930 - 3.4 or 3.5?

[engken]

I will be completing the normal bolt-on (1 bar spring, fuel enrichment, Kokeln intercooler, headers, muffler, K27 7200 etc.) modifications to my 930 over the next month. By next summer I would like to increase the displacement of the motor to either 3.4 or 3.5 liters. Does anyone have advice on which one would be best? I think the cost differential between the two is small enough to make this a real decision. I know the rule of thumb is to go as large as you can, but I want a little more information. I would like to end up with over 400 rwhp, but with decent reliability as well. I am asking now because I will purchase the parts needed for the rebuild on a monthly basis as my budget permits. I don't plan to replace the CIS. Any advice is appreciated. Thanks.

[Emission]

Maybe a stupid question... but why do you want to increase the displacement? You won't get much more horsepower in comparison to the gains you get with increased boost. In all honesty, there are plenty of 930's out there with stock displacement and 450+ horsepower!

Check out www.911turbo.com and ask the same question.

[Yargk]

One good reason to go to at least 3.4 liters has nothing to do with displacement. Ruf knew this back in the 80s. The stock cylinders on a 3.3 only have cooling fins on one side. If you go with a Ruf 3.4 you get more air cooling. At least I think I remember reading something about this in Bruce Anderson's book years ago. Please correct me if I am misrepresenting the info.

[5axis]

3.4 is a bolt on. 3.5 requires the case to be bored for the larger cylinders. BA's performance handbook covers this swap. I am guessing it's not worth the $$$'s. EFI swap is probably a better bang for your buck.

[arrivederci]

A set of RUF 3.4 P&Cs along with appropriate cams can make the motor pull a lot harder when off boost (a friend has this setup on his '79). With the other mods you mentioned and a set of cams, you should easily be over 400 hp at the crank.

[dean]

Originally posted by 5axis
EFI swap is probably a better bang for your buck.

I agree 100%. I heard that just going to EFI will give you 30-70 hp. I am going to go with EFI with my motor which is a 3.0l.

Sorry to not answer your question

[engken]

Originally posted by Blown
I heard that just going to EFI will give you 30-70 hp.

I guess I don't understand, but forgive me since I'm new at this. How does EFI deliver more horsepower? I am under the impression that the main improvement that EFI offers is increasing the amount of fuel to the engine that is needed at higher horsepower. How is switching to EFI going to add additional horsepower if my displacement and boost level is the same as it was prior to the switch? In other words, if the demand for fuel is the same, how does the EFI help?

[dean]

I believe that the EFI reduces the air restriction that the CIS causes and you get more airflow through the motor.

[Amac]

Ken,
I have experence with both 3.4 and 3.5, and EFI systems.
If cost is a factor, go with 3.4. Add a EFI system and you can
reach your HP goal with ease. This will stress a stock bottom
end a bit.
If "bling is your thing" 3.5 is more bullet proof. The cost can
get away from you, so be carefull.
When you talk about EFI, do you mean "Motech" or the more
costly "EFI" racing system?
You can gain 30 HP just by replacing CIS with a Motech EFI.
The HP Gain is due to the endless tuning options EFI gives
you.
Allways room for another "Big Dog"!!
Good Luck

[Bruce M.]

There's a third option, if your case is open--98 mm P&Cs from the 3.4 set (which has fins all around, as stated), and a 964 or 993 crank. Makes displacement 3.45.

As to why you want to increase displacement when you can get beaucoup power other ways, there's a simple answer--increasing displacement makes for an appreciable increase in low-end response. In my opinion and experience, the stroker is particularly effective in that regard.

Getting a true 3.5 out of a 3.3 case, as Anderson states, requires machine work.

As to efi, I'm not sure what system "EFI" is, but Motec is not exactly inexpensive. Although it is coming down in price (and getting even better in execution).

CIS is, in my opinion, a pretty *****ty fuel delivery system for present times, for many reasons. It won't allow for decent overlap on our cams, it won't flow enough fuel for high horsepower applications, it's imprecise and inefficient (at least compared to fuel injection that's properly dialed in). Fuel injection is the way to go for high horsepower applications (450 + at the crank).

[rick conrath]

Don't waste your money on the 3.5. You can get to your power target easily with a set of 3.4 pistons. The 3.5's are nice, but your money is better spent elsewhere, like your heads or even dyno time. Longevity is also a potential issue if you have to open up the case to accept the 3.5 spigot.
Rick
'78 930

[engken]

Based on your collective advice I am going to do two things: 1) Buy Bruce Anderson's book and 2) go with the 3.4. I plan to begin ordering the parts in July. I will provide updates and dyno results from my current bolt-on modifications soon. Thanks for all the advice guys.

[beepbeep]

I don't think 3.4 will help you much. CIS barn-door measuring device is biggest obstacle here. Just running speed-density EFI will give you as much (if not more) power as 3.4 P&C will give, plus aditional headroom to tune with.

It's much better bang for buck to port those narrow-intake heads, install hotter cams and EFI than to change P&C's...


My 5 cents...

[engken]

I do plan on porting the intake heads and installing the hotter cams at the same as 3.4 P&C's, just not the EFI.