| Author | Message | ||
     Ryker77 |
my bike was purchased with a high compression setup. CP 30 degree domed pistons at 12:1 ratio and stage 3 XB heads. I've read that the huge Andrews N80 cam allows some of the pressure to bleed off (62 degrees of overlap at .600 lift). I did ride the bike on pump gas with some octane booster for safety. Just slight pinging on take off --sometimes. would a .40 or higher headgasket help? the bike is allready apart so its not like I would be tearing it apart just to change head gaskets. I see that Cometic MLS has this listed "Available thickness: .027”, .030”, .036", .040", .045”, .051”, .054", .060”, .065”, .074” , .120" I guess those thicker sizes would be special order. (Message edited by ryker77 on December 17, 2005) | ||
| Ryker77 |
jp cycles has cometic CFM-20 gaskets in stock at .043, .051, and .059 CFM-20 A perforated steel core with an elastomer surface is ideal for head gaskets allowing heat to be drawn evenly across the gasket surface while providing maximum sealing characteristics when exposed to coolants and oils. | ||
| Ryker77 |
I guess I should mention my goal is: street pump gas with no pinging. not a race bike. | ||
| Ryker77 |
if my math is correct the .060 gasket would decrease the C/R by .5-.7 points. 11.8-12.0 that thick .120 gasket would decrease C/R down to 9.7-10.2 range | ||
| Ryker77 |
is this correct "Your engine summary is as follows: Bore 3.530 inches, stroke 3.812 inches, rod c-c length 6.926 inches, with a static compression ratio of 12.5 :1. Your camshaft specifications call for an inlet valve closing of 60 degrees ABDC (after bottom dead center). Your chamber volume is 53.16 cc's. With this camshaft your dynamic, or effective stroke is 3.06 inches. Your dynamic compression ratio is 10.19 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 216.79 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 0 PSI is 10.19 :1." I used this web site http://www.rbracing-rsr.com/comprAdvHD.htm | ||
| Ryker77 |
the only thing that I need to learn is what happens after my intake valve close and the exhuast valve is still open. intake 32 before TDC OPENS 60 after BDC Closes exhaust 66 before BDC Opens 30 before TDC Closes I'm just now learning about cams and doubt I can ever understand them. But it looks like that even after my intake valve is close on the piston upstroke the exhaust valve is still open for 60 degrees. which would drastically effect the C/R ratio untill higher RPMS... I guess I am understanding it??? which is why they say big cams are for higher RPMS. | ||
| Panic |
"the huge Andrews N80 cam allows some of the pressure to bleed off (62 degrees of overlap at .600 lift" 1. overlap doesn't bleed off anything. 2. you don't have .600" lift on overlap. 3. overlap is way more than 62 - that's at .053". | ||
| Panic |
Your pistons and head were set up to provide a specific tight clearance for squish. Adding head gasket thickness will reduce your static and dynamic CR, but will also make the knocking much worse. You have a limited range of choices: 1. better gas 2. reduced spark (= higher temperature, less power) 3. new pistons 4. remove metal from dome (and re-balance if needed) | ||
| Panic |
"it looks like that even after my intake valve is close on the piston upstroke the exhaust valve is still open for 60 degrees" No. | ||
| Ryker77 |
uggghhhh.. And after hours of reading I still don't understand. | ||
| Ryker77 |
Panic, I got the overlap based of 62 degress at .52 lift based on http://www.rbracing-rsr.com/camshaft.html "The industry standard is when the valve lifts .050 inches (fifty thousandths) off it's seated position. The idea in this is that we will start at an agreed on point where there is "measurable" flow, the assumption being that there is no meaningful flow in the first .050" of valve lift." | ||
| Ryker77 |
I think I'll just listen to what I've been told and use .040 head and .020 base gaskets. I've got an adjustable twintec igntion which should help. Plus if needed I can just run an octane booster. | ||
| Pammy |
Most all motorcycle cam specifications are based on .053 valve lift except Lein Weber, that base on .020. To correctly guage what true compression is. You need to know the dome volume of the piston, the cc's of the combustion chamber, the thickness of the head gasket, the bore and stroke, of course, and the deck height. Just as a side note, knowing about your situation, oil in the combustion chamber can cause 'pinging'. .040 and .020 is a guess really. A good guess but a guess none the less. I am sure I should have been clearer about it with you. If you are truly running 12:1 CR with a .030 headgasket and no other changes(base gasket being what you have now), your compression would =12:1 .040=11 ![: ]](http://www.badweatherbikers.com/buell/clipart/ohwell.gif) That's 11 to 7(I don't know where that emoticon is coming from) .050=11:4 .060=11:1 .070=10:8 .080=10:6 .090=10:4 .100=10:1 The thing is, to be precise, you need to know your true starting point. (Message edited by pammy on December 17, 2005) | ||
| Panic |
From Andrews: 62 overlap is @ .053". Actual lift on overlap .264" intake, .244" exhaust @ TDC. | ||
| Ryker77 |
note to others.. don't buy a built engine/bike unless you have solid sales receipts to know what you have bought. biggest headache ever!! | ||
| Ryker77 |
11.7:1 ration with a 30 degree dome and the n80 cam should be able to run on pump gas. If not then I'll use some octane booster. Just need to get through the next 9 months of school. Then if the bike needs tweaked, she will be on a trailer and down to cycle-rama. | ||
| Sarodude |
This is simple stuff.... A thicker gasket will reduce your compression. However, since HOPEFULLY that motor has been built by reputable folks who mind little details like squish bands, I'd go for the dome machining instead just as Panic suggested. Best case, increasing squish clearance will result in wasted air / fuel charge. Wort case is, like Panic suggested, that you'll have another contributor to pinging. Spend a little extra time, do it right, and you'll not give up too much. -Saro |
