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The answer is no anyway because quench height is a thing. Quench can be as important if not more important to limiting detonation than static compression.
The reason you want 8.6:1 compression is because you can run more boost and ignition timing on pumpgas. With the higher compression you will have to retard the timing more and actually make LESS power. Even with lower compression with a turbo at low rpm your car will feel much stronger across the whole rpm range. The turbo creates positive pressure in the pipes ( even when the boost gauge shows nothing ) and this will make your engine feel stronger across the whole powerband even when out of boost. Once you get your engine together you will see what we are all talking about and you will be glad you went with lower compression. My WRX runs 8.1:1 compression STOCK and it gets out of its own way just fine out of boost, better than a stock miata engine infact.
do you really want/need a daily driver with a “built” engine? If it’s your only car, you’re making a mistake.
are you willing to run an inferior piggyback ECU (will be a pain) or switch your ECU/injectors (and possibly other sensors) for every emissions test? You’re going to be best served with a stand-alone ECU making that kind of power.
Don't buy the high-compression ST's! Not only for the reasons mentioned, but because the valve relief cuts are too aggressive (IMHO) and have stress risers that will eventually fail from fatigue. How do I know...this happened to my engine which is now being built with low comp Wiseco's;
This is a crack propagating on the second exhaust valve relief cut on the same piston
This same failure happened on two pistons. I haven't looked at the other two, but I suspect that I'll find crack on both of those as well.
Moral of the story; don't use HC pistons on a boosted motor.
do you really want/need a daily driver with a “built” engine? If it’s your only car, you’re making a mistake.
are you willing to run an inferior piggyback ECU (will be a pain) or switch your ECU/injectors (and possibly other sensors) for every emissions test? You’re going to be best served with a stand-alone ECU making that kind of power.
why are you building an engine for 250hp?
Well my car's current engine has to be rebuilt anyways, and I figured why not just set the entire engine up for future power and reliability. 250HP is just my initial target for when I eventually do add a turbo on it. My first priority is getting the engine back in and running and passing emissions.
do you really want/need a daily driver with a “built” engine? If it’s your only car, you’re making a mistake.
The Miata has been my daily for the last 4 or so years and I haven't had any issues (besides the DIY maintenance I've had to do). I'd be interested to hear more as to why you think it's a mistake. Sure - the engine could grenade itself or something could go wrong. The worst case scenario is I just buy another car. I actually find it rewarding being able to daily drive something I've personally worked on and built, so for me it's worth the risk.
are you willing to run an inferior piggyback ECU (will be a pain) or switch your ECU/injectors (and possibly other sensors) for every emissions test? You’re going to be best served with a stand-alone ECU making that kind of power
When I do add a Turbo, I'll definitely be using an aftermarket ECU, possibly with upgraded injectors. The goal is to be able to easily switch back to OEM ECU and injectors when inspection is needed.
Don't buy the high-compression ST's! Not only for the reasons mentioned, but because the valve relief cuts are too aggressive (IMHO) and have stress risers that will eventually fail from fatigue. How do I know...this happened to my engine which is now being built with low comp Wiseco's;
This is a crack propagating on the second exhaust valve relief cut on the same piston
This same failure happened on two pistons. I haven't looked at the other two, but I suspect that I'll find crack on both of those as well.
Moral of the story; don't use HC pistons on a boosted motor.
I already ordered the 8.6:1 Supertechs. Hopefully I have nothing to worry about haha (probably not because I have low comp ones). Photos like this definitely don't boost my confidence (no pun intended).
Thanks Mat. I'd be interested to hear your reasoning behind your choice, just so I can understand the factors that go into deciding for myself
Here's the advice I received from Fab9. I'd just like to get some additional opinions from you guys on here if possible!
If you’re just running a standard, non-vvt engine car that’s equipped with an oil fed turbo I’d run 2 shims. The additional pressure will not cause issues, even before it’s turbod. If you will be adding other parasitic items to your oiling system, like an oil cooler – Then run 3 shims.
2 Shims would be 72 PSI on the engine. I'm planning on running the engine without any boost/turbo for probably 6-12 months.
08-15-2018 | 10:54 PM
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