Forged STOCK compression turbo build??
03-05-2009, 11:21 AM
#41
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There is a lot of useless crap being strewn about in this thread. The FACT is lower compression does NOT hurt your power output per pound of boost nearly as much as the OP claimed.
Lower compression not only allows you to run more boost, but it allows more timing advance at any given boost level. Running stock 10.3:1 compression on a built motor would be retarded with the safety margin and minimal power loss from dropping the comp to 9:1.
Why do FI cars run lower compression??? Because the safety margin advantage far outweighs the small power bump that you get from higher compression.
Lower compression not only allows you to run more boost, but it allows more timing advance at any given boost level. Running stock 10.3:1 compression on a built motor would be retarded with the safety margin and minimal power loss from dropping the comp to 9:1.
Why do FI cars run lower compression??? Because the safety margin advantage far outweighs the small power bump that you get from higher compression.
03-05-2009, 11:49 AM
#42
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am doing cams on mine this season.
Stock cams fall like a cliff above 6200rpm...you can see it in the VTR dyno.
With cams, all else being equal, I should make the same power on a little less boost.
We'll see.
[QUOTE=athenG;7034757You made mid 500whp just above 15psi with stock cams and I assume no headwork with your 8:5:1 piston. Imagine what power you will make with cams, headswork and increase redline.[/QUOTE]
Stock cams fall like a cliff above 6200rpm...you can see it in the VTR dyno.
With cams, all else being equal, I should make the same power on a little less boost.
We'll see.
[QUOTE=athenG;7034757You made mid 500whp just above 15psi with stock cams and I assume no headwork with your 8:5:1 piston. Imagine what power you will make with cams, headswork and increase redline.[/QUOTE]
03-05-2009, 07:26 PM
#44
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I've never heard of predetonation.
I think a few of you are mixing preignition and detonation into one all inclusive idea when in reality they are two different occurrences. They are related, and detonation can cause preignition but they happen at different times in the engine's cycle.
Detonation occurs after the sparkplug ignites the mixture. The rapid rise in pressure and increase in heat in the combustion chamber cause unstable areas, usually the farthest from the spark plug, to ignite. Hot spots can be a cause also. Instead of a nice pressure increase in the cylinder, this causes a very abrupt spike and this causes the noise you or your knock sensor hears, and broken ring lands. Detonation also destroys the boundary layer of insulating air that surrounds all surfaces of the combustion chamber. This causes the typical eroded appearence of the piston tops or combustion chamber, a decrease in EGT, and an increase in coolant temperature as the engine is absorbing more heat. Detonation can be a cause of preignition inducing hot spots.
Now, before anyone goes off the deep end about EGT decreasing during detonation, keep this in mind. Engine management systems that reference the knock sensor will at least retard ignition timing and possibly add fuel. This reduction in ignition timing is what causes the EGTs to rise.
Preignition is what happens when the air/fuel mixture is ignited before the spark plug fires. Rather than thinking of this happening 5* before the spark plug fires, think of it happening more like 90* before the spark plug fires, as the mixture is still being compressed. Preignition destroys parts very fast with no warning. Holed pistons, bent and broken rods, severe engine damage with no sign of any issue. I suffered an engine failure due to preignition, it wasn't pretty.
On the part of compression ratios and boost, it all works down to the dynamic compression ratio with boost taken into account that you can get away with on the fuel you have access to. Anywhere from 8:1 to 9:1 have been what most find to work best with pump gas and typical desired boost levels. I have run DCRs with boost of over 20:1 on pump gas with my DSM, but each engine's design dictates where the limit is on each fuel.
Using a dynamic compression ratio (w/boost) calculator, you can see the differences in data.
The following variables were used.
Bore 86mm (3.779")
Stroke 81.4mm (3.204")
Rod Length 144.2 (5.677")
Intake valve close 64* ABDC (this is the cam in the 0* advanced position)
With the stock DE static compression ratio of 10.3:1 the dynamic compression ratios were as follows at the listed boost levels.
10psi 13.80
15psi 16.63
19.2psi 18.98
The next use 9.0:1.
13.5psi 13.85
15psi 14.59
19.2psi 16.65
Now we can look at these numbers and see that on the lower CR setup, we don't get to the same DCR until 13.5psi. At that boost level there is surely a more dense mixture in the combustion chambers so more power will be made with proper tuning. I couldn't find a specific acceptable DCR for pump gas as again, it changes with engine design, but if 17 were our limit, you can see how the lower compression ratio lets you run a substantially more boost. That 4psi more boost will net you much more power than the 1.3 point loss in CR. Calculators using the stock HP and CR estimate about a 4% loss in power (before boost) from the CR change.
I think a few of you are mixing preignition and detonation into one all inclusive idea when in reality they are two different occurrences. They are related, and detonation can cause preignition but they happen at different times in the engine's cycle.
Detonation occurs after the sparkplug ignites the mixture. The rapid rise in pressure and increase in heat in the combustion chamber cause unstable areas, usually the farthest from the spark plug, to ignite. Hot spots can be a cause also. Instead of a nice pressure increase in the cylinder, this causes a very abrupt spike and this causes the noise you or your knock sensor hears, and broken ring lands. Detonation also destroys the boundary layer of insulating air that surrounds all surfaces of the combustion chamber. This causes the typical eroded appearence of the piston tops or combustion chamber, a decrease in EGT, and an increase in coolant temperature as the engine is absorbing more heat. Detonation can be a cause of preignition inducing hot spots.
Now, before anyone goes off the deep end about EGT decreasing during detonation, keep this in mind. Engine management systems that reference the knock sensor will at least retard ignition timing and possibly add fuel. This reduction in ignition timing is what causes the EGTs to rise.
Preignition is what happens when the air/fuel mixture is ignited before the spark plug fires. Rather than thinking of this happening 5* before the spark plug fires, think of it happening more like 90* before the spark plug fires, as the mixture is still being compressed. Preignition destroys parts very fast with no warning. Holed pistons, bent and broken rods, severe engine damage with no sign of any issue. I suffered an engine failure due to preignition, it wasn't pretty.
On the part of compression ratios and boost, it all works down to the dynamic compression ratio with boost taken into account that you can get away with on the fuel you have access to. Anywhere from 8:1 to 9:1 have been what most find to work best with pump gas and typical desired boost levels. I have run DCRs with boost of over 20:1 on pump gas with my DSM, but each engine's design dictates where the limit is on each fuel.
Using a dynamic compression ratio (w/boost) calculator, you can see the differences in data.
The following variables were used.
Bore 86mm (3.779")
Stroke 81.4mm (3.204")
Rod Length 144.2 (5.677")
Intake valve close 64* ABDC (this is the cam in the 0* advanced position)
With the stock DE static compression ratio of 10.3:1 the dynamic compression ratios were as follows at the listed boost levels.
10psi 13.80
15psi 16.63
19.2psi 18.98
The next use 9.0:1.
13.5psi 13.85
15psi 14.59
19.2psi 16.65
Now we can look at these numbers and see that on the lower CR setup, we don't get to the same DCR until 13.5psi. At that boost level there is surely a more dense mixture in the combustion chambers so more power will be made with proper tuning. I couldn't find a specific acceptable DCR for pump gas as again, it changes with engine design, but if 17 were our limit, you can see how the lower compression ratio lets you run a substantially more boost. That 4psi more boost will net you much more power than the 1.3 point loss in CR. Calculators using the stock HP and CR estimate about a 4% loss in power (before boost) from the CR change.
03-06-2009, 06:47 AM
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/\ nice.. much better explanation. So with stock compression (not that I'm going with it) the max boost will be around 15psi to stay under the limit of 17DCR...
I use the Wallaceracing Dynamic Compression Calc and plugged in some of the value you gave and this is what I got if I want to use a 9:5:1 piston
http://www.wallaceracing.com/dynamic-cr.php
Static compression ratio of 9:5:1.
Effective stroke is 2.49 inches.
Your dynamic compression ratio is 7.22:1 .
Your dynamic cranking pressure is 139.06 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 19.2 PSI is 16.65 :1.
V/P (Volume to Pressure Index) is 70
So if you say a good limit for our motor is 17DCR then a 9:5:1 piston should be capable of handling psi up to 20, I guess... Either way nice post.
I use the Wallaceracing Dynamic Compression Calc and plugged in some of the value you gave and this is what I got if I want to use a 9:5:1 piston
http://www.wallaceracing.com/dynamic-cr.php
Static compression ratio of 9:5:1.
Effective stroke is 2.49 inches.
Your dynamic compression ratio is 7.22:1 .
Your dynamic cranking pressure is 139.06 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 19.2 PSI is 16.65 :1.
V/P (Volume to Pressure Index) is 70
So if you say a good limit for our motor is 17DCR then a 9:5:1 piston should be capable of handling psi up to 20, I guess... Either way nice post.
Last edited by athenG; 03-06-2009 at 06:52 AM.
03-06-2009, 07:08 AM
#46
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Thanks BlinkerFluid, that's what I was trying to explain in much more simple terms....That is exactly why even in a race setup on race gas, you don't want high static motor compression.
rr_z33 read Blinker's post, race cars with boost do NOT use high compression motors for that very reason.
rr_z33 read Blinker's post, race cars with boost do NOT use high compression motors for that very reason.
03-06-2009, 09:04 AM
#47
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/\ nice.. much better explanation. So with stock compression (not that I'm going with it) the max boost will be around 15psi to stay under the limit of 17DCR...
I use the Wallaceracing Dynamic Compression Calc and plugged in some of the value you gave and this is what I got if I want to use a 9:5:1 piston
http://www.wallaceracing.com/dynamic-cr.php
Static compression ratio of 9:5:1.
Effective stroke is 2.49 inches.
Your dynamic compression ratio is 7.22:1 .
Your dynamic cranking pressure is 139.06 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 19.2 PSI is 16.65 :1.
V/P (Volume to Pressure Index) is 70
So if you say a good limit for our motor is 17DCR then a 9:5:1 piston should be capable of handling psi up to 20, I guess... Either way nice post.
I use the Wallaceracing Dynamic Compression Calc and plugged in some of the value you gave and this is what I got if I want to use a 9:5:1 piston
http://www.wallaceracing.com/dynamic-cr.php
Static compression ratio of 9:5:1.
Effective stroke is 2.49 inches.
Your dynamic compression ratio is 7.22:1 .
Your dynamic cranking pressure is 139.06 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 19.2 PSI is 16.65 :1.
V/P (Volume to Pressure Index) is 70
So if you say a good limit for our motor is 17DCR then a 9:5:1 piston should be capable of handling psi up to 20, I guess... Either way nice post.
I got 17.55 when using 9.5:1 with 19.2psi boost. Did you use the same intake valve close angle?
Just keep in mind guys, the DCR of 17 I used as an example is just a number I pulled out of the air for a solid comparison value, I didn't intend to make it seem as if 17 is limit for our engines as I don't know it. I just used 17 as IF it were the limit.
I just want that to be completely clear.
Last edited by BlinkerFluid; 03-06-2009 at 09:11 AM.
03-06-2009, 09:52 AM
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I did use the same value you just posted, as a matter of fact I did the 9:0:1 compression calculation just to make sure the calculator give the same value as the one you came up with. I did it again and I keep getting the same 16.65:1 ratio with the 9:5:1 piston
Oh well, gotcha on the 17 DCR #
Oh well, gotcha on the 17 DCR #
03-10-2009, 07:03 AM
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alright there is a lot being thrown around in the thread, and im honestly scared to enter and receive a blow to the head.
but i have had this same question, as i am in the infancy of my build, literally tearing it all down, and buying the rods/pistons this week, so a lot of research is going into this. I think the original poster of this thread is really just trying to figure out if stock compression, and FI is possible, and possibly, if it is safe.
After reading through, it seems like stock compression will work, and is fairly safe, with a modest build, and modest numbers. I personally would like to get about 400-420rwhp and call it a day, so this may be a viable option for me. It also seems like most people are saying, you're in there already, why not just spend the money, buy the pistons with a lower CR and be 100% in the clear for streetable applications, and if you want to go big later. Am i wrong?
I would really like to see some answers to the original question. Is it possible, with better rods, to run stock compression, and forced induction safely? Obviously this wouldn't be a good idea for crazy numbers, but for a basic, daily driver, 10-15psi, would this be a good idea, or are you setting yourself up for a blown engine further down the road?
but i have had this same question, as i am in the infancy of my build, literally tearing it all down, and buying the rods/pistons this week, so a lot of research is going into this. I think the original poster of this thread is really just trying to figure out if stock compression, and FI is possible, and possibly, if it is safe.
After reading through, it seems like stock compression will work, and is fairly safe, with a modest build, and modest numbers. I personally would like to get about 400-420rwhp and call it a day, so this may be a viable option for me. It also seems like most people are saying, you're in there already, why not just spend the money, buy the pistons with a lower CR and be 100% in the clear for streetable applications, and if you want to go big later. Am i wrong?
I would really like to see some answers to the original question. Is it possible, with better rods, to run stock compression, and forced induction safely? Obviously this wouldn't be a good idea for crazy numbers, but for a basic, daily driver, 10-15psi, would this be a good idea, or are you setting yourself up for a blown engine further down the road?
03-10-2009, 07:08 AM
#50
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alright there is a lot being thrown around in the thread, and im honestly scared to enter and receive a blow to the head.
but i have had this same question, as i am in the infancy of my build, literally tearing it all down, and buying the rods/pistons this week, so a lot of research is going into this. I think the original poster of this thread is really just trying to figure out if stock compression, and FI is possible, and possibly, if it is safe.
After reading through, it seems like stock compression will work, and is fairly safe, with a modest build, and modest numbers. I personally would like to get about 400-420rwhp and call it a day, so this may be a viable option for me. It also seems like most people are saying, you're in there already, why not just spend the money and be 100% in the clear for streetable applications, and if you want to go big later. Am i wrong?
I would really like to see some answers to the original question. Is it possible, with better rods, to run stock compression, and forced induction safely? Obviously this wouldn't be a good idea for crazy numbers, but for a basic, daily driver, 10-15psi, would this be a good idea, or are you setting yourself up for a blown engine further down the road?
but i have had this same question, as i am in the infancy of my build, literally tearing it all down, and buying the rods/pistons this week, so a lot of research is going into this. I think the original poster of this thread is really just trying to figure out if stock compression, and FI is possible, and possibly, if it is safe.
After reading through, it seems like stock compression will work, and is fairly safe, with a modest build, and modest numbers. I personally would like to get about 400-420rwhp and call it a day, so this may be a viable option for me. It also seems like most people are saying, you're in there already, why not just spend the money and be 100% in the clear for streetable applications, and if you want to go big later. Am i wrong?
I would really like to see some answers to the original question. Is it possible, with better rods, to run stock compression, and forced induction safely? Obviously this wouldn't be a good idea for crazy numbers, but for a basic, daily driver, 10-15psi, would this be a good idea, or are you setting yourself up for a blown engine further down the road?
The long answer is, if you are going for only 400-420whp, the stock motor will be just fine. If you are going for higher than that, then you definitely want the extra safety margin of STRONGER, lower compression pistons to complement your stronger rods. There's a reason that no one just replaces the rods, because the stock pistons are just as susceptible to damage as the stock rods.
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