JohnA Posted August 11, 2006 Share Posted August 11, 2006 Just a sidenote --- as the more experienced people here might know, piston ring sealing is not really related to crankcase pressure or even ring elasticity. (as I thought when I was younger and didn't know any better) The rings seal because the combustion gases fill the gap under their groove and they are pushed against the cyl wall. So the higher the cyl pressures, the tighter the rings push against the walls Quote Link to comment Share on other sites More sharing options...
Digsy Posted August 11, 2006 Share Posted August 11, 2006 Correct, but surely the rings dont know if it is a pressure acting from on top or a depression from underneath? If you could (in theory) apply a negative pressure from inside the crankcase, would that not work just as well as a positive pressure from on top? In any case, the % benefits of an additional half a bar pressure difference accros the rings are goign to be negligible on the power stroke, but maybe noticeable on the latter part of the exhaust stroke or the early part of the compression stroke? Quote Link to comment Share on other sites More sharing options...
JohnA Posted August 11, 2006 Share Posted August 11, 2006 ...maybe noticeable on the latter part of the exhaust stroke or the early part of the compression stroke? yeah, fair point. It wouldn't be much, but wouldn't hurt either, would it... Quote Link to comment Share on other sites More sharing options...
JamieP Posted August 11, 2006 Author Share Posted August 11, 2006 I don't actualy vent to the atmosphere, I run the pipe from the catch tank back into the intake, to ensure I get vacum pressure. The best thing you can do to stop FMS failure is change your oil pump and run one of our modified oil pumps combined with our breather system. I spoke at length with Larry from SP when designing our system and it has been tried and tested on a lot of high BHP cars. We keep both items on the shelf. You will be surprised how much the -12 lines and fittings cost when you start to add them up! HTH Kevin hi kevin... im building a new breather system now based on your car as i think your proberly the best qualified at this..I have all the parts now.. im just waiting on my cam cover to come back with the -12 fittings im intrested in the modified oil pump.. can you give me a price on one please.. regards jamie Quote Link to comment Share on other sites More sharing options...
Digsy Posted September 5, 2006 Share Posted September 5, 2006 Digsy, what do the engine designers in your place think about this? Right, I got to talk to our head of R&D today (ex Cosworth, SAE paper author, etc. etc) his opinions are as follows: 1) If you over scavenge the crankcase you can get a net increase in power, but not from the mechanism that you might think. Sucking the oil and lowering the air density in the crankcase has the effect of reducing crankcase windage - that is the aerodynamic resistance of the crankshaft as it rotates through the air. "Blading" the crank webs has a similar effect. As mentioned by CW, dry sumped engines can use the scav pumps to drop the crankcase pressure to achieve this effect. Pure-bred racing engines usually maximize this effect by having totally separate crank bays, each with a separate scav pump. A road going engine isn't really suited to crankcase evacuation using scav pumps (even if the engine in question were dry sumped, which it isn't). 2) You don't get any increase in pumping efficiency because although its easier to pull the piston down, its harder to push it back up again. There will be a marginal increase in pressure differentail across the rings. 3) In a road car application, in order to draw an effective vacuum using the exhaust, you would need a free-flowing exhaust and obviously no cats. You would also need to effectively separate the oil out of the blow by gas before you dumped it into the exhaust. Unless you didn't give a damn about emissions, which I'm assuming we don't. In which case you may as well run an open breather system - something you already know I'm against from a durability point of view. To summarise, apart from the windage improvement he didn't think it was worth the gains in a road car application. He actually said that in a race application, if you were really worried about oil carry over he'd prefer an open breather system with a big "puke tank" (oil separator or swirl pot). Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted September 5, 2006 Share Posted September 5, 2006 Thats interesting, and nice to know that my original *can o worms" opinions where not far off an "experts";) Quote Link to comment Share on other sites More sharing options...
JohnA Posted September 6, 2006 Share Posted September 6, 2006 Right, I got to talk to our head of R&D today (ex Cosworth, SAE paper author, etc. etc) his opinions are as follows: .... Thanks Digsy. I knew about the windage and the pumping efficiency. However, it is the closed vs open operation that I am interested in, and it looks like your guys favour the 'open' solution (for a 'race' application, lol) My experience indicates otherwise, but then again they may refer to race engines that seal well (5% leakdown perhaps) and are rebuilt often. Ours are kinda tired and only get rebuild when they develop a hole in a piston or the dipstick leaves a mark on the inside of the bonnet. Still, interesting to have their view. Quote Link to comment Share on other sites More sharing options...
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