MFI recalibration w/ compression increase
 

Does anyone know if you need to recalibrate the MFI pump when you increase compression?

I have a 2.4 E pump that I will put on a 2.4 motor with 2.2 E pistons and an E cam (I assume I cannot use the 2.2S components without having to recalibrate). In Wayne's book he says you must recalibrate the pump with compression increase but in a thread here he says you probably don't. Has anyone done this engine?

there's an easy way to test it... fire up the puppy and check measure adjust it ... there's a big chance you can get her to run smooth without recalibrating ... and it can't hurt to try

I don't think you'll have that much of a problem. I have a friend who's running a 2.4T pump on a hot rod E-S motor @~9.8:1 and it runs just fine.

I've never tried it, but my thought is, the MFI system meters the correct amount of fuel for the amount of air in the swept cylinder volume, i.e. the displacement. If you change cams, that affects how much air you have and how much fuel you then need. But once the intake valve is closed, the fuel-air mixture for that stroke is already determined, and what happens on the compression stroke shouldn't matter.

This is certainly an oversimplification: we know that compression ratio affects BMEP, temperatures, piston speed, gas flow, and the shape of the piston crown is a factor in detonation, all of which affect the amount of fuel.

MFI and higher compression
 

You do not need to have your pump recalibrated when you simply increase the comression ratio. I have done this several times. Latest time is on my 73S. I replaced the 84mm 8.5:1 pistons with J&E 9.5:1 85mm pistons. After MFI adjustment the thing runs super. In fact for anyone contemplating this change I highly recommend it. The original engine started to pull OK from 4000 rpm but really came alive at 5000. As all you S owners out there know this is what makes these cars so much fun. However the torque below 3500 was pretty poor. Now with the new 9.5s and using the old seat of the pants dyno I would say that it pulls at 3000 like it used to at 4000 and that the lower rpm torque makes the car much easier and more enjoyable around town. It still screams from 5000 to 7000 like nobody's business but it pulls very strong from 4000 (sort of like an SC motor in this mid range). Overall I can't think of a better upgrade this side of going to 2.7 P&Cs and it is a lot cheaper than that.
I am sure that had Porsche been able to market the 72/73 S in the USA with 9.5:1 compression like the older Ss had been the car would be considered even more of a legend than it is. It is a much more tractable car than before but still with the amazing top end.
Recall that Grady Clay had a very nice post on his 2.4E motor on which the only thing he changed was to install 2.8RSR P&Cs and he said it was one of the best street motors. He claims he changed nothing else on the motor and it ran its MFI fine.
Good luck, Scott

Hijack thread alert! But first let me chime in on the orginal question. The 2.4 E pump should work and you can fiddle around with it to tune for optimum running.

I have a 2.4S MFI motor and scotc's upgrade to hi comp. JE pistons sounds way kewl. A couple questions if I may....

Any mods on the heads for fitting the JE pistons?
Any dyno tests? before/after, how about just after?
How does it idle?
What plugs are you using?
Is gas mileage still sh**
What fuel do you run?
I found that 85 octane here with 10% alcohol in the mile high city is nominally ok. TIA

Regards,
Bill
Now back to the original thread
;)

One more data point - I have a 2.7RS pump on my 2.7 ++ engine (10.5:1 comp'n ratio, GE80 cams) and it tuned up fine.

more info on 2.4 High compression mod
 

Zotman72;
I hope this info is of interest to others and we are not being bores to talk about Ss rather than Es.
1)No mods made or needed to run J&E 9.5s. I just had the old Biral cylinders bored to 85mm. This was neccessitated(sp?) but a mouse nest on top of the left back due to long term storage. The clinders were slightly ovalized and the over bore seemed the best way to clean them up and get more compression.
2)No dyno test before or after. Compression before was 150-155 on all 6 before so it was a strong motor. All I can say is with my butt dyno. Having worked on and driven these cars since the early '70s (starting in Denver) I have pretty fair feel for them and this is a dramatic upgrade. You WILL notice the difference!
3) It idles smoothly at 750rpm
4) I'll have to check on the plugs tomorrow but it is whatever Bosch platinum is recommended for the 2.4 S. Nothing fancy needed
5) On a 800 mile trip last September all secondary mountain roads in NE Washington and BC I got 20-23 mpg. Before the conversion I was getting the same. This was driven briskly but not insanely. Usually around 65-80mph. But, I have never had the poor milage others report with MFI and this includes owning a 69S, 71E, 72T, this 73E and S.
6) I use what ever standard pump premium I find. Usually 91-92 octane with no problems. It seems very happy on this. Have not tried it on ethanol but at Colorado altitudes that should be fine.
Scott

Since I have one of Grady's 2.8s with Webers and are planning
a move to MFI, he and I have had this conversation. According
to Grady, they did shim the pump's barometer to simulate sea
pressure (or below) altitude. Remember, these cars we built in
Denver and ran on tracks in the mountains.

Hopefully Grady will chime in with better info ...