Reducing compression to up boost via VVT

Hi all, I'm running a 11.3:1 compression ratio engine and 8psi of boost. I've had an idea to reduce effective compression ratio (then increase boost) by using the variable valve timing. Has anyone got any real world experience of using this method and its effectiveness. I'm on the Porsche 987 m97 platform fyi.

Thanks

Sure you can reduce dynamic compression ratio by using variable cam timing to some extend but how far depends on some other things such as turbine AR, boost level, exhaust size, spark plugs, camshafts duration, fuel octane rating. It all boils down to amount of back pressure in the cylinder head that heats up the internal components and promotes the detonation. On a small size turbo it usually doesn't work well as the amount of back pressure is rapidly increasing as you go up with the boost level.

The other thing is to remember that "boost" is just one of the tools for gaining power/torque.

It's certainly possible to increase boost while reducing those values we're actually chasing - do some experimentation, if you like, but remember to fucus on maximising the power/torque and NOT the boost.

Ultimately, you really need to pull the engine and install lower compression pistons. Oh, is this one of the engines that has the problem of the bore surface failing - if so, might be worth taking care of that as well?

Maybe I should've elaborated more on increasing the boost in that case. Lots of guys fell into that trap with a small size turbo and exhaust even if the increasing boost is in good area of compressor map. One may think that in order to get more power it's very logical to increase the boost level since compressor map supports that idea but the real issue starts with small turbine AR resulting in raising back pressure and detonation. So instead of tightening LSA ( lob separation angle) of camshafts to increase overlap and reduce dynamic compression ratio for getting more power you have to do complete opposite thing which is spreading out LSA to combat the back pressure. As the result of that you increase dynamic compression ratio and that leads to detonation too. As I said - it doesn't work well for factory size turbos or close to it frame size turbos...

Thanks for the speedy responses guys 👌

We're developing the twin turbo setup in various guises, to provide different levels to the end users. The turbos we're running on the 2.7 engine are Garrett GT2560r 0.64 a/r which according to Garrett should flow 270 Bhp each so I think we've got the headroom in turbine flow for more boost. Thankfully the 2.7 engine doesn't seem to suffer the bore score issue like the 3.4. We've just recently got a 3.4 car up and running on a Syvecs S7 which we'll be pushing and gathering data to find the limit on the standard engine internals. This will then be stripped and built with a capacity increase, reworked heads, forged rods and lower comp pistons. (Liners will also be installed to resolve the bore score issue)

Current peak power we've seen on the 2.7 engine is 390bhp at 8psi however we've not touched the valve control in the tune yet.

Usually the bigger overlap is the more power you get at high RPM range but it also has it's own limits - once the overlap is too big you may get detonation issue and super high fuel consumption as part of air-fuel mixture literally goes out of the window instead of getting trapped inside of combination chamber. This point is as much as you can have influence on dynamic compression ratio with variable timing...

Ahh so what your saying is if I close the inlet valve later during compression I'll get some reduction in dynamic compression ratio but as a function of doing so it'll increase LSA and I'll start to lose charge? 🤔

Yes. It's always the act of balance. Only one correction- you will make LSA smaller to increase overlap, not bigger.

But definitely worth some dyno testing on the VVT mapping would you say? Really appreciate the input, any other areas you think we should focus on to optimise this twin turbo setup on the standard internals? Will be logging charge cooler & radiator temperature next as our intake temps are hitting like 65°C in warm weather. Considering water / meth to aid in this but have zero experience tuning with it, any pointers would be greatly appreciated 👍

Definitely it's worth trying on a dyno. Intake temperature is very important as well as EMAP for turbo but you should be fine on both since you're going with twin turbo setup and WMI...