Compression VS. Boost
Joined: Jan 2003
Posts: 10,868
Likes: 1,058
Compression vs Boost
Effective Compression Be sure and check out the dynamic effective compression page too.
((boost psi / 14.7) + 1) x motor compression = effective compression.
hope that helps
fltsfshr
Effective Compression Be sure and check out the dynamic effective compression page too.
((boost psi / 14.7) + 1) x motor compression = effective compression.
hope that helps
fltsfshr
Joined: Jan 2002
Posts: 5,069
Likes: 4
From: Barkhamsted
I think that what slimjim was trying to get to, to put it very simple is that wheter you run:
low CR + high boost or High CR + lower boost that either way one or the other, compression or boost will yield the same max power.
And Aus did answer the question and said to see which one you can run the most advanced timing before getting detonation.
An to go back to motor A + B, you wouldn't have to run as much timing on the motor with high CR + lower boost because of the high compression and therefore the benefit of added torque as a result.
Would it amount to the same power curve as motor B?
low CR + high boost or High CR + lower boost that either way one or the other, compression or boost will yield the same max power.
And Aus did answer the question and said to see which one you can run the most advanced timing before getting detonation.
An to go back to motor A + B, you wouldn't have to run as much timing on the motor with high CR + lower boost because of the high compression and therefore the benefit of added torque as a result.
Would it amount to the same power curve as motor B?
Registered User
Joined: Jul 2005
Posts: 1,670
Likes: 0
From: Gie
I did some more research and things are getting clearer.
Up to a certain point, the low boost/high CR and high boost/low CR motors will produce the same numbers. Off boost response will be significantly greater in the higher compression motor. This lends lower boost applications to yield the best results with stock compression. Medium to high boost applications provide too much pressure too soon in the compression stroke with the stock compression ratio, running the risk of detonation. For the same reason, timing can be advanced more in the lower compression motor to get a "better" and "earlier" burn. The same timing advnace in the high compression motor would result in detonation.
Thanks for the input!
Up to a certain point, the low boost/high CR and high boost/low CR motors will produce the same numbers. Off boost response will be significantly greater in the higher compression motor. This lends lower boost applications to yield the best results with stock compression. Medium to high boost applications provide too much pressure too soon in the compression stroke with the stock compression ratio, running the risk of detonation. For the same reason, timing can be advanced more in the lower compression motor to get a "better" and "earlier" burn. The same timing advnace in the high compression motor would result in detonation.
Thanks for the input!
Thread Starter
Registered User
Joined: Sep 2005
Posts: 1,195
Likes: 0
From: Tampa, FL
atleast were getting some responses... i understand what you guys are saying... its not about the motor's CR, but the effective compression in the combustion chamber... as clearly see in the chart... (excellent resource by the way, thanks)
i fully understand that off-boost power curve is better with higher CR... so we can start the discussion where boost picks up...
my question is, outside of the increase in the effective compression, is there any other stress inducing elements on the motor for a turbo application?
if not, taking the previous example of motor A and motor B both with 300 hp. (timing adjusted for 0 detonation)... even after starting where boost picks up, motor A will be faster because it gets to full boost faster.. (only to 8psi vs. 12psi)... and if i look at a dyno graph, will motor A have a bigger power band?... and motor A will be at no greater stress than motor B while in boost...?
if so, why is everyone suggesting lowering compression and adding boost for more power? why is every production F/I engine in the world produced with a low compression motor? if a car will perform better with higher compression + lower boost and be just as safe why are manufacturers making lower compression motors with higher boost pressures? im wondering if there's a reliability issue?
i fully understand that off-boost power curve is better with higher CR... so we can start the discussion where boost picks up...
my question is, outside of the increase in the effective compression, is there any other stress inducing elements on the motor for a turbo application?
if not, taking the previous example of motor A and motor B both with 300 hp. (timing adjusted for 0 detonation)... even after starting where boost picks up, motor A will be faster because it gets to full boost faster.. (only to 8psi vs. 12psi)... and if i look at a dyno graph, will motor A have a bigger power band?... and motor A will be at no greater stress than motor B while in boost...?
if so, why is everyone suggesting lowering compression and adding boost for more power? why is every production F/I engine in the world produced with a low compression motor? if a car will perform better with higher compression + lower boost and be just as safe why are manufacturers making lower compression motors with higher boost pressures? im wondering if there's a reliability issue?
Moderator
Joined: Oct 2000
Posts: 30,809
Likes: 15
From: Sydney
With high compression you reach your power ceiling earlier than with low compression. Sure A and B both produce 300hp, but A can't be turned up much more. And if you do want to up boost on A you will need to add fuel which is a no-no for emissions and fuel economy.
Personally I like the A option and that is what I have got. Stock engine with low boost, quick spool turbo. I will turn up the boost when I get more familiar with tuning but I'll probably use the boost controller to taper off boost in the top end to avoid problems. Only tuning will tell.
Personally I like the A option and that is what I have got. Stock engine with low boost, quick spool turbo. I will turn up the boost when I get more familiar with tuning but I'll probably use the boost controller to taper off boost in the top end to avoid problems. Only tuning will tell.
Registered User
Joined: Jul 2005
Posts: 1,670
Likes: 0
From: Gie
I think I can answer this finally.
Assume that both cars are tuned with the EXACT same timing advance. Motor A will make more power sooner than Motor B. This is due to its effective compression off-boost. Starts at 11 instead of 9.
It won't matter at low boost levels, but the timing can be advanced on both motors to yield even more power. Once we reach a certain effective compression, probably around 17 from what information I have gathered, larger timing advance on Motor A (the higher CR motor) will cause detonation sooner than the same timing advance on Motor B. Even if both engines reach the same effective compression, Motor A reaches pressures that will cause detonation sooner. Bigger power is easier to obtain with Motor B when the boost gets really boostin'
More research to come.
Assume that both cars are tuned with the EXACT same timing advance. Motor A will make more power sooner than Motor B. This is due to its effective compression off-boost. Starts at 11 instead of 9.
It won't matter at low boost levels, but the timing can be advanced on both motors to yield even more power. Once we reach a certain effective compression, probably around 17 from what information I have gathered, larger timing advance on Motor A (the higher CR motor) will cause detonation sooner than the same timing advance on Motor B. Even if both engines reach the same effective compression, Motor A reaches pressures that will cause detonation sooner. Bigger power is easier to obtain with Motor B when the boost gets really boostin'
More research to come.
Moderator
Joined: Oct 2000
Posts: 30,809
Likes: 15
From: Sydney
True, but doesn't really consider real world examples. Actually, I guess it is spot on for an SC with almost linear boost increase, but my turbo reaches peak boost before 4000rpm and stays there until readline. So turbo choice is very important.
