LiveandLetDrive

Hi, I'm getting a pair of Sportsman 2s (64cc, straight plug) and I need to know some things about what I've already got. Now this engine was rebuilt by the previous owner about 20k ago, but I don't think it was bored out, I think all the stock internals were reused. I'm trying to exactly figure out my static CR, so I need to know:

Deck height
Dish volume of piston
Top ring land height
and 72 or 76cc for stock head's chambers?

Thanks guys,
Chris

LiveandLetDrive

bump

burners

Deck height is 9.025"
I'm not sure if L48's had flat top pistons or not so I can't help you there.
I can measure a forged piston out of an L82 for the ring land height if that will help you.
My guess is 76cc heads but keep in mind that those heads can vary several cc's and it is usually larger.

Do you know the thickness of the head gasket or how far the piston sits in the bore? Those will be needed as well to calculate CR.

LiveandLetDrive

By deck height, I meant the recession of the piston in the block. What you're talking about I think of as block height.

I'm pretty sure they're dished, not flat top, to get 8.5:1. Thanks for the offer to measure, but I'm just looking for what I have to start with with this L48 short block.

I don't know the current head gasket's thickness, choosing the new one is actually the main goal of this post.

76 sounds right, thanks!

-Chris

Twin_Turbo

yeah, the piston is about .025" down the hole, if that's what you mean. The deck height is 9,025"

The L48 psitons are cast 4 valve reliefs and they are sort of like a flat top but the outer edge of the piston top is higher than the center, kind of like a squish band around it.
Dunno about ring land height or dish volume, what does it matter? It's not like you are running super thin rings and very high compression (with a lot of heat transfer) that would cause warpage of the lands.

The heads are 76cc chambers, 160 (or a little less) intake cc.

Pete79L82

You can not figure compression ratio "excatly" using GM advertised specs. There were too many manufacturing variables. But you can get close.

Deck hieght for an unmachined block is about 9.025
Dish volume is for a stock L-48 piston is about 13cc's
Ring land height for a stock L-48 piston is about .275 (varies alot)
Chamber size for a stock L-48 head (333882 casting) is 76cc's
Factory steel shim head gaskets were about .025 with a volume of 5.6cc's

This combination will give you a compression ratio of 8.184. GM advertised the L-48 as 8.2 CR.

If you use the Sportsman II heads at 64cc chamber and a .039 head gasket with 8.7cc volume on the same bottom end you will get a 8.887 CR.



[Modified by Pete79L82, 6:51 AM 4/26/2004]

L79vette

You really need to measure how far down the hole the pistons are before selecting a head gasket. If you are 0.025 down the hole, and then use a 0.039 gasket, you have a 0.064 quench distance, which is terrible. It will ping even though the compression isn't very high. If the block is not decked, you'll probably need to use a very thin gasket to get the quench distance to 0.040-0.045, which is what you want.

Quench distance is important to mixture motion in the combustion chamber, which reduces hot spots and detonation. If you need more info, go to the Speed-O-Motive website, they have a good article on quench.

My engine is at 10.2:1 compression with iron heads, but it never pings on pump premium because the quench is right at 0.040. This is running 40 degrees of timing advance, all in at 2500RPM.

LiveandLetDrive

Thanks TT and Pete. I had no idea the compression was that low, I thought it was 8.5:1? I know it varies a bit too. So I won't even end up with 9:1 after the change (with the head gasket you stated)

It looks like an .010 gasket would give me 9.5:1, can I go that thin? What issues will I run into?

-Chris

Edit: I know about quench, that's another reason I'd like to go with the thinnest gasket possible. .010 might be too small, if the pistons end up being less than .025 down the bore, but .015 seems like it should be safe. Even if they're only .020 down I'd have right at .035 quench. Close, but okay. You're right, I should probably wait and measure it for myself. Thanks!


[Modified by LiveandLetDrive, 11:47 AM 4/26/2004]

LiveandLetDrive

Ok, it's come to my attention that I may not be able to get a thin enough fiber/composite gasket, and to get these thicknesses I'll need metal. Also I've been told metal gaskets need a very smooth surface to seal right. My heads have this, but my block probably does not. What should I use?

Also, how do I know the compressed height of the gaskets I'm looking at? Summit doesn't seem to say, unless that is the number they are quoting.

-Chris

Twin_Turbo

It heavily depends on the cooling system, carbon deposits, fuel quality and the camshaft. CR doesn't say all that much, it's jsut a static number. Cylinder pressure is what it's alla bout and with a low static compression, a wimpy cam and a poor intake/exhaust it will most likely not detonate at all.

L79vette

Go to a parts store that features people who know what they are talking about. In my town that is NAPA. They can tell you about the various SBC head gaskets and their compressed height. You should also be able to get head gasket specs from Federal Mogul or Fel-Pro or something.

Dunno why your block would not be smooth on top. Mine was not decked (to preserve the stamp pad) and is using the steel shim gasket. They may be harder to put on, I don't know because I had someone else assemble the engine. The factory used steel shim gaskets for many years.

L79vette

Looking at the specs in his sig, I would expect it to ping at 0.064 quench. It might not, but why set it up to increase the possibility? Also, a good quench improves power by giving you a more uniform mixture in the combustion chamber.

Twin_Turbo

Of course it's better to deck and get the quench about right, although what's right is open to debate and getting a tight quench height(below .035 or so) with GM type tolerances and quality of parts.
It's true that steel shim gaskets on a non resurfaced deck are more prone to leaking, GM used them but they were isntalled after the blocks were freshly machined and absolutely stright. If the deck is not straight you're asking for problems.

LiveandLetDrive

What about sanding? Pardon my ignorance if that's stupid. What exactly is the condition of the surface that causes leaks? Overall flatness?

I don't have any knowledgeable parts places, even NAPA is going downhill

268HE (moderately wimpy), Performer intake, Hooker Comp Headers and duals with "non-mufflers"

-Chris


[Modified by LiveandLetDrive, 8:00 PM 4/26/2004]

MotorHead

Most factory motors are running far in excess of 40 thou piston to head clearance and run fine. A small block deck height is well above 9.025" so thousands of non decked motors running a standard gasket ( 0,039") have been running around for years with over 0.065" quench before the average guy even heard the word "quench" My new motor (406ci) was decked to the 9.025" standard deck height and with a 39 thou gasket puts my quench at 64 thou with 11:1 CR.

I am glad I did not 0 deck the block as my piston to valve clearance is right on the lower limit right now with .610" lift and flat top pistons. I am not worried about any pinging and will be running at least 35deg advance on pump gas ( I hope :lol: )


]


[Modified by MotorHead, 9:47 PM 4/26/2004]

Pete79L82

Did you have steel shim or compisition head gaskets in it now?


You have a couple of different choices for head gaskets

Fel-pro 1094 - .015 steel shim - CR 9.392 - QH .040
Fel-pro HS7733S2 - .020 steel shim - CR 9.287 - QH .045
SEC T11063 - .032 copper - CR - 9.058 - QH .057
Ferrea G50021 - .035 compisition - CR 9.000 QH .060
Fel-pro 1010 - .039 compisition - CR 8.887 QH .064

(all are fiquired on 64cc heads - 9.025 block - piston .025 in the hole - 13cc piston)

I agree thet the quelch height is important but with a low compression/ low horsepower motor I would not be that concerned about detonation being a problem.

With steel shim gaskets it is more important that the surfaces are flat than the surface finish. If the block has the original finish on it (which I assume it does) then sealing should not be a problem, provided that it is still flat.


Pete