I'm curious what you all think is a reasonable guesstimate of how much flywheel HP gain can be expected from increasing boost per psi in the 10-19 psi range.

I'm curious what the guesstimates are based on my set up, where I am not overly spinning the Steig ported Eaton, which is spinning approx 14,500 rpm @ redline, but I am using only a lower +4 pulley with the stock upper and I'm seeing 18-19 psi. While I understand people have previously had less efficient air flow tracks and were only able to get 17-18 psi with BOTH upper and lower pulleys, mine is breathing just fine as evidenced in my 0-100 video showing more power at higher revs and zero boost drop off.

Heat has not been an issue as I now have less heat soak than when totally stock.

This, of course is assuming IATs are well controlled by adding sufficient cooling capacity. For the record, my water/meth injection running a single 375 ml/minute nozzle before the TB is cooling the IATs an estimated 50-70 degrees F and having added the 12" pusher cooling fan made a big difference. I am also running a water wetter -go ahead, throw your tomato(e)s-, and I have an water misting inter cooler mist spray system which can drop the IATs an estimated additional 20 degrees F.

What say you? 9 flywheel HP per psi, or 10, 11, 12 ??

 

I'll guess at 6.9 fwhp/psi....

 

How do you come to this figure?

As a referrence, AVOS @ 23 psi was yielding 23 hp per lb of boost, and as I understand it, this was without injecting water/meth.

I understand the twin screw is much more efficient than an eaton, but I have added approx 70+ degrees F of cooling. I don't think the eaton is only 30% as efficient as the TS. 7 being 30% of 23.

631 AVOS dyno proven HP
370 stock
261 gain

24 psi - 11.6 stock = 11.4 psi gain

261 hp gain / 11.4 psi = 22.9 HP per lb of boost


If anyone else has dynos of increased pressured super chargers, please post your gain and calculation of HP per PSI of boost so we can zero in a little better. Right now we have I think both ends of the spectrum covered with the 23 HP+ per boost for AVOS, and a lower guesstimate of 7 for the Eaton.

I think my ultimate goal is to run about 15 psi on my system with only a mild +1.7 upper pulley and stock lower as a good balance of having added +5 psi overall boost, and maintaining more reliability at 15 psi of boost vs 18-19 makes me a little nervous as that is a lot more than it was designed for. If I already had changed the head gaskets to the 4.2s I wouldn't worry about it, but I don't really want to have to do that right now as I'm getting very busy with more important projects.

I'm trying to figure out if I can still run a 12.9 with only 15 psi with my setup. I know running it down the 1/4 mile is the only way to know for sure, but I was hoping to get a bunch on guesstimates on the HP side, just for general info. Given how well it ran with the 18 psi, I think I have a very good chance of breaking into the 12's with only a mild upper pulley.

 

1 psi of boost increase theoretically adds a maximum of 6.8% horsepower. In reality it's less due to parasitic losses and inefficiency.

 

So "max" of say 370 x .068 = 25 hp per psi of boost
So lets say AVOS's 21 hp per psi for the more efficient Twin screw compressor proves this to be true, and gives a benchmark in the real world.

Would it be a reasonable conservative guess then to say + 12-15 HP per PSI for the less efficient Eaton blower in my situation, where I have added water/meth injection+ other cooling, and I'm only using a lower pulley so I'm not over spinning the blower like people sometimes do with 3 lb upper and +4 lower pulleys do? I have reduced 2 lbs of vacuum on my intake side, and the blower is ported, and has been demonstrated via my 0-100 video to show higher boost (18 psi) at 5500-6000 rpm vs 4500-5000 rpm (17 psi), proving there is no drop off in boost, so my system is not wasting efficiency with an overly restrictive air path, thereby allowing the blower to, well....blow

A big part of my equation is that I am properly controlling the IATs, so, given this is 12-15 hp per PSI an accurate enough guesstimate if my boost ends up being in the 15-19psi range?

 

Next to added temperature (which is lower now due to the water/meth injections), the Eaton is consuming much more power at higher boost levels and also gets less efficient, which you can't change as its inherent to the design of the unit.

So the gains will get progressively lower per extra psi of boost.

Therefore if you want to know you need to go to the dyno.


The Twin-screw has a higher efficiency at a wider/higher boost level obviously, so very different performance characteristics which can't be compared. Also I haven’t dynod the 24 psi yet, the max measurement you use above was about 23 psi, and although I noticed some difference again with 24 psi, I don’t know what the final result on the dyno will be.

 

Avos,

Yes, all good points.

What I want to know is will the car run a 12.9 at the track. This I will find out at the track.

The 23 vs 24 makes the gains even higher per psi from 21 to 23 hp per psi.

I agree that the Eaton consumes more at higher rpm and boost. Steig calculated my blower to be spinning at only 14,500 rpm at redline with the stock upper and +4 lower. It is in this configuration that I am getting 18-19 psi. At 15 psi it will be spinning even slower, so still not terribly inefficient. I have proven unquestionably that adding the +4 lower added A LOT of power all the way to redline.

My biggest gains came from understanding the eaton is a blower and not a compressor, so while it does a decent job at moving the air, it does not compress it well. Applying this has manifested in me reducing 2 lbs of vacuum on the intake side as evidenced by +2 lbs of more boost when I did that, then I gained +1 more psi by injecting the water/meth before the blower, making it run cooler, and giving it denser air which was easier for it to move more efficiently, then I had the blower ported, which gave +1 psi more. Right now, with stock upper and lower pulleys, I'm at +4 psi over stock with the stock blower rpm.

My plan is to try a very mild +1.7ish upper pulley with a total of 15 psi of cool running boost and try to run a 12.9 at street weight.

I am hoping, not knowing, but hoping, this gives me approx 12.5 hp per psi average.

If this is the case, that if including my temperature control cooling mods, if I am able to be only 54% as efficient as the TS, then my +psi gain x 12.5 hp/psi = approx 104 more HP.

If I were running a +3 upper and +4 lower, the blower would consume exponentially more power than currently, but given the performance improvement, it it pretty obvious it is working very well.

Again, this calculation is to give me general info to see if I will be able to get into the 12's with "only" 15 psi. Obviously only running it at the track will tell for sure.

 

Yeah, my calculation was flawed. I'll try again.

I think we have to go back to the unboosted horsepower for the calculation...which I'll estimate is about 220 horsepower. If you boost that to the stock R engine pressure of 11.8 psi, you get 370 horsepower. If you boost it to 19 psi you get 504 horsepower. That works out to 284 horsepower due to boost or 14.9 horsepower per psi. That checks pretty well against the theoretical 6.8% of horsepower per boost. I suppose you could get to the theoretical number but I suspect that is very difficult in the real world...

So, if we input 4050 pounds...the stock weight of the car, 428 horsepower...the rear wheel horsepower from 504 at the flywheel (that assumes only 15% driveline power loss) you'd see a theoretical ET of 12.48sec @ 110 mph...according to the drag racing calculators out there. I usually do worse than the calculators say I should be capable of, but you may be a better driver...

 

Thank you for the effort

My -02 XJ8 is listed at 295 hp

With my XJR I am hoping for a 12.9 or better at approx 4200 lbs as I will race the car as it sits with at least a 1/2 tank of gas, little ol me driving it, and my trunk full of my usual stuff. I think the 370 HP is a fair estimate at the stock 10 psi as my gauge read stock. I ran about a 13.8 stock, so adding 5 psi of boost (if I run 15 psi) I think will put me right around the 12.9 or 13.0 mark. We will see I think the 12.48 is with a really hard and fast launch which I cannot do.

With the 18-19 psi the car feels fast enough for a mid 12.5-12.6, but is definitely slower at 15 psi. Still quite fast at 15, but not the same as 18-19.

 

But remember that your XJ8 has a much higher compression ratio than the XJR. You're starting at a lower level and boosting beyond the XJ8 output....

This is an interesting discussion. Please do post your strip results...and dyno numbers if you do them.

 

Good points.

I will try to post a video of the 1/4 mile when I can get out there
As you can tell from my very opinionated posts, I like video proof to reduce the wiggle room for possible exaggerations.