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Old 04-12-2012, 05:19 PM
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L98 heads iron+383 compression ratio?

Ok I'm building a 383 for a friend the factory motor is an L98 he got the crank with dome pistons and 5.7 rods he was told they'll be 10:1 something tells me it's going to be greater. Any ideas what compression will be they are the iron 64cc heads

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Old 04-12-2012, 06:31 PM
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It will be over 11:1, way to high for pump gas.
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Old 04-12-2012, 06:40 PM
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Quote:
Originally Posted by Cortrox93
Ok I'm building a 383 for a friend the factory motor is an L98 he got the crank with dome pistons and 5.7 rods he was told they'll be 10:1 something tells me it's going to be greater. Any ideas what compression will be they are the iron 64cc heads
Holding the head gasket at reasonable thickness to keep the squish/quench clearance from getting too wide the compression with domes will be pretty high.

A flat top piston on a 019 gasket with a .025 in the hole piston will easily crack 10.7. Figure that the L98 aluminum head runs around 58ccs but with a thicker .050 gasket the number looks to be around the same 10.7 but now the squish/quench is at .075 where you'd really like it more in the direction of .040 gasket and crown to deck clearance combined.

Zero deck this, mill the head a little, or use a pop up dome piston and you can see where this can get into trouble real fast.

The gooder news is that you need to compute the Dynamic Compression Ratio which is determined by how much stroke is lost to crankshaft rotation at the point where the cam shuts the intake valve. A hefty, late closing cam just might make these numbers look more favorable. the DCR needs to be in the 8 to 9 :1 range

But before anything you just gotta get the ccs of the dome. I you can id whose pistons, the data should be on line, otherwise you'll have to measure the dome volume.

On a long block I've done this a couple ways none are perfect but they are close and none are not without risk of getting crap into the rings but the chances of that can be minimized.

1.With one sides deck level bring the piston down the bore just enough to either put the dome's peak just flat with the deck or a little below. Measure the distance from deck to the flat part of the crown. Pack waxed string between the crown and the top ring to seal it up. Bow string from a sporting goods store like Cabella's works well it's waxed, strong and thick enough to fill the area during your life time. Then measure in a light oil like Marvel Mystery or ATF. That minus the math of bore area times depth to the crown will give a pretty good representation of the dome volume. Don't forget to remove the waxed string after draining the oil out.

2. Similarly position the piston as in number 1. Press and form a piece of heavy aluminum foil used for ovens or baking, not the sandwich wrap thin stuff, so the bore and piston are covered and the shape is pretty darn smooth to the surfaces and there are no holes or cracks. You might want to fill the crown to top ring space with the waxed string just to be sure nothing gets in there. Than either fill the space with plaster or with mold making RTV, you can get that stuff at hobby or machinists supply houses. This will make a model of the piston crown and dome which can be removed, turned over and measured for volume with a burette or graduated cylinder using light oil or since this won't risk getting into the engine as it could dissolve the wax on the sealing string if just poured into the top as the oil in number 1 above is --- use rubbing alcohol with some food coloring dye in it. You could but I don't recommend it, use a single malt Scotch, but I can think of better uses for that, but it does saturate and get into the tight places for accurate measurements.

Bogie
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Old 04-12-2012, 09:33 PM
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Well thank you guys for helping thats what I was thinking...now I have one more issue...he just looked online for his casting number and gave it to me, I went to put the heads on today, they Arent the heads he told me they were they are the infamous 462624 casting number, 76cc.....
Now projected compression?
Should I even use these boat anchors?
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Old 04-12-2012, 10:04 PM
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And I forgot to mention the pistons are -9.5cc
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Old 04-12-2012, 10:34 PM
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Disregard the -9.5 ha wrong engine, too many projects
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Old 04-13-2012, 06:31 PM
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Quote:
Originally Posted by Cortrox93
Well thank you guys for helping thats what I was thinking...now I have one more issue...he just looked online for his casting number and gave it to me, I went to put the heads on today, they Arent the heads he told me they were they are the infamous 462624 casting number, 76cc.....
Now projected compression?
Should I even use these boat anchors?
With no changes to the numbers I published yesterday the 76 cc head would drop the compression to 8.9 from the 10.7.

Bogie
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