S vs. RS MFI inj. pump in 2.7RS engine
 

I'm planning a 2.7RS upgrade on my E motor in a few months and I have questions about the injection pump. Is it necssary to use the RS space cam in the MFI injection pump or can an S-spec pump do the job? What's the downside of using an S-spec pump? Thanks for your help.

Cross-post: S vs. RS MFI inj. pump in 2.7RS engine
 

Also posted on the Engine Rebuilding Forum:

I'm planning a 2.7RS upgrade on my E motor in a few months and I have questions about the injection pump. Is it necssary to use the RS space cam in the MFI injection pump or can an S-spec pump do the job? What's the downside of using an S-spec pump? Thanks for your help.

Jim-
I'm facing a similar dilema with the short stroke 2.5E motor I'm building using a 2.2E pump. I called Gus Phister, and he indicated that I should be O.K. If your pump is a 2.4S, you should be O.K. Of course, you will need to adjust the mixture. Gus told me that, in my case, if I can't get the adjustment right, that he could make some internal adjustments for a few hundred dollars (NOT a space cam change). Also of interest, space cams are not interchangeable between 2.0, 2.2, and 2.4 pumps. I'm sure Gus would be glad to talk to you about your motor. We was very helpful with me. My semi-educated guess is that you should not have much of a problem, assuming that your pump is in good shape. The most crucial match is betwen cam and space cam. It sounds like you have a S space cam, so that should work.
-Scott

My concern is too lean a mixture, particularly if you expect
to track the car. You don't want it to lean out at 6800 rpm
on a hot day, e.g. like Summit Point a couple weeks past.

I thought that the RS used the 2.4 space cam, not a special one?

Re: Cross-post: S vs. RS MFI inj. pump in 2.7RS engine
 

It's World War II technology.

The RS space cam is different...

There is not much difference between a 2.4-S and RS motor...

The pistons and cylinders (same profile but 90mm)
The distributor and timing...
The space cam in the MFI pump...
and that's about it...

If all else is RS spec... pistons, cams (S-cams), heads(S), and throttle bodies(S) ... you will need more fuel distribution at given RPM's for the bigger displacement... therefore the different space cam...

The car will run... but not to RS performance through it's complete range...

Gus at Pacific Fuel Injection (650-588-8880) will change out the cam and recalibrate the pump for about $750 (if the pump is good)
His turnaround time last year (when I did the exact same thing) was 7 weeks.

Hope this helps...

Re: Re: Cross-post: S vs. RS MFI inj. pump in 2.7RS engine
 

Thanks John! :p

I'd try running it and richen up the part-throttle mixture on the dyno. Basically it's the same engine drawing 12.5% more air across the board. It's a little port constrained at higher rev's compared to a 2.4S which might make it run a little richer then an S in that range assuming the pump can be richend up by 12.5%. But what's it cost to try?

And to the others, thanks for your comments! I was hoping it would only be a matter of running a little rich since the space cam and engine cams would both be S spec. Maybe that's wishful thinking.

edit: just saw your response John L. Thanks!

Scott, that's what I was thinking but others (on the 911 forum) left me with the impression that it might not be appropriate to use an S pump set a little rich. Thanks for your comments!

Re: Re: Cross-post: S vs. RS MFI inj. pump in 2.7RS engine
 

LOL :D

If it's good enough for a 908/3...
http://www.dorkiphus.com/porsche/mod...3/IMGP2670.jpg
http://www.dorkiphus.com/porsche/mod...3/IMGP2671.jpg

Do *NOT* Cross post across forums. I will now delete one of these posts.

Read the forum rules next time please:

"DO NOT POST TOPICS IN BOTH THIS FORUM AND THE MAIN 911 FORUM - PICK ONE AND THEN POST."

http://forums.pelicanparts.com/911-engine-rebuilding-forum/95338-read-me-first-information-regarding-engine-rebuild-book-forum.html

-Wayne

Well said, Jim. I just found that funny :D

OK

Jim-
Gus told me that the issue would be that when set to run rich enough at high RPM, it may be the case that it is difficult to avoid an overly rich condition at idle. He thought that I'd get away with it, even though the change in displacement is greater in my case than in yours. I asked if people had used 2.2 pumps in RS motors, and he said that they had, and it seems to work. That's going from 2.2 to 2.7, so it is hard to imagine that you will have an issue. Unless this Gus guy doesn't know what he is talking about.;)
-Scott

Hi Jim,

2.4S distributor and 2.7RS distributor should be the same. Just the timing at idle changes slightly.

When fiddling around with the MFI pumps, don't waste your time with the idle mixture, use only the main rack to adjust.
Playing arround with the idle mixture has the negative side effect that you will change the axial position of the cam follower arm to the cam itself, and changing things too much will bring the pump completetly out of tune ( I am talking about smooth transition from low rpm upward)
Use only the main rack, as there everything is reverseble, and you are interested in high rpm mixture anyway.
A new 2.7RS cam is about 380 Euro, but the all 2.7RS engine is running fine at all rpm and throttle conditions.
You will definetely loose smoothness between 2000-3000rpm at almost closed throttle when using not the original cam.

Good luck

Andreas

I had the same conversation with Gus. That number doesn't include the price of the cam, 380 euros or about $466.

So, am I correct in understanding that upgrading a known good injection pump from T, E or S to RS-specs is $1,200+ ?