2.4 to 2.7 conversion on 73.5 CIS

[Rick Cabell]

Has anyone upgraded a 73.5 CIS 2.4 up to 2.7 during a rebuild and kept the original CIS intact?

[Tom F2]

Yes, I did this in the 1990s with no change to cylinder heads, ignition, or fuel injection. I switched to Dilavar head studs to accomodate the Nikasil cylinders.

The only concerns I recall are that the machine shop opened the cylinder spigots a little too far, so they weren't as snug a fit as I like. Also, there was lots of torque, but not as much gain at the top end. It made for a great autocross motor.

Silver sunroof coupe, I'd love to know where it is today.

[E Sully]

I would think you could, as many of the parts were similar for the '74. The main differences I see is the ports on the 73.5 were smaller than the 2.7 heads, and the air flow meter changed.
The camshafts for the base 2.7 were the same, and you could possibly use the CIS S camshafts for a little more top end. I actually bought a reasonably priced good used set of CIS S cams along with a '74 air flow meter,0 438 120 004, thinking I may want to use them in mine one day.

[Rick Cabell]

Tom, thanks for the quick reply.
I have done the same and can’t get the engine to run right, undrivable.
A 73.5 car is the only CIS that has the injectors in the cylinder head. In 74 they went into the intake runners. Another Porsche engine guy told me that when you go to a 2.7 CIS that the piston dome has a different shape and the injectors in the heads won’t let the fuel atomize properly.
I remember doing this conversion decades ago and not having a problem, maybe I am remembering wrong.
Any other comments would be appreciated.

[Rick Cabell]

E Sully, thanks for the info.
Many don’t know that in 74 only there was a choice of 2 engines. A normal, or a S. The normal engine had the same 30mm intake ports as the 73.5 car. The part number for the 74 normal has the same part number for the FD and WUR. The only change between the 2 engines is the 74 has the injectors in the manifold pipes. And in 74 they added a thermo time switch for cold start.
More comments welcome.

[E Sully]

If you could give a few details on what was done it might help. If you are using the 73.5 heads and injectors, the problem could be bad injectors if you did not use new. Have they been tested for leaks and spray pattern? In this thread a reasonably priced Mercedes replacement injector is available, it's main difference would be a slightly higher opening pressure. I have read a few 928 owners using these with good results. Panama911, has used it successfully in his modified 73.5, Post 55 1973.5 911 fuel injector
I bought a set from Pelican very low priced. I will install them next year after fixing a few other things on my car first.

[Rick Cabell]

Thanks again Ed for the comeback. So here is the story.
Car sat 30 years and customer wanted to restore it. Car went to body shop and I got the motor.
Took it all apart for evaluation. Pistons and cylinders were junk and 2.4 CIS new ones were not available, so just go up to 2.7.
Case off to Ollie’s for surface grind and machining case to accept new cylinder. Heads off to the valve job shop and open heads to accept larger cylinders.
Fuel distributor and WUR off to FIC for rebuild.
Months later parts are back and engine assembled. Engine starts fine, kind of a lumpy idle. Go to drive off and car falls flat on its face and is bucking, undrivable.
Check injectors for spray pattern and not that good. So new Injectors from Porsche.(stupid price).
Nope, still a problem. Testing says throttle compensator on throttle body bad, order and install , adjust a new one. Same.
Weeks go by as I check everything when I have the time in between other job.
To keep this post short, I’ll fast forward. I found another 73.5 owner who let me use his car for testing and it a car that runs great. I have swapped every part of the fuel system and ignition system over to the problem car and it has done nothing. Took the muffler off and ran it to see if there was a exhaust restriction, as mice love 911 mufflers that have been sitting around. While muffler was off I decided to pull the chain covers and recheck the cam timing.
That’s why my original post asked if someone has done this before with no issues. I have removed many 2.4 cars injection systems before but customers always wanted a carb conversion.
So looking for someone who took their 2.4 CIS car and just went to 2.7 P&C’s and retained the original CIS. All comments welcome.
BTW, if you push the throttle down and clear 3-4 grand the car takes off like a rocket, sounds ok, but not right.

[E Sully]

I'm not sure what you have checked, but I always start by looking at the spark plugs. Are they wet, carbon coated, etc. I also do a compression and leak down test before I first fire it up. That also gets a little oil flowing. I was a bit upset when I found a slight leak on one of the valves on my '86 engine. It's never fun pulling heads off a fresh assembly.
I am sure you verified all the CIS pressures or you wouldn't have found the bad TPR, and swapping parts the other good injection system didn't help. It should run with that set up. The only change between the 73.5 and '74 system was the air flow meter, but it should run with the original. New injectors installed, although I would still check the spray pattern and flow. New old stock could still be old. I can't see issues from the location of the injectors. Leaking cold start valve could cause issues. It does start, stumbles, then higher rpm goes. Does it sound like it's rich, or a cylinder isn't firing right?

[PeteKz]

If it stumbles at low RPM, but takes off at high RPM, check for vacuum leaks. Some hose not connected? Broken airbox?

[Tom F2]

I used my original 2.4 CIS camshafts and fuel injection system without any change, including the injectors positioned where the MFI injectors would normally be. I ran a set of used 2.7S-CIS Mahle pistons and cylinders. (Which I got for free!) Car ran great. Somewhere I have a videotape of the car auto crossing and lifting that inside front wheel way off the ground. I don't recall any adjustment problems with the CIS but it was a while ago! Life was so simple back then. Hah!

[Rick Cabell]

Thanks guys for posting.
Ed, the plugs look pretty good.
I have checked and set the CO at 2.5%, and checked cam timing, and experimented with different ignition timing.
I did have the thought the injector position with 2.7 set up might require the injector moved up to the intake runner position, but Tom F2 pours water on that.
I did a smoke check on air box and didn’t see any leaks. Car idles consistently. Wouldn’t a leak in air box cause the idle to serge?
With no load on engine,(sitting), you,punch the throttle and it revs great. But try to drive away and it bucks so bad you have to stop. if you punch it and it takes off with plenty of power but doesn’t sound 100% correct.

[PeteKz]

If you did a smoke test and verified no intake leaks, and you checked the fuel injector delivery, I would start suspecting an ignition problem. What is your ignition system and what timing are you running? Distributor possibly internally sticking?

[Rick Cabell]

Smoke test done. I have put the injectors in test tubes and lifted sensor plate. Good amount, but not all exact. Spark plugs wires ohm check ok. CD box fresh from Ashlock.
Swapped it out anyway. Timing per book 5 degrees after TDC.
I might removed the whole intake system in the car and put in on the bench for examination.
Distributor went to Ed Fall before he retired for full rebuild.
Moved ignition timing and CO some and it made no differance. I am missing something.

[Tom F2]

I'd swap out the fuel distributor if you have a spare on hand, in case the rebuilder made a mistake. The uneven injection quantities with new injectors makes me suspect the fuel distributor.

[HarryD]

Originally Posted by Rick Cabell

Smoke test done. I have put the injectors in test tubes and lifted sensor plate. Good amount, but not all exact. Spark plugs wires ohm check ok. CD box fresh from Ashlock.
Swapped it out anyway. Timing per book 5 degrees after TDC.
I might removed the whole intake system in the car and put in on the bench for examination.
Distributor went to Ed Fall before he retired for full rebuild.
Moved ignition timing and CO some and it made no differance. I am missing something.

Timing at idle is easy to set but not as important as timing at 5-6000 rpm. Should be 35-36 degrees. Idle timing is whatever it ends up being. This article is helpful. Early 911 Distributors - Carbs-Fuel-Ignition (CFI) - Repair & Sales

[PeteKz]

Emphasize what Harry said. Check that the timing is all in by 4000 RPM without the vacuum advance connected. I'll differ a bit in recommending that you stick to 30 degrees mechanical advance, until you have everything running right and can tweak a few more degrees out of it. Vacuum advance should add about 10 degrees more at cruising RPM and throttle settings. Your distributor should not have a vacuum retard.

[Rick Cabell]

Timing seems good. Keep in mind that just idling it doesn’t sound right.
Rev the engine in the driveway and it sounds good, until you drive away, then the bucking starts and you have to push the clutch in to stop it.

[E Sully]

The Tech Spec book recommends CO of 1.5-2%
I will say that I've had to re-condition my '73.5 and '86 distributors to get the advance weights and vertical shaft play in shape. I don't believe the distributor is the problem, especially since it was rebuilt.
PeteKZ there is a little advance after 4,000. I thought I'd share a page from the tech spec book, along with a couple of TSB's.

[PeteKz]

Ed, I know the spec book shows advance out to 6000 RPM. However the shops that recurve dizzys make the advance all-in by 4000, usually lower.

At any rate, if it is working per the spec book, it should run smoothly at low RPM and moderate throttle. But it’s not, so the question is why? I’m still leaning towards something related to the ignition, but if you have double-checked all of it, then i don't know what to look at next.

Got any other hypotheses?

[E Sully]

So it has been smoke tested for leaks, rebuilt fuel and ignition distributors. CO level at 2.5. As I mentioned earlier, I think it's a bit on the rich side. I would try bringing it closer to 1.5 and see how that runs. Generally popping out the intake is rich, exhaust is lean.
What spark plugs are you running? Copper Nickel is recommended. No platinum or other exotic metals. NGK recommends BPR7ES. Bosch no longer has a plug for the '73, but recommends the 7992 WR5DC for the 1974 2.7l. I personally prefer NGK.
I would do a flow and spray pattern check on the injectors, along with making sure the cold start valve is not dripping. From what I can gather, injector flow within 5% of each other should be ok.
How original is the fuel system? Just to verify the CIS parts, fuel distributor should be 0 438 100 004, WUR 0 438 140 008 or 129, air flow meter 0 438 120 002, 004 is '74 which should work with increased displacement. The air plate is set level, and moves smoothly up and down when you push it by hand?