# Lexus 1UZFE Compression Ratio with Different Gaskets

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I have been working on compression ratios with the 1uz.

Hope the info I have put together comes in handy for some of you.

I have cc’d everything and this is what I ended up with.

First we need the cylinder displacement

Pie x r2 x L =

3.142 x 1914.0625 x 82.5 = 496.13

Or alternatively 3969cc divided by 8 = 496.125

So lets say 496cc for each cylinder.

Now we need the volume of the combustion chamber.

I have cc’d my standard chambers and came up with 43cc

Next is to cc the standard piston at TDC, which gives a volume of 5.1cc

Now we add these 3 volumes together to get the true combustion chamber volume

Which is a total of 55.9cc.

So as we all know CR is obtained by adding the combustion chamber volume to the

cylinder displacement and then dividing by the combustion chamber volume.

(496 + 55.9) divide by 55.9 = 9.8:1

I have checked and double checked my cc measurements to make sure of the actual compression ratio as I assumed the advertised cr of 10:1 was correct, however I can assure you that my engine is 9.8:1.

If anyone else has done these measurements and disagrees with my findings please pm me to discuss this so I can correct this post.

My particular engine is a front sump, hydraulic fan, Jap spec, Celsior (ls400) model Year unknown and was in excellent original condition.

The heads were thoroughly cleaned to remove carbon from the combustion chambers before any measurements were taken.

As I am turboing my engine a lower cr is more desirable so here are a few formulas for obtaining some different ratios.

I have also reshaped my combustion chambers to gain more volume but will explain this later on and factor it into the calculations.

All calculations are also based on an engine retaining standard pistons.

Therefore the new total combustion chamber volume = 43+5.1+9.6=57.7

Cr = (496+ 57.7) divide by 57.7 = 9.6:1

Therefore the new total combustion chamber volume = 43+5.1+12=60.1

Cr = (496+ 60.1) divide by 60.1 = 9.25:1

Therefore the new total combustion chamber volume = 43+5.1+14.2=62.3

Cr = (496+ 62.3) divide by 62.3 = 8.96:1

As far as I know 2.364mm or .093 is the thickest available copper gasket so I will go no further with gaskets.

Next section: Relieved combustion chambers and thicker gaskets.

Another way to reduce the compression ratio is to remove material from the combustion chambers; however, this job is only for those with a steady hand, a keen eye and a basic understanding of combustion chamber shapes.

This said it is also not beyond the scope of most home enthusiasts such as the members of this forum. But if you don’t feel confident and are not experienced with a die grinder please do not attempt this mod.

Step 1 is to cover the cylinder head face with contact or similar this will help to protect it encase of a slip and make it easier to see your marks.  Scribe the cylinder shape onto the head using the old head gasket as a template, be accurate. This will give you a reference line that under no circumstances should be passed through when grinding.

Then remove the contact from within the circles with a scalpel or hobby knife. The remaining tape will help to protect the head surface from accidents. Most of the material to be removed comes from the sides of the chamber and between the exhaust valves. Do not remove any material from above the radius line of the intake valves and only a small amount from between them. The amount I have removed only adds up to a volume of 2.7cc so please do not go crazy with your tool.

It is probably possible to remove more material but as I did not have a spare head to cut up to measure the water jacket closeness I erred on the side of caution. I did however use a piece of bent wire inserted into the water jacket to help me gauge the remaining thickness.

When you have ruffed out the combustion chambers it is a good idea to cc them to equalise the volumes before spending time polishing them.

My final combustion chamber volume (head only) was 45.7cc an increase of 2.7cc

The effect this has on CR is discussed next.  Therefore the new total combustion chamber volume = 45.7+5.1+7.8 =58.6

Cr = (496+ 58.6) divide by 58.6 = 9.46:1

Therefore the new total combustion chamber volume = 45.7+5.1+9.6 =60.4

Cr = (496+ 60.4) divide by 60.4 = 9.21:1

Therefore the new total combustion chamber volume = 45.7+5.1+12 =62.8

Cr = (496+ 62.8) divide by 62.8 = 8.89:1

Therefore the new total combustion chamber volume = 45.7+5.1+14.2 =65

Cr = (496+ 65) divide by 65 = 8.63:1

Now the final thing to take into consideration is if you have to machine the head.

When I did my heads I covered the surface with tape to protect them and even with the utmost care still managed to mark the head surface in one spot (it only takes one).

So now we need to subtract the amount of volume lost through machining.

The cut depth required to remove my mistake was only 10 thou or .254 which meant a loss of 1.52cc from chamber volume. Demonstrated in the next formula.

Minus 1.52cc for machining

Therefore the new total combustion chamber volume = 45.7+5.1+14.2-1.52 =63.48

Cr = (496+ 63.48) divide by 63.48 = 8.81:1 I hope this information comes in handy for some of you.

All the best

Trev

(mycarhasposessedme) 