I have thrown a rod in my turbo 1.8 mk1 and am getting a new engine.
The plan is to get 250-300 hp with manley forged rods and a new thicker head gasket, plus an engine rebuild with new gaskets and seals and all that. I assume this would do it? I was making about 250-300hp when i threw the rod at 1.5bar, in 5th gear. (It was awesome, I won).
Any tips?

 

Yes forged rods are 100% needed if you ever want to make 250-300whp reliably. The thicker head gasket is not needed, I honestly don't even know where you would find one for a miata. For all the gaskets you want to go OEM. They will cost a little bit more, but the quality is worth it. Replacing rear and front mains 6 months after you put the motor together because the shitty seals are leaking sucks major donkey dick.

 

Cool, thanks. You sure the thicker gasket won't be better by making less compression in the cylinders? I've heard a lot about how lower compression is better for turbo cars. And the gaskets are widely available. For instance, 949racing has them.

 

For the 1/2 point drop in compression I'd save my money. IMHO

 

I had not realized 949 sold different thickness HG. The amount of difference in the thickness will not matter enough to change your compression in any serious amount. If you are sticking with stock pistons, I would recommend finding factory 94 pistons as they are the lowest compression stock ones.

 

Maybe 15-20 years ago when tuning software hadn't evolved into what it is now, and lowering compression in forced induction engines was used as a "safety buffer", But nowadays you are perfectly fine running 9:1, or even 10:1 for that matter.

 

Thicker head gaskets also reduce quench area, the near-but-not-quite interference between the face of the piston and the flat surfaces of your cylinder head at the top of the combustion chamber.

Depending on who you believe, keeping the quench tight may provide anti-detonation capabilities due to epic turbulence created at TDC when the air and fuel mixture between the piston face and the flat parts of the head FORCE the mixture into the center of the chamber. The turbulence theoretically helps atomize stray "droplets" of fuel to avoid hot spots, and centers the resultant ignited charge over the piston center rather than allowing it to happen at the "edges".

There's a ****-ton of theory out there about quench and its usefulness, and while I do not buy ALLOFIT, the basis of the theory seems reasonable. In my ancient, shitty Tiburon build, I had Ross build me a custom set of forged 9.5:1 pistons which only dished at the center where the valve reliefs were cut. I then decked the block so that, when cold, the quench distance between piston face and the flat pads of the cylinder head combustion chambers were 0.035". Stock quench distance in that motor was roughly measured at 0.065", give or take. Thus, my decking of the block combined with the very slightly dished pistons ended up with a compression ratio that was damned-close to the stock 10.3:1.

With the expansion of the forged pistons under load, with the piston "rock" from the wall clearances along with the terrible 1.5:1 rod ratio geometry in that motor, the combined movement + expansion of the piston resulted in something closer to 0.010-0.015" clearance while running.

I could feed that motor timing for days, and it got really good gas mileage. It also had terrible blow-by because the piston-to-wall clearance was something absurd like 0.004" as defined by Ross for the power target I was aiming for.

That was probably way too much BlahBlahBlah from a n00b, but thought I'd at least offer.

I'd agree with the folks above; never go thicker head gasket.

 

Thicker head gaskets are only to be used if your custom pistons sit proud from the block at TDC. They are not to be used as a bandaid to lower compression.

 

Your fuel choice, power goals, and purpose for the car will dictate the best CR. Very generally speaking, E85 = 10:1 and sometimes more is fine. Pump 93 and <200whp street use = 9.5 or less. Pump 93 and >250whp street use = 9:1 or less. Obviously many variables effect these broad generalizations. Crappy gas, small turbo vs large, restrictive exhaust or not, small or large IC, aggressive timing or not, track or street, reroute or not, good radiator and ducting or not - these things would always effect my recommendations.

Edit: Running lower compression ratios allows you to more easily achieve MBT on detonation limited pump gas. It also allows you to bring more molecules of air and fuel into the combustion chamber without detonation at the same timing.

 

Thanks for all responses. Im going with engine rebuild and forged rods! That should do me.