Page 1 of 2

flycutting for compression ration

Posted: Mon Apr 07, 2008 5:08 am
by colinD
im building a 2110 FK8 1.4/1for a street car( bug)
i want a compression ration of around 8.5 to 1

my deck height is currently 1.25mm ( about 50 thou i think in old money)

ive bought a set of CB mini wedge ports as i want something a bit on the milder side of wild

thing is with the mini wedge ports the combustion chambers are huge at 69cc giving a CR of 7.8 /1
i cant reduce my deck height much really

so i would think flycutting the heads is the way forward
will this damage the chamber shape of my nice new expensive cnc ported heads


and is there a way to work out how much to fly cut them by?

Posted: Mon Apr 07, 2008 6:54 am
by Tom Notch
Level the head with the chamber up. Pour in the desired CCs using the liquid of your choice. Mark the liquid level. Measure the depth. Now flycut down to near that depth and do the CC thing all over again. Repeat as required.

Posted: Mon Apr 07, 2008 7:15 am
by Bugfuel
Tom Notch wrote:Level the head with the chamber up. Pour in the desired CCs using the liquid of your choice. Mark the liquid level. Measure the depth. Now flycut down to near that depth and do the CC thing all over again. Repeat as required.
use liquid with weak surface tension. Water for example will 'bubble up' in the chamber an give a false reading. Break the tension by mixing some laundry detergent in the water.

The chamber shape won't be affected much at all.

Posted: Wed Jan 28, 2009 9:57 pm
by 1 Chance Racing
I'm thinking about heads too. Doesn't CB flycut heads when you by them if you know the cc's you need?

Re: flycutting for compression ration

Posted: Tue Feb 09, 2010 8:32 pm
by sparkmaster1
They will for $100.00. I can tell you that if you have a Berg Semi Hemi head that is opened for 90.5's and it had 59cc's... if you take out .120" you will have 48cc's :P
Tim

Re: flycutting for compression ration

Posted: Mon Nov 01, 2010 10:00 am
by Arnolds64
I just did mine. They were 60cc took out .060 and they are now 50cc. I am going to 9.8 using an Engle 130.
CB has a chart to show what different cuts will do. It is basic though, so it is only close. They did not have an engine exact in size to my 2017 so I split the difference. However .050 will add about 1 to your compression ratio. I need almost 1.5 and I should be close. Going with a .040 deck but I do have H beam rods though and can get by with that on the street. You might consider 9.1 with the K8 as it is close to the W120 at .050 duration and they run well at 9.1. 8.5 is what a mild 110 runs, what I had and the K8 is a far cry from that.

Pic Link of mine after fly cut. These were previously open up allot for CC volume and running a tight deck. Look almost Semi Hemi. Worked pretty good.

Image

Re: flycutting for compression ration

Posted: Tue Nov 02, 2010 10:57 am
by Bugfuel
the cc reduction vs. flycut depth depends on the shape of the combustion chamber.
.100 on one head may yield 10cc, while on another head it could do 15cc.
This is no place for guesswork and ballpark figures.

Re: flycutting for compression ration

Posted: Thu Nov 11, 2010 12:49 pm
by Arnolds64
Bugfuel wrote:the cc reduction vs. flycut depth depends on the shape of the combustion chamber.
.100 on one head may yield 10cc, while on another head it could do 15cc.
This is no place for guesswork and ballpark figures.
I did my Homework before I decided to do this and totally understand what you are saying. I knew the shape of the chamber was the unknown in how it would turn out. Without much of a Squish pad in them I thought that another .010 over .050 would maybe compensate and it worked. I think sometimes you can over analyze and just need to pull the trigger. Money is tight nowadays but this is a hobby and sometimes you have to gamble. What I find interesting is how with me going to an W-130 cam people are telling me it will be a bear to drive on the street and it will eat up my lifter bores. I drove a friends Chevelle several times on the street years ago with a 402, ZL-1 Cam the same one they used in Can-Am Racing cars back in the day, Closed Chamber High Perf Chevy Heads with huge ports and a single 4bbl carb on top of a Tunnel Ram manifold. Talk about crazy! You did have to rev it some to get it going then you had to hang on. Parking was not easy and this was his daily! LOL! So I think I can handle the bug. Allot of the cons come from those putting them in non stroker engines whereas mine is. The bore thing is interesting in that I had all these people telling me to sleeve the bores and all and the 1.4.1 cams would be easier on them. Then Shawn Proffit, Manxracer1 on the samba tells me he has run one in 1914 for like 25 years and puts them in all his street cars and see no damage. He is a long time drag racer in Cal. as well and right now running a Manx that is a 500hp Turbo'd car. I think I would take his word for it after speaking to him as well. Sometimes you just have to go for it. LOL!

Re: flycutting for compression ration

Posted: Thu Sep 27, 2012 2:22 pm
by andy198712
Anyone tried the gene berg or scat borig tools to do this?

Re: flycutting for compression ration

Posted: Thu Mar 28, 2013 1:48 am
by Marc
andy198712 wrote:Anyone tried the gene berg or scat borig tools to do this?
Only when totally desperate (night before a race and no machine shops open). By the time I got both holes flat & level one was .009" deeper - rather than keep chasing it and risk making things worse I just shimmed that cylinder out more. It's really a job that should be done on a mill.

Re: flycutting for compression ration

Posted: Fri Mar 29, 2013 4:41 am
by andy198712
I think they're alot better for cutting cases then heads.... Must play with it more though

Re: flycutting for compression ration

Posted: Wed Sep 04, 2013 4:52 pm
by dragvw2180
Back in the day I bought one , alot of setup time and I used a drill press to drive it, ended up putting chatter marks on the combustion chamber deck . I still use the cutters but use them on a mill. Mike McCarthy

Re: flycutting for compression ration

Posted: Wed Sep 04, 2013 5:19 pm
by Max Welton
Why not have the cylinders cut down to reduce deck? Any competent machine-shop should be able to do that.

Max

Re: flycutting for compression ration

Posted: Wed Sep 04, 2013 5:39 pm
by dragvw2180
He has .050 deck now, how much more would you raise it without the piston hitting the heads ?

Re: flycutting for compression ration

Posted: Wed Sep 04, 2013 6:08 pm
by Max Welton
I'd take it to .040 for a CR of ~8:1. Quite safe with good parts and balance.

Max