Head Gasket

[NCGermerican]

Hi All. I blew my head gasket this weekend and I searched the 944 Turbo forum section but couldn't quite find an answer to my question. I'll try to keep this brief but I feel the following information is key to getting the most informed answers:

1987 951
170K miles
Evergreen Raptor Turbo, Tial 38mm WG (.8bar spring), LR MBC (all installed in the past few weeks)
Fabspeed downpipe (Jet-hot coated), Fabspeed exhaust with "performance cat"
Innovate WBo2 Gauge, Autometer Boost gauge
3-Bar FPR
LR MAXX DME and KLR chips
F9 Sport DME w/ ODB+
Rebuilt KLR
Valvoline VR1 20w 50 (changed often)
Peak Blue Coolant (Porsche pre-1996)

Compression test 1-145; 2-90 ; 3-145; 4-145
Pulled the head yesterday and confirmed HG failure on cyl 2 (cylinder was also wet and noticeably cleaner)

HISTORY: Car purchased Nov 2022. Vehicle has pretty good documentation from 2000 to now. Head was rebuilt in 2016. Unfortunately, the receipt was one of those diner style "let me take your order" written receipts from wherever the work was done - it didn't even list the name of the shop. One of the line items was written "skim head", but it didn't say HOW MUCH was skimmed. A Widefire HG was used, which failed. When I received the car it had an eBay style MBC, with NO aftermarket chips (or CV), so I'm assuming it was running pretty lean on boost - and for who knows how long. I removed the eBay MBC, put in a new CV and plumbed to factory routing. The car ran pretty good, but there was always a hesitation when just getting into boost. I replaced all fluids when I got the car and it never overheated under my ownership in the past year. It was upon installing the new turbo/WG/MBC setup and putting the pedal to about 1/2 throttle where I immediately had a loss of power, the motor bogged down and my AFR went to around 16/5-17. Before that, idle was between 14.7 - 15 and under boost it would go to around 12.5

The purpose of this post is "which HG should I get?" My main conundrum is that I don't know how much was skimmed in the first head rebuild. I'm going to drop the head at a machine shop and I'm sure they might skim it again. I'm afraid that enough material will have been removed to not allow for another factory style HG (including WF HG). If so, that leaves me with aftermarket or "performance" HG's. I watched Tom's video where he compared a few HG's for his project but it mainly compared the different brands and types but not to calculate which thickness to go with.

My goal for the car is for a RELIABLE 300-350 HP. I have a Vitesse MAF kit coming in a few weeks. This is not a track car - more for "spirited" runs with the local PCA chapter - I need to keep up with the Cayman S's on the straights. Other than that, it will be driven on sunny days to work, local cars & coffee meetups/car shows. I know the Cometic metal HG's come in various thicknesses and I have no idea which thickness I need - or if they are just overkill.

I truly appreciate any help and advice! I don't want to have to go through this again for a very long time.

[cda951]

As ever, the factory service manual is your friend, see the attached picture of the page from the manual detailing the wear limit dimension for the cylinder head. The measurement "A" shown in the diagram is referencing the tab that protrudes from the right front corner of the cylinder head.

The 951 104 374 50 thicker (1.4mm) head gasket is still available from Porsche at just over $100 list price. The "wide fire" head gasket that is popular to install on modified 951s was originally meant for cylinder heads that were modified to accept O-rings to enhance sealing, and do not offer an advantage for a head that has not been modified as such.

The factory wear limit for any cylinder head is stipulated because both camshaft timing and compression ratio is altered if the head is too thin (valve to piston clearances are also affected, but this is not a worry in a low-compression engine like a turbocharged 951). There are plenty of 944s out there running on cylinder heads that have been skimmed too much, but it is always worth checking and considering any time a cylinder head is removed.

If your measurement is close to the factory spec, and the cylinder head is true and flat, and everything is torqued to factory spec with new hardware, the head gasket choice is trivial when compared to zeroing in on your tuning and eliminating whatever hesitation issue you had. You need to choose a lower initial boost level and make sure everything runs very well and in a safe AFR range before turning up the boost, or you will blow the next head gasket, or worse, no matter which type that is.

EDIT: just re-read your post, because of the larger turbocharger you need more fuel, as in larger fuel pump and injectors, and the ability to tune your engine management to suit. I went VEMS standalone engine management because I wanted to do so myself and have more flexibility for ignition options and extra sensors and datalogging, and have had great results. Others have not had so much success with the VEMS, it is a bit of a rabbit hole. I would recommend contacting Vitesse/ @Fast951 , he has for many years successfully modified the factory 951 DME (same thing should work on FTech version) to be quite tunable for setups like yours. Dyno tuning is also recommended!

[Fejjj951]

The best HG available is the Vulcan Cut Ring. I have 1 available if your are interested. The thickness is .063 which is probably close to what you need.

[NCGermerican]

As ever, the factory service manual is your friend, see the attached picture of the page from the manual detailing the wear limit dimension for the cylinder head. The measurement "A" shown in the diagram is referencing the tab that protrudes from the right front corner of the cylinder head.

The 951 104 374 50 thicker (1.4mm) head gasket is still available from Porsche at just over $100 list price. The "wide fire" head gasket that is popular to install on modified 951s was originally meant for cylinder heads that were modified to accept O-rings to enhance sealing, and do not offer an advantage for a head that has not been modified as such.

The factory wear limit for any cylinder head is stipulated because both camshaft timing and compression ratio is altered if the head is too thin (valve to piston clearances are also affected, but this is not a worry in a low-compression engine like a turbocharged 951). There are plenty of 944s out there running on cylinder heads that have been skimmed too much, but it is always worth checking and considering any time a cylinder head is removed.

If your measurement is close to the factory spec, and the cylinder head is true and flat, and everything is torqued to factory spec with new hardware, the head gasket choice is trivial when compared to zeroing in on your tuning and eliminating whatever hesitation issue you had. You need to choose a lower initial boost level and make sure everything runs very well and in a safe AFR range before turning up the boost, or you will blow the next head gasket, or worse, no matter which type that is.

EDIT: just re-read your post, because of the larger turbocharger you need more fuel, as in larger fuel pump and injectors, and the ability to tune your engine management to suit. I went VEMS standalone engine management because I wanted to do so myself and have more flexibility for ignition options and extra sensors and datalogging, and have had great results. Others have not had so much success with the VEMS, it is a bit of a rabbit hole. I would recommend contacting Vitesse/ @Fast951 , he has for many years successfully modified the factory 951 DME (same thing should work on FTech version) to be quite tunable for setups like yours. Dyno tuning is also recommended!

Thank you very much for the detailed response! Yeah, I need to get a FSM for this car. I printed one out for my 87 S and need to do it for the 951 as well.

I truly believe that the HG was already compromised when I purchased the car (bought it out of state w/o PPI), and then the Evergreen just finished it off. I should have done a compression test when I got it - that's my mistake that I won't make again. I'm going to have the head checked over the next week or so and will then measure to see what thickness of HG I can go with.

I believe the Vitesse MAF kit comes with a few MAPs out of the box and it sounds like John is willing to help provide new ones based on data logging. I went with the "stealth" kit so I can keep the factory airbox for the most factory look possible. That's why there's a lead time as they are made to order - he said I should have it late this month. I bought new injectors from him as well. I debated going VEMS but I was looking for as much "plug and play" as possible. Once I get it dialed in, I'm hoping to "set it and forget it".

[NCGermerican]

Just an update (I hate threads with no conclusion or resolution), I ended up going with a Graphite Composite HG from 944Store. I had the head rebuilt with new valve springs, retainers, seals, keepers, etc. The valves were all cleaned and tested and the head surface was skimmed. The machinist also noted that the surface was NOT skimmed properly whenever the last HG was replaced. He said it was "wavy, as though someone just used a dremel to clean some portions of the surface instead of leveling the whole thing". He also said that the valve guides were most certainly original (176K miles) despite the service records showing they were replaced previously. Word of warning, just because a car may have documentation of a major service or repair, it doesn't mean it was done properly - especially when the receipt looks like a diner order ticket that doesn't even list the name of the shop.

I put everything back together again and started up the car. I pressure tested the coolant system and there were no leaks. I also installed a steam vent kit to help prevent future HG issues.

I'm currently installing the Vitesse MAF/MAP kit w/ injectors. Once that's done and fired up, I'll do another compression test to see how it does. Prior to replacing the HG it was 1-145, 2-90, 3-145, 4-145.

[Tom]

Just an update (I hate threads with no conclusion or resolution)...

Thanks for the update! And I agree. The only thing worse is finding an old thread somewhere with your exact question (like which wire goes where), only to have someone say "thank all -- I figured it out" without giving the answer.

Fingers crossed your car is all set for thousands of miles now! I've seen heads (and decks!) like that -- where someone goes to town on it with roloc or scrotchbrite disk on a power tool, not realizing how soft aluminum really is...