No problem Andy, it's just that I've seen first hand the damage that the wrong head gasket can do to a motor. Here's what I believe is the correct method:
I want to give some incite as to how to properly determine the head gasket thickness needed to achieve your target compression ratio during your build. I am putting this together to help those building 1.5JZ’s, but the principals apply to all builds. It takes more time than just buying parts right from the start, but it will help you get the results you want to achieve.
First, it all starts with planning; you have to decide what it is you’re after. You can’t buy the parts that someone else has and expect the same results they got. I was guilty of this years ago, and it cost me a lot of time, heartache and most of all, money. For instants, Jay Hayes (Suprahero) motor and mine are VERY similar, yet completely different. He has a bigger turbo, more aggressive cams and he runs methanol, but the rest is almost the same. Both cars make a lot of power, but they make it very differently. I think one of the main reasons for the huge difference comes down to two things: the prior planning and compression ratios.
Simply said, Jay wanted gobs of top end horsepower that would scream when he got into the high boost/RPM with giving into lag a little, I wanted lots of power (torque) as quick as I could make it. I think it’s easy to say that we both made our goals. Jay’s car screams and would crush mine overall because he makes a lot more power, but I think I could hold my own for a while because I make boost quickly (positive boost at around 2200 RPM, 17psi by 3000 RPM).
So, lets get into planning. If you want a top end beast, you need to look at lower compression and bigger turbo A/R’s. A torque infested street motor that builds boost fast needs higher compression and a smaller turbo A/R. There are a lot of other factors involved in planning your build, which can be discussed forever, but this is all I want to get into for now.
Now that you have decided the power output and delivery, let’s talk about how to achieve it…..the first time. This is the method I used.
First, before buying a ton of parts, I had to buy a spare (used) 2JZ-GE piston and have it cc’d. Jay had the motor, so I needed another one. If I recall correctly, the machine shop I took it to measured it at 9cc for the dish, and 11cc for the entire piston, including the area above the rings.
After I got the motor and head from Jay (he had the machine work done by his guy), I measured the piston-to-deck height and cc’d the heads combustion chamber. The pistons sat 0.005mm below the deck. This WILL vary because of the amount of cut that needs to be made to bring yours to true. My 1JZ head combustion chamber cc’d at 41cc (again, working off of memory). Again, the combustion chamber cc’s WILL vary depending on the amount of the cut.
CC’ing the head really is cheap and easy to do. All you need is a piece of Plexiglas big enough to cover the entire combustion chamber, Vaseline, a syringe measured in cc’s and some vegetable oil. Drill a hole in the center of the Plexiglas, put a light coat of Vaseline around the edge of the combustion chamber to “seal” the Plexiglas to the deck of the head, then fill the chamber with the vegetable oil while making note of the amount needed. Do all six chambers and average them out. There shouldn’t be too much of a variance between the six, so if there is, figure out why. Too easy.
Here is a good example that explains the process in more detail:
http://www.steigerperformance.com/products/sp90005.html
Now that you have all of the known measurements and your decided compression ratio, you can plug in this data into a compression calculator and play with the head gasket thickness to determine the thickness needed to hit your target compression ratio. There are a ton of them online, but here’s one that I used:
http://www.csgnetwork.com/compcalc.html
Enter Bore/Stroke Designation Type
1 = Inches 2 = Millimeters 86
Enter Cylinder Bore Size 86
Enter Piston Stroke Length 86
Enter Head Gasket Bore Diameter 87
Enter Compressed Head Gasket Thickness 1.1
Enter Combustion Chamber Volume In CCs 41
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') -11
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : .006
Calculated Engine Compression Ratio 9.53:1
Had a stock 2JZ-GE head gasket been used (0.3mm), the compression ratio would jump to 10.28:1. A stock 1JZ head gasket (1.3mm) would have yeilded 9.36:1. Still very high for turbo applications.
You can cheat some to save time. I bought a 1.6mm head gasket in advance, and after I had the required thickness, I took it to a machine shop and had them mill it down appropriately. If you take this route, make sure to measure it before you leave the machine shop. It took them all of two hours to get it all done and they only charged me $25 for doing it. Not bad at all in my opinion.
There you have it. Time intensive, but accurate and conpletely neccesary. As a disclaimer, DO NOT TAKE THE NUMBERS ABOVE AS ACCURATE!! Some of them are takren from memory of measurements I took over 2 years ago. DO YOUR OWN MEASUREMENTS!!!!
Also, as for which head gasket to use for a 1.5JZ swap. There was a thread started years ago on another website that said the 2JZ head gaskets won't seal a 1.5JZ properly and it caused an uproar. In laymens terms, the thread said that a 2JZ MHG would not seal the head-to-block oil passage and would leak oil, so a 1JZ MHG had to be used. I won't get into it any more than that, other than saying my 1.5JZ uses a 2JZ MHG and it's sealed up great, and I know Supra hero does as well with no sealing issues. In my opinion, and it's my opinion, the 2JZ MHG is well suited for 1.5JZ use.
