Lowering Compression Ratio

[miked]

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]

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]

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

[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?

[uls]

lower compression with thicker head gasket

[miked]

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.

[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.

[arsen]

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

As far as I know, using a shorter valve is an engine rebuilder fix if the valve seat has been refaced. (Same thing if the cylinder bore was honed, the engine rebuilder will use an over-sized or bigger diameter piston)

If the valve seat has been refaced or the hole for the valve seat has been re-bored, the valve will sink deeper into the cylinder head and the valve stem protrusion will become higher.

Especially for overhead cam type, possible na tumukod ang valve stem sa cam if the height of protruding stem is way above the specs. Thus the valve could not be shut-close completely.

For rocker arm type, the force exerted by the partially depressed valve spring might not be enough to produce a tight fit if the height of protruding stem is way above the specs. Thus leak might happen.

[niky]

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.

[arsen]

As far as I know, using a shorter valve is an engine rebuilder fix if the valve seat has been refaced. (Same thing if the cylinder bore was honed, the engine rebuilder will use an over-sized or bigger diameter piston)

If the valve seat has been refaced or the hole for the valve seat has been re-bored, the valve will sink deeper into the cylinder head and the valve stem protrusion will become higher.

Especially for overhead cam type, possible na tumukod ang valve stem sa cam if the height of protruding stem is way above the specs. Thus the valve could not be shut-close completely.

For rocker arm type, the force exerted by the partially depressed valve spring might not be enough to produce a tight fit if the height of protruding stem is way above the specs. Thus leak might happen.

In addition, its not commonly known the exact tolerance of how much bigger the valve seat MUST BE with respect to the hole in the cylinder head. Besides, the correct way of installing the valve seat is to dip it into a liquid nitrogen, until the diameter matches the size of the hole. Bagay na wala ang karamihan sa mga engine rebuilder. Therefore using a shorter valve is the next reliable option after machining the valve seat. The next reliable engine rebuilding ay palit na dapat yan ng cylinder head.