7.5:1 too low???
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Joined: Dec 1969
Posts: 10,745
From: Buffalo, New York
Lonnie is a smart guy and experienced. I have read his posts for a long time and learned from them. So, none of this personal. Just a discussion of the facts as I see them and my own experiences with a large number of blower cars.
Let's take a hypothetical motor that has a displacement of 700cc/cylinder and a 70cc combustion space. The CR will be ~10:1. Operating NA, even if the VE is 100%, it can't take in more than a 700cc intake charge (yes, I do know that NA VE may exceed 100% but I am simplifying to make a point). If we raise the CR by decreasing the combustion space to 60cc it still takes in the same volume (and mass) of intake charge. The new CR is ~11.7:1. The mass of the intake charge is the same, so there is the same amount of fuel to burn. The combustion efficiency is higher due to the greater compression, and the power is therefore higher. In fact, it will be ~2% higher. But the mass of fuel available to burn is fixed by the displacement and the VE.
How is forced induction different? Well, I am going to have to ignore a lot of fine points, but bear with me. Under boost, the density of the intake charge is higher. And we are no longer limited to 100% VE. In the NA situation, you generallly cannot take in a volume of intake charge > the displacement, as that would mean a VE of >100% Yes, as I stated above, I know that the best NA motors do operate above 100% VE over a narrow rev band, but that's not relevant to the illustration and most street or street/strip motors never see 100% VE at all. So, let's take our hypothetical motor at 11.6:1 CR and put a blower on it. As we know, blower motors operate far above 100% VE. So, we fill the 700cc swept volume PLUS the 60cc combustion space with the intake charge. The intake charge is denser as well. So, the mass of the intake charge is MUCH larger (as is the cylinder pressure just before spark ignition) and with a given level of combustion efficiency, the motor will make a lot more hp. Now, let's drop the CR back to 10:1 by enlarging the combustion space to 70cc. The volume (and mass) of the intake charge is now going to be at least 10/760 = 1.3% greater. The actual mass of the intake charge at 130% VE would be at least 13/760 = 1.7% greater than with the lower CR and that's not even taking the higher charge density into account. Yes, the combustion efficiency will be lower than with a higher CR, but the mass of the intake charge will be greater. In fact, in the ranges we are talking, they nearly cancel out and there is minimal hp loss with decreased CR in a blower application.
So, the way all this works in the dynamic environment of a running engine is a lot more complicated. But the above is the basic theory that supports the observation made by me and by many others: lowering the CR in a boosted motor has much less effect on power output than it does on an NA setup. A very simple way to explain this is that the cylinder pressure just before spark ignition in an NA motor decreases in direct proportion with CR but that in the case of forced induction, the decrease is much less due to the "decoupling" of displacement and the mass of the intake charge.
"Drivability" is subjective. It amuses me to see people worry about "drivability" on a board where nobody thinks there is such a thing as cam that is too big! My point is that lowering the CR by 1 - 1.5 will not make the car unpleasant to drive. The change, even in an NA motor, isn't huge though it's noticible. But if it's got a big blower, you WILL see increased MAP even at very light throttle. This partially compensates for the lower CR, and of course, you can always open the throttle a little more. And a lower CR allows even MORE peak boost. This, in turn, means that you will see more positive MAP at even lighter load/smaller throttle openings due to the pullied up blower.
I have never tested a combo where the only change was lowering the CR. Lonnie's example doesn't blow my argument however. A ~4.5:1 decrease in CR is huge. But even there, it did go faster with enough boost to compensate. We are talking about much smaller differences. So, there isn't a huge difference either way, but if you have a big enough blower and the correct cam choice, more boost and a lower static CR is a better choice to a point. I don't know "how low is too low". But we are running a bunch of pump gas combos with big boost and no intercooling reliably and making good hp with static CR in the ~8:1 range. Race cars that run race gas are a different animal as the boost limit for a given CR is much higher.
In the specific example being discussed. A lot depends on the blower being used. If you are already maxing out your blower, lowering the CR will hurt just as much as it does NA because the mass of the intake charge will be "fixed" by the blower capacity.
So, here's my guidelines for pump fuel.
6psi - stock compression
10psi - ~9.5:1
15psi - ~8.5:1
20psi - ~7.5 - 8.0:1
Rich Krause
Let's take a hypothetical motor that has a displacement of 700cc/cylinder and a 70cc combustion space. The CR will be ~10:1. Operating NA, even if the VE is 100%, it can't take in more than a 700cc intake charge (yes, I do know that NA VE may exceed 100% but I am simplifying to make a point). If we raise the CR by decreasing the combustion space to 60cc it still takes in the same volume (and mass) of intake charge. The new CR is ~11.7:1. The mass of the intake charge is the same, so there is the same amount of fuel to burn. The combustion efficiency is higher due to the greater compression, and the power is therefore higher. In fact, it will be ~2% higher. But the mass of fuel available to burn is fixed by the displacement and the VE.
How is forced induction different? Well, I am going to have to ignore a lot of fine points, but bear with me. Under boost, the density of the intake charge is higher. And we are no longer limited to 100% VE. In the NA situation, you generallly cannot take in a volume of intake charge > the displacement, as that would mean a VE of >100% Yes, as I stated above, I know that the best NA motors do operate above 100% VE over a narrow rev band, but that's not relevant to the illustration and most street or street/strip motors never see 100% VE at all. So, let's take our hypothetical motor at 11.6:1 CR and put a blower on it. As we know, blower motors operate far above 100% VE. So, we fill the 700cc swept volume PLUS the 60cc combustion space with the intake charge. The intake charge is denser as well. So, the mass of the intake charge is MUCH larger (as is the cylinder pressure just before spark ignition) and with a given level of combustion efficiency, the motor will make a lot more hp. Now, let's drop the CR back to 10:1 by enlarging the combustion space to 70cc. The volume (and mass) of the intake charge is now going to be at least 10/760 = 1.3% greater. The actual mass of the intake charge at 130% VE would be at least 13/760 = 1.7% greater than with the lower CR and that's not even taking the higher charge density into account. Yes, the combustion efficiency will be lower than with a higher CR, but the mass of the intake charge will be greater. In fact, in the ranges we are talking, they nearly cancel out and there is minimal hp loss with decreased CR in a blower application.
So, the way all this works in the dynamic environment of a running engine is a lot more complicated. But the above is the basic theory that supports the observation made by me and by many others: lowering the CR in a boosted motor has much less effect on power output than it does on an NA setup. A very simple way to explain this is that the cylinder pressure just before spark ignition in an NA motor decreases in direct proportion with CR but that in the case of forced induction, the decrease is much less due to the "decoupling" of displacement and the mass of the intake charge.
"Drivability" is subjective. It amuses me to see people worry about "drivability" on a board where nobody thinks there is such a thing as cam that is too big! My point is that lowering the CR by 1 - 1.5 will not make the car unpleasant to drive. The change, even in an NA motor, isn't huge though it's noticible. But if it's got a big blower, you WILL see increased MAP even at very light throttle. This partially compensates for the lower CR, and of course, you can always open the throttle a little more. And a lower CR allows even MORE peak boost. This, in turn, means that you will see more positive MAP at even lighter load/smaller throttle openings due to the pullied up blower.
I have never tested a combo where the only change was lowering the CR. Lonnie's example doesn't blow my argument however. A ~4.5:1 decrease in CR is huge. But even there, it did go faster with enough boost to compensate. We are talking about much smaller differences. So, there isn't a huge difference either way, but if you have a big enough blower and the correct cam choice, more boost and a lower static CR is a better choice to a point. I don't know "how low is too low". But we are running a bunch of pump gas combos with big boost and no intercooling reliably and making good hp with static CR in the ~8:1 range. Race cars that run race gas are a different animal as the boost limit for a given CR is much higher.
In the specific example being discussed. A lot depends on the blower being used. If you are already maxing out your blower, lowering the CR will hurt just as much as it does NA because the mass of the intake charge will be "fixed" by the blower capacity.
So, here's my guidelines for pump fuel.
6psi - stock compression
10psi - ~9.5:1
15psi - ~8.5:1
20psi - ~7.5 - 8.0:1
Rich Krause
Registered User
Joined: Aug 1998
Posts: 520
From: Perryopolis, Pa
Rich,
You are absolutely correct, without a doubt, that you can make more ultimate power with lower compression & higher boost.
But.... all things being equal, the higher compression will make a better driving car. Also, at lower boost levels (approx 12# & lower), the higher compression car will make more power (assuming that you do not repulley the blower to compensate).
If running a 230 or bigger cam, high stall converter & various other non-daily driver type components, drivability in my opinion is already less than ideal. I know that many view this type of drivability as "acceptable", but compared with the stock vehicle, it is a big step backwards. Most are a tolerable tradeoff for the level of performance the average enthusiast is trying to achieve. Given the opportunity for the best of both worlds, I'll take it when possible. The guys with a M6 will appreciate the low end as it is nice to actually use 6th gear besides at highway speeds. My car will accelerate from 900rpm in 6th (35mph) without hesitation in 6th w/ 3.42's where it was unhappy under 1200rpm when stock.
My main reason for this post was to say do not run less compression than you have to.
To reply to the original posted question..... consider 8.5:1 or higher if you only plan to have 15# boost & you should be happy.
As you recommended 8.5:1 for 15#, if non-intercooled, I totally agree. I would say 9:1 at 15# intercooled on a LT1 is very safe. I wouldn't suggest under 8:1 unless planning well over 20#.
9.6:1 on the LS1's also work well w/ 12# intercooled.
You are absolutely correct, without a doubt, that you can make more ultimate power with lower compression & higher boost.
But.... all things being equal, the higher compression will make a better driving car. Also, at lower boost levels (approx 12# & lower), the higher compression car will make more power (assuming that you do not repulley the blower to compensate).
If running a 230 or bigger cam, high stall converter & various other non-daily driver type components, drivability in my opinion is already less than ideal. I know that many view this type of drivability as "acceptable", but compared with the stock vehicle, it is a big step backwards. Most are a tolerable tradeoff for the level of performance the average enthusiast is trying to achieve. Given the opportunity for the best of both worlds, I'll take it when possible. The guys with a M6 will appreciate the low end as it is nice to actually use 6th gear besides at highway speeds. My car will accelerate from 900rpm in 6th (35mph) without hesitation in 6th w/ 3.42's where it was unhappy under 1200rpm when stock.
My main reason for this post was to say do not run less compression than you have to.
To reply to the original posted question..... consider 8.5:1 or higher if you only plan to have 15# boost & you should be happy.
As you recommended 8.5:1 for 15#, if non-intercooled, I totally agree. I would say 9:1 at 15# intercooled on a LT1 is very safe. I wouldn't suggest under 8:1 unless planning well over 20#.
9.6:1 on the LS1's also work well w/ 12# intercooled.
Last edited by Lonnie Pavtis; Jun 10, 2004 at 08:16 AM.
Moderator
Joined: Dec 1969
Posts: 10,745
From: Buffalo, New York
Originally posted by Lonnie Pavtis
Rich,
You are absolutely correct, without a doubt, that you can make more ultimate power with lower compression & higher boost.
But.... all things being equal, the higher compression will make a better driving car. Also, at lower boost levels (approx 12# & lower), the higher compression car will make more power (assuming that you do not repulley the blower to compensate).
If running a 230 or bigger cam, high stall converter & various other non-daily driver type components, drivability in my opinion is already less than ideal. I know that many view this type of drivability as "acceptable", but compared with the stock vehicle, it is a big step backwards. Most are a tolerable tradeoff for the level of performance the average enthusiast is trying to achieve. Given the opportunity for the best of both worlds, I'll take it when possible.
My main reason for this post was to say do not run less compression than you have to.
As you recommended 8.5:1 for 15#, if non-intercooled, I totally agree. I would say 9:1 at 15# intercooled on a LT1 is very safe. I wouldn't suggest under 8:1 unless planning well over 20#.
9.6:1 on the LS1's also work well w/ 12# intercooled.
Rich,
You are absolutely correct, without a doubt, that you can make more ultimate power with lower compression & higher boost.
But.... all things being equal, the higher compression will make a better driving car. Also, at lower boost levels (approx 12# & lower), the higher compression car will make more power (assuming that you do not repulley the blower to compensate).
If running a 230 or bigger cam, high stall converter & various other non-daily driver type components, drivability in my opinion is already less than ideal. I know that many view this type of drivability as "acceptable", but compared with the stock vehicle, it is a big step backwards. Most are a tolerable tradeoff for the level of performance the average enthusiast is trying to achieve. Given the opportunity for the best of both worlds, I'll take it when possible.
My main reason for this post was to say do not run less compression than you have to.
As you recommended 8.5:1 for 15#, if non-intercooled, I totally agree. I would say 9:1 at 15# intercooled on a LT1 is very safe. I wouldn't suggest under 8:1 unless planning well over 20#.
9.6:1 on the LS1's also work well w/ 12# intercooled.
Rich
Registered User
Joined: Aug 1998
Posts: 520
From: Perryopolis, Pa
AdioSS,
You are now not talking about street motors for sure now.
N/A alky motors are usually between 13:1 to above 16:1
Forced induction is generally above 10:1. The evaporation rate of alcohol & the inherent cooling effect produced requires some additional compression & the corresponding heat to work well. Keeping fuel vaporized can be a problem without enough combustion chamber heat.
I have no direct experience w/ running on straight alcohol, but have seen some TA/FC motors with compression in the 10:1 range while running a big screw type charger.
Nice thought if you can afford it. Twice the fuel consumption & all the corrosion problems associated with alky prevent me from experimenting with it.
You are now not talking about street motors for sure now.
N/A alky motors are usually between 13:1 to above 16:1
Forced induction is generally above 10:1. The evaporation rate of alcohol & the inherent cooling effect produced requires some additional compression & the corresponding heat to work well. Keeping fuel vaporized can be a problem without enough combustion chamber heat.
I have no direct experience w/ running on straight alcohol, but have seen some TA/FC motors with compression in the 10:1 range while running a big screw type charger.
Nice thought if you can afford it. Twice the fuel consumption & all the corrosion problems associated with alky prevent me from experimenting with it.
Thread Starter
Registered User
Joined: Sep 2003
Posts: 546
From: Boerne
Well in all reality I am going to use a T-trim because I don't really want to wait on having the turbo setup, though it's not out of the equation. So say I want to run a T-trim on my 383 and I want to make out the blower and this will be intercooled. What compression would you suggest. I want it to make power until it is inefficient. Thanks
Registered User
Joined: Mar 2002
Posts: 3,082
From: San Juan PR
8.8:1
I'd suggest more or less that
I was running 9:1 and 10psi... I went down to a bit only 8.8 to gain a bit more top end when i strapped the ys-trim or even max out the t-trim...
The t-trim should max at around 700rwhp... The supercharger is noticeably bigger than a S-trim.. if the s-trim has done from 600-634 (intimid8) i think the T has 100HP more.
I'd suggest more or less that
I was running 9:1 and 10psi... I went down to a bit only 8.8 to gain a bit more top end when i strapped the ys-trim or even max out the t-trim...
The t-trim should max at around 700rwhp... The supercharger is noticeably bigger than a S-trim.. if the s-trim has done from 600-634 (intimid8) i think the T has 100HP more.
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