A bit of information before I ask my question.. Currently my combustion chamber sits at 55.8cc, and I already have the oversize valves. My compression is 8.8 right now, and with a 290-94H cam by dynamic is a measly 6.5. I am looking to mil .060 off the head to get my goal of 10:1ish compression. My piston sits 0.078 in the hole, .030 overbore, and I have a 10.18cc dish The heads are in progress of being removed from the vehicle and taken to the machine shop. Is there any limit as to how small a combustion chamber can be on a sbb? If I mill .060 off and it dips into the 48ish cc combustion chamber range will that cause any issues? Thank you for your time.
I know that a few people will say you should tear the engine down and mill the block instead of the heads but I know you are not up for that and neither would I if the engine was running nice especially. I know that the NHRA Super stock guys are running 48 CC heads as that is the min spec for their class... I am sure you can go smaller than that, how much smaller I am not sure. The main question is how much milling is required to get to the 48 CC from 55 CC. I do not have the answer to that. The engine will be much happier with the higher compression!
Thanks Sean, that's a sigh of relief about having the cc's under 50. And you are correct in saying that everything else about the motor works so well, that to me it does not make sense to tear apart in any way beyond taking the heads off. Hope someone has some input on how much to remove.
With that much milling you will need to check intake to see if it needs to be milled to match.With your specs I got 11.616 to 1 with what you have in 1 calculator and 9.18 in summit racing calculator.
Good info on the intake side. I've budgeted and if it needs to happen then so be it. Cross that bridge when we get there I reckon. I have been using the calculator that the wizard recommends and its .01 away from the fact sheet the machine shop gave me on the motor from the original build. As it stands now of course.. I plugged a few numbers in and only changing the combustion chamber and a .040 gasket I got .. 50cc 9.57:1 48cc. 9.77:1 Or the thin steel shim .020 50cc 10.1:1 48cc 10.3:1 Educate me please, not trying to make another mistake and not listen to the advise given
I'd recommend cleaning up the block if you are thinking of using steel shim head gaskets. Any imperfection can allow a leak.
As a rule of thumb I only ever used the steel shim gaskets with 68-71 blocks and heads. I know you said you were using 73 heads so I do not think the steel shim gaskets will work but I could be wrong. I would just mill an extra 20 thou and use the thick gaskets. Were you using the steel shim gaskets before? Post 43 shows the issue with using shim gaskets on the later engines: http://www.v8buick.com/showthread.php?92424-Project-started/page2
Running some numbers for you: With 3.83 bore, 3.85 stroke, 55.8cc head chamber, effective dish volume 10.18cc, deck clearance of .078, and compressed gasket thickness of .040, your compression ratio comes out to be 9.23:1 with 354.85 CID. That camshaft in a Buick 350 uses 110* LSA, putting intake valve closing at 71* ABDC assuming a 4* advanced cam install. This puts your static compression ratio requirement for 8:1 dynamic at 10.56:1. You will achieve exactly 10.5:1 compression if you shaved your heads to 44cc. My best guess is 5cc = .020 on Buick 350 heads (it's pretty close, though I don't have exact numbers). You'd need to remove about 12cc from heads to achieve 44cc, or about .048 off the heads. It's important to keep in mind that overboring will increase compression if the dish size remains unchanged. Also, with your dished pistons sitting that far in the hole, taking that much off the heads shouldn't be an issue with valve clearance on the pistons.
Keep in mind that you may need adjustable or custom length pushrods after taking so much off from the heads.
I am not currently using the steel shim. The ones my shop used measured .047 (not sure if compressed measurement or not) Correct me if I am wrong, but were those problems with the steel shim dealing with the extra cooling in the later style heads? I did plug those extra holes before throwing the heads on. I guess next step is once the heads are off I'll match the head and the gasket and post up some pictures. I'm highly curious. The motor has about 2000 miles and 2 years after a professional rebuild. Really trying to avoid taking the motor out and apart. And would hope it would hold form for a bit. But then again, im wrong 80% of the time. Understood.. Was planning on going the custom route once I have measurements. As well as measuring for TA composite gaskets. For curiosity sake, I had planned on tossing the heads back on and using some modeling clay to check valve to piston clearance before I started any of this. I do not think its anywhere close now, but if it is then I guess this is all for not. So moving on for me would be step 1 to get the heads to the machine shop.. ask for 45cc combustion chambers or how ever much they are comfortable with and let them figure out how much to take off? Ill assume that since I'm lopping off a nice chunk Ill have to get the intake side done, but when the milling is over we'll cross that one. Pictures coming soon with the steel shim comparison, those headers are a pain to get off..
How well does it run? What advance do you have on the cam? I measured felpros head gasket before after crush and got .043. So that's the spec I measure by most of the time
Since you have plugged the center coolant holes in the heads you can use the steel shim gaskets as far as I know. ---------- Post added at 02:05 AM ---------- Previous post was at 02:02 AM ---------- based on what we know so far you should be able to safely mill off 50 thou if using the steel shim gaskets and 70 thou if using the thicker gaskets and just let the head CC end up where it ends up. You can still CC the heads when you get them back so you can calculate the compression however it would save the shop from having to mess with them. This way they would just mill them the amount you ask for and you are on your way.
It runs well at higher RPMs, lower RPMs it still runs decently but can tell its a bit sluggish. The cam is installed 1* advance, for a 111 LSA. Thanks for the gasket info, I didn't order, measure, or install this one. Great! We have a plan then, hopefully get them to the shop this week or early next.
1* advance will put DCR even lower, but increasing compression is good even if you don't get it to 8:1.
Thoughts: Excessive milling on the deck side of the head may create sealing problems due to warpage under fastener load. How much is too much? I don't know Already mentioned, intake ports and bolts may no longer be aligned properly. Rather than milling the intake manifold, mill the intake side of the heads. That way the heads and intake will still work on other engines. Devon
Thanks for your reply Devon, milling the intake side of the head is the way I will go. Is there any sort of formula, or do I need to mock up the intake to see how much to mill from the head? Heads are off, aside from the green lake beneath the car everything went fine. Took the head off and put the .020 gasket in place, observed the intake and exhaust valve cycle and the valves did not touch the mound of clay I put on the piston. So with this knowledge, I will proceed. Had some carbon buildup on the pistons, here are the plugs. I thought the heads would be cleaner after a few thousand miles, but I guess exploding fossil fuels isn't the cleanest of processes. My old gasket and the .020 steel shim matched up really well. The only way to improve would be to enlarge the oil return hole in the steel shim. Now the real question.. both my old used, and now .020 gaskets have the areas indicated blocked off. The motor ran a constant 180* with the old gaskets in. Is this the correct way?
Yes the gaskets are both fine like they are, this is the way they are supposed to be. I added some info for you: ---------- Post added at 06:36 PM ---------- Previous post was at 06:26 PM ---------- There is a formula to determine how much to mill the intake side however I prefer to just mock it up with gaskets and then take the heads back if the intake sealing surface needs to be milled to correct the fit. Just a quick mock up will show if anything needs to be done or not. I am having my intake milled this week to make it fit better, and in this case I am having it angle milled because it needs it. All combos are different so have a look at the fit after the heads are back from milling, and go from there.
Great news, was pretty dumbfounded when I took the heads off. Going to match the oil return hole and leave the rest as is. More to come on the intake side after milling
