Thread: td5 head gasket thickness, compression ratio and performance

I know of one Mechanic that replaces every Td5 head gasket With the 3 hole version

This is mere theism without actual knowledge of the result.

I think the drop in compression ratio will allow a littlemore boost to be run without any detrimental effects that the same amount ofextra boost would cause to an engine without the compression ratio lowered.

The extra boost with the added capacity should still onlyprovide the same peak cylinder pressure but the pressure could remain higher asthe extra air would allow extra fuel but there will be extra space the burn theextra fuel.

The extra cylinder capacity might provide a very marginalamount of extra power (with the added boost) but at the cost of economy asthere will always be the requirement of the extra fuel to provide the samecylinder pressure to keep the motor turning over.

I think the end result would be a rise in the best power butat a loss of best economy.

This is would only be for a motor with some form of precylinder compression such as a turbo or supercharger. A naturally aspiratedengine on the other hand will make more power by raising the compression ratherthan lowering it.

Out of interest. Mazda did some engine years ago that was basicallya supercharged engine but it’s difference was they used the supercharger tocreate a huge amount of boost then didn’t close the exhaust valve and open theintake valve until the piston was half way up it’s stroke because thesupercharger had already done half the compression. The motor’s claim to famewas how smooth it was as the supercharger provided a constant load creatingcompression then the piston was only left to finish the compression off. In aconventional engine the motor will speed up at each firing of a cylinder andthen slow as it compresses the next cylinder.

Happy Days

Ok,

It wasn't initially Mazda's idea but they used it in the KJ-ZEM motor.

[ame="http://en.wikipedia.org/wiki/Miller_cycle"]Miller cycle - Wikipedia, the free encyclopedia[/ame]

Interesting. Obviously a huge cost of power in a conventional motor is the compression stroke. A big chunk of power that's made by the power stroke is sucked back up by the compression stroke. So the idea that a roots or other positive displacement supercharger also sucks up engine power as it needs the engine to drive it to compress air but then the piston doesn't have to do so much work.

I don't follow F1 much but I heard they banned supercharges because they became too fast, then they banned turbos because they became too fast so where do these ideas get tested and developed now?

Happy Days.

It's not the compression ratio that's of concern, it's the loss of squish.

Originally Posted by Dougal

It's not the compression ratio that's of concern, it's the loss of squish.

Can you explain some more?

Happy Days

Originally Posted by joel0407

Can you explain some more?

Happy Days

Squish is the piston almost meeting the head and squeezing almost all the gas into the central bowl. Creating a lot of turbulence and minimising dead volume.

If you use a thicker gasket it reduces Squish, resulting in more dead volume, less turbulence and worse air fuel mixing.

That said I have intentionally used the thickest head gasket on a diesel before. It was an engine I didn't care about.

LR specifiy the thickest gasket when the correct gasket is unknown for the piston-head clearance so i would think that it would still be with in spec.

Originally Posted by loanrangie

LR specifiy the thickest gasket when the correct gasket is unknown for the piston-head clearance so i would think that it would still be with in spec.

@loanrangie - do you have a reference in a manual where you saw this?