Sorta useless post, how much power per PSI of boost (at least, in my case)

[Pyro]

Yes, I agree the turbo pumping air is a whole different animal. I guess that could be covered with compressor map theories. But a piston engine is a positive displacement pump. And I feel the engine would pull in air more effiecently because of the positive manifold pressure. This could help make up for intake restrictions found in pure NA situations.

 

I believe that "simple formula" that no one likes, is in Corky Bells turbo book. I need to check out that book again!

 

But here is a site from HotRod tech files that claims "15 psi effectively doubles engine displacement". First part of page 1.

 

http://www.hotrod.com/techarticles/engine/113_0312_turbo/index1.html

[mobythevan]

But a piston engine is a positive displacement pump.

 

But it only pumps (pushes) on the exhaust stroke. Otherwise it creates a vacuum in the cylinder that the pressure on the intake side fills.

 

The one case that has always interested me that seems to go against the formula is the northstar boosted engine. Since I haven't built one myself I have to guess that the actuall testers did more mods than they admit to. But the engine stock is 300hp (flywheel I'm sure), 10psi boost yields 650hp (again flywheel I'm sure). Should take more like 16psi boost to make that. The builders as I remember claim all stock components just add boost. Does the northstar just have a head, cam, valve setup that compliments the boost? David Vizard claims in his boost books that when an engine is built with the correct compliment of parts for the given type of boost (super, centrif, turbo, etc) that you see a much wider margin between it NA hp figure and boosted hp figure. That is what I get from reading his books anyway. Maybe some of the people with higher numbers that Bastaad remembers had some other mods, maybe they didn't. But I can tell you that it is hard for me to remember which hp numbers from this site go with which persons build and what stage of the build they were on at the time of dyno.

 

Anyway, interesting thread.

[SleeperZ]

Yes' date=' I agree the turbo pumping air is a whole different animal. I guess that could be covered with compressor map theories. But a piston engine is a positive displacement pump. And I feel the engine would pull in air more effiecently because of the positive manifold pressure. This could help make up for intake restrictions found in pure NA situations.

 

I believe that "simple formula" that no one likes, is in Corky Bells turbo book. I need to check out that book again!

 

But here is a site from HotRod tech files that claims "15 psi effectively doubles engine displacement". First part of page 1.

 

http://www.hotrod.com/techarticles/engine/113_0312_turbo/index1.html[/quote']

You are correct, Corky Bell starts with the simple formula you describe, but he takes into account density changes, mostly due to temperature from turbo and intercooler inefficiencies, and also engine volumetric efficiency without explanation, just "pumping loss" expressed as a percentage.

[Sean73]

I think Corky' Bells pressure ratio formula is only intended a simple guildine for planning purposes, such as injector & fuel pump sizing, compressor sizing, etc... For that, it works well. He never claims for it to be an accurate theoretical formula.

 

Most of the time, it gets you in the ballpark, if you know the RWHP of your car at zero boost. If you don't know the base HP accurately, the error gets multiplied the more boost you run. If you're off by 10 HP in the base HP, you're off 20 HP at 14.7 psi.

 

Since you guys disagree on the base HP of the L28ET by +/- 25 HP, this translates to +/- 50 HP at 14.7 psi, so no wonder there's no agreement on Corky's formula!

 

Not knowing the base HP accurately aside, the real question is: Does power rise linearly with boost? If it does, then a simple formula like Corky's should work fine, assuming you know the right inputs-- a BIG assumption.

 

Bastaad, I looked at your dyno charts on your website:

 

1) 200 HP / 234 TQ @ 10 psi (P=1.68 )

2) 210 HP / 270 TQ @ 12 psi (P= 1.82)

3) 233 HP / 303 TQ @ 14 psi (P=1.95)

 

Comparing runs 2 and 3 with the first run:

 

+8% boost = +5% HP and +15 % TQ.

+16% boost = +16% HP and +29% TQ.

 

HP is anamolously down 3% in run #2. HP probably peaked early due to some other problem. But, with the exception of this data point, the relationship between boost and HP/TQ looks pretty linear to me! Given this empirical linear relationship, we could probably guestimate with ease how much HP & torque you'll make with higher boost (all other factors being equal).

 

I noticed your HP dropped in the second 2 runs after 4100 RPM, but pulled to 5000+ RPM in the first run. If you could have tuned your A/F ratios, I think you would have yielded more consistent HP results.

 

"All other factors being equal" assumption is an important thing to remember when applying Corky's formula. Increasing boost does not lead to predictable power increases, when lot's of other variables are changing (, ie. A/F ratio, charge temperature, IC efficiency, etc...). The formula works when implemented correctly.

 

So to answer you're original question, Bastaad, 16% more boost got you 16% more HP compared to your baseline run. 'nuf said.

[Pyro]

I think the biggest reason the formula didn't work so well with Bastaad525 engine is during the 7 and 10 psi there was no IC and in the 12 psi run there were A/F issues (too lean).

 

Both issues, no ic and/or lean fuel ratios messed up the formula real badly. Air temps and fuel ratio need to be consistant for it to work.

 

However, the 14 psi run, with an IC and more fuel pressure bought the engine's power up closer to where the formula estimated it should be at. 233hp at 14 psi means 120hp base hp.

[Guest bastaad525]

actually the 14psi run isn't even a good one to use for this discussion. The problem with that run was that my boost would hit 14psi at 3000rpm and then would DROP almost exactly 1psi per 1000rpm, down to 10-11psi by the end of the run (6000rpm). That is a big part of why my hp peaks at much lower RPM, because boost and torque fall off very soon/fast.

 

I've been bouncing back and forth with how I have the boost controller set up. There are two ways to run it, and I couldn't decide which to go with. I either set it up so that it is getting the boost signal from the stock location on the turbo housing, which results in almost no spike, but then it has the problem with the boost falling off. OR I'll get the signal from the intake manifold, which gives a bit more spike, like 2psi, but after that boost stays steady to redline. It was really stupid of me to go back and forth like that, and has made it even harder for me to draw useful comparison from my many dyno runs, since they are just about evenly split between me running the MBC one way or the other every time I tested.

 

I finally decided I'd put up with the spiking so I could have the consistent boost to redline, and that's how it has been set up and will remain. It hits 13psi and STAYS at 13psi all the way to redline, so I assume my torque curve stays higher longer, and should mean it's putting down a bit more power, than what I got on that 14psi run. I can only estimate, with some basic math just looking at how much torque I have gained or lost on average, for 1psi, from several dyno test'n'tune sessions. I drew up a new dyno graph from that 14psi run, with what I THINK the torque curve would look like with a nice constant boost, based on those numbers. This is why I say I SHOULD be putting down 240-245hp, as opposed to 233hp from that 14psi run. And I am being very conservative there, the first time I redrew the graph, I added 15ftlbs of torque for each 1psi lost (so 15ftlbs at 5000rpm, 30ftlbs at 6000rpm) and the HP numbers came out MUCH higher. But I figure my turbo is not going to add that power linearly as revs increase so I didn't either. After I adjusted the torque curve in that way, the hp peak is right at 5000rpm where it's supposed to be (and where it WAS on my first few dyno tests) instead of at 4200rpm. It's all just wild *** guesstimation, but I feel confident that I'm probably VERY close, and I'm not going to go spend another $75 at the dyno just to find out. At least not for a little while

 

Here is my slightly altered dyno sheet

 

 

As you can see, I put the torque LOWER at 3k (since it's 13psi instead of 14), then they are even at 4k (since boost had fallen down to 13psi by 4k), and then is 15ftlbs higher at 5000rpm (15ftlbs is the LEAST amount of torque I have seen gained for 1psi of boost on my motor). Again, maybe just wild *** guesstimation but it's based on experience from (lot's of) other dyno runs.

 

The second set of numbers you list was at only 210hp because I was running WAY too lean, 12psi intercooled with still 100% stock EFI. I shouldn't even have bothered dynoing at that point, but I was curious how much difference the I/C made. I also had the boost controller set up the same on that day as on the 14psi runs, with the boost falling off, but at that point I WANTED it that way, since I figured less boost at higher RPM was safer since I KNEW I was running it lean. So that also probalby accounts for the low hp vs. high tq number there as well. That run was before I had my RRFPR. By the way, Pyro, based on my own experiences, I HIGHLY doubt that JUST adding an I/C to an otherwise bone stock 280zxt motor at stock boost will add 30hp to the wheels. In my case adding an I/C AND some boost didn't even come CLOSE to 30hp.

 

I had done another run at 12psi on a dynapack dyno, it was between the 2nd and 3rd runs you listed. But that time I had the boost controller hooked to the manifold instead, so it was 12psi steady, and also had added the RRFPR so wasn't running lean anymore, and there I made 226rwhp. It seems everyone I've talked to who knows anything about Dynapack say that, like the Mustang dyno, the Dynapacks tend to give lower numbers than dynojet dynos. All the other runs I've ever done were dynojets.

 

And Pyro, listen to Sean73, he is THE MAN when it comes to turbo math... I still have a text file on my desktop with all kinds of formula's and calculations that he gave me. I"m not gonna argue about that formula anymore, it seems we're almost all in agreement that it CAN be accurate, but generally is not since it doesn't account for the variables. But sure, it is a good ballpark estimate. It's NOT perfect though.

[Tony D]

Ahhh, we have digressed into the 'dynamic' compressor feeding a 'positive displacement' compressor discussion...

 

There is actually a lot of information on that system, as in industrial compression systmes it is very common. Two different typse of compressor feeding a single system for a higher pressure with greater efficiency that could be achieved with one system or the other... (hmmmm, does this sound familiar?)

 

I get into nasty arguments over the "vacuum" versus "pressure" arguement regarding cylinder filling. All I can say at this point is that if an intake system was TRULY working on "vacuum" they would be designed VERY differently! Even on N/A engines, the intakes are UNIVERSALLY designed using pressure-flow dynamics. It is indeed atmospheric pressure PUSHING air into the cylinder. If you follow with the Turbocharger, you see that the VE increase on NON-BOOST areas where the turbo is only aiding ABSOLUTE PRESSURE flow, you see the torque get pumped up, and realize the pumping losses are affected even when the turbo is not pumping enough air to go to positive boost. It is STILL boosting even while still in a "Vacuum" state by the compound gauge in the cockpit...

 

I digress. I go to sleep now.

[MJLamberson]

I am about to bump a VERY old thread, why you ask? because there are many many people asking questions about this sort of stuff these days, and this was a very good read. It doesnt matter who's formula I agreed with, they all excited me about my build. Enjoy.

[jc052685]

bump again

[240zwannabe]

the theory behind it is...take your desired psi divide it by 14.7 to get that into a barometric measurement, add one since your above sea level then multiply that by base hp. it will give you an approximate hp. obviously with mods like rrfpr, pistons, blah blah blah your base hp will be higher.

 

for example: 10psi/14.7 = .68+ 1 = 1.68*180 = 302 thats theoretically a stock l28et on .68 bar's power....just a theory so i do not claim it to be correct