Well, changing the head gasket alters squish and affects combustion volume on a 1:1 ratio. Machining the head deck surface does not affect squish and affects the combustion volume about 60-70% of the same relative change in the squish due to the quench areas of the head. From a tuning standpoint, these two modifications act to different ways and should be approached separately. Rocky Stargel Team Stargel
Rocky, can you expound on your last statement? I've always thought that the goal of either method was to raise compression. What affect does changing the squish area have?
There are complete chapters in engine books about this very thing, but I will try to hit the high points. Squish: Has a greater net effect on compression than head milling. Affects the rate of acceleration of the intake mixture towards the center of the piston. Affects efficiency of combustion due to potential mixture trapping. Distance is mostly governed by piston contact at the quench areas by rod stretch as well as detonation depending on fuel. Head Volume: Has a lesser net effect on compression than squish height alteration. Attention must be paid to the sometimes complex interface with the shape of the piston crown, distance to valve seats, piston contact at the raised areas of the piston as well as detonation. Without writing a dissertation here, you tend to set an ideal squish size for a given engine depending on a number of factors including; piston mass, rod strength, rpm ceiling, bore size, fuel spec, application. You set the squish size first by machining the deck surface or changing gasket thickness, or both. Then once this is done, you examine the head to determine how much material you can remove while paying consideration to piston clearance, piston to valve clearance as it affects cam timing, valve seat interference from machining, and overall compression ratio with regard to fuel as well as rpm dependent combustion efficiency (power curve). Make sense? Nope, it doesn't to me either! :up: Rocky Stargel Team Stargel
So what in your opinion is better to do for higher horsepower? I'm assuming a thinner head gasket is about $150.00 + labor. As for decking the head am I right to say it's about $1000 for a complete job. What do most pro-racing bikes have, decked heads or thinner gaskets? (I'm not even sure a decked head is SS legal?) Sorry in advance - I'm no engine guy! Thanks:up:
Like I tried to convey in my earlier ramblings...you do both. Usually a machine shop will charge less to deck a head than most kit head gaskets cost. A professional race bike will usually have both. If given the choice, you get more gain from tightening the squish in most cases than cutting the head. Yes, machining deck surfaces is SS legal. When given the time, I machine everything and use a stock gasket. Why? Because deck heights vary somewhat in production engines, and stock gaskets are cheap ($20 or so) and readily available. BTW, you don't pull the center section when a restrictor is present, but you can certainly pull the top or bottom in a three layer gasket. You could then use spray copper or spray paint. In most cases it probably wont last quite as long as a stock three layer. Of course all this depends on it you have adequate squish or we could get into some good old metal on metal contact. Merry Christmas, Rocky Stargel Team Stargel
I think I'm getting it!:up: Thanks for your help! I can't wait to build another bike and try alot of these tricks of the trade!:up: Happy New Year!!
Ah, that's because I've never worked on one with a restrictor. We've been doing the removing a layer deal for a long time on SV's and Kawi 650s. Good info Rocky, now go comb your hair
rocky i dont think some are really reading/understanding your statments. 1. you set the squish FIRST (at least imo) to whatever deck height you have found works the best (hp/tq AND reliability..since you can run 0 deck height) ohh..deck height is distance from top of block (NOT HEAD) to the piston at true tdc. This is done with either a thinner head gasket or machining the top of the crankcase/cylinder block. every bike is different, hell i even run different squish depending on rider, does he engine brake a lot? does this guy never let the bike warm up first (some cases "grow" as much as .009" once warm!!) 2. then personally decking the HEAD (machining the cc smaller) to increase compression is 100% dependent on bike, size, gas, rider, track, etc. a lot of times for someone new, degree your cams FIRST to what you want..then check your ptv, then you will see how much you have to play with...ie extra room. then set your deck height (ie thinner head gasket) to what is meant to work best, THEN if you still have more "room" in your numbers, thats what you can (if engine needs) remove from the head by surfacing it. Honestly its NOT just "throw in a thin head gasket" and your golden....IMO. anyone, seriously anyone can strip/rebuild an engine. the key is the little things that come with experience (some times not good ones..lol) and the most important is the tuning in the end imo anyways. *i used caps to show the important words
I have a motor I just took apart, built by someone else, that had a stock gasket minus the top layer. Why wouldn't this last as long? Something to do with sealing to the surface of the head? I guess because the oil restrictor is still there no other problems? Thanks,
Some motors now are too tight from stock to pull a layer. There are reasons many HRC gaskets are thicker than stock. Many times backyard bob's customer has dropped a motor into me with the pistons banging into the head and the big end bearings shot - new pistons, new rods, new shells, ching-ching thankyou sir. Piston to head clearance can only go so tight on an engine. As can piston to valve. Both can be related but can be changed seperately. Deck the head = tighter on one, deck the block (or thinner gasket) = tighter on both. If anyone here can't get their head around this simple concept and is trying to work on an engine, then this is the least of your worries!!!
Thanks for the explanation Rocky; it was actually very well said. The tricky part I assume is figuring out what tolerances are needed to get the power characteristics you're looking for.
yep, exactly Tim. to scare some people, we had our 2007 R6 superbike set up SO tight on the squish if you cold rev'ed it up when not fully hot (engine hot to touch) the pistons would hit the head...but we knew the "real" squish was very different once it was up to temp..never had an issue. Ive seen several newer bikes that to be honest the std .018" head gasket would not be enough IMO for safe squish. like you said, thats why some are thicker. When we order our gaskets from honda say for the 2008 CBR1000RR we get the .55-.75 HG and .15-.25 base. then you have a lot of adjustability and if you know more than say 3 engines, you know that the new CBR1000RR will take a bit different squish then the 07 CBR1000RR..hmm...cylinder block vs crankcase block....
I am happy to be winning the essay portion of the pageant! Thank you. :up: My tip of the day? Ginger afros make more hp. Rocky Stargel Team Stargel
Had an 07 r6 superbike motor installed in my 08r6 (freshening up the 08r6 ss motor). While tuning, we noticed the strongest pull was always the 1st when the bike was just getting fully to temp. Usually lost a hp with later runs. Could this be due to the cases growing a tad as noted in some earlier posts?
Well i have found the same thing on the world supersport r6's i build and develop so during development we tried some crazy shit that really worked. We build them with different than stock clearances. The bores get honed out half a thou oversize - might as well take up all the clearance allowed!!! The main bearings i run 2 sizes thinner if i can and same with the big ends, nice and loose. Then i run a 5W/30 oil and mix in with that a product called Belesta from Belzona which coats all the internal surfaces and cuts down on the friction drastically. Remember there is about 40+bhp of friction in a 130bhp motor!! Then after the engine has had about 1.5 hours of steady run time on the dyno using this coating, we swap out the oil for a 15W/50. On track we tape the radiator to make the engine run about 30 degrees hotter. This causes the piston to take up the slack. We pick up 0.5cc and a bunch of power. Currently running 152bhp in WSS trim on Motec ECU with factory cams on stock pistons. So Ed is right on the money with getting clearances right. I wouldn't be surprised if the sneaky dog is doing exactly this and that is why his motors are so powerful.
wow. I wonder if this would also explain why the motor turns over much slower (like the battery is weak) when the 2mm over motor is hot. Perhaps the cylinder wall clearance is slightly tighter when hot? It is no wonder that if a builder is clueless you could end up with a pile of junk, or a motor that makes less power than from the factory.
