I subscribe to Cycle World magazine primarily to read Kevin Cameron's monthly one page TDC column, and to try to absorb detailed technical articles he writes that appear in about every other issue.

In the most recent issue (March?) Kevin addresses coolant velocity in cylinder heads. According to KC, turbulent high velocity coolant is vastly more effective in removing heat from combustion chamber "domes" than lazy slow moving coolant. Worse yet, is any area of stagnant coolant that rests against the domes' coolant side surfaces with slow or no movement which can create dangerous detonation causing hot-spots. 

Kevin describes large coolant volume cylinder heads as antique "bathtub" design. Conversely, he describes modern sled cylinder heads (like some SkiDoo models) as having a "shrink wrapped" appearance where the diecast cylinder heads' outer covers closely mirror the shape of the combustion  chambers. Shrink-wrapped covers create small but ample passages for high velocity coolant flow over the combustion chambers.

Some of the aftermarket snowmobile cylinder heads I see on the dyno look like bathtubs to me.  Most "billet"  head covers are large rectangular structures that appear to hold lots of coolant, exactly the opposite of what would create high velocity turbulent coolant flow over the internal surfaces of the combustion chambers.

On typical dyno tuneups especially this time of year it's more time (and cost) effective to dyno sled engines with their own coolant, letting the 7.5hp roof mounted dyno room blower cool the engine between dyno runs. To maintain consistancy, I like to measure head surface temp with an infrared gun before each dyno run. On most sleds, head surface temp runs about 20 degrees F lower than coolant temp, but this is not as important as doing each dyno run with similar engine and pipe temperature.

What I notice on some aftermarket cylinder heads is that ending surface temp (after 10-15 second dyno runs) is sometimes much higher than what we usually see with stock heads. In the past I have dismissed this difference in being caused by different material finish (shiny machined and anodized or powder coated billet instead of die or sand cast surfaces) causing different infrared gun readings. But after digesting Kevin's  info I'm thinking that we may be  experiencing the bathtub syndrome.

How bad is this?

My favorite example of stagnant coolant was Tim Bender's experience with an Exciter FIII oval racing engine nearly 20 years ago. Kevin Cameron had suggested to Tim that the golf-cart-like transfer ports on the Exciter engine limited its potential. So Tim decided to widen the engine to allow room for larger transfer ports that he would create out of material welded on to the sides cylinders. This meant having the Crankshop build him a wider crankshaft, then saw a crankcase and cylinder head in half, then weld in an inch or so of material to allow bore spacing to be wider, accomodating normal-size transfer ports.

Tim's problems began when the one-piece cylinder head was widened and welded back together. The Exciter 570's coolant normally enters the front of the head in the center, then is forced around each combustion chamber surface, then rejoins as it exits the rear center of the head at the thermostat housing. However the widened and rewelded head provided  an unintended short-circuit for the coolant straight through the center of the head from front to back without being forced around the combustion chambers. Unbeknownst to Tim and me, this allowed stagnant water to lay on the combustion chambers instead of flowing over them. Trouble was lurking.

On the dyno, even with lots of water flowing to cool the engine, we never could create low BSFC without detonation. But the engine made more HP than before and Tim was anxious to test the engine in his race sled before going to the annual big oval race at Eagle River.

I went with Tim to a nearby frozen lake where the night before he had snowblown an oval track on the shallow end. When he began doing laps with his dyno-tuned engine Tim encountered detonation with the same jets, same gas as we had dyno'd with the day before. Jets that were dandy for 15 seconds on the dyno were causing deto on the track after 20 seconds. The 48mm carbs required about 15 sizes larger jets to be deto-free, and that extra fuel drowned the HP added by the larger transfers. This was perplexing to both of us. At the end of the day, Tim's high HP wide engine was no faster than the narrow golf-cart Exciter race engine he had run previously. Why the deto? With his other race engines, winter dyno jets were within a few sizes of what he needed for 20 laps.

That evening, one of us remembered the widened cylinder head. I like to think that I came up with the solution to the problem, but it probably was Tim. One of us called the other, and we discussed the possibility of a problem with combustion chamber coolant flow. Tim went to the shop at 10pm, and upon inspection found that there was an open passage in the widened area for coolant to flow directly from the front center head inlet to the rear center outlet! He mixed up some Devcon epoxy filler and goobered that unintended passage closed. As intended, coolant would once again be forced to the outsides of the head, pass over the chambers then rejoin at the rear before exiting the engine.

The next day back at the lake, Tim was able to jet down all of the 15 sizes (and then some) and the HP was back, deto was gone. More races would be won.

Bottom line--stagnant coolant was surely the culprit. According to KC, when coolant boils and creates a steam pocket anywhere around a combustion chamber, detonation is sure to follow quickly even with the safest A/F ratio.

I'm not suggesting that all billet replacement heads are low velocity bathtub design--in each case I don't know what sort of internal passages were created by the person who programmed the milling machine to carve out the "tub". But my opinion is that before an aftermarket head is installed in might be good to compare coolant volume between the stock head and the replacement head. If the replacement head has larger capacity than the shrink-wrapped stocker I would question the design and ask why the volume is greater. If the answer is "to help cool the engine".....

The factories' sled engine designers surely use dyno software that measures coolant flow in GPH, and that figure along with temperature rise in the coolant around each combustion chamber is necessary to correctly design the cooling passages in any cylinder head, either OEM or replacement.

When it comes to snowmobile cylinder heads, cool-looking and big is not necessarily better.