“Enthusiasts for silicone based fluids tend to oversell it rather a bit.”
“First, we need to distinguish between DOT-3/4/5 (a set of specifications) and silicone (a material on which some fluids are based). The DOT specs are based on the concept of wet and dry boiling points. the dry boiling point is applicable when fluid is fresh (which it really ought to be when you’re racing) and the wet boiling point is applicable after the fluid has been exposed to moisture and has had the opportunity to adsorb water. The minimum values for the wet and dry boiling points is specified for each DOT- level, and are higher for 5 than 4, and higher for 4 than for 3. Note that these are _minimum_ values, and there is no constraint on by how much a manufacturer may exceed them. Thus, Ford Heavy Duty is a DOT-3 fluid with an _Outstanding_ dry boiling point comparable to that of AP550 racing brake fluid. It is rated DOT-3 because its wet boiling point is mediocre; since racers are supposed to change their fluid out regularly anyway, we may infer that Ford Heavy Duty is an appropriate fluid for the track, and so it is.”
“The issue of glycol vs. silicone is one of implementation; until recently DOT-3 and DOT-4 fluids were generally implemented with glycol, and silicone was the only fluid that worked as a DOT-5 fluid. apparently in Europe, though, Valvoline is marketing a glycol based DOT-5 fluid; I’ve never seen this in the states but a scandinavian member of the italian-cars list reported that he had obtained some.”
“The first problem with silicone has to do with the concept of boiling point. Boiling point is a very good metric for evaluating glycol based fluids because it roughly represents the conditions under which glycol turns compressible. It is a very poor metric for evaluating silicone based fluids because they turn compressible at a lower temperature than that at which they boil. It is not clear that a DOT-5 silicone fluid has anything to offer relative to a good DOT-4 fluid once this is taken into account (at least with regards to the boiling point.)”
“The second problem is the difficulty of pouring silicone fluids without getting air bubbles into them; systems with silicone based fluids generally have poorer pedal feel as a result.”
“The third problem is that while silicone fluids have good wet boiling points (which should be irrelevant to your race car anyway, right?) silicone does nothing to keep water out of your brake system. The difference is that with silicone fluids, any water that gets in will simply pool up in the low spots of the brake lines. The best defense against water is fresh seals all around.”
“The fourth problem is that when you have a brake system that has had glycol in it for any length of time, proper conversion to silicone requires that you rebuild the entire braking system, replacing all rubber seals with new ones. This is a lot of work.”
“In summary, silicone is a good choice for your collectors classic that sits for long periods and is never driven very far, provided you’ve done a complete braking system renovation. Otherwise, stick to proven fluids, like Castrol LMA or ATE SL for street cars and Ford Heavy Duty or AP550 for race cars (and actually, the Castrol and ATE will probably work pretty well for a few racing applications as well).”
To continue to belabor the silicone brake fluid question … Just discovered a brochure in my pile of karting information from the Ripley Engineering folks … they make braking systems for karts. Here’s what they say about silicone brake fluid: “unsuitable for racing applications (for these three reasons): 1. At approximately 275 degrees, the additives used to compound silicone fluid begin gassing off resulting in air in the lines. 2. At elevated temperature, silicone fluid has four times the compressibility of glycol-based fluids (leading to) increased pedal travel and a spongy pedal. 3. At elevated temperatures, silicone brake fluid will expand significantly. ”
But, according to another source, the “Brake Handbook” by Fred Puhn, the federally mandated standard for brake fluids are as follows: dry boiling point:
DOT 3 = 401 degrees F; DOT 4 = 446 DOT 5 = 500
Wet boiling points are approximately 120 degrees less in each instance. The article goes on to say that the only drawback to DOT 5 is a sometimes spongy pedal “after exposure to high temperatures” because of DOT 5’s compressibility under high temps. Again, this is not likely to happen in most forms of kart racing (with the exceptions as noted below).
Ripley’s brochure goes on to give these helpful hints:
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