Compression ratio and tuning

[Nevyn04]

I have an '87 z31 turbo. Mostly stock aside from a turbo timer and manual boost controller. Looking to rebuild the engine, replace the timing chain, water pump, and all seals. Considering increasing compression while I'm at it with new pistons. Thinking 9.5:1 or 10:1. After a compression increase should I have the ecu tuned or will adjusting timing be enough? Sorry if it's a dumb question. I'm fairly certain it needs to be tuned. If so I'll hold off until I can put a Nistune in. Any replies are greatly appreciated.

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[Nevyn04]

Surprised at the lack of answer...
Anyway, talked to a speed shop in town, Relentless RPM. They said with just an adjustable fuel regulator and the Air/Fuel guage i have i'll be good. Now by adjustable i assume they mean i need a rising rate regulator. Because static pressure set at a good rate while not under boost would be very lean under boost. Just my thinking, am i right? Also, suggestions on good rising rate regulators?

And as far as increasing compression with VG30e pistons, they are 87mm. So they should fit. But will the valves clear? I definately don't wanna finish the build just to bend my valves and have to open it up again. I'm only looking to hit about 250-300whp right now. I think the compression bump and 12psi intercooled should get me there.

[TheRealSy90]

The factory fuel pressure regulator is rising rate. You need to get the Ecu re tuned for stoich fuel mixture with the higher compression, possibly requiring bigger Injectors. As far as piston clearance you need to check the clearance manually with putty most likely.


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[[email protected]]

Increasing compression will lower your boost threshold, there's a reason the majority of turbo engines don't go above 9:1

However, your 87T motor has 7.8:1, so just going to non-turbo 9:1 pistons will be a free bump in power, you can get away with using premium fuel and intercooler with no ecu/injector change, but to maximize it, tuning would be wise

[Nevyn04]

I know keeping your compression low allows more boost. And the reason we prefer boost is because it can be cooled after compression whereas engine compression cannot. But, my stock turbo is only efficient up to about 15psi. So i figured a decent compression will allow me to make the most of that limited boost level. But i guess 9:1 will give a decent bump, all the better without requiring new injectors or ecu, and allow for later turbo upgrade and more boost.

Eventually I plan to run Nistune for tuning.

Reason i'm looking to rebuild the motor now is because not sure when the last time the timing chain was replaced and have valve cover gaskets leaking oil. Nothing serious. But while getting into it I figured replacing all the seals so I know when everything has been done would be a good idea, and just feel that 7.8:1 compression is way too low.

So my issue now is to find out if the NA pistons will clear my valves. Anyone have experience with this swap?

I can buy the NA pistons new aftermarket, the ones I found come with floating wrist pins. I'm not sure if my motor is "W" series or not. If it is not and it has press fit pins can I use the pistons with floating pins?

[TheRealSy90]

Umm, there's 12.5-1 cars running 30+psi. It's all in the fuel and the tune. You CAN have high compression and boost.


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[Nevyn04]

Understood. But I'm referring to streetability issues, this car is and will continue to be a daily driver. So the wear caused by the heat of that much compression with boost on top would require constant rebuilds, and I don't know about you but i'd prefer a decent lifespan in my engine.

[[email protected]]

You don't need anything fancy, you could even use a set of z32tt pistons and rods for your build for a middling 8.5:1 or so

Might be cheaper to buy a good condition 9:1 engine in it's entirety, possibly a vg33 from a '98 quest, and swap over your accessories

... btw they use timing _belts_

[Nevyn04]

Chain, belt, you know what I mean. Was thinking I could get the NA pistons and use my current rods. But would floating pins fit press rods or does it even matter? Not sure if the rod itself varies based on the pin...

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The floating style was used in 88+ w-series blocks, so if you got those pistons and have a pre-w block, you also need new rods... most aftermarket forged pistons are designed for the larger floating pins too

The cheapest bet is junkyard, as long as the pistons aren't pitted/melted and no bearings spun or seized, they'll be fine with a hot tanking... unless you want to go forged?

[Nevyn04]

I thought about forged but since stock can handle close to 500 HP didn't, see the point.

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Keep in mind that 400hp from a vg30 isn't like 400hp from an sr20, you'll have gobs and gobs of torque, and with 9:1 you'll start with a lot more out of boost too

[Nevyn04]

I have a buddy that likes to say "HP sells cars, torque wins races." Lol. I'll look into those pistons and rods then. Not needing a tune means I can get it done sooner rather than later. Then save for new injectors and a Nistune ecu.

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