85 C10 Cylinder Heads Assembled

[jbreeves85C10]

One thing I am confused on...ok more than one :-) I ordered the 190/68cc heads and they came back with a call stating they didn't ship those anymore so they gave me the 200's at 64cc for the same price with the PBM valve upgrades. My true flat top 030 pistons have 8.62:1 compression and they said the valve upgrade and 64cc heads would make it 10:1. Is there anything I should be concerned about with this change in compression? The cam is .467 lift and heads have max lift of .550 setup. Being the dummy here...what does compression affect that I should be concerned with...other than blowing stuff up? Is a 8.62 to 10 jump huge?
Thanks,
JB

[rich weyand]

You will need to run 93 octane gas.

[jbreeves85C10]

Thanks Rich..! You guys are awesome. I'm learning :-)

[rich weyand]

You'll also have to see where knocking starts on the timing.  Some guys over in Corvette forum are running 10:1 at 36* of total timing with 93 octane, but you may need to back off to 34* or 32* to keep from knocking.  Depends a lot on the dynamic compression ratio of that cam.  It may be late enough on the intake valve closure that you can run the full 36*.

I should elaborate.  The static compression ration (here 10:1) is the volume of the cylinder with the piston at bottom dead center divided by the volume of the cylinder with the piston at top dead center.  Dynamic compression ratio is the volume of the cylinder with the piston located at the point where the intake valve finally closes completely divided by the volume of the cylinder with the piston at top dead center.  Dynamic compression ratio is always less than the static compression ratio, and the dynamic compression ration will determine at what manifold vacuum and ignition timing that knocking will occur.

[jbreeves85C10]

I found out where my confusion was I think. They said the head is 64cc but the valve upgrade makes it 68cc which should give a tad over 9:1 compression. Does that sound right?

[rich weyand]

That sounds right.  64 cc would be about 9.5:1.  With 68 cc you might even be able to get away with regular gas, or at least no worse than 91 octane.

[jbreeves85C10]

I'm glad y'all are educating me :-) I was wondering about the gas and if I needed to go with the MSD Blaster kit or if that would even help.
Thanks and have a Happy Thanksgiving..!
JB

[nlauffer]

Are these aluminum heads?  Aluminum will be a little more forgiving on compression. 
How does a valve upgrade change combustion chamber cc?
Are you absolutely sure you have flat top pistons?  I can't remember if you said you had this thing apart yet.
If I remember right, some of the "flat top" pistons with 4 valve reliefs had some dish cc's.  Maybe someone can chime in with better info on this.

[nlauffer]

Maybe I'm a little slow, but I just realized this is the same motor you posted in your 85 piston post, with the picture of the piston with "valve clearance" issues.  Did you decide to keep the current pistons in the block and only replace heads and cam, and other little goodies?

I believe you will be fine on compression.  68cc with those pistons should put you no more than 9.5:1 I think.

Also, listen to Rich on cam selection and such.  He has been a great help to me. 

[jbreeves85C10]

Yes. I lightly dremeled the dings to get burs off and hit it with a soft wire brush brush. It looks fine and the compression seems to be good on all pistons. Heads, cam, and other goodies replacement is correct :-)

Rich,
This is the cam i went with for aluminum heads 68cc 9.5:1 compression, max lift .550, PBM valve upgrade, roller rockers, push rods.
Summit Racing Part Number:SUM-K00132
Cam Style:Hydraulic flat tappet
Basic Operating RPM Range:2,000-5,500
Intake Duration at 050 inch Lift:222
Exhaust Duration at 050 inch Lift:222
Duration at 050 inch Lift:222 int./222 exh.
Advertised Intake Duration:280
Advertised Exhaust Duration:280
Advertised Duration:280 int./280 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:0.467 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:0.467 in.
Valve Lift with Factory Rocker Arm Ratio:0.467 int./0.467 exh.
Lobe Separation (degrees):110

Did I do ok? I can always change but thought that was a good choice. I will lose a little low end torque but may change converter.
Thanks,
JB

[nlauffer]

With that 700R4 and possibly 3.08 rear gears, running down the highway in overdrive will be on the bottom end of the power for that cam.  Maybe even a couple hundred rpm less.  This is just me looking at numbers and not actually running the calculations.  You might not get the full potential of that motor unless you are up in the RPM range.

One thing I noticed when trying to decide on cam selection was that the HP & TQ curves stayed the same shape.  They just moved up the RPM range as I increased cam size.  Basically just moving my usable power higher in the RPMs.  It took awhile for me to admit to myself that my actual usable RPM range was gonna be from idle to 3000.  Only rarely seeing foot to the floor and not over 5000.  So I changed my thinking and went with the cam that fit my range. 

Unfortunately I do not have my pickup assembled to let everyone know how the cam choice went.  You might want to look into figuring out your rear gear ratio.  I had thought about changing mine from 3.08 to 3.73 but I have a TH350 and I do want a little highway help.  If I change to the 700 I might put the 3.73s in, but for the time being they will just sit on my shelf.

Just some food for thought. 

Try to get some numbers figured out.  Like gears, compression, head flow and use them in something like camquest or just run them by Rich.  It might help make sense of any future decisions and how they affect the total package.

[jbreeves85C10]

I want to say I did all of that and I used camquest. 68cc, 9.5:1 compression with 700r4 and 3.73 gears. 200cc head flow...I think that is the head flow number right? Edelbrock performer dual plane intake. holley 650 carb. hedman headers to straights. stock converter...may upgrade.
Is that about right on the numbers?

[nlauffer]

If you know 9.5 is CR for sure use it. Otherwise go to somewhere like summit and use the calculator. You will need head gasket thickness, bore, stroke, distance above piston to deck surface, and comb cc. This will give an accurate number. You can use some known numbers. Like bore 4.030, stroke 3.48.

I'm coming back from a hospital transfer and phone is dying. I'll post more in a bit.

[rich weyand]

Just back in town from TG. 

When talking cams for these engines, I recommend one of four of the Comp cams: 12-300-4, 12-231-2, 12-235-2, 12-238-2.  I chose the first one.  nlauffer chose the last one.  They are in order of increasing lift.  They are all torquey, and give great performance in the 1000-3000 rpm range where we drive most often.

Remember that, when you take your foot off the brake and punch the gas at the light, the engine is at 500 rpm.  You can go to a 2000 or 2500 rpm torque converter, which will let the engine rev up before the torque converter kicks in, but this is harder on the transmission.  That will help the launch when you are in a hurry, but it's an all-the-time thing, so daily driving it will not be as pleasant.

The 2000-up operating range on that Summit cam is a dead giveaway.  I would want something that says at least 1500-up; 1200-up is even better for a truck application, IMO.

[rich weyand]

You probably want to read through this thread:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=26206.0

And read this article:
http://www.crankshaftcoalition.com/wiki/How_to_choose_a_camshaft