shaved head

[stephensimmons]

I got a quick question for someone. If you shave your head to build up compession i know it lowers your head to the block which would offset your timing a litte. Now dose it advance or retard your timming when you do that?

[cubert]

It will retard it, if i remember correctly.

[cygnus x-1]

Yep, it will retard the cam and therefore the ignition timing. You can rotate the distributor to correct the ignition timing but you need an adjustable cam pulley if you want to correct the cam timing. The change is not huge though, roughly 2 crank degrees per 1mm you take off the head.

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[stephensimmons]

Ok thanks for the imfo!

[stephensimmons]

Whats the most you can shave off a head without messing it up?

[cygnus x-1]

Whats the most you can shave off a head without messing it up?

Depends on who you ask. If you go by the indicator dots on the under side of the head it's about 0.050". I've heard of people doing up to 0.060" though without issue. I'm planning to try 0.080" on the head I'm currently building. I haven't done it yet but I did try a test cut at 0.080" on a chopped up head that I'm using for R&D. Here's a picture of it:







If you do this you will want an adjustable cam gear to get the cam timing back to where it's supposed to be. It would also be wise to check the piston to valve clearance, but from what I can tell it will still be ok. The only other possible issue is that the oil jet may stick up too far into the pocket in the head. This is an unknown still but if it is a problem, either the oil jet will have to be cut down or the pocket in the head will have to be made deeper.


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[Tomisimo]

yes to everything cygnus said, but a small correction, for each MM you shave you have timing retarded by 3°, there fore you have to have adj.cam gear.

[stephensimmons]

Ok thanks alot for the imfo guyz!

[cygnus x-1]

yes to everything cygnus said, but a small correction, for each MM you shave you have timing retarded by 3°, there fore you have to have adj.cam gear.

How do you get 3 degrees? Diameter of the cam pulley is 125mm, circumference is 125 * pi = 392.7. To take up an extra 1mm the cam pulley will rotate 1mm of it's circumference, so 1 / 392.7 = 0.0025465 of a full 360 degree turn. 0.002565 * 360 = 0.9167 degrees. That's in cam degrees though so multiply by two to get crank degrees. 0.9167 * 2 = 1.833 degrees.


Regardless, the timing will change. If you only take 1mm off then you could probably live with it. More than that and the low end torque will start to suffer due to the retarded cam. High RPM torque would improve though.


I was going to mention before, the casting looks plenty thick enough to take off 2mm (0.080"). Slightly more might be possible but the risk of clearance problems and head warping increases. 2mm off would get you to 11:1 CR with stock pistons, maybe a little less with the valve job and some deshrouding work in the chambers. 11:1 and higher you will likely not be able to run pump gas without pulling way back on the timing. Cam selection will be more important too. With high static CR you need more overlap in the cam to lower the dynamic CR at low RPM so that it doesn't tend to detonate.


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[Tomisimo]

Ahh, my bad. Maybe it was for B16/18 DOCH engines.. was reading a article on that same matter. Was considering to shave mine B20.

[stephensimmons]

Well i just picked up a a20a3 and 5 speed tranny today and bout to start tearing it apart so i can start my build! Anyone can tell me how to post pics? Im bout to search to see if i can figure it out.

[dacantu]

1 take the picture
2 upload the pictures in photobucket.com
3 when you see a picture you want to upload, click it (in photobucket still)
4 the picture will maximize and there will be some codes in the bottom left of the screen
5 click the one that says direct link (it will say *COPIED)
6 (back in 3geez.com) when you go to write a message paste it into the message and voila.

[Tomisimo]

yes.. or use image link location and post it true this image you have on your reply screen

[stephensimmons]

Thanks alot!

[stephensimmons]

Well im getting started today on my 10.50 a20a3! Hopefully I'll have it done buy the east coast meet in N.C. http://i754.photobucket.com/albums/x...mons/89a-2.jpg

[stephensimmons]

How do you get the pics to come out larger?!

[stephensimmons]

yes to everything cygnus said, but a small correction, for each MM you shave you have timing retarded by 3°, there fore you have to have adj.cam gear.

I dont know if anyone knows it or not but but I just got word that we (the a20's) should have an adjustible came gear just for US in about 3 weeks!

[2oodoor]

How do you get the pics to come out larger?!

photobucket changed something since I used it, I discovered this week that if you click on SHARE there will be several formats there to choose from for sharing.. look for IMG (full size) ..highlite it and copy paste. to the insert image block (delete the http because your copy will have that already, then paste it.

k where in sc are you? close to columbia?

[import racer]

How does shaving the head change the timing?

[MessyHonda]

How does shaving the head change the timing?

when you shave the head the cam will not spin the same as the crank anymore because there is less material...it does not change it al whole lot but enough to throw it off factory specs

[cygnus x-1]

when you shave the head the cam will not spin the same as the crank anymore because there is less material...it does not change it al whole lot but enough to throw it off factory specs

Yeah, It's kinda hard to explain without pictures, but the cam and crank have to be synchronized with each other. That is they both need to be at TDC at the same time. Because they are connected to each other with a toothed belt, the distance between the cam and the crank becomes critically important for them to stay synchronized. When you mill the head, it changes the distance between the cam and the crank. It's not much, but it's enough to create a little bit of an offset between when they each cross TDC. The cam will lag behind the crank by an amount directly related to how much the head is milled. If it was possible to mill enough off, you could just reset the cam pulley by one tooth and everything would line up again.


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[stephensimmons]

photobucket changed something since I used it, I discovered this week that if you click on SHARE there will be several formats there to choose from for sharing.. look for IMG (full size) ..highlite it and copy paste. to the insert image block (delete the http because your copy will have that already, then paste it.

k where in sc are you? close to columbia?

Im bout an hour south of columbia im near charleston.

[2oodoor]

How does shaving the head change the timing?

It puts the head closer to the block (pulleys closer) so that in turn makes the original marks on the cam gear slightey inaccurate. So again in turn, the valve opening in relation to pistion TDC is inaccurate as well. Compensate for that by using adjustable cam gear.
Very small matter really but when you are dealing with fuel (induction) charges and efficeint combustion inregards to picky performance tuning, small details matter.

[2oodoor]

Im bout an hour south of columbia im near charleston.

Im about 2.5 hr from ya then, Augusta GA right on the Carolina border. I go to Columbia all the time.

[stephensimmons]

Im about 2.5 hr from ya then, Augusta GA right on the Carolina border. I go to Columbia all the time.

Yea arnt you just north of I20 on 520?