Forum » General Engine Building Discussion » My Frs rebuild— 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

My Frs rebuild— 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

General Engine Building Discussion

Forum Posts

Courses

Blog

Tech Articles

Talk about engine building here. New products, tricky questions or showcase your work - If it's engine building related it's welcome here.

= Resolved threads

Page 1
Author
121 Views

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 most power with out having to change my forced induction!! thanks for any input and knowledge!!!

Attached Files
  • 40A8DA11-24A0-48DD-B936-FF5B763D1B92.jpeg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

hello in my experience you will not lose much peak hp but the mid-range torque will suffer a bit

i would recommend the cams you mentioned

Not sure what think! many people have said that by dropping my compression from 12:5.1 to 10:8.1 and increase my stroke by a 100cc that I’m going to lose a lot of power and that by upgrading my rotrex from a c30 to c38 blower with smaller pulley will give me around 3-4 psi over my current set up and should bring my power back to what it was before changing compression. So all this work and no gains??

Power, and torque, is going to be roughly in proportion to the air being used to burn the fuel. Since you're using the same supercharger, and drive ratio, the engine is going to be burning roughly the same amount of fuel.

Don't over-think the whole boost thing as, without actually checking, you're not going to know if the lower parasitic losses from reduced compression resistance is greater, or lesser, than the affect of the larger capacity engine and/or compression ratio. Especially as you may be able to run more timing, too.

There is a school of thought that a slightly bigger exhaust, advancing it a little, can help. Advanced to reduce 'blow though' during overlap which can bleed off boost, and as the more highly compressed charge should burn faster, the useful energy from combustion has been used and there will be a stronger exhaust pulse for tuning the exhaust side of the engine. However, that's kinda controversal and while some swear by it, some swear at it.