surfacing the head to increase compression
07-10-2004, 02:33 PM
#33
Nordschleife Master
One of the cool things about old Ducati singles is the cam shaft-drive with two sets of helical-cut bevel gears. You adjust the timing by shimming the gears in and out.
07-10-2004, 02:37 PM
#34
Race Director
Originally posted by Peckster
One of the cool things about old Ducati singles is the cam shaft-drive with two sets of helical-cut bevel gears. You adjust the timing by shimming the gears in and out.
One of the cool things about old Ducati singles is the cam shaft-drive with two sets of helical-cut bevel gears. You adjust the timing by shimming the gears in and out.
Bikes have the coolest technology. I have no real interest in owning one, but I'm fascinated by them, both the technology and those who race them (either I don't have large enough jewels or I have too much sense - don't know which).
07-11-2004, 02:04 AM
#35
Campeck Rulez
Rennlist Member
Rennlist Member
Originally posted by Peckster
Why?
Why?
but after 18 yrs of coolant corroding the surface, heat and pressure stretching and bending the metal, blah blah blah.......its just if it's never been done its prolly a good idea.....unless you meant why as in why i said excuse me 
07-11-2004, 02:23 AM
#36
Burning Brakes
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Originally posted by M758
Had heard of done on 944-spec cars since rules allow 11:1 compression. There is only so much to be machined before there is nothing left.
Gain is a little hp. How much is varried, but I have seen as little as Zero to as much a rumored 10 hp.
Draw backs are many. Firstly you will need higher octane gas. Stock compressrino 944-spec can run on pump gas 91 oct. (mine does) shaved head cars typically run race gas and some need it to prevent detonation and headgasket failures.
Had heard of done on 944-spec cars since rules allow 11:1 compression. There is only so much to be machined before there is nothing left.
Gain is a little hp. How much is varried, but I have seen as little as Zero to as much a rumored 10 hp.
Draw backs are many. Firstly you will need higher octane gas. Stock compressrino 944-spec can run on pump gas 91 oct. (mine does) shaved head cars typically run race gas and some need it to prevent detonation and headgasket failures.
What are they stock 8.5 or 9???
10 : 1 will run perfect on premium
07-11-2004, 04:02 AM
#38
Race Director
"um....shaving .020 off the head requiring you to run "race gas" um....not gonna bump up the compression that much"
Yeah, 0.020" is nothing! Might increase compression by 0.2 maximum. I've shaved 0.060" off my 951's head and compression only increased by 10psi, or from 8.0:1 to about 8.4:1. Shaving it another 0.020" to a total of 0.080" would need angle-milling to not run into the intake-valve. Even then, I doubt you'd bet a full point increase in compression. On the NA cars, maybe, and that would take you 10.5:1 from 9.5:1 on the pre-'87 cars.
The difference in timing after taking 0.060" off the head is about 5-degrees retarded, which is just about perfect on these cars. It moves the torque-peak up about 1000rpm and really flattens out the torque-curve. You no longer lose 25-30% of your peak-torque by redline, only about 10 -15%. And HP goes up with more high-end torque as well.
Yeah, 0.020" is nothing! Might increase compression by 0.2 maximum. I've shaved 0.060" off my 951's head and compression only increased by 10psi, or from 8.0:1 to about 8.4:1. Shaving it another 0.020" to a total of 0.080" would need angle-milling to not run into the intake-valve. Even then, I doubt you'd bet a full point increase in compression. On the NA cars, maybe, and that would take you 10.5:1 from 9.5:1 on the pre-'87 cars.
The difference in timing after taking 0.060" off the head is about 5-degrees retarded, which is just about perfect on these cars. It moves the torque-peak up about 1000rpm and really flattens out the torque-curve. You no longer lose 25-30% of your peak-torque by redline, only about 10 -15%. And HP goes up with more high-end torque as well.
07-11-2004, 11:32 AM
#39
Race Car
Originally posted by azndrvr447
hey dave....so your'e saying you actually lost hp above 5500 rpm?
hey dave....so your'e saying you actually lost hp above 5500 rpm?
07-11-2004, 11:50 AM
#40
Three Wheelin'
Actually the amount gained is not linear due to the shape of the combustion chamber. So the first .020" might increase the compression ration by .25 points but the next .025" would only increase it by .15 points.
If i remember correctly the maximum you can remove with the stock cam on an N/A engine is .060". With my aftermarket cam I was only able to remove .040" and still have enough clearance.
If i remember correctly the maximum you can remove with the stock cam on an N/A engine is .060". With my aftermarket cam I was only able to remove .040" and still have enough clearance.
07-11-2004, 11:54 AM
#41
Race Director
Originally posted by david fracolli
Actually the amount gained is not linear due to the shape of the combustion chamber. So the first .020" might increase the compression ration by .25 points but the next .025" would only increase it by .15 points.
Actually the amount gained is not linear due to the shape of the combustion chamber. So the first .020" might increase the compression ration by .25 points but the next .025" would only increase it by .15 points.
07-11-2004, 11:55 AM
#42
Burning Brakes
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Makes you think of restricter plate racing (NASCAR). Back in the 80s I think it was Robert Yates team, they built a smaller displacement engine to go along with the smaller holes at the carb. They made more power and better gas mileage with a smaller engine.
07-11-2004, 12:27 PM
#43
Three Wheelin'
That would be correct if the volume that was remove stayed constant with the amount of material removed. The point I was trying to get across was that do to the shape and design of the combustion chamber the decrease in volume of the chamber is not directly proportional to the amount removed.
For example the first .010" might decrease the volume 2cc, the next .010" another 2cc but the 3rd ..010" would only decrease the volume 1.5cc.
If I was at home I could tell you exactly how much the volume of the combustion chamber decreases with each .010" removed as I had a local machine shop do exactly this on a junk head. I had them measure the volume of a stock head and then remeasure after every .010" removed.
For example the first .010" might decrease the volume 2cc, the next .010" another 2cc but the 3rd ..010" would only decrease the volume 1.5cc.
If I was at home I could tell you exactly how much the volume of the combustion chamber decreases with each .010" removed as I had a local machine shop do exactly this on a junk head. I had them measure the volume of a stock head and then remeasure after every .010" removed.
07-11-2004, 03:35 PM
#45
Race Director
Originally posted by david fracolli
That would be correct if the volume that was remove stayed constant with the amount of material removed. The point I was trying to get across was that do to the shape and design of the combustion chamber the decrease in volume of the chamber is not directly proportional to the amount removed.
For example the first .010" might decrease the volume 2cc, the next .010" another 2cc but the 3rd ..010" would only decrease the volume 1.5cc.
If I was at home I could tell you exactly how much the volume of the combustion chamber decreases with each .010" removed as I had a local machine shop do exactly this on a junk head. I had them measure the volume of a stock head and then remeasure after every .010" removed.
That would be correct if the volume that was remove stayed constant with the amount of material removed. The point I was trying to get across was that do to the shape and design of the combustion chamber the decrease in volume of the chamber is not directly proportional to the amount removed.
For example the first .010" might decrease the volume 2cc, the next .010" another 2cc but the 3rd ..010" would only decrease the volume 1.5cc.
If I was at home I could tell you exactly how much the volume of the combustion chamber decreases with each .010" removed as I had a local machine shop do exactly this on a junk head. I had them measure the volume of a stock head and then remeasure after every .010" removed.
Right-o!
However, since CC volume is in the demoninator it has greater effect upon the equation. IOW even though the CC volume shrinks at a decreasing rate, the CR make go up at a faster rate still.
If you have actual figures, I'd love to see them. It would be an interesting reference and tell us for sure what is going on.