Lowering Compression Ratio

[SiRbossR]

i'm still debating myself on this. i was actually fiddling with the idea. the story is a 4d56 turbodiesel with a 136.5 mm long valves (early models). if ever, want to try out the shorter valve (along with the camshaft of course). a friend is offering a surplus CH at a very friendly price. i'm not interested with the head casting(some cracks maybe) but actually with the valve train assembly (130 mm valves) i.e. longer valves to be replaced with short ones.

as per Uls' thicker head gasket, can't find a "thicker" gastket. the brands available around my place seem to be of the same thickness.

Changing the valves wont affect your compression, it will only affect the amount of A/F mixture that goes in to the cylinder.

To lower your compression, you can use two gaskets in between the CH and the Block.

[sirkosero]

what are the benefit/s of having a lower compression? isnt it that a higher compression for gas engine should be better?

[arsen]

i'm still debating myself on this. i was actually fiddling with the idea. the story is a 4d56 turbodiesel with a 136.5 mm long valves (early models). if ever, want to try out the shorter valve (along with the camshaft of course). a friend is offering a surplus CH at a very friendly price. i'm not interested with the head casting(some cracks maybe) but actually with the valve train assembly (130 mm valves) i.e. longer valves to be replaced with short ones.

as per Uls' thicker head gasket, can't find a "thicker" gastket. the brands available around my place seem to be of the same thickness.

Specifically for 4d56 pala to.

If the original valve is protruding on the cylinder head, there should be a corresponding recess on the piston head.

If your engine displacement is 2477/4 that will be around 620cc per cylinder.

If your compression ratio is 21:1 the original volume of combustion chamber will be 620cc/21-1 or around 31cc

If you will replace your original cylinder head with recessed type valve and if you will still be using the same recessed piston, you can estimate the change in the volume of combustion chamber as follows:

Intake valve dia = 40mm, volume of recess in CH will be (pie/4*40^2)*(136.5-130), that will be around 8,000mm3 or 8cc of additional chamber.
Exhaust valve dia = 34mm, volume of recess in CH will be (pie/4*34^2)*(136.5-130), that will be around 6,000mm3 or 6cc of additional chamber.

Your new combustion chamber volume per cylinder will now be 31cc + 8cc + 6cc or around 45cc

Your new compression ratio will now be (620+45)/45 or around 15:1.....

[miked]

thanks for your replies.

here's the scenario: same pistons (& block), same cylinder head....only the valves (maybe cam and springs too) to be replaced if ever. afaik, all 4d56 have the same recess on the pistons. be it NA or turbo versions.

[niky]

Shouldn't have much effect, then. What will matter is the shape of the cam profile, since I assume those shorter valves are mated to taller buckets and/or different cams.

[miked]

after some research (of course taking these discussion here too), it (shorter valves) won't physically change the amount of air compressed. the most possible solution for a lower CR are the dished pistons. a lower CR would also mean lower hp/torque but engine will run cooler. problem is no available dished pistons. a double gasket is also unreliable....cancel the plan then.

[niky]

If the purpose is to lower operating temps, why not just go with a small intercooler? Like maybe a stock side-mount or front-mount from another vehicle. Simpler. Less complicated installation.

[miked]

If the purpose is to lower operating temps, why not just go with a small intercooler? Like maybe a stock side-mount or front-mount from another vehicle. Simpler. Less complicated installation.

have to change the plumbing. turbo outlet is about an inch to the intake manifold.

[niky]

Yup... you'll have to add a bit of piping, but it's more easily reversible and less risky than cracking open the engine to change out the cams.