Does Boost Blow the motor?
ccrain, you are 100% correct and I feel sorry for you getting bashed by so many people that just follow the crowd and don't understand what's going on.
The whole straw comment...that made me
The 'straw' in this case is our engines. That is a FIXED constant. The straw is not changing size.
Everyone sees a compressor map of a big turbo and they see that at 20 psi it can flow a crapload of air and just assume that's why a big turbo makes more power as the same psi as a small turbo. The fact is that the compressor map is just that...a map for that compressor. You fit your engine to it by calculating the CFM that your engine (irregardless of the turbo) can flow.
As you said, there are two main reasons: lower VE and reduced temps...that's it. Plain and simple. CFM is going to be the same between the smaller turbo and the bigger turbo at the same boost pressure when connected to the same engine (at a constant VE..in actuality the bigger turbo's hotside will increase VE). It's the VE and temp difference that changes the mass airflow, with the bigger turbo having a higher mass airflow than the smaller one.
Everyone get's hung up on comparing the stock turbo at it's choke limit, like 23 psi at redline, where it's spitting out flames basically. The CFMs the same....it's just a very, very hot CFM with a very low VE, which results in a lower mass airflow for the smaller turbo. The bigger turbo still has a relatively cool charge, along with a bigger hotside (bigger VE).
A good test would be to run a stock turbo or 50 trim or something small and a huge turbo like a 35r, 40R, etc, at 6psi and see how much difference in power there would be (keeping timnig, AFR, etc constant). My bet would be it would be pretty small...actually I can probably figure it out exactly.
The fact of the matter is that airflow (CFM) through an engine is dictated by the following equation:
Airflow (CFM) = PR[RPM*V.E.*Cid/3456]
PR=Pressure ratio=(boost in psi+atmos(psi))/atmos(psi)
RPM = RPM of engine
V.E. = volumetric efficiency at RPM being measured
Cid=cubic inch displacement= 122 for our 2.0L engines
To get pounds of air: n (lbs/min) = P (psia) x V (CFM) x 29 / (10.73[ft3·psi· °R-1·lb-mol-1] x T)
P=atmospheric pressure
In the CFM equation, the only thing that changes CFM for the bigger turbo is VE. For the corresponding mass airflow, the only thing that changes for the bigger turbo is reduced T (due to increased compressor efficiency).
I would have to look at the compressor maps of two turbos, but I bet there is even a boost level where the smaller turbo would produce more power than the bigger turbo, due to it being more efficient at that PR(keeping RPM, timing, AFR) the same.
I don't know why so many people don't grasp this concept...but to have 20 people coming in here laughing at you and bashing you with stupid comments is just stupid.
JohnBradley is the only one who tried to explain things. But, I think he isn't even taking into the account that the engine is a fixed constant...the CFM stays the same, with respect to VE. It's mass airflow that is changing for the reasons mentioned.
Edit: There are other equations that are involved as well, such as IC efficiency, which is compounded and makes the hotter outlet temps of the smaller turbo make even less power, but this is as simple as I can state it for easy understanding.
The whole straw comment...that made me
The 'straw' in this case is our engines. That is a FIXED constant. The straw is not changing size.
Everyone sees a compressor map of a big turbo and they see that at 20 psi it can flow a crapload of air and just assume that's why a big turbo makes more power as the same psi as a small turbo. The fact is that the compressor map is just that...a map for that compressor. You fit your engine to it by calculating the CFM that your engine (irregardless of the turbo) can flow.
As you said, there are two main reasons: lower VE and reduced temps...that's it. Plain and simple. CFM is going to be the same between the smaller turbo and the bigger turbo at the same boost pressure when connected to the same engine (at a constant VE..in actuality the bigger turbo's hotside will increase VE). It's the VE and temp difference that changes the mass airflow, with the bigger turbo having a higher mass airflow than the smaller one.
Everyone get's hung up on comparing the stock turbo at it's choke limit, like 23 psi at redline, where it's spitting out flames basically. The CFMs the same....it's just a very, very hot CFM with a very low VE, which results in a lower mass airflow for the smaller turbo. The bigger turbo still has a relatively cool charge, along with a bigger hotside (bigger VE).
A good test would be to run a stock turbo or 50 trim or something small and a huge turbo like a 35r, 40R, etc, at 6psi and see how much difference in power there would be (keeping timnig, AFR, etc constant). My bet would be it would be pretty small...actually I can probably figure it out exactly.
The fact of the matter is that airflow (CFM) through an engine is dictated by the following equation:
Airflow (CFM) = PR[RPM*V.E.*Cid/3456]
PR=Pressure ratio=(boost in psi+atmos(psi))/atmos(psi)
RPM = RPM of engine
V.E. = volumetric efficiency at RPM being measured
Cid=cubic inch displacement= 122 for our 2.0L engines
To get pounds of air: n (lbs/min) = P (psia) x V (CFM) x 29 / (10.73[ft3·psi· °R-1·lb-mol-1] x T)
P=atmospheric pressure
In the CFM equation, the only thing that changes CFM for the bigger turbo is VE. For the corresponding mass airflow, the only thing that changes for the bigger turbo is reduced T (due to increased compressor efficiency).
I would have to look at the compressor maps of two turbos, but I bet there is even a boost level where the smaller turbo would produce more power than the bigger turbo, due to it being more efficient at that PR(keeping RPM, timing, AFR) the same.
I don't know why so many people don't grasp this concept...but to have 20 people coming in here laughing at you and bashing you with stupid comments is just stupid.
JohnBradley is the only one who tried to explain things. But, I think he isn't even taking into the account that the engine is a fixed constant...the CFM stays the same, with respect to VE. It's mass airflow that is changing for the reasons mentioned.
Edit: There are other equations that are involved as well, such as IC efficiency, which is compounded and makes the hotter outlet temps of the smaller turbo make even less power, but this is as simple as I can state it for easy understanding.
Ok glade we got the answer to turbo efficiency.
BUT back on topic, If its the tq that blows the motor would tuning for less tq make it possible to run more boost to get more hp at the same tq?
BUT back on topic, If its the tq that blows the motor would tuning for less tq make it possible to run more boost to get more hp at the same tq?
but then have more top end hp by adding back in timing, which would increase tq but not more than the peak tq. So you would get less peak tq with less hp in the area with more hp after that.
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I think it's accepted that when people talk, in the context we are, about torque killing a motor, they are speaking of peak torque, where cylinder pressure is the greatest.
When people in this context talk about HP, they mean making decent torqu at high RPM.
When people in this context talk about HP, they mean making decent torqu at high RPM.
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It's all about VOLUME of air. The reason bigger turbos make more power & torque, at the same boost as a smaller one, is simply that they deliver more air. Assuming proper tuning of course. Tuning is just optimizing the fuel and timing to match with the airflow being delivered. Got to have the AIR first. Thats why we spend so much money to get it.
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It's all about VOLUME of air. The reason bigger turbos make more power & torque, at the same boost as a smaller one, is simply that they deliver more air. Assuming proper tuning of course. Tuning is just optimizing the fuel and timing to match with the airflow being delivered. Got to have the AIR first. Thats why we spend so much money to get it.
I'm not even gonna get into this again!
It's all about VOLUME of air. The reason bigger turbos make more power & torque, at the same boost as a smaller one, is simply that they deliver more air. Assuming proper tuning of course. Tuning is just optimizing the fuel and timing to match with the airflow being delivered. Got to have the AIR first. Thats why we spend so much money to get it.
So, instead of just reiterating everything over and over, let me ask you a question, since you seem to think it's based on the VOLUME of air and a bigger turbo can flow a bigger volume of air when attached to a fixed volume engine.
How much VOLUME of air can you flow through a 2L engine (per 2 revolutions to keep it easy)?
Hint: The answer is in the question
Let's make it even simpler...how much VOLUME of air can you fit in a 2L metal container?
Hint: Again the answer is in the question.
Does it matter what is filling the engine or container with the air? Is the volume of the container changing? Can the volume of your engine change? If so, you have a pretty special engine...one of those new warping balloon-like ones. Maybe get a patent for a variable displacement engine.
As explained the volume of the engine is a constant. The volume airflow per unit time can change when/if the VE is greater for the bigger turbine compared to the smaller turbine, but not simply because a bigger turbo can flow more volume of air based on it's compressor map. It's the mass airflow that is changing, which is dependent on the greater VE (if any) and the reduced temperature (if any).
Last edited by l2r99gst; Feb 26, 2010 at 12:23 PM.
It took a while for you to get it, so why not drop some additional gain knowledge on another forum member....
The larger turbo has 'more airflow' in the sense that it flows and produces more dense air flow and not just blowing hot air into your motor like the stock turbo does @ 23 PSI... There are many other reason why a larger turbo produces more power than a smaller turbo at the same given psi, and they've been thoroughly covered in this post...
Some analogy's that were used, (some people take them so literally
) is the straw, the balloon, etc... The basic fact is that a small turbo @ 23 PSI is not producing the same power producing airflow that a larger turbo is at the same @ 23 PSI.
The larger turbo has 'more airflow' in the sense that it flows and produces more dense air flow and not just blowing hot air into your motor like the stock turbo does @ 23 PSI... There are many other reason why a larger turbo produces more power than a smaller turbo at the same given psi, and they've been thoroughly covered in this post...
Some analogy's that were used, (some people take them so literally
) is the straw, the balloon, etc... The basic fact is that a small turbo @ 23 PSI is not producing the same power producing airflow that a larger turbo is at the same @ 23 PSI.
Last edited by Philthy748; Feb 26, 2010 at 02:42 PM.
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Now I will concede that I may have been wrong by saying that the difference was nominal between the two in all scenarios. I think, as others stated in this thread, on a very souped up high VE motor, 20 psi might be a very high boost pressure to achieve for a stock motor because of the large airflow demands.
That said, the scenario we were speaking about was always assuming both turbos were within their efficiency range. And 20 psi for a stock turbo on souped up, big cam motor might not be in the stock turbos efficiency range.
Well it's all here to read. l2r99gst explained it perfectly. But in essence, the answer is the same as my very first post... reduced back pressure, and cooler charge air.
Now I will concede that I may have been wrong by saying that the difference was nominal between the two in all scenarios. I think, as others stated in this thread, on a very souped up high VE motor, 20 psi might be a very high boost pressure to achieve for a stock motor because of the large airflow demands.
That said, the scenario we were speaking about was always assuming both turbos were within their efficiency range. And 20 psi for a stock turbo on souped up, big cam motor might not be in the stock turbos efficiency range.
Now I will concede that I may have been wrong by saying that the difference was nominal between the two in all scenarios. I think, as others stated in this thread, on a very souped up high VE motor, 20 psi might be a very high boost pressure to achieve for a stock motor because of the large airflow demands.
That said, the scenario we were speaking about was always assuming both turbos were within their efficiency range. And 20 psi for a stock turbo on souped up, big cam motor might not be in the stock turbos efficiency range.
