L-81 piston shape?

[Little Brother]

I am looking at new heads, cam, intake and headers. However, in order to ensure I get the proper "quench" with the new heads, I'd like to know the shape to the original L-81 pistons before I dissemble. Are they flat, partially dished (only on the combustion chamber side) or fully dished (across the entire top except the very perimeter)? If fully dished, and with the low compression that's what I'm guessing, I'll probably be looking into new pistons, and then of course while I have the bottom end apart.... $$$... $$$...

Thanks for any help!

 

[gedmeyer]

How about a pic?

I have plenty more engine pics at my website, under the "Projects-383 Stroker" page.

http://gedmeyer.l81vetteregistry.com/

-gedmeyer

 

[Tom Bryant]

The stocks were probably a 21cc dish. You can see the dish in gedmeyer's photo. I just finished a .030 over 350, now 355, with Silv-O-Lite flat top hyper pistons that ended up .040 in the hole without decking the block. I used a .020 steel shim head gasket and ported 76cc smogger heads. The heads were actually 78 cc (measured) and after polishing and deburring the chambers I had them milled to set the final cr at 9.3:1. Don't be tempted to get too small of chambers on your new heads. You can probably use 68CC with a thick composite head gasket of about .040 compressed thickness to end up with not more than 9.5 cr with iron heads or 10.5 with aluminum heads and an even smaller chamber, maybe 64cc, with a thinner head gasket and still use pump gas without an additive.

A rule of thumb: due to the superior heat transfer properties of aluminum you need 1 point higher compression ratio more than iron heads to make the same power, all else being equal.

Tom

 

[GerryLP]

The stocks were probably a 21cc dish. You can see the dish in gedmeyer's photo. I just finished a .030 over 350, now 355, with Silv-O-Lite flat top hyper pistons that ended up .040 in the hole without decking the block. I used a .020 steel shim head gasket and ported 76cc smogger heads. The heads were actually 78 cc (measured) and after polishing and deburring the chambers I had them milled to set the final cr at 9.3:1. Don't be tempted to get too small of chambers on your new heads. You can probably use 68CC with a thick composite head gasket of about .040 compressed thickness to end up with not more than 9.5 cr with iron heads or 10.5 with aluminum heads and an even smaller chamber, maybe 64cc, with a thinner head gasket and still use pump gas without an additive.

A rule of thumb: due to the superior heat transfer properties of aluminum you need 1 point higher compression ratio more than iron heads to make the same power, all else being equal.

Tom

Tom,

Just curious...when milling the heads for a smaller cc's chamber -Did it require to also shave the intake manifold? In some situations this is necessary. I've been quoted as much as $120.00 to do this if I decided to follow the deck/head shaving for increase of CR (opposite of swaping pistons, for example).

GerryLP:cool

 

[Tom Bryant]

I depends on how far you go. If only around .020 to gain a half point then probably not. It is a good idea to set the manifold on the engine without gaskets after the heads are on and measure with feeler guages to determine the gap. Intake gaskets will make up for some gap. If you have had to deck the block and mill the heads you are moving the heads a considerable distance in towards the crank centerline and you will not only have a larger gap but port missalignment. Then cutting the intake is necessary. Also shorter pushrods are in order.

On the engine I just finished I didn't deck the block. If I would have it would have taken a .040 cut to put the pistons at zero deck height, assuming that was what I wanted. Then the heads would not need milled and I would have had to use the dished pistons to end up at the 9.3:1cr I wanted. If I would have had to mill the heads .010 to clean thm up then a thick head gasket would have been in order to peep the CR down to what I wanted.

Using the compression ratio calculator in the tool box (click on tech center in the tool bar above) you can play with all of the different combos of chamber volume, deck height. piston dish volume, head gasket compressed thickness volume ect to determine fairly closely where you will be with various combos.

 

[GerryLP]

Tom,

As far as terminology or definitions go, could an L81 that has been blueprinted in every respect except only for bored over cylinders be clasified as "blueprinted"? I am looking at options on how to rebuild my original L81 engine. I am not sure, but there is a good chance that my machinist will recommend boring-over the cylinder bores (the engine has 145,000 miles). This would blow-out the stock specifications.

GerryLP:cool

 

[Tom Bryant]

I'd agree with that. With all those miles it will have some ridge at the top of the cylinders that will need to come off. Also you will want to be sure the cylinders are round and don't have any taper. Since blueprinting just means building to exact specifications you can say your engine is blueprinted. It's still an L81. It just now has perfect cylinders.

Tom