Forum » General Tuning Discussion » how much power will I loss when dropping Compression from12:5.1 to 10:7.1 on a centrifugal supercharger that peaks @14psi and runs on E85

how much power will I loss when dropping Compression from12:5.1 to 10:7.1 on a centrifugal supercharger that peaks @14psi and runs on E85

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I have Subaru FA20 engine that was stock motor cr 12:5.1 with a c30 rotrex centrifugal supercharger peaking at 14psi on E85.I was able to reach MBT no problem on e60 and above . I made 356whp and 253wTQ 180 whp over stock see dyno sheet attached below blue line was high boost pulley at 14psi. I ran this power level for 3years and 100 1/4 mile passes plus 10,000miles of hard street driving before breaking a rod and putting to holes in my block. I am now putting in a stroker motor I went from a 86mm crank to 90mm crank and a 86mm piston up to 86.5mm making it a 2.1L stroker motor. But I decreased the Compression from 12:5.1 to 10:8.1 and my supercharger is max out @14psi. How much hp will I lose cause of the drop in Compression ? Will stroking it and the adding 100cc of displacement plus better forged internals make up for the loss of Compression ? Also my stock cams are 255 Intake duration@11.00mm lift and Exhaust is 252 duration@11.00mm lift would it be a good Idea to add HKS cams ? HKS cam specs are 260intake duration@11.7mm of lift and the exhaust is 266 duration@11.7 of lift. is this a good cam for the stroker/supercharger or does the stock cams seam better fit? I know there is a lot of questions hear but I'm trying to put together an engine that's going to make more power with out having to change my forced induction!! thanks for any input and knowledge!!!

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Ok, let's make some estimations here. When you made CR 10.8 instead of 12.5 at 14 psi you lost about 2 percent of potential maximum power. When you changed your engine displacement from 2 to 2.1 liter you added 5 percent to it thus potentialy consuming 5 percent more air than before that might result in 5 percent of addition power if tuned properly...

I'm not a pro with supercharger at all, but I see it differently, and please correct me if my assumptions are wrong.

You added displacement. Your supercharger is maxed out at 14 psi, which mean pressure will probably go down as it can't flow more air than that, but it needs to fill more volume. So basically, the quantity of air entering the engine will be the same. Same air flow = same fuel qty = same power. But you also lowered compression, which means probably less power.

What do you think?

Ambient pressure will still fill the additional displacement by air and charger's pressure will probably not drop much maybe a percent or two so overall he will stay about the same power he had before ( potentialy). Adding hks camshafts will shift powerband to the right accros the entire rpm range which is a little bit of trade off - less torque at low rpm zone and more power at the middle and top - it is the question of this car priorities weither to install them or not. If good throttle response is absolute priority stock camshafts should be kept, if maximum power is ultimate goal then hks camshafts to be chosen...

Boost pressure is a measure of the resistance to flow - in this case into the cylinders. The greater the volume there to accept the air mass, the less boost - this is, incidently, why I have a BIG problem with the use of "volumetric efficiency" by 'tuners' instead of mass efficiency, which is a more accurate term.

However, a small increase in the blower rpm should restore the boost level.

I’m using the smallest pulley that is made for my supercharger . Boost will drop a small amount but the blower may be able to flow easier with less heat and back pressure. Making the air charge more dense. I’m trying to make more midrange and top end power. I mainly drag race. Since I can’t speed the blower up would a small shot of nitrous be advisable to make up up for lower compression and larger displacement? I would go turbo but I don’t have the funds right now cause of building the motor. Nos kit is lot cheaper

NOS is definitely the option and cheapest way to go faster. HKS camshafts are beter choice in case of NOS installation as they will reduce dynamic compression ratio thus reducing knock threshold.

Another option may be to use a larger diameter crank' pulley, if available. If not in current production, you may be able to get the manufacturer to machine up an oversized one for you?

Be mindful that increasing overlap significantly on a supercharged engine may lose you a fair bit of torque across a lot of the rev range. Turbocharged engines with large well matched turbos respond very similarly to NA engines in terms of cams, you need to be a bit more carefully when there is a significant pressure difference across the engine.

How do figure out what the overlap is on a DOHC and does variable cam timing change this?

Overlap is the number of degrees between the intake opening and the exhaust closing - ie. if the intake has the opening from 30 BTDC, and the exhaust has the close point at 35 ATDC, there will be 65 degrees of overlap.

Yes, if either, or or both, camshaft(s) have variable timing or, in some instances lift, the effective overlap will change.

Thank you for the info that makes perfect sense.