Learning to rebuild a '73 2.4s

[Topocalm]

I recently acquired a 1973 2.4s motor which I plan to disassemble, inspect and refurbish where necessary.

I have never done this before. My motivation is nothing else but the pleasure of learning how these motors are desinged and rebuilding them. I have done a lot of reading on this forum, youtube (thanks Kav) and of course Wayne's book. In preparation I also purchased a set of original factory workshop manuals which I am reading. And I am very grateful for the great advice that is available on this forum.

My goal is to keep the motor completely original (or return it to original depending on what I will find) but end up with a motor that is thoroughly checked and renewed where necessary. I do not plan for mods.

Oh, and ultimatly I plan to put it into my 1973 2.4T (CIS!).

I will try to document the process here including the challenges and costs involved with suche a project in order to add to the collective knowledge.

The motor itself was sold to me out of a car - so no proof how it runs - and without any papers.

I did a leakdown test with the following results:

Cylinder 1: 10%
Cylinder 2: 28%
Cylinder 3: 8%
Cylinder 4: 12%
Cylinder 5: 16%
Cylinder 6: 10%

As a next step I will send the MFI Pump to a specialist shop for testing and start the disassembly process of the engine.

[911sportomatic]

thanks for sharing you're job

[cmcfaul]

Gonna need a new heat exchanger for the MFI to work properly. The hose to the MFI thermostat goes to no where. Supposed to connect to the heat exchanger. When you get a new heat exchanger, need to make sure it is for a car with MFI

Chris
73 911 E - Rebuilt the engine a couple times. Know a thing or 2 about MFI

[Topocalm]

Indeed. I am surprised the MFI can be run without it - my understanding ist that the termostat will cause the MFI Pump to run a richer mix when cold and leaner when hot. So without the connection to the heat exchanger it would normally run too rich even when the engint is hot...?

Originally Posted by cmcfaul

Gonna need a new heat exchanger for the MFI to work properly. The hose to the MFI thermostat goes to no where. Supposed to connect to the heat exchanger. When you get a new heat exchanger, need to make sure it is for a car with MFI

Chris
73 911 E - Rebuilt the engine a couple times. Know a thing or 2 about MFI

[Topocalm]

I had some time tody to start taking the engine apart. How much fun was that.



The good news is that the substance of what I have seen today looks quite good.

The velocity stacks and throttle bodies look very solid:

I read that play in the MFI control rods is to be avoided. I am wondering how much play is normal and ok. Most of the rods on this engine have some play in the ball joints. I made a little video

The cylinders seem to be original, the heads from a 1972. There do not seem to be any case savers in place....








Will be interesting to see the full status of the heads, cylinders, pistons and studs soon....

[cmcfaul]

Originally Posted by Topocalm

Indeed. I am surprised the MFI can be run without it - my understanding ist that the termostat will cause the MFI Pump to run a richer mix when cold and leaner when hot. So without the connection to the heat exchanger it would normally run too rich even when the engint is hot...?

There are a million jerry rigs to make it run OK. It will run best if set up the way porsche designed it. Dont let anyone tell you differently.

Chris

73 E

[Topocalm]

I made quite some progress - so far what I find looks quite good. I am curious to see what the measurements will look like once I get to it....

Looks like I have the 930 "semi-upgrade" chain tensioners.

[cmcfaul]

Your chains are definitely stretched and she has been running rich. Someone has messed with the cams so not the first time she has been taken apart.

Been down this road a few times. Lots of options.

Chris
73 E
89 Carrera

[Topocalm]

Chris,

Rich might be due to disconnecting the MFI thermo switch ...
How do you determine the chain stretch (I will replace them but still curious) - and that the cams have been altered ? I will remove and post some pictures this weekend...

[cmcfaul]

Very good guess the rich running issue is because of the disconnected hose to the thermostat. The MFI never knows the car is warm and essentially runs with the choke on.

The cam end is slightly buggered. It appears to have been wrenched on. The crowsfeet wrench and cam socket is difficult to work with and easily slips off when adjusting or setting cam timing.

Your tensioners are extended about as far as I have seen indicating that your chains are stretched. Not sure if you intend to split the case or not. It is possible to replace the chains without splitting the case.

when its all done a AFR (air fuel ratio) gauge is very helpful in dialing in the MFI. One can be permanently installed or just stuck in the tailpipe for short trips to see how she is running when hot, cold, idle, part and full throttle.

Chris
73 911 E

[Topocalm]

Ok I am almost done with the disassembly.

First observations (I yet have to measure all pistons and cylinders):

Firstly pistons:
1. They all have these wear markings on the side which surprised me. Is that a sign of lack of oil pressure? They also show "burn-marks" on the top sides. What does this all tell me - are they re-usable?






2. Unless I am measuring completely wrong the size is the second oversize with 84,55 mm. I have the factory manuals and they state the largest 2nd oversized pistons 2 KD 84.520-84.530. Might be that I have still some oil and dirt that makes me measure 0,02mm more.

3. The piston is a "CPS" - seems like an Italian outfit


This also explains the size, they make up to 84,6 mm!


Zylinder:
1. They have these 'markings'. I is also a 'diamond' pattern from a previous refurbishement (?) visible?





Clearance:
1. I measure 0,1mm clearance between piston and cylinder. Workshop manuals state 0,15mm as max clearance, Wayne in his book 0,2mm. This should be ok.

I will measure all again (and yet have to measure ovality of the cylinder), but was keen to get some opinions on it.


Then: I opened the case!

First thing that surprised me: All surfaces are coated by a 'laquer'-like hard film that seems to be old residue. It does not wipe off readily, but I have not tried with solvents. Is this normal?

[cmcfaul]

I would not reuse those P & C's Wear could be related to rich running condition (gas washing the oil off). The brown sludge is likely from using conventional oil and or too infrequent changes.

Chris

[Trackrash]

Hard to tell from the pictures, but the scoring on the cylinder walls is a concern. Weird that the cross hatch is still clearly visible at the top of the cylinder. It must have been overheated or run with dirty oil.

Could it be possible that the cylinders were bored oversize and fitted with aftermarket pistons? Seems to be the case.

You may be able to "deglaze" or lightly hone the cylinders and still be in spec on the piston to cylinder clearance. I know the wear limit is around 0.008" but I would hesitate to assemble any motor I cared about with more than 0.004" piston to cylinder clearance.

The key measurement on the piston is the side clearance on the top piston ring. If it is 0.004" or more the pistons are done.

[targa72e]

The specs you quote are for factory P&C. Aftermarket pistons (particularly if forged) may require different clearance (typically larger) . The cylinders you have pictured are biral which is cast iron with cast aluminum cooling fins. So you would want to find the clearance recommended for cast iron which does not expand as much as aluminum. The scuffing on the pistons could be a result of too little clearance.

john

[cmcfaul]

Nickacell, either new jugs or have yours plated.

Chris

[Topocalm]

Ok I got around to measuring the cylinders and pistons. The cylinders are all without ovality and from 84,61 to 84,63 mm.

More interesting are the pistons. I measured:
- 84,54mm in the middles of the skirts where the manual says to measure. This is also the place where the cylinders are scuffed (see the pics above)
- 84,30mm on the edges of the skirts (about 45 degrees from the first data Point)
- 84,08mm above the piston rings

So the scuffing Marks the places where the pistons are more than 2/10 mm wieder than the rest of the cylinder. They are literally deformed !

I guess I will def not use these again.

I would like to reuse the mahle cylinders. I could get them bored and honed and use JE 86mm pistons I think (?). I assume this slight increase in volume does not screw up the balance with cam and MFI..?

[Mixed76]

Pistons are meant to be barrel shaped. Whether yours are in spec I can't say, but the difference in diameter at different heights is on purpose.

Sent from my Nokia 7.1 using Tapatalk

[Topocalm]

Thanks for the comments on the pistons. The cylinders clearly have been bored oversize in the past...

I got round to taking out one of the camshafts today. And I found this number on the shaft: 901 105 103. 0R. My PET for 1973 tells me the number should rather be 901 105 183 01.

[cmcfaul]

Not sure what the number means as far as what the cams are for but they need reground or replaced. Pros and con:
Lots of power in modern profile cams. DC 30's with higher compression is a no brainer and the engine Porsche should have built.

Cons: If you replace the Cams or have them ground to a different profile they wont match the cam in the MFI and will never be perfect. You can get it close but never perfect. For example you can have it adjusted for fully warmed up and optimal WOT but may have a lean spot at 2k RPMs. You adjust and fix the 2K RPM lean spot and it is slightly out of whack someplace else. LIke I said you can get it close everywhere but will never be perfect. Ultimately you Learn to keep it away from know lean spot during warmup or wherever it's slightly out of spec.

I went with DC 30 route and a permanently AFR meter. For the first month it was go for a ride. Watch or log the AFR. Come home, make adjustments and repeat. I always kept track of any adjustments I made so I could return to baseline if needed.

Result was a screamer. Car made incredible power all the way to 7300 RPM's an beyond. A huge increase in performance. But not plug and play.


Chris
73 E.

[targa72e]

By the casting number it looks like the cams are 906 cams. Some more info here.
906 Cams?
They have pitting so will be need to be repaired/reground. If they are indeed 906 grind you might want to go with something less aggressive. If the MFI system is stock 2.4S then S cams would be a good match.
john