Mucus72 Posted March 16, 2014 Share Posted March 16, 2014 Hi all, I need a little advice. The guy I bought my 7 from a few years ago recommended Castrol React Performance DOT4. Instead of just buying from OPIE as usual, I asked my local motor factors to supply some, but they had none on the shelves and then went into a big chat asking me why I am not using DOT5.1 as apparently I'll get a better feel, and it's less likely to boil on a hard hot track day and it's fully synthetic and doesn't need replacing as often because it's characteristics are a lot less hygroscopic. I still have my Castrol DOT4 on order for a quickie bleed before Silverstone track day next weekend, but now I'm wondering if it's better after that to replace fully with DOT 5.1. An old post on here was inconclusive, so looking for advice and thoughts from all you lovely POBC? Link to comment Share on other sites More sharing options...
Dr Slotter Posted March 16, 2014 Share Posted March 16, 2014 DOT 4 is plenty for something the mass/speed of a Caterham. I wouldn't bother. Link to comment Share on other sites More sharing options...
paul jacobs Posted March 16, 2014 Share Posted March 16, 2014 I have an idea that Dot 5.1 has silicon in it, which is to be avoided at all costs, as it is not compatible with normal fluid, and can damage seals etc. Link to comment Share on other sites More sharing options...
Roadsport06 Posted March 16, 2014 Share Posted March 16, 2014 5 has silicone 5.1 is glycol based like DOT 4. Link to comment Share on other sites More sharing options...
Area Representative IanW Posted March 16, 2014 Area Representative Share Posted March 16, 2014 Not sure I agree with the boiling point. I changed to 5.1 for my track day at Llandow last year and had very soft brakes after a couple of sessions. Bled than over lunch but the softness came back when leaving the circuit in the evening. Normal feeling resumed later. I'm changing back to DOT 4 before my next track day. Link to comment Share on other sites More sharing options...
Pendennis Posted March 16, 2014 Share Posted March 16, 2014 Then you did something wrong as DOT 5.1 is suited for higher temperatures thus track days and circuit racing. I've managed to cook DOT 4 but not DOT5.1 Link to comment Share on other sites More sharing options...
Graham Perry Posted March 16, 2014 Share Posted March 16, 2014 Have a look in the archive as we have discussed this several times. The most focussed of motorsport grade brake fluids are generally Dot 4. Link to comment Share on other sites More sharing options...
7 wonders of the world Posted March 16, 2014 Share Posted March 16, 2014 Quoting Paul Jacobs: I have an idea that Dot 5.1 has silicon in it, which is to be avoided at all costs, as it is not compatible with normal fluid, and can damage seals etc. DOT5 is a silicone based fluid which can be used in conventional systems but existing glycol fluid should be flushed out, it will not damage the seals if the system is designed for normal DOT fluids, it is less hygroscopic that normal DOT fluids, but offer less in the way of system lubrication and is more compressible than DOT. Seal damage / swelling thro fluid cross contamination can occur if DOT fluids are used in systems designed for mineral fluids, LHM etc and vice versa. Many racing fluids offer a marginal improvement in WET boiling point which is a comparison designed to show what the boil point would be after an average 2 years service based on absorbing a water content of circa 2.5%, however they offer significant improvement in new DRY boil point, since the fluid in many race applications is changed every event the WET figure is to all intense an purpose irrelevant. There will also be a useful improvement in compressibility over a normal road biased fluid. Edited by - 7 wonders of the world on 16 Mar 2014 22:18:30 Link to comment Share on other sites More sharing options...
Grim Reaper Posted March 17, 2014 Share Posted March 17, 2014 Forget about using DoT5 (silicone) fluids, they will not give a better performing system. Too many downsides to them (such as retaining bubbles of air) for them to be worth the change. Go for a good DoT4 such as Shell ESL or Donax YB for a lower viscosity fluid, or the ATE Super Blue Racing for one that is easy to see flushing through vs the usual clear fluids Link to comment Share on other sites More sharing options...
gundersen Posted March 17, 2014 Share Posted March 17, 2014 Quoting Pendennis: Then you did something wrong as DOT 5.1 is suited for higher temperatures thus track days and circuit racing. I've managed to cook DOT 4 but not DOT5.1 My understanding is that the MINIMUM requirements for DOT 5.1 is higher than DOT 4, but there is nothing prohibiting a high quality DOT 4 fluid having a higher boiling point than say a chap DOT 5.1. Link to comment Share on other sites More sharing options...
Roger Ford Posted March 17, 2014 Share Posted March 17, 2014 If you look at the specs for Castrol React, it exceeds all the temperature requirements for DOT 5.1, yet it's DOT 4. Why? Is it that DOT 5.1 has some other requirements beyond temperatures, or is it simply that Castrol think there's more market for DOT 4 and if they called it 5.1 people wouldn't buy it to put in a vehicle that specifies DOT 4? Link to comment Share on other sites More sharing options...
Pendennis Posted March 17, 2014 Share Posted March 17, 2014 Could well be right Roger. I have used standard DOT4 (don't know it's origin), but cooked it on my race car so had to flush, now use AP551 and do not see any temperature issues. PS. I used DOT5 on our go karts for years, never had a seal issue and never had moisture ingress Link to comment Share on other sites More sharing options...
Grim Reaper Posted March 17, 2014 Share Posted March 17, 2014 DoT 5.1 is the same standard set as DoT 5 but in a mineral form. The spec that Castrol React is failing is probably the viscosity spec which is 900mm2/s for 5.1. 5.1 is designed for ABS systems where fast flow is desirable to maximise ABS modulation capabilities. If you think your brakes are slow to release, try a thinner fluid. That's why I suggested the Shell ESL/Donax YB. It has a viscosity of 675mm2/s which is about as thin as they get. Link to comment Share on other sites More sharing options...
Mucus72 Posted March 17, 2014 Author Share Posted March 17, 2014 This thread has been an interesting romp through lots of your views, and as usual, thanks to you all. It helps me think about what to do. I am going to stick with Castrol React DOT 4, and just bleed before the next track outing, then check specs of Shell/Donax DOT4 and the odd DOT5.1 before I bleed the existing through and fully replace with another option for later in the year. Knowing that there is more than one 'right' answer and stay with Glycol based products means I can have fun testing 'before' and 'after' and see if I can spot (or even remember!) the difference. All I really know is that during warmer ambient track days last year my pedal was going 'long' near the end of bigger track stints with Castrol React. Link to comment Share on other sites More sharing options...
FramerateUK Posted March 18, 2014 Share Posted March 18, 2014 My old Dot4 was fine on most tracks but I cooked it at Llandow a couple of times (very mushy pedal feel!). I've just had it flushed and replaced with 5.1, and I'll be testing it this Saturday at the Taffia trackday. I'll report back on this thread to let you know how I got on. Just to note, on my tin top Megane 250, I switched the stock DOT4 for some RBF600 (Also DOT4) and it made a world of difference. So I think it's more down to the quality of the fluid than the rating! Link to comment Share on other sites More sharing options...
Roger Ford Posted March 18, 2014 Share Posted March 18, 2014 I believe Castrol SRF is considered one of the best racing brake fluids, but you'll pay handsomely for the privilege of using it. I stick with React. Beware of confusing "pad knockback" with fluid problems. If you run over kerbs, particularly if your wheel bearings are slightly loose, your disks will push the pistons in (via the pads) and you'll have long travel the next time you use the brakes. Link to comment Share on other sites More sharing options...
craigyb Posted March 19, 2014 Share Posted March 19, 2014 I used RBF 600 last year, then when widetrack fitted they put standard dot 4 in. Pedal definitely felt more positive with RBF600 Anyhow have a TD at Sivlerstone on Sunday, so will see how it gets on. Most likely back to the RBF600 after that Link to comment Share on other sites More sharing options...
FramerateUK Posted March 27, 2014 Share Posted March 27, 2014 Just a quick update (a little late but I only just remembered). Huge improvement in braking at Llandow since my move to Dot 5.1. So much so I didn't get any fade at all and completely forgot it was even an issue. I'd mostly say it's due to new fluid, but it certainly made a huge difference and I've got no complaints about the 5.1 on road or track. Link to comment Share on other sites More sharing options...
Mucus72 Posted March 27, 2014 Author Share Posted March 27, 2014 FramerateUk. Which DOT5.1 did you use on the Taffia track day please? Link to comment Share on other sites More sharing options...
FramerateUK Posted March 27, 2014 Share Posted March 27, 2014 No idea sorry. I trusted my usual mechanic to put what he thought was best in there while he did the service for me. Link to comment Share on other sites More sharing options...
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