Question for engine experts
 

I've heard that the heads on 99-01 3.4L engines and 00-01 3.2L engines are the same...
My 99 996 C4 has oil in the coolant, so I'm guessing cracked head. My plan is to swap in a used 3.4.
So, if the bottom end on my old 3.4 is good, can I put my Boxster S 3.2 heads on and get some added displacement?? (Hopefully a good port/polish is in the budget).

J.fro, Have you checked the heat exchanger?

Oil in coolant = not nessecarily cracked head. Could just be blown Head Gasket.

Cracked heads and failed head gaskets are actually fairly rare and are usually the result of severe overheating.

If the everything was fine until you found intermix, then I agree with Gilles and suggest checking the oil cooler as they are known to fail with this result.

oil cooler is one common cause, they use coolant to oil sandwich cooler

Hello,

cylinder heads. From the part number the heads seems to be different – so there might be differences, so you should check that in very detail (even if the 986 parts number says 996 in the beginning):
986S - bank 1: 996 104 001 09
986S - bank 2: 996 104 002 09
996 - bank 1: 996 104 003 05
996 - bank 1: 996 104 004 05

Also the prior part numbers don't match.

It's pretty unusual that cylinder head gaskets break in this engines. Mostly it's a cracked cylinder – open deck design. If you have access to an endoscope camera you can check it with pulled sparks.

The 996 doesn't have a combined water / oil cooler. The oil cooler system is completely separated.

Regards, Markus

I've heard the 3.2 and 3.4 use the same heads from a few different people.

Sorry, but every 996 ever made uses an oil to water heat exchanger/cooler, just like the Boxster, because the 996 uses the same basic engine layout.

JFP, do you have insight regarding 3.2 & 3.4 heads?
I've sent an oil sample to Blackstone for analysis... we'll see what they have to say.
I found the oil in coolant (just a bit of Choco on the top of the coolant in the reservoir) as I inspected a couple of days after making a 400 mile trip from VA to SC. I was running Dash Command for the duration of the trip, just watching data. As I got into traffic, coolant temps climbed from a highway temp of around 180 to a max of 223. I noticed that I could smell coolant. I turned on the AC to get the fans going, and that bought temps down to around 210.
It'd be nice if it's just the heat exchanger, but I'm still guessing heads. If the bottom end is still good and the 3.2 heads will work, then the 996 gets a salvage motor and the 986 gets a hot rodded 3.4. And I'll be functionally broke for a good while.

If you think you have a cylinder head issue, try running a leak down and/or bore scoping the suspect.

You can interchange 3.2 and 3.4 cylinder heads within certain bounds and caveats: Both engines need to be 5 chain designs to allow this to happen as there were no 3 chain 3.4 engines. The 3.2 heads will increase the compression ratio a small amount due to their very slightly smaller chambers, and both use same camshafts. There is also some difference in the intake arrangement.

3.6 heads are not usable because of the VarioCam+ .

Ups, sorry, i've meant something different. For example Mercedes cars have a combined water/oil cooler in front of the car. This is one cooler element that has 2 chambers for both fluids. The front coolers of the 986 / 996 are separated.

Regards, Markus

We had a failed oil cooler on our 2.7 when doing a refurb. It's easy to pull off and test with an air gun and 1 rubber plug. Bubbles showed up instantly and we knew we had the culprit. This made the fix simple but getting the oil out of the system was a bear!!!

The heads are the same casting including the bolt hole bosses. Depending on application (3.4 or 3.2) holes are drilled & taped for bolt on accessories & combustion chambers are finished with 3cc's less volume for the 3.2L to maintain compression ratio Valvetrain is the same.

^This. And I think the 996 heads will have the other holes for the intake manifolds.

I'm not an engine expert but I play one on the internet. There was a comment above that the heads don't crack often. Actually cracks around the valve seats and plug hole aren't that uncommon. My train wreck had one cylinder head that had been previously welded, and I've seen plenty of photo evidence and statements to come to the conclusion that it shouldn't be discounted.

As JFP (who is an engine expert) mentions, do a leakdown.

Thanks for all the info.
Never done a leak down, but I've read about the process. What should I be looking for?

I thought I blew my head too. It happened to be the oil cooler. I replaced it with the bigger S model

Hello,

i would recommend a simple compression test of every cylinder first. So you'll see if there is a cylinder that is out of specs, which will be a good indicator.

If you have a endoscope camera you can inspect the cylinders. But it is difficult to inspect the heads with a cheap one, because for that you need to direct the camera into the direction from where you put in the camera. Also you need to interpret what you see there. ;)

The combines oil & water cooler is on top of the engine. There are round rubber seals below the cooler. Sometimes they fail. So if it is oily or wet there, this can be an indicator that a seal failed. Also the cooler can fail internally.

Regards, Markus

(corrected post from original text)