13

A well-known fact when adding forced-induction to an engine originally designed for naturally-aspirated applications, is that it may be necessary to reduce the compression ratio to counter the increased risk of pre-ignition/detonation.

A cost-effective way to do this is to use a thicker head gasket. I came across an article which makes a rather interesting claim (emphasis my own):

A thicker gasket will reduce the compression ratio by a small fraction, probably only by .1 or .2. This is by far the easiest method of reducing compression but the risk is gasket failure and the gains in lower compression are minimal.

The physics behind this assertion is not clear to me. My questions are:

  • with all other factors held constant, is it true that a thicker head gasket is likely to fail?
  • if so, what is it about a thicker head gasket that makes it more prone to failure?

    Is it just that there is more surface area for the combustion gases to interact with the gasket, causing the head gasket to erode more quickly?

    Or is it that a thicker head gasket will bulge more under compression, creating regions of higher stress concentration in the process?

  • 1
    great question +1 – DucatiKiller Mar 10 '16 at 19:53
  • 2
    Intuitively it makes sense. There's more non-metal exposed to pressure. Whether that is the proximate cause is ... uh ... your original question. ;-) – Bob Cross Mar 10 '16 at 19:56
  • @BobCross right... I'm interested in the failure mode(s) - the how and the why – Zaid Mar 10 '16 at 20:00
9

with all other factors held constant, is it true that a thicker head gasket is likely to fail?

Yes. A thicker head gasket impacts how headbolt torque is distributed between the block and the heads.

Take two pieces of bread. Squash them with your hand. Now put cheese and ham inside and squash it again. The first thing to move under the pressure is an ingredient and not the bread. Same thing with thicker gaskets.

Thicker gaskets will usually call for longer-ish head bolts and higher torque.

if so, what is it about a thicker head gasket that makes it more prone to failure?

You are adding more space between two parts that need to seal under constant pressure and changes in temperature.

I stopped doing this and went with the water injection route. It works better for less money.

6

It turns out that multi-layer head gaskets can be OEM as well.

In this Hot Rod Garage video the presenter makes mention of the LS9 head gasket, which consists of 5 layers.

This would seem to indicate that it is possible to have a reliable multi-layer head gasket, so thicker doesn't necessarily mean more failure-prone.

3

Depends on the quality of the gasket and installation, I have done this and had no failures, I do suggest re-torquing the head bolts after a few hundred miles and a some heat cycles of the engine, then do it again in a year or so.

They also make head gasket spacers, they are solid metal and do the same thing, you can use a stock head gasket with these spacers, they do not make them for all engines though.

  • I have a friend that had one made for his 4 cylinder suzuki hyabusa when he added a supercharger to it in order to reduce the CR. Is that what you have used them for (the spacer)? – DucatiKiller May 2 '16 at 23:43

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