Lowering Compression Ratio

**miked** · March 12th, 2012 03:51 PM

I've been thinking what will happen if I replace a 136.5 mm valve with a 130 mm valve (intake and exhaust valves):
Compression ratios will surely go down but what will happen with the following?

1. hp & torque band (will it adjust dramatically? will it widen or shorten?)
2. starting characteristics (cold)
3. operating temperature (this will go down for sure but how drastic?)
4. what else?
5. do i need to change the valve springs too?

Theoretically higher CRs will result to higher efficiencies but the pressure will surely go up too. same goes with the operating temperatures.

waiting for your replies everybody. thanks

**niky** · March 12th, 2012 04:02 PM

Valves won't lower compression ratio. Bigger valves will allow more air in, which should help make more power if the valve ports are enlarged to take the new valves.

**miked** · March 12th, 2012 04:11 PM

i mean the length of the valve. 136.5mm is protruding while a 130 mm will be flush with the head.

**niky** · March 12th, 2012 04:22 PM

Same same. Valve lift will not change static compression but it could alter dynamic compression depending on the overlap and how fast the intake charge enters.

I don't understand how you're going to implement that. The amount of valve lift is dependent on the cam profile. Unless the cam is severly undercut, then longer valve stems means that your valves will not close completely. But if this modification takes the cam profile into account and the valves do close completely when they ought to, the higher lift should give you more power.

Whether it lowers compression will depend on how large the overlap is between intake opening and exhaust closing. If and only if there is considerable overlap, more lift will lower effective compression.

You'll notice a loss of low end torque, cold starting will get worse, and the engine might run hotter in traffic due to improper combustion. But that's only if the extra lift is too aggressive.

And, of course, if the extra lift is too aggressive, you run the risk of bending a valve against the piston at high engine speeds. Better valve springs will be a must.

But higher lift means more air, which means more power at high rpms... if and only if the valves themselves don't flow enough air to match the intake and exhaust ports ability to move it or the cylinder's ability to suck it up. And if the ports themselves are not undersized and providing a restriction. It's sure you'd see some gains, but I can't say how much.

But I really don't understand how you are going to run longer valves without changing cams. Seems the wrong way around. Those of us who fiddle with this stuff change the cams, as this is a surer way of getting a performance boost.

What engine is this, and who's selling the valves? What brand of valves are these and what is the performance claim?

**uls** · March 12th, 2012 04:32 PM

lower compression with thicker head gasket

**miked** · March 12th, 2012 07:15 PM

Originally Posted by niky

Same same. Valve lift will not change static compression but it could alter dynamic compression depending on the overlap and how fast the intake charge enters.

I don't understand how you're going to implement that. The amount of valve lift is dependent on the cam profile. Unless the cam is severly undercut, then longer valve stems means that your valves will not close completely. But if this modification takes the cam profile into account and the valves do close completely when they ought to, the higher lift should give you more power.

Whether it lowers compression will depend on how large the overlap is between intake opening and exhaust closing. If and only if there is considerable overlap, more lift will lower effective compression.

You'll notice a loss of low end torque, cold starting will get worse, and the engine might run hotter in traffic due to improper combustion. But that's only if the extra lift is too aggressive.

And, of course, if the extra lift is too aggressive, you run the risk of bending a valve against the piston at high engine speeds. Better valve springs will be a must.

But higher lift means more air, which means more power at high rpms... if and only if the valves themselves don't flow enough air to match the intake and exhaust ports ability to move it or the cylinder's ability to suck it up. And if the ports themselves are not undersized and providing a restriction. It's sure you'd see some gains, but I can't say how much.

But I really don't understand how you are going to run longer valves without changing cams. Seems the wrong way around. Those of us who fiddle with this stuff change the cams, as this is a surer way of getting a performance boost.

What engine is this, and who's selling the valves? What brand of valves are these and what is the performance claim?

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.

**SiRbossR** · March 12th, 2012 10:49 PM

Originally Posted by miked

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.

**arsen** · March 13th, 2012 01:16 AM

Originally Posted by miked

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** · March 12th, 2012 07:26 PM

Originally Posted by miked

i mean the length of the valve. 136.5mm is protruding while a 130 mm will be flush with the head.

i meant recessed not flush.

**niky** · March 12th, 2012 09:50 PM

Hard to say what the effects will be without knowing the details of the head.

It's possible that the shorter valves are mated to larger shims or higher lift camshafts or something...

If it's the later model, the shorter valves are probably for durability at higher rpms versus the older valves... But if the newer head has more recessed valve seats, perhaps the head and valves will yield lower compression that will also allow more boost.

Unless you have access to data on compression ratio differences between the two versions of the 4D56, as well as data on their rev limits, you can't say for sure.