964-C2

Maybe a stupid question.. But which cylinder(s) usually run hottest/leanest/richest/coolest in a 951? I am thinking different air flow because of intake design, +++

Van

Well, I'd think that #4 runs the hottest... because it's the farthest away from the water pump. The coolant in the jacket will have warmed up a bit by the time it gets to #4. I personally have seen more head gaskets fail at #4, too.

txhokie4life

LR has a steam vent kit designed to relieve an air pocket they believe is created near the #4 cylinder.

Mike

lart951

Well i have seen blown hg in #4 even with the steam vent, it has a tendency to go lean perhaps the intake design?

jerome951

When I burnt an exhaust valve, it was in #4....

86 951 Driver

Has anyone thought about reverse cooling this engine? to keep the head cooler?

Oddjob

Agree w/ above posts, it seems to be more common to burn valves on #4 and more head gaskets leaks around #4.

blade7

I'd be inclined to fit a EGT probe in no.4 exhaust and tune to that.

michaelmount123

Here's some food for thought, and perhaps to generate more theories:

1. Coolant flows from the H20 pump to the back of the block, then up into the head at #4 and back toward #1 where it exits back to the radiator. Look at a head gasket and you'll see this.

2. I've had a 951 manifold on the flow bench and each intake runner flows the same.

Thom

Me too and there was not 2 runners that flowed the same.

refresh951

Michael - As coolant around the cylinders flows in the opposite direction, do you think coolant in the head plays a much more dominate role wrt overall cylinder temperature?

michaelmount123

I don't know whether the head contributes more to cylinder cooling, but I would treat both as equally important. If the concern is providing an environment that minimizes detonation and head gasket failures, I'd say lean fuel conditions are the biggest contributor, followed closely by cooling issues.

Here are a few other thoughts that may be of some interest:

- There's no need to cool the lower portion of the cylinders. Porsche applied this to the 3L block design. The shallow water jacket around the cylinders also provides significant additional support. I think the 2.5 block guys that are filling their blocks are on the right track.

- If coolant flows too rapidly, it can't absorb heat and remove it efficiently. Restrictors in the coolant outlet increases coolant pressure (a good thing for other reasons) and slows the flow. Note the increased pressure is internal pressure that has nothing to do with the pressure cap nor pressure in the coolant reservoir.

- I fought head gasket problems in my own 944 race engine for far too long. I finally figured out the thermostat was closing on long straights, and it opens so slowly (try it in a pan on the stove) the reduced cooling would cause detonation coming out of the first turn (slow speed, reduced airflow, t-stat closed, full throttle) until it got open again. BTW, don't just remove a 944 thermostat. It's a sophisticated little devil that opens and closes passages as it operates. If it's removed, one of the passages needs to be blocked.

- Everyone had trouble with Cometic head gaskets at one point. It was a design issue that was first fixed on the 2.5 gaskets, and later on the 2.7/3.0 gaskets. The fix was to widen and lower the embossing. I use factory torque settings, but when completed I go back in sequence and completely back off each nut and retorque it to the final setting. Go to #1, back it off and torque to spec, go to #2, back it off and torque to spec, etc, etc. You'll find that the studs early in the sequence will be much looser than those later in the sequence.

MM

333pg333

Interesting thoughts/observations Michael.

So did you customise the thermostat?

Have you tried larger headstuds and if so what torque changes, if any, did you make?

ps...be very interested in your take on another thread.
https://rennlist.com/forums/944-turb...ad-design.html

michaelmount123

Agreed, but the issue is whether the flow characteristics are causing a lean #4 cylinder which I believe is a wives tale. My flow bench showed similar flow (you're right, not same) in all four runners. They were within a few percent of each other. A cylinder lean enough to cause detonation would mean it flowed significantly better than the others. My testing did not show this in any of them.

refresh951

This is awesome information. Thanks for sharing here. Two thoughts come to mind for me:

1 - I need to better understand the entire cooling system and its operation in various conditions.

2 - The idea of lowering and widening the embossing on the Cometic makes a ton of sense. In my view the embossing is absolutely necessary as it keeps the head sealed during flex (it acts like a spring). The key seems to be keeping the embossing from being over stressed which results in fatigue fracture at the base of the embossing while still keeping the head sealed during peak flex. Lowering and widening the embossing will reduce stress significantly and extend the fatigue life. I am going to call Cometic this week and discuss this with them. As Sid has mentioned, they seem to be willing to make changes at little to no cost. Proper modification will require some trial and error but we should be able to come up with some reasonable first cuts on the dimensions.