2.2 'S' cams

Randy, "backpressure" may not have been the correct term ... maybe "restriction" is more appropriate.

As far as the exhaust pulses causing disruptions on MFI cars, you are correct. A set of headers with built in reversion cones will, however, make the MFI immune to muffler/megaphone alterations because the reversion cones eliminate the aforementioned exhaust pulses.

A set of these headers are expensive and hard to find, but on an MFI car they are worth their weight in gold.

PS: Anyone have a set of 914-6 reversion cone headers? ;)

Quote:

---

Originally posted by Mr9146
PS: Anyone have a set of 914-6 reversion cone headers? ;)

---

I would love to be able to find a set of these, or even a source for the cones themselves so I can weld them into my headers. I think they are even more effective on carbed engines than on MFI. I posted on Rennlist about it awhile back and no one could come up with a source. Anyone?

TT

Marco,

That is a good suggestion. The car does have stock heat exchangers/headers and I have inspected them for any obvious leaks and haven't found any, but I will double check that again.

I did talk with a person at Webcams today that knows Porsche engines and cams. I explained I had a receipt that showed my car has a set of Webcams with a part # 05-261. She confirmed that a set was sent to the shop that the receicpt was from in that same month (1/00). I explained the problem with the performance of the engine. She inquired if the P/C had been changed. I said that I have no record that they were changed from stock. She explained this could be the problem because the P/C need to a higher compression than stock for these cams to work correctly. She also suggested to check the cam timing and to determine if they have been retarded. I also tracked down the owner of the car during this time and he states that the P/C, nor heads, were changed when the new cams were installed. He also thought the shop was putting in stock cams though and said the car never really ran correctly after that work was done.:(

Guess I need to find an experienced Early Porsche shop in the Chicago area to help me get this engine running correctly again. Hopefully sending the cams back to Webcams for a stock 'S' grind and then proper installation will get this engine running like it should.

Brian, is the engine carburerreted or MFI'ed?

Marco,

It has MFI. All those components have been rebuilt along the way.

Brian, from my experience, you don't need high compression to run S cams ... but it helps if you want to make a significant amount of power. Improper compression will not result in flat spots that you mentioned earlier.

You say it's a dog ... have you checked your injection pump timing? That's key.

You also said that it has factory heater boxes, but what about the muffler? The heater boxes are fine if they are stock, it's the muffler that should be the focus of your search. A stock 911 muffler will suffice, but aftermarket companies rarely tune their mufflers for MFI cars - Dannsk may be the only exception to this rule.

Also, make sure that the rack in the pump is set correctly on the idle circuit. You can adjust the main circuit and idle circuit on an MFI engine independently of one another. You should futz with the idle circuit to make sure it's set at the correct mixture.

On the other hand, their is an easier way to richen up the idle circuit if you're willing to sacrifice top end performance for low end drivability - just richen up the main curcuit. This will, by default, richen up the entire rack.

There are a few other ideas that I have regarding how to fix your problem, but check these out and if they don't work let me know.

-Marco

Marco,

I do have a stock muffler in the car. The Webcam rep. said that with the custom grind done to the cams in the car the pistons have to be higher compression than stock to work properly. That was not done. So that has to be part of the probem.

Yes I do need to have my mechanic check the enrichment of the idle and the main circuit again. I don't know about setting the rack in the pump correctly. Is that an adjustment that is made to tune the car or is that set during a rebuild of the pump? Thanks.

Brian,

I looked up the cams you have in your car and compared them to some Porsche profiles and seemed to me that the cams you have are closer to a 906 cam than they are to a S cam. I'm no expert (actually far from it) but I wouldn't think they would be much fun on a street car. I think your on the right track considering a cam replacement.

The comment the lady at Webcam made about the pistons kind of bares this out in that when you have a cam with more overlap you have to raise the compression ratio a bit to compensate.

The RPM's climbing slow suggest a timing issue that may be a combination of the distributor problem as well as the cams poor low range performance.

I say get the S cams and set the car up stock.

Bobby,

Looks like an off-season top end rebuild. You and the rep from Webcams are saying the same thing. I really don't understand cam terminology and what the exact differences are, but she mentioned the overlap difference as a reason to up the compression. For my own knowlegde could you explain the differences in the specs. of the stock vs. the 05-261 cam.

For instance the spec sheet says for 05-261 is:
Valve lift is .476/.450
Duration in deg. is 284/270
Duration @ .050 in deg. is 264/248

How do those compare to the stock 'S' cam? Thanks.

Brian,

First, Bobby and the Webb Cam lady are on the right track. What I can impart based on personal experience is that 906 cams in a streetcar are a BEAR to deal with unless you have a big displacement motor. They are very very peaky, and they don't make any significant power below 3600 RPM; the big displacement motors mask this peakiness with their ability to make torque and, thus, allow the car to remain driveable. And, by "big displacement" I mean 3.0 and up. S cams are worlds better for street use.

As far as MFI is concerned, that is where you will want to look to clean up the power delivery - i.e. get rid of the flat spots. During the rebuild process, Pacific FI will set the rack more to the middle ground as different users require different tuning. In your case, if you or your mechanic hasn't futzed with it since you got the pump back, this would have a lot do with the idle and power delivery. Setting the rack is a tuning procedure, not a rebuild procedure.

To boot, because the pump runs off the cam timing, if the cams are improperly timed the pump is, by default, also out of time.

Tuning MFI is tricky, but when you get it perfect you never have to mess with it again and it rewards you with a completely unique driving experience. Keep the updates coming.