To start my name is Kyle, I work at a machine shop that does mostly head work and know how things inside the head can change with the easiest effort. (1-920-261-1850 is the number as for Kyle in the machine shop if you have a hard time believing).
This is for all the people out there that want to go through and build any head for any car.
CC'ing a head is the best way to figure out what you need to do to get anything and everything you want out of the head. This post will be about the ecotec head but any head rebuild can be done off this info.
GM from factory has a 3 CC difference in any year of ecotec head. Now people are saying when you don’t put in a set of OE style valves (dished the same way) it throws the cc's of the combustion chamber off. This it does but it is SO easy to change its easy to fix.
Reason why the valves are not dished as much when they are aftermarket is for the fact that the company wants a valve that will not mushroom under high RPMS. When I mean mushroom, this is what is really is... You have the seating part on the valve (weakest part of the valve) actually distort from the original form and start sinking into the head it self. When you have Valves getting pushed up and down as fast as those things do stock it’s alright. Stock OE limiter is roughly 6750 rpms. When you do either of two things.... Start spinning the engine up higher then stock, or you put stiffer springs you ... your likely hood of mushrooming the valves is allot higher. This is way the OE valves can get by with having as much of a dish as they do. Now, add the higher RPMS, and the stiff springs to keep the valves from floating and this is why they designed the less dished valves. More material there, which is stronger will nearly (not always) keeps the valves from mushrooming.
Now how can you get past the added on average 2cc (I did the test today) in the combustion chamber? Easy, you can go through and get the seats in the head resurfaced/ touched up, or you can get a thicker head gasket.
How can resurfacing the seats in the head change it? If you look at a bare head you can see the seat plain as day. Now look at your valves compared to the seats. When you open up the seats roughly .005 all the way around you mostly evened out that 2 cc's.
Now this is true with getting larger valves. There are .050 mm and 1 mm over sized valves out there. I HIGHLY recommend you do NOT get the 1mm over sized, due to the lack of seat material in the head them selves, plus any shop will want to shoot you if you say you want to. You will need 16 seats cut into the head... NOT FUN and expensive. .050 can get by with in the head fairly easy by just grinding down the seats them selves. When you use the larger laves its even easier to get the CC's to level out because they sit in the head allot farther then OE.
Now how can you plain aka deck a head and not change the CC's in the actual combustion chamber it self it you will not be able to change it there is nothing you can do with out getting a new head (brand new not reconditioned). People do it all the time and on stock engines it does no harm. People get very picky about boost due to higher compression changes timing, what octane and a few other things. When you deck a head, they make thicker head gaskets. Cometic (even though they are hated by few and loved by many) makes up to a .051" head gasket; Stock L61 is roughly .037". In all of my 3 years working at the shop I have never had to take anything more then .21" off a head. And when I did have to take that much off it was a stingray racing 327 that over heated, blew the head gasket so bad it hydro locked the internals. Make sure if you get a head resurfaced/ decked that the shop measures the difference (mark were you measure and use a mic/ caliper and measure before and after.
Any time you ever do anything to a head that is performance based, you should do these things:
1. CC the head to figure out if you need to adjust the seats and/or valve surface,
2. Deck the head if reusing OE (I don’t care what you might have heard from the local speed shops, if you have a warped head it WILL leak, maybe not right away but it will allot sooner then you want it to.
3. Check clearances to determine head gaskets
4. Trust the set up.
Even if you adjust the hole thing ( throw valves in with out CC matching, deck the head, use OE gasket) It will MAYBE raise compression by .35 MAYBE ( not tested but still manageable)
When you are in the head, there is always away to level out each factor by changing another.
Now this test is from the Compression calculator online:
Variable all in mm … .020 over bore, flat top pistons w/ valve reliefs (estimating the CC’s of that right now but that will stay the same through out the calculations)
Stock
Enter Cylinder Bore Size: 86.38
Enter Piston Stroke Length : 94.6
Enter Head Gasket Bore Diameter: 87.00
Enter Compressed Head Gasket Thickness: 1.02
Enter Combustion Chamber Volume In CCs: 56
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') : 0.02
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : 0.00
Calculated Engine Compression Ratio: 9.93532974459375:1
Total Displacement Volume: 62.0435716859585 cc
Adding 2cc’ to the head combustion chamber
Enter Cylinder Bore Size: 86.38
Enter Piston Stroke Length: 94.6
Enter Head Gasket Bore Diameter: 87.00
Enter Compressed Head Gasket Thickness: 1.02
Enter Combustion Chamber Volume In CCs: 54
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-'): 0.02
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : 0.00
Calculated Engine Compression Ratio: 10.2329579333806:1
Total Displacement Volume: 60.0435716859585 cc
Adding .051” head gasket to stock:
Enter Cylinder Bore Size: 86.38
Enter Piston Stroke Length: 94.6
Enter Head Gasket Bore Diameter: 87.00
Enter Compressed Head Gasket Thickness: 1.31
Enter Combustion Chamber Volume In CCs: 56.00
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') : 0.02
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : 0.00
Calculated Engine Compression Ratio: 9.69376289096718 :1
Total Displacement Volume: 63.7675283417703 cc
Adding .051 head gasket and 2cc the combustion chamber:
Enter Cylinder Bore Size: 86.38
Enter Piston Stroke Length: 94.6
Enter Head Gasket Bore Diameter: 87.00
Enter Compressed Head Gasket Thickness: 1.31
Enter Combustion Chamber Volume In CCs: 54
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-'): 0.02
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : 0.00
Calculated Engine Compression Ratio: 9.9752623498082 :1
Total Displacement Volume: 61.7675283417703 cc
Now you can even get ride of even more then this or level it all out with taking away material in the valve seat head!
Hope this helps people out with building a head to work with what they want!
