Supercharging Discussion: Boost Pressure vs. Altitude

[Filin M3]

Originally Posted by Bob MG

Two words.....volcanic ash

no actually three...Fuck Volcanic Ash

The same shit (was trying to upgrade to VT2-600 and 95mm pulley...)

[Bob MG]

Checked boost pressure today.

My calculations were 100% correct

Sea level pressure 1019hpa
5300 ft pressure 839hpa (this can be worked out for any altitude)

The percentage of air pressure at 5300ft compared to sea level is *
derived by 839/1019 = 0,82

I lose 18% at my altitude.

This is the ratio you must apply to the boost a pulley generates at *
sea level.

95mm pulley
6 psi x 0,82 = 4,92 psi

On my runs with the boost gauge connected, the maximum recorded boost *
was 0,33 bar = 4,78 psi
This proves my theory beautifully.

92mm pulley
7 psi x 0,82 = 5,7 psi
7,5psi x 0,82 = 6,1 psi

Based on what we saw with the 95mm pulley boosting at the bottom of *
the 6-6,5 range, I believe the 92mm will boost at 7

808E90 (Ricky) dynoed at 440HP with 4,5psi and he had full ericsson *
exhaust.
Mine dynoed at 443 HP with 4,78 psi, and i have a full akrapovic. *
Makes sense.

Therefore on a SC, power is directly proportional to amount of boost, *
regardless of altitude, making the effective delta gains compared to **
stock car, much higher at altitude

[RussRamz]

This is a great discussion. I have learned quite a bit, but I don't understand how (if at all) supercharger systems on these cars compensate for cooler than normal temperatures. A standard atmosphere is 15 C at sea level. If the supercharger system is designed for X amount of boost maximum, are there any safe guards for cooler than standard days at sea level, or do you just have to monitor the boost level yourself so that limits aren't exceeded? At altitude, temperature really effects the atmospheric pressure. For instance, here in El Paso, I am sitting at 3,948 feet. A standard day at this elevation would result in a temperature of approximately 7 C. This doesn't happen in the Summer months and we usually see 30-35 C in the afternoons. Right this instant at 10:43 AM with a temperature of 27 C, dew point of 3 C and a barometric pressure at 30.13 results in a density altitude of roughly 6,075 feet!

Now on the other hand, place yourself at sea level, winter months and you can see -2000 feet which would raise boost pressure possibly above design limits, no? What's the safeguard for this? I'm asking because I don't know and want a car designed to function in all types of environments (hence my reluctance to supercharge).

[Bob MG]

Bud the temp variation is slight and the ESS kit allows for variation of around 1 psi either way anyway

Also ECU's adaptation would also take care these normal variations

I have done the blower on two of my M3s and had no issue ever.

In winter my temp are around 5-15 deg and summer 20-35 deg so similar to your temps

I have seen a drop in perceived performance now that temps have warmed up.

[Bob MG]

Also of you chat to AJ from ESS the different pulleys are rated at a varied boost pressure

Eg a 92mm is 7-7,5 psi (depending on density altitude affected by temps)

[M3PO]

Originally Posted by RussRamz

This is a great discussion. I have learned quite a bit, but I don't understand how (if at all) supercharger systems on these cars compensate for cooler than normal temperatures. A standard atmosphere is 15 C at sea level. If the supercharger system is designed for X amount of boost maximum, are there any safe guards for cooler than standard days at sea level, or do you just have to monitor the boost level yourself so that limits aren't exceeded? At altitude, temperature really effects the atmospheric pressure. For instance, here in El Paso, I am sitting at 3,948 feet. A standard day at this elevation would result in a temperature of approximately 7 C. This doesn't happen in the Summer months and we usually see 30-35 C in the afternoons. Right this instant at 10:43 AM with a temperature of 27 C, dew point of 3 C and a barometric pressure at 30.13 results in a density altitude of roughly 6,075 feet!

Now on the other hand, place yourself at sea level, winter months and you can see -2000 feet which would raise boost pressure possibly above design limits, no? What's the safeguard for this? I'm asking because I don't know and want a car designed to function in all types of environments (hence my reluctance to supercharge).

SC boost varies with density/pressure. Temperature and altitude both effect density/pressure. Therefore, the ambient temperature will effect the boost pressure. The SC spins at a fixed RPM (unlike a turbo) governed by it's mechanical connection to the engine. It has no control over the boost pressure that it's producing. The engine must be able to "absorb" the differences in boost pressure by adjusting fueling and timing.

[M&M]

Bob was 100% right. I have driven down to sea-level & logged boost at both elevations & the boost is around 2 psi more with the same pulley at sea-level vs 5000ft.

[M3Denver]

Great post for us ppl living in Colorado !!!

[Skooterbrah]

Been doing some research. @ROMAN@ESS is there anyway to purchase a smaller pulley then the supplied one Andrew @ AR Design ordered for my Car? We are running the ESS 625 kit, but are only managing 526hp corrected (mid 400's iifc uncorrected) and are trying to find a way to up the boost a pound or 2 safely to get the power on par (or close too) with the 625 guys at sea level (500 uncorrected up here would be great) For reference, he is a dyno of our pull up in Denver. Car will never see sea level, or leave Colorado even.

*EDIT*
On a side note, we have a vortech pulley at the shop (3.6" 91.4mm) not sure on the size difference between that and the 625 pulley. We noticed that there is a slight offset in the 625 supplied pulley, so we haven't attempted to try this one out yet. Figured I would touch base with you first to see what the options are.

[Verify]

Originally Posted by Skooterbrah

Been doing some research. @ROMAN@ESS is there anyway to purchase a smaller pulley then the supplied one Andrew @ AR Design ordered for my Car? We are running the ESS 625 kit, but are only managing 526hp corrected (mid 400's iifc uncorrected) and are trying to find a way to up the boost a pound or 2 safely to get the power on par (or close too) with the 625 guys at sea level (500 uncorrected up here would be great) For reference, he is a dyno of our pull up in Denver. Car will never see sea level, or leave Colorado even.

*EDIT*
On a side note, we have a vortech pulley at the shop (3.6" 91.4mm) not sure on the size difference between that and the 625 pulley. We noticed that there is a slight offset in the 625 supplied pulley, so we haven't attempted to try this one out yet. Figured I would touch base with you first to see what the options are.

thats good power, corrected.

just curious, as Ive never seen it before, what is the N/A power uncorrected?

[Skooterbrah]

Originally Posted by SSRE

thats good power, corrected.

just curious, as Ive never seen it before, what is the N/A power uncorrected?

Yea corrected is decent, but the uncorrected was around 46x iirc. Uncorrected pre-boost with full exhaust, drop in filter was 305hp i believe. Fuck this thin air.

[signes]

Agree! I was getting 305-310 whp here stock.

Will be curious to hear what you find with the blower. If you go with a smaller pulley to increase boost, what happens if/when you choose to go down to sea level?

[Skooterbrah]

Out of curiosity what are the boosted M's up in Colorado/5k+ altitude dynoing at?

I've made peak power of 526hp/343tq peaking at 6psi. (ESS 625 kit)
Before that, bone stock i was 309, and drop in filter and catless/resonator delete xpipe and oem exhaust mod 349.

All runs were done at Finishline

[Bob MG]

I at 5500 ft altitude and I have a VT2 625 kit boosting at around 7,8 psi

To achieve this boost at my altitude I run a custom 650mm crank and 95mm S/C pulley set up from VT3 kits with a 2130mm belt

[Malek@MRF]

Originally Posted by M&M

Ok I see I emphasize the word QUANTITY and everyone keeps harping on about the ratio. Of course the ratio is the same. There is still 80/20 Nitrogen vs oxygen. But there's less O2 and less Nitrogen. You may end up climbing up to the top of Everest and you may end up getting 2 02 molecules and 8 Nitrogen molecules in each gasp of air (not actual numbers, I'm just trying to illustrate that the ratio stays the same).

The ratio will always remain the same, unless you are using this M3 somewhere other than planet Earth.

This "quantity" you are referring to is known as D.A. (Density at Altitude).

Bob MG is correct, all he will have to do to compensate for his altitude is very basic, and he is already on track to doing it. Superchargers and turbocharger systems do not respond the same way in regards to D.A. The turbo will continue to compress air without the wastegate opening until the target pressure is acheived, it does not care what altitude it is at. The CF supercharged car will generate boost as a function of crank speed and SC pulley speed. Alter the ratio between these two, and you effectively alter the boost pressure (given that D.A. does not change along with ambient temperature). The SC car will be affected by D.A. because it does not have a wastegate boost control system, it is predefined in pulley size and RPM. The higher up you go, the less boost the SC car will generate. Much like in NA form, the SC car will suffer power at higher altitude.

[Hawker30]

Originally Posted by Skooterbrah

Out of curiosity what are the boosted M's up in Colorado/5k+ altitude dynoing at?

I've made peak power of 526hp/343tq peaking at 6psi. (ESS 625 kit)
Before that, bone stock i was 309, and drop in filter and catless/resonator delete xpipe and oem exhaust mod 349.

All runs were done at Finishline

Just got mine Dyno'd in Denver. My best run was 534hp/346tq/6.68boost.

[Skooterbrah]

Originally Posted by Hawker30

Just got mine Dyno'd in Denver. My best run was 534hp/346tq/6.68boost.

Awesome, what kit/fuel?

[Skooterbrah]

Originally Posted by Skooterbrah

On a side note, we have a vortech pulley at the shop (3.6" 91.4mm) not sure on the size difference between that and the 625 pulley. We noticed that there is a slight offset in the 625 supplied pulley, so we haven't attempted to try this one out yet. Figured I would touch base with you first to see what the options are.

We threw on the 3.6" pulley and replaced the 625 (3.8") today. We saw about 1 psi increase (from 6.1 to 7.0psi) Managed to pull 565hp/362tq at 7psi (previous 526hp/343tq at 6psi) AFR's were low 11's past 6k rpm. After throwing some ideas around we are gonna aim for 9psi peak. Apparently the DMU will auto-compensate fuel if needed, and with the lower static compression up here in Colorado, 9 should be plenty safe and SHOULD net me right around 600-625 corrected (500ish uncorrected)




Next step after the 3.2" pulley is converting the car to E85. Just trying to figure a way to tune the car once we throw in a larger pump and injectors to handle to E85. At that point I would love to see 650+ on 9-10psi.

[Skooterbrah]

A little fun post pulley install haha. Clearly need some sticky drag radials now!

[Skooterbrah]

Swapped pulley to a 3.2" (81mm) and saw 9psi on dyno today and managed to jump from 565 at 7psi and 613 at 9psi.

[Bob MG]

Originally Posted by Skooterbrah

Swapped pulley to a 3.2" (81mm) and saw 9psi on dyno today and managed to jump from 565 at 7psi and 613 at 9psi.

You are doing it wrong.

Smaller than 88mm causes the belt to slip. To get this done correctly you must change your crank to 650mm and then you need to only go to around 92mm to achieve the same level of boost.

Also 9psi is too high for the tune you have and you are asking for crap

[Skooterbrah]

Originally Posted by Bob MG

You are doing it wrong.

Smaller than 88mm causes the belt to slip. To get this done correctly you must change your crank to 650mm and then you need to only go to around 92mm to achieve the same level of boost.

Also 9psi is too high for the tune you have and you are asking for crap

No slip with a 82mm, We changed the belt, and she holds fine. How is 9psi to high for the tune? The DME has compensated across the board for fuel, and am still sitting in the 11's AFR's. Keep in mind, this is being done at 6k elevation.