I started on my top end rebuild with new cam today. When I got the head off I was a bit surprised at what I saw. The pistons which are the dish style also had a taper around the perimeter which would further increase the volume at TDC.
To find out what volume the dish pistons had, since they are not being replaced and I want an accurate computation of my CR for the cam, i decided to cc the pistons while in the cylinder. I filled the taper around the perimeter of the piston with grease to seal the rings then put grease around the cylinder on the head to seal the plexiglass to the block. Used a syringe and filled the void. I did it twice and came up with 21 cc's and 21.2 cc's.
Next I measured the distance down the cylinder the piston rested. that was .025", so stock. The cylinder diameter is 4.002 in, so it's stock too.
I computed the volume of .025" down a 4.0" hole and came up with 5.1cc's. That would mean that the piston dish is 16cc's roughly.
Did Chevy make a 16cc piston for the corvette?

So if this is correct and I figure another what?... 3cc's maybe for that taper around the piston down to the top ring, then I have 24cc's at TDC before the gasket. I plan on using the .015 felpro gasket so that's another 3.1cc's now I'm up to 27cc's. Add that to a 64cc head and I have 91cc's.
Using summit racing's compression calculator I added that 3 cc's around the piston top to the 64cc heads and came up with 8.85:1.
I was hoping for at least 9.3:1. Dang it! Hard to get any decent compression without decking the block or changing the pistons. I maybe could get the heads shaved a little to bring it up some more I guess. I'm not sure at this point how much I could take off and still clear the valves. would .015" be safe? Wouldn't gain me much. Maybe .025". That would be like decking the block right? This would be off a new SHP dart 64cc head.

Am I figuring this area around the piston about right at 3cc's?

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If you do shave them check into angle milling. From what I've read it will increase flow.

 

If you haven't bought heads yet.
http://www.summitracing.com/parts/tf...make/chevrolet

 

I always heard that the dish volume on stock L48 pistons was 15.6 CCs. So your testing that revealed 16 appears pretty spot on.

With all the verifiable CC chamber volume you have, what is the CR with the stock 76 CC heads? Probably with the factory shim gasket 8.0 or 8.2:1. The factory rating of 8.5:1 is usually overstated.

Would be interesting to see what your old head chambers CC out to.

The '71 and '72 Lt1 engines along with the L82s were rated at 9.0:1, but using your tests I bet they actually spec'd out to below 9 to 1.

What cam are you planning? XE262 or something like it, you will probably be ok.

I also understand for each .006 of head milling equates to about the removal of 1 CC of chamber volume removal. Going beyond .015 or .020 will likely require the intake manifold ends to be milled as well to avoid mating surface issues. Someone probably knows the limits for this.

 

Those are nice heads no doubt, baout another $300. Not sure if the angled plugs are going to work with my headers.

I haven't bought anything yet. Just seeing what I got for sure first.


The gasket I pulled out was a steel shim type and measured at .017. I haven't cc'd the head yet but if 76cc's and add in 3cc's around piston perimeter, stock compression would be 7.91:1.

The cam I'm gong to use would be fine as low as 8.5:1 but would like higher.
Forgot about head to intake mating, glad you brought that up. I suppose that 1cc would depend on the chamber shape/size or is that for the stock heads?

 

the Isky 264 megacam would work nice with the low CR.
214/214/108

 

..."I maybe could get the heads shaved a little to bring it up some more I guess. I'm not sure at this point how much I could take off and still clear the valves. would .015" be safe? Wouldn't gain me much. Maybe .025". That would be like decking the block right? This would be off a new SHP dart 64cc head."...



Because of intake face-to head face mismatch issues that are common with smallblock Chevys I would personally NOT recommend this (unless you want to spend the next six months or so taking your motor apart a couple more times to try and figure out why it's using so much oil).

Suffice to say I have extensive personal experience with this issue, and the minimal increase in compression that you'd get from milling your heads really isn't worth the risk of subsequent intake manifold oil suction issues that would result. Those who haven't personally witnessed this tend to scoff at the idea that it could even happen, while those such as myself who have gone through the anguish of trying to correct the problem once it HAS happened know it's very real indeed.

You would be time and money ahead in the long run by simply fitting a set of flattop pistons...

 

Thanks, ya that sounds like a real PITA. Not what I want to accomplish on this top end. So I guess I'll start looking for smaller combustion chambers if I want more compression.
I noticed on the heads at summit the smaller the combustion chamber the smaller the intake runner. More like 170 cc instead of the 180 cc on the dart SHP's. Kind of thought that it was displacement that dictated the intake runner size. Maybe these heads are designed more for 327's and 302's?

 

Stock 370 HP LT1 heads are 165 CC and they run 7000 rpm with a solid cam. 175CC will be fine for a sub 375-400 HP 350 with a hydralic cam.

 

Thanks 63mako, I just did some homework after that last post and it looks like intake runner size more determines flow at high lifts and rpms instead of displacement. So yes completely fine for my application of 350 hp target power. I guess I could try it at 8.83 CR. It might reqire a rethink on the cam and that might end up loosing HP in the end. I still want to hit that 350 HP mark.

 

You ever look at any of the Vizard grinds made by Lunati?
He's got a 219/219 on a 108 that only calls for 8.5:1 CR. recommends 1.6 rockers for a lift of .549. this is with roller cam. It has an overlap of 54*.
My DCR is now computed at 7.2. Good news is that may allow me to run 85 octane. Although that will probably become a moot point once the EPA gets around to regulating ethanol into all the fuel in my area. Currently I can get 85 or 91 ethanol free.

 

Just got done ccing the heads. I cc'd three of the head volumes one of them twice to see if I could get repeatable results.

The results are as follows;
78.5 cc
79.5 cc
80.0 cc I did this one twice to make sure. It was 80cc's
Keep in mind these measurements were without cleaning any carbon out of the chamber.
So GM's 76 cc heads are not 76 cc heads.
In the Manual it states a CR of 8.5:1 for the L-48... that's optimistic.

Using the CR calculator at Summit racing if I negate the area of taper on the circumference of the top of the piston and use 79 cc head volume I get a CR of 7.91:1.
However if I include the area around the top of the piston which I measured this morning and computed at 3.9 cc's,79cc for the head .017 gasket then the compression for this 1977 L48 works out to be a wopping 7.67:1.

Here of some pics of how I cc'd the heads.

Pic of air hole in plexiglass. I fill until it comes out this hole.
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Use this one twice. 70cc's
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Use this one to fill remainder. total 80cc's
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I gotta hand it to you, never seen anyone CC the pistons in the hole. Couple of things that come to mind is deck hieght. I doubt whether GM ever make a square deck height on on four corners, ever. That is the same height on both sides and from corner to corner. I have seen them all over the place, the main reason why you want to get a new engine decked.

Second, piston manufacturers will give you the specs. ( CC's + or - ) at zero deck, for instance a flat top with valve reliefs they will say 5cc or what ever.

This has got to be a first for me and I've been around a long time (I know it's been done just never seen it ) . It does suggest though that you nailed it and should be proud of yourself

 

Figured that was the case. So you always have to account for the distance down the cylinder. In stock form that is only 5ccs though so not too big of a penalty there.

Appreciate the complement. Surprised nobody else has posted it up before.
I'm not overly concerned about the deck irregularity as I figure the gasket and new heads will acommodate the inconsistancy.... I hope. But then again my experience on SBC's is nil so far, would hate to have leaks after it's all done.

 

I agree on the 264 Isky Mega cam. Had one in my 76 L82 and had much better low and mid range than the L-82 cam. On my carbed 84 C4 I am using the Summit 152123(made by Dart) large valve cast iron heads with 72 cc chambers. The 84 C4's came with the L82 forged pistons so I figure I am around 8.8 c.r or so with these heads. They made a difference in the idle, I was using a Crane 274H06 218-218 106 LSA cam and the idle was more crisp. 274 cam has a pretty good lope to it but still works good on a low c.r. engine. I just installed an Isky Z-20 solid lifter cam 228-228 .450 lift and had Isky grind it on a 108 LSA rather than the standard 112 LSA. Really nice mild cam for a low c.r. 350. Idles with 16" of vac. at 900 and 14.5 at 700 and has tons of low and mid range. This is a very old school cam but easy on the valve train and you can use your stock springs too. Bottom line is that the heads and cam made a big difference in power and the solid lifters sound great.

 

I'm not sure how to compare the duration of a solid cam to a hydraulic cam. 228 sounds like a lot of duration (is that about 284 or so advertised?) in my application. I plan on using hydraulic roller cam on this one. The cam I'm looking at now has a 270/270 advertised which is a 219/219 on a 108 lsa. It has a fairly agressive ramp (compared to others i've looked at) and total lift of .514 with 1.5 rockers. Should make about 14" to 16" of vac at 800 rpm, or so I've been told.

 

The lash setting on solid cam takes away a bit of the duration as well as lift

 

So a 228/228 solid lifter would be like a 218/218 hydraulic cam?

 

when checking a solid cam, don't start measuring until the lash is used up. Since lash varies(some cams have a tight lash) the answer varies.
4-6 degrees would be a starting guess. so a 228 solid might be like a 222-224H subject to actual measuring.

 

The Z-20 cam has an advertised duration of 268-268 and acts like a 218 cam on a 108 LSA. Most solid cams I have run in the past act about 10* smaller than a similar hydraulic. Hot Rod did a cam comparo test awhile back and the Comp DEH 275 cam, 219-229 cam had almost the exact same dyno numbers up to 5500 as the 282S solid cam at 236-236 solid cam. I think Comp figures that a solid acts 6-10* smaller. They say a Comp 270S 224-224 solid cam will act similar to the old 268HE 218-218 cam. A 228-228 hydrualic would be a bit much for my combo but the Z-20 is a good match.