Is boost pressure directly proportional to BHP?
Discussion
Hi All
Been reading threads on boost pressures recentl (996TT mostly but this post applies to all turbo cars) and got me thinking of a question.
Generally 996 TT X50's run at 0.9 bar and have 450bhp.
Now someone like DMS will take a standard car up to around 500bhp & when asked, they say boost will be 0.9 bar.
So 450 & 500bhp on the same bar ? I know that the X50 car uses different turbo's, but I assume this is only so that the higher boost can be run in greater safety than on the standard turbo's.
Maybe this answers my own question & boost bar is not directly related to bhp?
If it is, then a standard car runs 420bhp at 0.8 bar and an X50 car runs 450bhp at 0.9 bar, so the 30bhp extra = 0.1 bar increase. So a 500 BHP car has 80bhp over a standard car, so around 0.3 more bar, so boost should be 1.1 bar on cars with 500bhp, but all the tuners say their 500bhp cars run no more than 0.9bar (1.0 occasionally on overboost)
Sorry for the long thread - comments welcome.
Ro.
Been reading threads on boost pressures recentl (996TT mostly but this post applies to all turbo cars) and got me thinking of a question.
Generally 996 TT X50's run at 0.9 bar and have 450bhp.
Now someone like DMS will take a standard car up to around 500bhp & when asked, they say boost will be 0.9 bar.
So 450 & 500bhp on the same bar ? I know that the X50 car uses different turbo's, but I assume this is only so that the higher boost can be run in greater safety than on the standard turbo's.
Maybe this answers my own question & boost bar is not directly related to bhp?
If it is, then a standard car runs 420bhp at 0.8 bar and an X50 car runs 450bhp at 0.9 bar, so the 30bhp extra = 0.1 bar increase. So a 500 BHP car has 80bhp over a standard car, so around 0.3 more bar, so boost should be 1.1 bar on cars with 500bhp, but all the tuners say their 500bhp cars run no more than 0.9bar (1.0 occasionally on overboost)
Sorry for the long thread - comments welcome.
Ro.
Higher input pressures do generally mean higher horsepower figures but if the boost goes much above 1 bar you start to run into problems with detonation, sealing, etc.
Think of a bigger turbo vs a smaller turbo like the hoses the fire brigade uses vs the hosepipe in your garden. The pressures might be similar but the volumes they flow are different.
Think of a bigger turbo vs a smaller turbo like the hoses the fire brigade uses vs the hosepipe in your garden. The pressures might be similar but the volumes they flow are different.
This isn't quite right - you can run more horsepower by running a greater volume of air at the same pressure, or by making the engine use the air more efficiently.
So a pair of small turbos will produce a given volume of air at a given pressure, whereas a pair of larger turbos could supply a larger volume of air at the same pressure.
More air + more fuel = more power.
Also, you'll often find manufacturers peak boost is not at peak revs, so a tuner could change the boost map so that once peak boost arrives, it stays til the rev limiter. This would also increase power.
Are you with me?
VS
So a pair of small turbos will produce a given volume of air at a given pressure, whereas a pair of larger turbos could supply a larger volume of air at the same pressure.
More air + more fuel = more power.
Also, you'll often find manufacturers peak boost is not at peak revs, so a tuner could change the boost map so that once peak boost arrives, it stays til the rev limiter. This would also increase power.
Are you with me?
VS
Chipping the car isn't just about the waste-gate control (which is what is allowing an increase in boost) but also the whole ignition mappings - just like chipping normally aspirated car.
Interestingly, my 996TT (not x50) is modded by DMS and I see 1.4 bar from the standard turbos very occasionally. Through out the rev range it'll normally hits 1.2 or 1.1bar between 3-4k rpm then settle at 1.0 bar through the rest of the rev range.
To get the 1.4 bar, I was holding @ 1/3 throttle @ 4750 rpm, as the traffic cleared, I floored it and that's when I saw the 1.4 bar.
DMS tell me that the standard turbos fitted to the 996TT are rated at 1.6 bar constant use by their manufacturer.
Since, having the DMS magic, the whole pickup is sharper even at lower RPM.
Gerry
Interestingly, my 996TT (not x50) is modded by DMS and I see 1.4 bar from the standard turbos very occasionally. Through out the rev range it'll normally hits 1.2 or 1.1bar between 3-4k rpm then settle at 1.0 bar through the rest of the rev range.
To get the 1.4 bar, I was holding @ 1/3 throttle @ 4750 rpm, as the traffic cleared, I floored it and that's when I saw the 1.4 bar.
DMS tell me that the standard turbos fitted to the 996TT are rated at 1.6 bar constant use by their manufacturer.
Since, having the DMS magic, the whole pickup is sharper even at lower RPM.
Gerry
If you got 30bhp from 0.1 bar increase a 0.5bar increase would not give 150bhp.
The further you get away from atmospheric pressure, the more you need to overfuel the intake charge and the less efficient the charge becomes.
Up rating the intercooler will help keep it all cooler, but all other fixes such as bigger injectors and retarding the ignition reduce the efficiency of the resultant burn and the last 5% of power costs 95% of the budget. Lowering the compression ratio as you increase the volume of charge inducted would be the next logical step as you can maintain final compression within acceptable limits, burning more intake charge and getting more power with more internal control. But off boost performance really suffers.
All turbos come with a chart that shows their efficiency at a certain pressure and volume of air pumped. Bigger turbos will pump huge volumes at relatively small increases in charge temp but you can grow a beard waiting for them to spool up. Cars fitted with these monsters tend to be very binary to drive. I wouldn't like to drive one that suddenly went from 150bhp to 500+bhp.
The further you get away from atmospheric pressure, the more you need to overfuel the intake charge and the less efficient the charge becomes.
Up rating the intercooler will help keep it all cooler, but all other fixes such as bigger injectors and retarding the ignition reduce the efficiency of the resultant burn and the last 5% of power costs 95% of the budget. Lowering the compression ratio as you increase the volume of charge inducted would be the next logical step as you can maintain final compression within acceptable limits, burning more intake charge and getting more power with more internal control. But off boost performance really suffers.
All turbos come with a chart that shows their efficiency at a certain pressure and volume of air pumped. Bigger turbos will pump huge volumes at relatively small increases in charge temp but you can grow a beard waiting for them to spool up. Cars fitted with these monsters tend to be very binary to drive. I wouldn't like to drive one that suddenly went from 150bhp to 500+bhp.
roshambo said:
911 habit.
DMS tell me that they modify their non-X50 TT's for 0.9 / 1 bar max....1.4 seems very very high.
Ro.
Guess I'm a lucky guy then!
They will say this, to give the minimum guarantee of what they can achieve / managing customer expectations – And it will vary from car to car.
Also, I did say very occasional I get 1.4, mostly I see 1.0 and 1.1bar. – I’m happy with it - Fantastic!
>> Edited by 911 habit on Monday 22 August 15:13
Within the typical mechanical limits of a standard 996TT, boost pressure increases will result in a proportional increase in torque as a result of higher cylinder pressure after combustion. However power is a function of torque and revs, therefore only if you increase the torque at higher rpm (by any method) will you get more power.
As an example JC's 993GT2 made around 540bhp & 800Nm at 1.0 bar, still made 535bhp at 0.95 bar but peak torque was down to 750Nm, at 0.9 bar it made 525bhp & 710Nm. The reason for this apparent discrepancy is due to the efficiency of the turbo's (& cams), which would only just sustain 0.97 bar at 6250rpm for peak power. Dropping the max boost level by 0.1bar therefore has minimal effect at max power rpm but has significant influences on the torque peak at 4000rpm where the turbos are running more efficiently and are easily capable of sustaining well over 1.0 bar.
Does this help?
As an example JC's 993GT2 made around 540bhp & 800Nm at 1.0 bar, still made 535bhp at 0.95 bar but peak torque was down to 750Nm, at 0.9 bar it made 525bhp & 710Nm. The reason for this apparent discrepancy is due to the efficiency of the turbo's (& cams), which would only just sustain 0.97 bar at 6250rpm for peak power. Dropping the max boost level by 0.1bar therefore has minimal effect at max power rpm but has significant influences on the torque peak at 4000rpm where the turbos are running more efficiently and are easily capable of sustaining well over 1.0 bar.
Does this help?
ninemeister said:
Within the typical mechanical limits of a standard 996TT, boost pressure increases will result in a proportional increase in torque as a result of higher cylinder pressure after combustion. However power is a function of torque and revs, therefore only if you increase the torque at higher rpm (by any method) will you get more power.
As an example JC's 993GT2 made around 540bhp & 800Nm at 1.0 bar, still made 535bhp at 0.95 bar but peak torque was down to 750Nm, at 0.9 bar it made 525bhp & 710Nm. The reason for this apparent discrepancy is due to the efficiency of the turbo's (& cams), which would only just sustain 0.97 bar at 6250rpm for peak power. Dropping the max boost level by 0.1bar therefore has minimal effect at max power rpm but has significant influences on the torque peak at 4000rpm where the turbos are running more efficiently and are easily capable of sustaining well over 1.0 bar.
Does this help?
Whilst, as you know I am still flumoxed as to how you achieve 800NM at 1 bar when my fully built RS Tuning twin plug motor only manages an engine dyno measured 736NM with 1.2/1.3bar, I am not sure I follow your reasoning. Surely if the turbos can only manage 0.97bar @ 6250 then they are at their limits and will be producing hotter air than if they were bigger units (say K24/26 like mine) which can easily produce the 1.2bar bar @ 6250rpm and their efficiency will be poor. At the lower end at 4000rpm, the turbos are nearer to their sweet spot and so surely small differences in boost should make little difference to the torque/power at this lower level (since they are well within their operating range and small boost differences should not heat up the air as drastically as at the top end)whereas using your numbers the 0.1bar decrease in boost pressure results in a 50hp drop at 4000rpm whereas at the top end 6250rpm where as you say the turbos are struggling and near their limits so should be producing hotter and hotter air, the drop is only 15hp - Now I've confused myself
I think the point is, that the power figures were being produced in a relatively poor efficiency area of the turbo's i.e. high rpm. So the turbos are not contributing much to that peak bhp figure. So dropping boost isn't going to affect the peak BHP figure. However peak torque is produced lower down the rev range and in a much more pleasent area of the turbos efficiency map. i.e. power at those rpm levels is more sensitive to boost.
Or simpler.... At 7000rpm you get 10 'relative presure' units per 0.1 bar of boost (air if you like), and at 4000 rpm you get 100. So clearly an extra 0.1bar is worth a lot of peek torque...
>> Edited by ultra violent on Monday 22 August 19:16
Or simpler.... At 7000rpm you get 10 'relative presure' units per 0.1 bar of boost (air if you like), and at 4000 rpm you get 100. So clearly an extra 0.1bar is worth a lot of peek torque...
>> Edited by ultra violent on Monday 22 August 19:16
ultra violent said:
I think the point is, that the power figures were being produced in a relatively poor efficiency area of the turbo's i.e. high rpm. So the turbos are not contributing much to that peak bhp figure. So dropping boost isn't going to affect the peak BHP figure. However peak torque is produced lower down the rev range and in a much more pleasent area of the turbos efficiency map. i.e. power at those rpm levels is more sensitive to boost.
Or simpler.... At 7000rpm you get 10 'relative presure' units per 0.1 bar of boost (air if you like), and at 4000 rpm you get 100. So clearly an extra 0.1bar is worth a lot of peek torque...
>> Edited by ultra violent on Monday 22 August 19:16
OK thanks I think I've grasped it - the turbos are working there nuts off right at the top of their range so small decrease in boost has little effect since the air is already v hot and there are other things contributing to the peak hp ? Following this logic, this motor could see 980NM at the 1.2bar I run at 4000rpm - now that would be something !
I started to read compressor maps from here
www.stealth316.com/2-3s-compflowmaps.htm#fm
These are not KKK, but you should get the idea, could not find public KKK maps on a quick google, only commerical site
>> Edited by madou on Monday 22 August 21:40
www.stealth316.com/2-3s-compflowmaps.htm#fm
These are not KKK, but you should get the idea, could not find public KKK maps on a quick google, only commerical site
>> Edited by madou on Monday 22 August 21:40
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