I have been puzzled as to why on turbo cars with boost set at a stable rate the horse power falls off after the peak, usually just as full boost is acheived. I think it is due to VE of the motor, it decreases as rpm increases, even for boosted applications. So if you know your motor can take 400ftlbs at 4000rpm it should ba able to take 400ftlbs at 7000rpm, right? But with a stable boost setting you loose power as rpms rise because not as much of the compressed air can be crammed in to the combustion chamber. So wouldnt you want to ramp up the boost pressure as rpms increase to counteract the decreasing VE. So start at 10psi at 4000rpm and end up at perhaps 12psi at 7000rpm? Sounds like good theory to me, any input or counter points are appreciated?

 

That sounds cool but can that be done?

 

you would have to find a way to variate the boost controls

 

the AVCR does that I think, but never messed with one so not 100% on the functionality. but I'm pretty sure it lets you scale out boost pressure over a range of RPM

 

Yea the E01 lets you do this also. I have see one Supra dyno where the plot was flat from peak to rpm limit. JWT needs to get a freakin move on with my dam ECU, 16 weeks and counting.

 

Believe me its going to be a long wait. Some people get lucky and get their ecu's within 2 weeks. Others get theirs in like 1-3 months, I believe the ones with basic P.O.P have the fastest turnaround.

 

We must have shared brain waves today, as I started talking to Bags (Kirk)about this very thing earlier. I don't see why this wouldn't work, but in order to play with some computer models I have, I'd need to know what the stock VE is for both the VQ30 and VQ35. Anyone have any clue?

 

I was hoping you would see this..

If you and matt could swap notes I bet you two could make 1 hellua of a motor

 

This is possible as a matter of Fact I have the AVC-R and it is RPM specific for boost. the only question here then is..... Lets say 10psi is the threshold of safety for a motor; meaning you could only run 10psi safely without your engine going boom, then after the boost has hit its peak, you would have to start incrementing the boost higher in order to keep that flat curve. Would the threshhold of safety be the same throughout? Or would you be able to boost highe since the VE is now lower?

 

If you max engine limit is 10 PSi, then that's it. Just b/c VE goes down that doesn't mean that you can run higher pressures and temps (aka boost) over your engine safe maximum to try and compensate for the loss in cylinder efficiency. It's a complex dynamic called the law of diminishing returns, and it's slightly different for every motor (even two of the same kind). But if your motor was fully built (say 20+ PSi capable), then you could start at 8 and work up to 15-16 PSi in an attempt to keep the HP/TQ curve flat...

 

Do you know if your egt's drop when your power starts to? If so, this would seem like a safe theory to me, which should be the first concern. If this were true, then i would say to up your boost until you reached your egt range you want to stay away from.

 

I won't claim to know the answer to all the formulas/VE etc.. but from my more simple approach... Why would you not run your max-safe boost the entire time?

If it is a matter of how much hp the engine can make, I would understand. But, if it is a matter of how much boost the internals can take (which it almost always is)- I'd say it is a dif story. Why not hold the maximum amount of safe boost, across the band instead of building up to it? I know the curve may not be as flat (efficiency not perfect throughout), but area under the curve would be greater.

I understand that VE needs more boost as rpms' go up - but lets take this to a more extreme situation. A car that runs 30psi. Lets say perfect VE is 3000rpm@5psi, 7000rpm@30psi. Although that is the most efficient, would you not make more total power(area under the curve) with 3000rpm@30psi, 7000rpm@30psi?

that is my quick thoughts, maybe I have lots to learn

 

i read somewhere that peak torque is where the engine is running most efficiently. i'm not sure if this will help you guys out or not trying to figure out the VE. i think i read that on how stuff works website. i'll try finding it again.

 

Hal, it depends on what your goal is. If you want max HP all the time, then running the highest engine safe PSi all the time would be the way to go I guess. If you just want 350 WHP all the time, then this idea has merit. And I'm not entirely sure that max PSi all the time would be more area under the curve over this method, but it could very well be. Imagine having the same HP and TQ from 2500 to 7000 RPM...I find that rather appealing.

I'm also guessing that a variable rate turbo (VATN Technology ) would probably be the most effective way to accomplish this task. Asking a stock turbo to do some of the things we're talking about would be very difficult, at best.

 

The "boost" level is not the limit, its the stress on the internals etc. caused by the power you are generating. Wouldn't a nitrous'd or NA motor have the same limit. Obviously with boost you need to fuel it, cool it and control detonation etc. but power is power, no matter how it is produced. Think about it this way, as VE decreases cylinder pressures during the compression stroke should drop because the cylinder is not being filled as well. The motor may be spinning 7000 rpm but it is not taking in as much air per cylinder per revolution as it is at 3500rpms, even with boost due to a lower VE, so less power will be produced per cylinder combustion at 7000 than 3500. So up the boost to counter act the loss of VE.

 

Yes thats it!

 

Would you have to increase fuel for the added boost at the rpm boost is increased?

 

You're right about less air being in the cylinders at higher RPM (I'm a doofus and forgot), but at 7000 RPM the inertial load of the connecting rods/bearings is huge (vs. 3500 RPM). Adding more boost at these levels could very well be beyond the limits of any stock connecting rods strength. But then again, maybe not. If you double the mixture in the chamber at any RPM, the overall peak pressure would only rise by 20% or so. What you would have to know is if that 20% peak would push the internals beyond their maximum load bearing capabilities...

 

Yes, the same rules apply...more boost/air, more fuel...

 

thanks for clearing that up


matt you and justin ( quicksilver ) need to have some BBQ at my house one weekend and school my ***..lol


Yall seem to be free thinkers. I am glad justin lives nearby





And BTW mardi, I did NOT forget the only reason I have a working VI is because of your efforts. THANK YOU

 

i am not a turbo guy but if the turbo kits have a external wastegate couldn't you get a progressive controller (aeromotive fuel pump controller) and wire it to the wastegate? more voltage at lower rpms keeps the wastgate open(with a variable pressure sensor /greddy)less volts closes it up. its all controlable-

 

a wastegate operates off vacume not voltage but the boost controlers they are talking about which controls the vacume going to the wastegate might be able to what they want them to

 

hes correct the external wastegate works off vacuum. The EC boost controller pulsates the wastegate thru vacuum. It sometimes makes a swoosh swoosh noise bleeding air into the atmosphere. Sounds weird!

 

I know this is off-topic put what source for vaccum do you use to hookup boost controllers, blow-off valves, and wastegates? I heard that the intake manifold was the best place, I am just wondering, maybe if someone has a picture, I really appreciate. I am trying to get the kinks out of the way.