JRSC C38 with an RS Pulley

[Irace86.2.0]

Originally Posted by RevoG

Good morning to all. Just found this thread and it seems to be what I am looking for. I have a question and hope someone on here can help my out. I'm running a JRSC kit with a C38 SC and a RS pulley. Built engine with 10.0:1 compression and stock heads and cams (upgraded valve springs) and running on 98 pump gas.

My question is at 7500rpm I get only 0.81 bar of boost (11.7Psi). But I do not understand how that is possible. Where does the rest of the 5+ Psi go?

What changes can I make to up the boost and power levels? (No, changing fuel is not possible since I can't get e85 here)

Thanks

What’s your power output/dyno results now? Your limits will be the limits of the supercharger (rpms)—not boost. Boost is a measure of resistance, so anything that reduces resistance might reduce boost without necessarily reducing power; in fact, power can increase. For example, a header can reduce boost, yet increase power.

[RevoG]

Originally Posted by CSG Mike

Is your boost linear or leveling off? Check for mechanical boost leaks.

Boost is linear, no boost leaks

[RevoG]

Originally Posted by Irace86.2.0

What’s your power output/dyno results now? Your limits will be the limits of the supercharger (rpms)—not boost. Boost is a measure of resistance, so anything that reduces resistance might reduce boost without necessarily reducing power; in fact, power can increase. For example, a header can reduce boost, yet increase power.

Currently around 320 WHP on a mainline dyno

[Irace86.2.0]

Originally Posted by RevoG

Currently around 320 WHP on a mainline dyno

That number isn’t bad for your fuel and compression ratio. By “98 pump gas” you mean it is 98 RON, equivalent to our 91 octane in the states, right? These guys made similar power on the HBP, which is what you are hitting.

Did you do a larger intercooler too when going with this pulley? What’s your IATs? I’m just wondering if you are getting heat soak.

Can you do a boost leak down test too?

[tomm.brz]

98 ron is decent fuel here, more like 92oct
95ron is the shitty 91oct

[RevoG]

Originally Posted by Irace86.2.0

That number isn’t bad for your fuel and compression ratio. By “98 pump gas” you mean it is 98 RON, equivalent to our 91 octane in the states, right? These guys made similar power on the HBP, which is what you are hitting.

Did you do a larger intercooler too when going with this pulley? What’s your IATs? I’m just wondering if you are getting heat soak.

Can you do a boost leak down test too?

Yes, larger intercooler was installed. IAT is around 2-4C higher than ambient for normal driving. Also installed a water /meth kit to run on track to keep the temps down.

Did a boost leak down test and all good there.

[ML]

Originally Posted by RevoG

Yes, larger intercooler was installed. IAT is around 2-4C higher than ambient for normal driving. Also installed a water /meth kit to run on track to keep the temps down.

Did a boost leak down test and all good there.

I have had very similar experience with the HBP and only seeing no more than 9.3 psi. I get that psi isn’t everything but like you that large of a drop in observed vs marketed pressure is very concerning at worst and interesting at best.

[mrg666]

RS pulley with high compression and 93 octane gas is not practical for street/daily setup. It only makes sense with lowered compression and/or race gas. To me, it only makes sense for a track dedicated race car with a built engine. I don't think tjis is just a Rotrex limitation. Even a turbo or PD pump will be unpractical to go far beyond 300 hp for a street setup since this is a 2L 4 cylinder engine after all. PD or turbo can feel faster on the street and drag strip due to low rpm torque but not on the track.

[Irace86.2.0]

Originally Posted by mrg666

RS pulley with high compression and 93 octane gas is not practical for street/daily setup. It only makes sense with lowered compression and/or race gas. To me, it only makes sense for a track dedicated race car with a built engine. I don't think tjis is just a Rotrex limitation. Even a turbo or PD pump will be unpractical to go far beyond 300 hp for a street setup since this is a 2L 4 cylinder engine after all. PD or turbo can feel faster on the street and drag strip due to low rpm torque but not on the track.

He has a built engine with lower compression. In many ways, a lower static compression engine should be capable of much more power or effective compression than a stock static compression built motor with E85 with the understanding that there will be some trade off in response. Are you saying 10.0:1 is still too high of compression for pump gas? There are many turbocharged cars that come with this setup from the factory (for the street) like the FA20F engine in the WRX, which has 10.6:1 compression.

[Irace86.2.0]

Originally Posted by RevoG

Yes, larger intercooler was installed. IAT is around 2-4C higher than ambient for normal driving. Also installed a water /meth kit to run on track to keep the temps down.

Did a boost leak down test and all good there.

What power did you make with meth, or was it the same or similar because your static compression already maxes out your timing?

If everything is dialed in perfect then perhaps there is just a drop in pressure across the IC or drop in resistance in your system. It is only reasonable to assume that two engines pushing a supercharger the same rotations or amount of air with the same timing advance will produce the same power, but if one has less compression then that one will make less power. Here is your problem:

https://jacksonracing.com/product/fr-sbrz-rs-upgrade/

The Jackson Racing RS Upgrade is designed for the FR-S/BRZ C38 System owner ready for all the boost! This limited production product takes the Rotrex C38-81 unit to maximum impeller speed and gives track users the ultimate power upgrade. The RS upgrade is a RACE/TRACK ONLY product and should only be installed on vehicles with fully built engines. Contents include the C38 RS bracket and RS Boost pulley. Performance/boost can vary depending on the engine build.

If the impeller speed wasn’t at its limit then a larger crank pulley could be used if the supercharger pulley couldn’t get any smaller, but this could offer diminished returns because the impeller speed could go out of its efficiency range and have problems grabbing air because it is moving too fast. I don’t know what the efficiency/compressor map looks like for this unit.

[mrg666]

Originally Posted by Irace86.2.0

He has a built engine with lower compression. In many ways, a lower static compression engine should be capable of much more power or effective compression than a stock static compression built motor with E85 with the understanding that there will be some trade off in response. Are you saying 10.0:1 is still too high of compression for pump gas? There are many turbocharged cars that come with this setup from the factory (for the street) like the FA20F engine in the WRX, which has 10.6:1 compression.

All I am saying is that it is a slippery slope increasing power after a certain level; increase boost, tune loose timings, lose power, increase boost again ... and the engine needs to be stronger, octane requirements reach race fuel, fuel efficiency starts dropping drastically ... One way to increase boost further when ignition timing becomes difficult is to reduce compression. I am not sure but 10:1 compression should be enough with RS pulley. But you know that decreasing compression reduces power. That makes more boost necessary to increase the power again. We are back on that slippery slope. It is just not practical after a power level to keep increasing the boost for a given engine. And I think we are close to that limit with most of our typical FI kits. I think the manufacturers see the similar engineering limits, that might be why WRX, STI, Type R, Golf R are all near similar power levels with similar engine capacities.

Edit: I was checking how to make most power with a 4 cylinder engine and I found M139 2L 4 cylinder engine at 400+ HP.
- 30 psi twin turbo
- 9:1 compression
- special electric water pump
- closed deck extra strong block and other components
I did not see the exhaust manifold, camshaft, valve train design, but those should be extra special too.

So it is possible ...

[Irace86.2.0]

Originally Posted by mrg666

Edit: I was checking how to make most power with a 4 cylinder engine and I found M139 2L 4 cylinder engine at 400+ HP.
- 30 psi twin turbo
- 9:1 compression
- special electric water pump
- closed deck extra strong block and other components
I did not see the exhaust manifold, camshaft, valve train design, but those should be extra special too.

So it is possible ...

It is definitely possible. Evos and WRXs have been making over 400...500hp for years:

https://www.evolutionm.net/forums/ev...our-stuff.html

Yes, Top Fuel drag cars use race fuel, but nevertheless, they often have even lower compression like 6.5:1. In theory, the lower the compression then the higher the effective compression ratio can be. The trade off is lag and efficiency. Some drag cars use nitrous and anti-lag to fill the void left by low compression.

[mrg666]

Originally Posted by Irace86.2.0

It is definitely possible. Evos and WRXs have been making over 400...500hp for years:

https://www.evolutionm.net/forums/ev...our-stuff.html

Yes, Top Fuel drag cars use race fuel, but nevertheless, they often have even lower compression like 6.5:1. In theory, the lower the compression then the higher the effective compression ratio can be. The trade off is lag and efficiency. Some drag cars use nitrous and anti-lag to fill the void left by low compression.

My valve springs failed several weeks ago and I had to get them replaced outside warranty. It was expensive and I thought about replacing the short block while the engine is already out and taken apart with a low compression built block from AIG so that I could increase the boost. But I didn't do it in the end. I love this car so much that I want to keep it as my daily driver. I thought it was not going to be reliable enough for a daily driver. I wish I could buy another 86/BRZ just to experiment with the second.

[Irace86.2.0]

Originally Posted by mrg666

My valve springs failed several weeks ago and I had to get them replaced outside warranty. It was expensive and I thought about replacing the short block while the engine is already out and taken apart with a low compression built block from AIG so that I could increase the boost. But I didn't do it in the end. I love this car so much that I want to keep it as my daily driver. I thought it was not going to be reliable enough for a daily driver. I wish I could buy another 86/BRZ just to experiment with the second.

Several people have dropped compression without any negative effects. It really depends by how much, but yea, there are definite benefits to dropping compression. If there weren’t benefits then high compression forced induction motors from the factory would exist and be prevalent.