Hi all, looking for a sanity check on my SCR/DCR calculations and head milling plans. According to threads here, 8.6-8.7 is optimal DCR for LS3.

I'm putting a CamMotion Titan 3 I'm my LS3. They suggested milling a maximum of 0.020", on an otherwise stock motor. I'll send my 821 heads to TSP to be ported and milled.

Advertised durations on the Titan 3 are 281/291 @ 0.006", .621/.604 lift. LSA is 113+3.5. I'll be using Cometic 0.040" thickness head gaskets. That will reduce the amount of head milling I can do by .010"-.011", but if the valve is sunk into the head, that should counteract the head gasket and increase the amount I can mill by the sink amount, right?

Assuming 68cc heads stock, my math says that with a 0.040" head gasket and that cam, my static compression would be 11.1:1 and dynamic would be 8.4:1 at sea level. Google says I'm 909ft above which puts my dynamic close to 8.2.

Milling to 66cc (0.016" if my conversion is correct) would bump me to 8.43 DCR at altitude. This would only work with my head gasket if the valve is sunk, which I've heard TSP sinks them about 0.010". If that's the case I could probably even go a couple more thousandths. Otherwise, I'm probably limited to milling 0.010" which would put me around 11.25 SCR, 8.56 DCR, and 8.38 DCR at altitude.

Any suggestions and insights would be appreciated.

 

and what your quench?

 

I'm embarrassed to admit I have not calculated. I will accept my lashings now.

The only thing I can find online says stock deck clearance is 0.007 out of the block. That would make my quench 0.033. I'll check with a caliper tomorrow.

If it is indeed 0.007 out, 66cc's might be pushing it. DCR would be 8.77, 8.59 at altitude. 66.5 would put me at 8.72 and 8.53.

 

You're cutting it close. Measure all eight cylinders. You may also need two different head gaskets.

 

​​​​​​Yeah, I see that the recommendation is for a quench of 0.035, however it seems like many people run these thin Cometic head gaskets on stock rotating assemblies. How is it that they can get away with it? Am I missing something?

 

A stock gasket is .051, so that leaves some room.

 

Yeah, makes sense. It just seems that there are a several examples of people running this head gasket, putting their quench between 0.030 - 0.035 depending on deck clearance. And from what I've read, tighter quench is better. The risk is the piston kissing the head, but I also read that the aluminum block should expand and give me approx 0.005. I don't know how true or accurate that is, but that's another factor I hadn't considered

 

Alright. Got a dial indicator and stand and attempted to measure my deck clearance so I can calculate quench.

First, the stand is hard to use because it's magnetic and aluminum is not so I've had to get creative. Long story short, I'm consistently measuring my #1 piston as 0.007 IN THE HOLE. Everything I've read says they should be out of the hole at TDC so I'm guess I'm slightly ahead or behind TDC.

Can I thread the crank bolt back in and use it to turn the crank without the balancer on? My only apprehension is that it threads slightly further in without the balancer. I obviously don't want to damage the crank so I wanted to ask first.
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EDIT: I'm thinking about the balancer incorrectly and that it doesn't sit between the bolt and crank. Should be fine to turn the crank with the bolt threaded in.

 

Stack enough washers on the bolt to simulate the thickness of the balancer. Once you get the bolt tight, it shouldn't be that hard to turn the crank. You can use a straightedge to see if it's actually in or out of the hole at TDC.

 

Alright, I got my measurements. 0.007" out of the block.

This would put my quench at 0.033" cold and makes my calculated compression ratios go up.

​​​​​So now my question, based on the information in post #1 of this thread is:
Do I mill my heads by 1cc, putting me at 11.39 static, 8.66:1 dynamic and 8.48:1 dynamic @ altitude? Do I push it another 0.5cc to hit 11.46 static, 8.72 dynamic and 8.53 dynamic @ altitude?

If I leave the heads alone, I'm at 11.25, 8.56, and 8.38.

 

I think you'll be fine with a .010 mill and .040 head gasket. That's what I'm running and it's been fine for over 30K miles. I see your static at 11.2:1 and dynamic at 8.5:1. If Cam Motion says you can mill .020 and not get into PTV issues with the thinner HG, I don't see a problem with it. Should bump your static up to 11.4 and dynamic up to 8.7. You'd be a little more sensitive to a bad tank of fuel. I just went through that with my rig. Had to add octane booster or drive like an old man....which I am....but, try not to act like it

 

Thanks for the feedback, that's what I was hoping for. I've heard about people keeping some octane booster on them for such emergencies.

In addition to Cam Motion saying a 0.020" mill is safe, Texas Speed confirmed that after a valve job, sinking the valves 0.010", I'd be safe for another 0.010 on the mill if desired. That's basically a wash with the head gasket, but good to know I'll have that little extra clearance.
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That's pretty tight for quench

 

Maybe not quite as tight as you think. Smarter guys than me state that the aluminum block swells with heat. So, .007 out of the hole will be less than that with a warm engine. Lots of street SBE engines use the .040 head gasket without issue. Twisting it north of 7000 rpm could be different.

 

This is my understanding as well. I'll get some thermal expansion out of the aluminum, and I've read a lot of posts of people using this 0.040" head gasket. 0.007" out of the hole seems to be a pretty common deck clearance as well.

Spin also confirmed to me in a private message that 0.033" cold quench would be fine so that makes me feel better about it too.

I don't think this motor will see even 7k RPM, but who knows how much I'll flirt with disaster in the moment? 😂