So I have been doing some research and found out some interesting things abou tthe relationship betewwn boost, cubes and airflow that I just thought I would share. Now though I thought I was pretty thurough with my research I do not swear by this as being 10000% fact so if someone with more knowlege than I would like to chime in please do. Except Ron. In short what I found that an increase in displacement or a port/polish job will actually REDUCE the effective boost that is in your cylinders. So lets say that you are running a blower with 6psi of boost on your 6.1L life is grand and you are getting about 20hp per lb of boost. You then swap your motor for a 7.0L stroker and port and polish the heads and intake but do nothing to the blower output. You are no longer making 6psi in the cylnders and are not getting that same 20hp per psi either. Your effective #s in the cylinders may now only be 4psi and 15hp per psi. Make sence?? this is due to the fact that the blower must move a certain amount of air and pressurize the cylinders. Well physics 101 taught us all that when you increase volume and flow rates if you do not increase air volume as well you will loose pressure. Common sence now but not thought about that often I think. Any comments or considerations?? Now the question is that if you know the flow rate of your F/I system and the RPM- boost rate for your engine system how do you calculate the increase of blower rpm you need to keep the same boost lvlin your cylinder settup? Clear as mud? good. duscuss.
This is why they have different size blowers, a Novi 1500 may hit 15lbs on a 6.1, but it will not hit that on a 426. The same goes with a turbo. To further simplify, lets look at this. 62-1 Fullsize / 65 .96 A/R (Turbonetics Turbo) 625 hp (maximum amount of hp able to be produced) This may be 10 lbs of boost on a 6.1, but it will only be about 3 lbs of boost on a 7.0, but either way, you will get 625hp. Now thats assuming you are maxing out the Turbo/Supercharger. Here is another Turbo to look at. HP-70 / 68 .81 A/R 650-800hp This Turbo can be ran at 650hp with 10lbs of boost on a 6.1, then you can change to a 7.0 and run up to 800hp with X amount of boost. So you can see that depending on your specific application, you may already be maxxed out, or you may have room to grow. With you already making 630rwhp, you may not be able to go much higher with the Novi 1500.
Check out this site, it may have some useful information regarding your questions. http://www.enginelogics.com/cmaps.html
Thanks. I researched it and the Novi1500 is rated to flow about 1200cfms of air and is rated at 825hp. I am just concerned that I am not making my correct boost in the cylinders due to the porting and bore over.
825hp x .80=660rwhp. I would say you are pretty close to maxxed out. I think you might be overcomplicating the specifics. 8lbs of boost is 8lbs of boost no matter what. The only thing different is that if your exact setup was on a stock engine, it would probably be making 10-12 lbs of boost. Your engine cant build as much boost because you are getting the air out faster than a stock engine. Its like trying to blow up a raft with a hole in it, it will take really long with a small pump and at some point, the raft will not fill up any further even with the pump on becuase you are releasing the air as quick as its coming in. Now, if you put that same pump on another raft with a smaller hole in it, it will fill up more that the raft with the bigger hole and have a higher psi. So for the raft with the big hole, the only way to get more psi in there is to put a bigger pump(supercharger) or use a smaller raft(engine). I know that is a dumb analogy, but maybe it makes sense to someone else. lol.
Lol not bad Andy. But I have always had this same settup on the blower but what the hell when I come to you guys for a tune we can just check the boost lvl then, and up it alot.
ha sorry I'm just picturing Andy blowing up a raft with a hole in it.. but using no pump.. Sorry Andy just made me laugh (but pretty good analogy)
Or, another way to look at it is, boost is a restriction in the system. The supercharger/turbo is trying to push air into the engine. How effectivly the engine can USE the added air flow will is give you boost(back pressure to the supercharger) You can and will make more power with less boost on a more efficient system(intake,heads, cam, ci, exhaust)
Ok, I got most of that, well actually only a little. So far I think: A blower blows air. When the blower blows more air than is being exhausted it builds pressure. If it's blowing more air than is being exhausted it should continue to build increasing pressure until something pops. This isn't the case since the blower is not perfectly sealed? not perfectly efficient? expecially centrifugal ones? The air sort of builds to some pressure and then the back pressure just increases the "leak" back through the blower? That about right? So if you just spin the blower faster this max point is higher. Spinning the blower faster is limited because they're only designed to spin so fast? Is that right? So if you up the cubes you either need a larger blower or spin the one you got faster. On turbos: Is it correct that you usually have more capacity and a blowoff valve to relieve it? but since it's exhaust driven to go past that you have to turn more rpms and move the power to higher revs? ...I'm so confused... and is Ron so slow because he's blowing into a small tube and doesn't have the lungs for it?
Yes Ron is so slow because he is Ron and its not really his fault, just how it is. You are correct about the logic you posted. Blowers blow air a certain amount per rpm depending on the blower. This intern creates boost pressure in the motor when the engine cannot release the air fast enough therefor making HP. When you increase the size of your motor or its flow rates dramatically you REDUCE the effective boost int he engine. You must then push moe air, either with a bigger blower or more RPMs.
So it would seem that tolerance between the blade and the housing (and/or blade to blade) would be about the same independent of the the size of the blower? So wouldn't a larger sized blower be more efficient than a smaller one? What is the decision between getting a larger blower or spinning a smaller one faster?
Yes a larger blower should be more efficient because it moves more air per rotation. The decision point is for 1 space to install the blower unit and of course cost. Im sure there is a point where the size of a blower could be just too large for a smaller engine and the parasitic loss would be too great to make it efficient but even so I think generally bigger is more efficient.
The supercharger/turbo needs to be sized right for the engine and expected air flow/HP it will be used for. If you spin a small blower to fast it becomes inefficient. You can also overspeed them. Given the fact that the compressor wheel spins in the neigborhood of 50,000 rpms. It's best to stay inside it's design parameter. As far as air backing up and leaking,,, you can wreck a compressor wheel with cavatation.(sort of like a prop in water) The whole blower will shake and vibrate. Not something you want to repeat often.
