turbo cam
08-16-2010, 06:12 PM
#81
But Mike, the cylinder isn't closed. It has a leak (exh valve). Making that cylinder bigger, the leak bigger, or making the compressor smaller can all result in zero restriction/boost.
Envision a tiny turbo for a little briggs motor and now use it to feed a 600 ci mountain motor. How much boost/restriction do you suppose that you'll have? I'm going to guess, zero and you probably wont even get out of vacuum (because of that pesky leak).
Envision a tiny turbo for a little briggs motor and now use it to feed a 600 ci mountain motor. How much boost/restriction do you suppose that you'll have? I'm going to guess, zero and you probably wont even get out of vacuum (because of that pesky leak).
08-16-2010, 07:28 PM
#83
In a process plant, it is very common to have a positive displacement compressor feeding another positive displacement compressor (I've seen up to 6 in series). If you guys walk in and tell them that the 2nd positive displacement compressor is really just a restriction, they're going to laugh you out of the plant.
08-16-2010, 07:43 PM
#85
Repeat after me. . . positive pressure IN the cylinder is a GOOD thing.
. . . that's because a Positive Displacement device is pumping air AWAY from the compressor. . . not because you got rid of a "restriction".
In a process plant, it is very common to have a positive displacement compressor feeding another positive displacement compressor (I've seen up to 6 in series). If you guys walk in and tell them that the 2nd positive displacement compressor is really just a restriction, they're going to laugh you out of the plant.
Not trying to be a smart ***. Perhaps restriction is an over simplified term for what is happening but it is the easiest way for people to understand it. You are over complicating it for the sake of a semantics argument and definitely confusing the noobs.
08-16-2010, 07:45 PM
#86
In a process plant, it is very common to have a positive displacement compressor feeding another positive displacement compressor (I've seen up to 6 in series). If you guys walk in and tell them that the 2nd positive displacement compressor is really just a restriction, they're going to laugh you out of the plant.
The 2nd compressor IS a restriction since it adds NOTHING to Mass Flow. It only increases the pressure and slows down the flow rate.
If your talking 1 cylinder, but you are dealing with a V8. All the pulses are dampened to the point to where its a continuous flow thru the motor as a whole system. By the time 1 cylinder is filled, another is opened creating a new "leak" to the closed system.
08-16-2010, 07:49 PM
#87
BTW Mike, the First compressor in the series determines total mass flow. Unless you add in mass after it externally (via some kind of checkvalve or whatever) the total air mass will not change.
08-16-2010, 08:44 PM
#90
Do you have a shop air compressor? When the compressor is running and you're building pressure, do you consider the tank a restriction? If so, then I simply give up trying to explain this to you.
Once upon a time, someone was running 8 psi boost (plenum). Only 6 psi was getting into the cylinder. He replaced the heads and cam and it dropped to 7 psi boost. He concluded that he removed a restriction, which I agree with. However, now, 7 psi was getting into the cylinder and he gained power. He extrapolated this to say that ALL boost was a measure of restriction, which is incorrect.
A restriction doesn't do work, nor is it closed-ended. A crappy intake port or small throttle body is a restriction. Positive pressure could indicate a restriction (in the case of overly restrictive heads), but it could also just indicate a higher density charge freely entering the cylinder without restriction.
Speaking of which. . . if you put a larger throttle body on your boosted engine (removing a restriction), would the boost increase or decrease? As I said earlier, I believe that where boost is measured throws people off. If you could measure it in the cylinder at IVC, you would find that removing restrictions in the valve timing and port actually increase, rather than decrease boost.
A small hole with high frictional losses is a restriction. Packing high density air into a cylinder is not a restriction. These 2 processes are fundamentally different. To build on INTMD8's post, boost is more correctly viewed as extra atmospheric pressure available for power. If atmospheric pressure were 30 psia instead of 14.7, would you say that atmosphweric pressure is a measure of restriction??? Or for that matter, is our 14.7 psia a measure of restriction???
Last edited by engineermike; 08-16-2010 at 08:51 PM.
08-16-2010, 09:23 PM
#91
Man engineers! LOL
All nice theories. Problem is we have seen big numbers, hp and fast ets and mph from various cams since FI seems to largely compensate for cams, heads anyway.
What might help is some mag like Hitech performance doing some tests with like normal split, reverse split, no split and some lsa variations and guess some lifts as well. Course then could still argue that on a different setup with different backpressure etc that a different cam would have still be better.
Does it really matter that much.I think its most important to pick a cam with qualities you like. Do you like lope or not. Do you want really high lift but then you will be changing out valve springs all the time or might get valve float.
Do you want an easy to tune cam with no low speed surging or you don't care.
Few hp here and there not going to matter all that much in the big turbo builds like Big birds or mine. When you have access to way more hp than you can ever possibly hook up especially on the street it becomes just waste of time worrying about little thing like perfect cam. But cams do affect powerband and that is worth some thinking about.
And if you do make more power at less boost that can sometimes be advantageous too.
And thats also why guys put on good heads on fi builds ,they are more efficient you can make more power at less boost.
But hey theorize away. Just remember most engineers can't explain how bumblebees fly!
All nice theories. Problem is we have seen big numbers, hp and fast ets and mph from various cams since FI seems to largely compensate for cams, heads anyway.
What might help is some mag like Hitech performance doing some tests with like normal split, reverse split, no split and some lsa variations and guess some lifts as well. Course then could still argue that on a different setup with different backpressure etc that a different cam would have still be better.
Does it really matter that much.I think its most important to pick a cam with qualities you like. Do you like lope or not. Do you want really high lift but then you will be changing out valve springs all the time or might get valve float.
Do you want an easy to tune cam with no low speed surging or you don't care.
Few hp here and there not going to matter all that much in the big turbo builds like Big birds or mine. When you have access to way more hp than you can ever possibly hook up especially on the street it becomes just waste of time worrying about little thing like perfect cam. But cams do affect powerband and that is worth some thinking about.
And if you do make more power at less boost that can sometimes be advantageous too.
And thats also why guys put on good heads on fi builds ,they are more efficient you can make more power at less boost.
But hey theorize away. Just remember most engineers can't explain how bumblebees fly!
08-16-2010, 09:26 PM
#92
Someone tell me if my thinking is wrong here. I want lots of power 800+ through a TH400. So to get that I would rather use a big turbo that flows a lot vs a small turbo that I would need to run high boost numbers to reach that goal. So in a case like that the big turbo should make my number at a much lower boost than a smaller one that needs to be run hard to get my number. Saving me from pushing water on stock casting heads.
08-16-2010, 09:34 PM
#93
Once upon a time, someone was running 8 psi boost (plenum). Only 6 psi was getting into the cylinder. He replaced the heads and cam and it dropped to 7 psi boost. He concluded that he removed a restriction, which I agree with. However, now, 7 psi was getting into the cylinder and he gained power. He extrapolated this to say that ALL boost was a measure of restriction, which is incorrect.
A restriction doesn't do work, nor is it closed-ended. A crappy intake port or small throttle body is a restriction. Positive pressure could indicate a restriction (in the case of overly restrictive heads), but it could also just indicate a higher density charge freely entering the cylinder without restriction.
Speaking of which. . . if you put a larger throttle body on your boosted engine (removing a restriction), would the boost increase or decrease? As I said earlier, I believe that where boost is measured throws people off. If you could measure it in the cylinder at IVC, you would find that removing restrictions in the valve timing and port actually increase, rather than decrease boost.
A small hole with high frictional losses is a restriction. Packing high density air into a cylinder is not a restriction. These 2 processes are fundamentally different. To build on INTMD8's post, boost is more correctly viewed as extra atmospheric pressure available for power. If atmospheric pressure were 30 psia instead of 14.7, would you say that atmosphweric pressure is a measure of restriction??? Or for that matter, is our 14.7 psia a measure of restriction???
08-16-2010, 11:28 PM
#94
Someone tell me if my thinking is wrong here. I want lots of power 800+ through a TH400. So to get that I would rather use a big turbo that flows a lot vs a small turbo that I would need to run high boost numbers to reach that goal. So in a case like that the big turbo should make my number at a much lower boost than a smaller one that needs to be run hard to get my number. Saving me from pushing water on stock casting heads.
You can get around lag some with various strategies for off the line. Six speed cars can use antilag and two steps , autos do pretty good with just two steps and proper stall. Both manuals and autos can use nitrous to spool up. And the newest turbo designs have things like HTA that give quicker spool than the older designs letting you run bigger turbo.
and as you said smaller turbos have to work harder possibly overspinning and dying prematurely on bigger engines. Pushing water I thought read though was less pure boost number and more tuning issues.
Still less boost should still be easier on things like couplers maybe on plastic intakes like my fast. I like a sort of middle ground. Smaller twin turbos for pretty fast spool up and broad power band yet their combined airflow still gives tons of potential hp more than I will ever use. Traction is my problem not power.
08-17-2010, 07:36 AM
#95
Someone tell me if my thinking is wrong here. I want lots of power 800+ through a TH400. So to get that I would rather use a big turbo that flows a lot vs a small turbo that I would need to run high boost numbers to reach that goal. So in a case like that the big turbo should make my number at a much lower boost than a smaller one that needs to be run hard to get my number. Saving me from pushing water on stock casting heads.
I have learned that you should also pay close attention to ALL losses in the system. The intercooler can kill your e/i pressure ratio, as can a marginal muffler or downpipe.
Mike
08-17-2010, 07:47 AM
#96
Someone tell me if my thinking is wrong here. I want lots of power 800+ through a TH400. So to get that I would rather use a big turbo that flows a lot vs a small turbo that I would need to run high boost numbers to reach that goal. So in a case like that the big turbo should make my number at a much lower boost than a smaller one that needs to be run hard to get my number. Saving me from pushing water on stock casting heads.
An oversized turbo will need more exhuast to build boost. This is not the chicken and the egg here. We know that before we can make boosted power with a turbo, we must first make NA power to get into boost. The time between NA power and boosted power is lag.
So, an oversized turbo will need more RPMs in order to get the pressure on the hot side required to spin the turbine to make the boost. If you are just looking for max power on a dyno, then a big turbo will peak out the highest. We are talking shut wastegate, max boost, highest number possible.
On the other hand, an undersized turbo will spool very quickly and give you more power under the curve (at a low rpm) at the cost of reduced max boost.
Note that I used the words undersized and oversized. Again talking extremes to make a point. In reality, there are lots of compressor/exh housing combos and a wide range of different ways to skin the cat. It all depends on the combo. I think it's best for someone that is new to this to find a recipe that works and stick to it. At least it gives you a base line of where you should be so that you know if YOUR combo is working. Trust me when I tell you that once it's all together and running is once you start working out the bugs. It's not the end, it's the beginning .Having a slightly under/over sized compressor/exh housing will be the least of your problems for a while. By the time it's all worked out, you'll have a much better grasp on it and will likely want to modify or change the turbo anyways.
08-17-2010, 07:55 AM
#97
Having 14.7 psia on the intake and exhaust side of an engine is NOT indicative of a restriction, but having 19.7 psia on the intake and exhaust side IS indicative of a restriction??? How is it any different? So, if our atmospheric pressure rose 2 psi, then suddenly every NA engine would have 2 psi worth of restriction in our engines?
Last edited by engineermike; 08-17-2010 at 08:16 AM.
08-17-2010, 08:14 AM
#98
Less the restriction with better cam and heads, and the boost (as measured in the plenum) goes down, but boost (as measured just beyond the intake valve) goes up. The difference here is removing a restriction results in zero dP. Removing restrictive intake runners, ports, valves still results in positive pressure because the cylinder is pressurized even with no restrictions leading up to it.
The motor absolutely can ingest air at the same pressure at which it is supplied. You guys are confusing density and flow.
There is where you are flat wrong (perhaps the root of all this). Density=PV/RT. Density is directly proportional to Pressure. Therefore, Density is proportional to Boost. Period. Boost is not a measure of restriction . . it is a measure of the density of charge.
So, to recap, you believe that putting a larger throttle body on a supercharged combo would result in a net loss in boost pressure? You sure? Wanna think that through some more?
Ok, sooo. . .
Having 14.7 psia on the intake and exhaust side of an engine is NOT indicative of a restriction, but having 19.7 psia on the intake and exhaust side IS indicative of a restriction??? How is it any different? So, if our atmospheric pressure rose 2 psi, then suddenly every NA engine would have 2 psi worth of restriction in our engines.
The motor absolutely can ingest air at the same pressure at which it is supplied. You guys are confusing density and flow.
There is where you are flat wrong (perhaps the root of all this). Density=PV/RT. Density is directly proportional to Pressure. Therefore, Density is proportional to Boost. Period. Boost is not a measure of restriction . . it is a measure of the density of charge.
So, to recap, you believe that putting a larger throttle body on a supercharged combo would result in a net loss in boost pressure? You sure? Wanna think that through some more?
Ok, sooo. . .
Having 14.7 psia on the intake and exhaust side of an engine is NOT indicative of a restriction, but having 19.7 psia on the intake and exhaust side IS indicative of a restriction??? How is it any different? So, if our atmospheric pressure rose 2 psi, then suddenly every NA engine would have 2 psi worth of restriction in our engines.
08-17-2010, 09:25 AM
#99
Having 14.7 psia on the intake and exhaust side of an engine is NOT indicative of a restriction, but having 19.7 psia on the intake and exhaust side IS indicative of a restriction??? How is it any different? So, if our atmospheric pressure rose 2 psi, then suddenly every NA engine would have 2 psi worth of restriction in our engines?
A 350 CI engine at 6000 with 100% VE moves approximately 607 CFM. I dont care if you have 40 psi of atmosphere, the motor will not move air any faster. So if the turbo is ingesting 900 CFM of air, and the motor is only moving 607 CFM, how is the motor not a restriction? If it wasn't a restriction, the motor would move 900 cfm of air, and the boost gauge would read 0.
