Curious about Experiences with Cracked 993 Cylinder Heads used in Racing Engines

Mahler9th · 10:02 AM

I am curious about experiences folks may have with 993 cylinder heads in racing engines, specifically related to cracking. The crack in this 1995 993 head occurred recently (#6). Cause undetermined. Application is 3.8 liter racing engine, rev limit of about 7300 rpm, and with Haltech FI.

Mahler9th · 08-04-2009, 10:06 AM

Re-sized Picture:

rsscotty · 08-04-2009, 11:46 AM

We had one do the same with the crack going all the way up through the intake port. This one was #2 cylinder on a 993 RSCS engine with stock Motronic. We just replaced the one head since they are indeed pricey. The car is a Club racer with headers and Flowmaster muffler.

Steve@Rennsport · 08-04-2009, 01:18 PM

Mike,

Does your Haltech log cylinder head temps?

What are your AFR's @50% load to WOT, 5K on up?

Mahler9th · 08-04-2009, 04:28 PM

Interesting. I have heard that other 1995 heads have cracked in the same manner. This was #6. I had the same thing happen in #1 a couple of years ago. Don't know the head temp nor the mixture- the builder/tuner has all of that. The mapping is good-- don't suspect a problem with that. Interestingly the first failure came one event after a really hot weekend. We were up at Fernley and it was between 100 and 105 all weekend. The next race weekend at Sears Point in more typical fall conditions is when it cracked. We had a few PRC cars with engine problems that ran that event... and "we" said we'd stay parked in the heat in the future... yeah, right.

This one cracked at Sears Point in late June in 100+ ambient heat.

One of my expert friends generally described some welding mods he makes to help with this-- he is a well-know specialist. Another friend worries about the welding leading to increased crack susceptibility, as he has seen this happen.

I have some secret airflow mods to bring the coldest possible air to the fan... hmmm... I guess not cold enough...

Mahler9th · 08-04-2009, 04:35 PM

Oh, and by the way, I found it kind of funny (not too funny though) that my #6 looks just like the cracked rsr head Bill Verberg posted on the 993/964 forum some time ago...

racing97 · 08-04-2009, 06:35 PM

Do the welding on the sides, the casting is not beefy enough and doesn't offer much support for the larger bore on 3.8. The picture shows the expansion leaks also somebody in So Cal manufactures a stepped oring that fills the cyl gasket gap and is machined into the head surface by 1.25mm and helps prevent leakage much like the 935 utilized. This will eliminate 90% of the problem but even yet sometimes they crack through the inlet port.

regards

Bill Verburg · Bill Verburg's Garage

993 heads are known for cracking when used on big bore race engines(3.8 up)

they need to have the lower fins welded to reinforce this area

Steve@Rennsport · 08-06-2009, 04:08 PM

I'd be really tempted to try Grady's "Rubbermaid Solution" in cases where I ran a 3.6 (or 3.8) in high ambient temperatures such as those.

Mahler9th · 08-06-2009, 07:36 PM

Thanks Bill and Steve.

I don't think that welding is the answer. I spoke with one expert machinist, and he does not like to weld these heads. Nor does my engine builder/friend. These guys are experts.

Another expert machinist/friend does believe in welding. Will have to ask Jerry what he thinks.

In any case, I will not have the replacement welded...

One of my contacts has a feeling that it's the 1995 heads that sometimes crack.

Mahler9th · 08-06-2009, 07:49 PM

I vaguely remember the Rubbermaid Solution... Will have to back and read about it. What I really should do is stay parked when it is that hot. What I want to do is buy some later heads and have my friend Mike sculp them into at
rt... Just for fun...

Mahler9th · 08-06-2009, 07:49 PM

That is sculpt them into art

JoeMag · 08-07-2009, 11:05 AM

I too have heard of the welding. ...especially on heads that have been cut a few times.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by Mahler9th

Thanks Bill and Steve.

I don't think that welding is the answer. I spoke with one expert machinist, and he does not like to weld these heads. Nor does my engine builder/friend. These guys are experts.

Another expert machinist/friend does believe in welding. Will have to ask Jerry what he thinks.

In any case, I will not have the replacement welded...

One of my contacts has a feeling that it's the 1995 heads that sometimes crack.

The east coast shops disagree, must be Ca. weather

MBruns · 08-07-2009, 05:18 PM

We run the 964 heads on our HSR 3.8, started with a low time set and welded the areas like the pic and machined the late style seal surface and after about 20 hrs. we split one from top to bottom, on inspection we found cracks in many areas near the welding that were not far behind, the current set we did no welding and is almost there time wise and no issues, when we pull it apart I will let you know how it worked. When AJR ran the 993 RSR's if they didn't start the 24 with brand new heads more that likely they would not make it. one year early in the 24 one cracked so bad that it burn't thru the chain box and fried the rails, changed the head and continued on and cracked one more with only a few hours to go, we just unplugged the injector and finished on 5 cyls.

Mike Bruns JBRacing.com

Mahler9th · 08-11-2009, 06:28 PM

Ted R. has the replacement head now, and he is of the do not weld school. He showed me an interesting museum of dead heads, including one from a 935... Always nice to visit Ted.

My two failures are at about the 60 hour mark give or take...

Anyway, it sounds like there are a variety of opinions and approaches.

I am curious about the relationship between ambient/engine temps and cracking failures experienced by folks as well as whether folks have seen cracking failures of later 993 heads.

Thanks everyone for info thus far.

Raceboy · Raceboy's Garage

Have you thought of cooling the combustion with additional fuel when driving in hot weather? The head in the photos could've make benefit from richer mixture. It may cost a few ponies but at least the head won't crack.

CliffBrown · 08-13-2009, 01:44 PM

May not be related but it's all I can offer.

Heads developing cracks in high horsepower air-cooled aircraft motors are not uncommon and normally related to (1) thermal shock or (2) overly lean condition in some cylinders.

(1) is cured with changing operating habits.

(2) is helped with an EGT probe for each cylinder's exhaust to detect faulty injectors, or just enriching all cylinders.

turboteener · 08-19-2009, 07:27 AM

What does the top of the piston on the corresponding cylinder look like?

You can weld aluminum heads, but it is a very time an labor intensive process to do it properly. First the head must have all the steel taken out. That means seats, guides, studs, etc. Once all those are out, the head needs to be pre-heated in an oven and then welded. Once welded they should be put back in the oven and slowly cooled. Once they are cooled send them out and have them annealed and heat treated back to T-6 standard that they started with. Once they have been heat treated, you will need to verify and correct any dimensional changes that will have occurred during the process. That means the chambers, seats, cam housing mounting points, valve guide holes etc will need to be verified and corrected if necessary.

Mahler9th · 08-22-2009, 10:49 AM

Thanks. Obviously some folks have had success with some type of welding, others have not. Interestingly Bill's post suggest that this is a standard practice in the East-- I am curious about who is doing it and what success they have had...

I spoke to my friend/engine builder last week and we discussed the full monty in terms of heat treatment, et cetera.... all of that is just not worth it to me for my application. But is seems to make sense. And locally, one of my machinist friends says he has done it successfully.

Thanks to Steve, I re-read the info on the rubbermaid solution... may fool around with that if I do some warm weather testing later this year. I am curious about the delta t I might experience in our dry heat.

In any case, my replacement head is being prepped and I will just go on down the road with that. My next priority is getting some 993 rsr Mahles into my engine and getting the JE's out before they crack and fail. No more JE's after that. Then maybe some replacement heads.

I am still curious as to whether anyone has seen '96-98 993 heads crack.