How to Lower Compression ????
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How to Lower Compression ????
Hello Im trying to find out how to lower the compression with a set of 460ci pistons, the pistons state 11-9-1 compression with 75cc heads, is it possible to use a thicker head gasket to get it down near 11-5-1 or 11-1, camshaft is a Solid flat tappet with 640/650 lift duration @ 050 is 260-266 ad duration 296/308 RPM range is 3400 to 7000. I will be using alloy heads, any advice........... ( Its a D9TE Block- Mid 80's )
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The Pistons are Keith Black part number KB137 in 040 bore size, I dont think my block has been zero decked as there is a lip on the top of my bores about 3-4 mm long- As it was a crate motor from the U-S-A , i removed it from my car last year with only 3'000 km on the engine, original plans were to turn it into a 557ci but i am going to build it into a Hi-performance 460ci for now ( Checked the KB website it states- Crank to deck clearance 10.32 )
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Any Ideas or comments ?????? Im guessing that extra length in my bores will bring the compression down a bit, I decided that I did not need a 557ci stroker with around 650 HP and sold the kit, i will be more than happy with a 450-500HP 460ci engine, I will be using my Victor intake 1050cfm 2 circuit Dominator, Edelbrock 61649 heads & the stock crank & Rods ( Football Rods ) as I just have to decide on pistons & a camshaft..............
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If so, that will make a lot of difference in the CR over the 75cc small chamber heads.
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Hello Dialtone my heads do have 75cc chambers, They are the Edelbrock 61649 with the large Cobra Jet ports, ( Edelbrock do make a 95cc chamber head ) could i get away with 10-5-1 compression with a high lift long duration camshaft, as i know you have to match cam & pistons....................
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The Pistons are Keith Black part number KB137 in 040 bore size, I dont think my block has been zero decked as there is a lip on the top of my bores about 3-4 mm long- As it was a crate motor from the U-S-A , i removed it from my car last year with only 3'000 km on the engine, original plans were to turn it into a 557ci but i am going to build it into a Hi-performance 460ci for now ( Checked the KB website it states- Crank to deck clearance 10.32 )
3 mm down the hole is .118 inches, figure if it's 3 or 4 mm, .125 down the hole....1/8 inch, maybe more.
IF that's the case, you have some fixin' to do to get your engine to run right and have the compression you are looking for. And it would come in a lot lower than your figures methinks....
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Yes it has a lip on top of each bore as the D9TE ( Mid 80's Block ) has a longer bore than the earlier 460 blocks, Im sure the block was decked before putting it together - yes i will have to have the lip machined off before fitting the new pistons, My 460ci has only 1'500 miles on it so its in A1 condition, So your saying because of the lip or longer bore the 11-9-1 compression will come down a bit ???????
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Yes, if your piston is down the hole, compression drops.
Enter your numbers in this calculator with and without the difference.
Engine Compression Ratio (CR) Calculator
Also, having sunken pistons and no quench area in your combustion chambers makes for poor performance as well.
Enter your numbers in this calculator with and without the difference.
Engine Compression Ratio (CR) Calculator
Also, having sunken pistons and no quench area in your combustion chambers makes for poor performance as well.
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Checked a few things... my block is 10.32 deck height, those KB pistons state for use with 10.32 deck height block so these pistons should not be down the bore as with the cast pistons, What thickness head gasket should i use to bring compression down, As i will order that howards solid cam & the 11-9-1 pistons from summit, Thanks for the ECU but i dont really know how to use it ???????
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I'm confused. What "lip" are you talking about? Picture?
Either the pistons are down the hole, or they aren't.
The calculator in the previous link will allow you to vary the head gasket thickness to see what effect that has on CR.
Your block should look like this:
http://vorhaben.net/images/460_Build...k/IMG_1225.jpg
FLAT across the bores.
A down the hole piston is going to look a little like this:
http://vorhaben.net/images/460_Build...y/IMG_1365.jpg
But for the CR you are talking about, it should come up almost flush, if not exactly flush, with the top of the deck.
So the "lip", is it sunken pistons?
On edit: OR are you talking about a lip INSIDE the cylinder at the top of the piston or piston ring stroke, which would show wear in the cylinders?
Either the pistons are down the hole, or they aren't.
The calculator in the previous link will allow you to vary the head gasket thickness to see what effect that has on CR.
Your block should look like this:
http://vorhaben.net/images/460_Build...k/IMG_1225.jpg
FLAT across the bores.
A down the hole piston is going to look a little like this:
http://vorhaben.net/images/460_Build...y/IMG_1365.jpg
But for the CR you are talking about, it should come up almost flush, if not exactly flush, with the top of the deck.
So the "lip", is it sunken pistons?
On edit: OR are you talking about a lip INSIDE the cylinder at the top of the piston or piston ring stroke, which would show wear in the cylinders?
Last edited by 85e150; Oct 17, 2010 at 12:18 AM. Reason: Another question.
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OK, so the lip is 2 or 3 mm down the bore. How far down the piston is the top of the top ring?
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