S or RS curve. You can certainly make it work, by adjusting the main rack for the wider throttle openings, and then adjusting the idle to get that as close as possible. But driveability may not be perfect, and you may end up with some lean spots and some rich spots in the rpm band.

Generally speaking, a T pump works fine in an E, and an E pump works fine in an S or rs. But a T pump in an S or RS is pushing the limits as far as performance coupled with driveability are concerned.

To sum things up. It will work just fine, but don't expect it to drive like a CIS or EFI car, or even Webers for that matter. It will never be "perfect" but may very well be close enough, with some time and effort invested in fine tuning.

obrut · 09-21-2003, 06:18 PM

Many thanks for the insight Tyson

What to do???

obrut · 09-21-2003, 06:27 PM

ps: Tyson - would 10.5:1 pistons and GE-80 cams make the tuning/drivability issues worse??

(and future twin plugging...)

Tyson Schmidt · 06:56 PM

Damn, that sounds like a healthy engine you're building there!

I don't think that will make much of a difference above what you're already going to be dealing with.

One thing to keep in mind though, is that even with the proper space cam, MFI is going to be a compromise in the street driving department. You have to set them very rich (about 7-8% CO at 2500-3000 rpm) to eliminate the part-throttle lean surge that is pretty much inherent in these cars with the fuel we have today. Today's fuel is much more volatile than what they had when these systems were new. So even under ideal circumstances, you have to compromise on the mixture adjustment.

One option, if you are unable to make the car behave like you want, is to buy just the space-cam and install it yourself. When/if you get to that point, I can guide you through it.