[stewbets]

I’ve been searching this relationship out for the last couple weeks as I’m thinking about upgrading my intercooler, but I can’t find any answers that really satisfy me. And what I mean by that is that we know our cars target a specific AFR and we also know the density of air changes with the temperature, so it seems completely logical to me to consider the density differences to predict the HP gains (without factoring in timing which could likely be advanced for even more power). The rule of thumb seems to be a 1% HP gain for every 10 degree drop in IAT, but this is not consistent with my math.

Air density at specific temperatures and standard pressure:

40 degrees - .079 lb/cu ft
60 degrees - .076 lb/cu ft
80 degrees - .074 lb/cu ft
100 degrees - .071 lb/cu ft
120 degrees - .068 lb/cu ft
140 degrees - .066 lb/cu ft
160 degrees - .064 lb/cu ft

Air density at specific temperatures and 20 psi of boost:

40 degrees - .187 lb/cu ft
60 degrees - .180 lb/cu ft
80 degrees - .174 lb/cu ft
100 degrees - .167 lb/cu ft
120 degrees - .162 lb/cu ft
140 degrees - .156 lb/cu ft
160 degrees - .151 lb/cu ft

Referencing the above numbers, let’s consider the theoretical difference in air density at 80 degrees vs. 140 degrees:

At standard pressure - .074 / .066 = 1.12 times more density in 80-degree air than 140-degree air

At 20 psi of boost - .174 / .156 = 1.12 times more density in 80-degree air than 140-degree air

The air pressure does not change the density ratio. It remains 1.12.

Next, let’s assume this car makes 500HP at 140 degrees IAT. How much is it theoretically making at 80 degrees IAT?

(500HP at 140 degrees) x 1.12 = 560HP at 80 degrees

The accepted rule of thumb would yield a 30HP increase (1% per 10 degrees), but my math yields a 60HP increase (2% per 10 degrees). This is a very significant difference. My question is, am I missing something? The math seems really simple, and beyond the extra air and fuel being burnt, more timing should be achievable to make even more power.

The reason I chose a 60-degree delta is because I’m currently seeing a 70-degree rise in temperature over the course of a ¼-mile run with my 5” eBay intercooler. I was hoping to get that down to ~10 degrees. And I realize this 60HP gain would not be across the entire pull since the IAT gradually rises and becomes more of an issue, but I would think it would average out to be about a 30HP gain over the whole run. This may be why the rule of thumb is 1% for every 10 degrees. It may simply be a rule for ¼-mile calculations. If so, that makes perfect sense to me. Anybody have some wisdom to add to this theory?

[_N54]

My Wisdom: 5" intercooler is useless. Run a 7" minimum. Below is a log with Meth + 5" intercooler and Meth + 7" intercooler. Ambient temp on both logs were around the same temperature.

5" IC + Meth: https://datazap.me/u/n54tt/n54-e50-m...ata=3-14-15-22

7" IC + Meth: https://datazap.me/u/n54tt/log-16184...ata=3-14-15-22

[stewbets]

Originally Posted by 6266N54

My Wisdom: 5" intercooler is useless. Run a 7" minimum. Below is a log with Meth + 5" intercooler and Meth + 7" intercooler. Ambient temp on both logs were around the same temperature.

5" IC + Meth: https://datazap.me/u/n54tt/n54-e50-m...ata=3-14-15-22

7" IC + Meth: https://datazap.me/u/n54tt/log-16184...ata=3-14-15-22

Your second link isn't working for me. I don't at all disagree that a 5" intercooler is pretty worthless on a moderately aggressive tune. I've got a 7.5" stepped intercooler at home and I already gutted the necessary bumper plastic for it, but I'm still waiting on some silicone couplers for it.

[Dave92N54]

Originally Posted by stewbets

Your second link isn't working for me. I don't at all disagree that a 5" intercooler is pretty worthless on a moderately aggressive tune. I've got a 7.5" stepped intercooler at home and I already gutted the necessary bumper plastic for it, but I'm still waiting on some silicone couplers for it.

7.5 stepped is awesome, it's what I've been running for the last 2 months and I consistently see a couple degree IAT drop during a 3rd gear pull. I'm running stage 1+ 93 so not crazy but i'd imagine on 2+ it'd probably sit stable or drop a little.

Highest it's been here this year has been 80 so far so as it gets hotter, time will tell.

[stewbets]

Originally Posted by Dave92N54

7.5 stepped is awesome, it's what I've been running for the last 2 months and I consistently see a couple degree IAT drop during a 3rd gear pull. I'm running stage 1+ 93 so not crazy but i'd imagine on 2+ it'd probably sit stable or drop a little.

Highest it's been here this year has been 80 so far so as it gets hotter, time will tell.

Good to hear! I'm curious if you noticed any lag, lower boost pressure, or higher WGDC when you switched to your 7.5".

[Dave92N54]

Originally Posted by stewbets

Good to hear! I'm curious if you noticed any lag, lower boost pressure, or higher WGDC when you switched to your 7.5".

Well, I was completely stock (even stock tune) before the 7.5 so I can't really say. I went FBO in one weekend lol. I'll say you can definitely feel boost hit, it's not as linear as it used to feel but again I was on stock tune so I've attributed that to just being tuned.

I'd definitely be curious to hear what you may find or notice though! If there is a pressure drop or slight lag, it will most likely be barely noticeable. I'd say it typically takes me 400rpm or less to go from vacuum to full spool.

[stewbets]

Originally Posted by Dave92N54

Well, I was completely stock (even stock tune) before the 7.5 so I can't really say. I went FBO in one weekend lol. I'll say you can definitely feel boost hit, it's not as linear as it used to feel but again I was on stock tune so I've attributed that to just being tuned.

I'd definitely be curious to hear what you may find or notice though! If there is a pressure drop or slight lag, it will most likely be barely noticeable. I'd say it typically takes me 400rpm or less to go from vacuum to full spool.

I see. That sounds like fun. It certainly sounds like you're not lagging with your setup. I'll keep you posted on my results.

[_N54]

Originally Posted by stewbets

Your second link isn't working for me. I don't at all disagree that a 5" intercooler is pretty worthless on a moderately aggressive tune. I've got a 7.5" stepped intercooler at home and I already gutted the necessary bumper plastic for it, but I'm still waiting on some silicone couplers for it.

It should work now.

[Jeef]

if you ran alcohol fuel, would IAT matter as much? let's say we ran e85-e90 fueling or straight methanol.

[stewbets]

Originally Posted by Jeef

if you ran alcohol fuel, would IAT matter as much? let's say we ran e85-e90 fueling or straight methanol.

Sure, it would be a little less of an issue, but the whole purpose of my math was to evaluate how much more air is entering the engine at a lower IAT, and that should lead to a very specific increase in power regardless of the type of fuel being used.

[Bimmer_Engineer]

I think the bust in the logic here is that you're assuming the horsepower output is directly proportional to the mass of the air moving through the engine. There are a lot of variables at play here, mass throughput of air is one of them. IAT is a proxy for mass throughput because air at a lower temperature but constant pressure is more dense. If you're trying to convince yourself to go to 7.5 inch stepped then the IAT rise you're seeing on a 1/4 mile pull would be plenty enough of objective evidence for me even if the theoretical doesn't exactly pan out.

After you have some data with the 7.5 inch IC you could also try and update you calculations and see if you can make sense of the relationship with a more complete data set.

[stewbets]

Originally Posted by Bimmer_Engineer

I think the bust in the logic here is that you're assuming the horsepower output is directly proportional to the mass of the air moving through the engine. There are a lot of variables at play here, mass throughput of air is one of them. IAT is a proxy for mass throughput because air at a lower temperature but constant pressure is more dense. If you're trying to convince yourself to go to 7.5 inch stepped then the IAT rise you're seeing on a 1/4 mile pull would be plenty enough of objective evidence for me even if the theoretical doesn't exactly pan out.

After you have some data with the 7.5 inch IC you could also try and update you calculations and see if you can make sense of the relationship with a more complete data set.

But I think the relationship between the mass of air and the HP output is proportional, everything else being constant. Do you know otherwise? There's a thing called brake-specific fuel consumption that is used in predicting fuel needs as well as evaluating properly running engines (a 4-stroke engine should burn .5 pounds of gas for every HP). I'm going 7.5" stepped no matter what. I was just interested in understanding the math and the theory and how accurately it translates to real life. I mean, there are several tuners out there that claim a more efficient IC doesn't even add any HP. That's crazy.

[Saif2018]

Originally Posted by stewbets

But I think the relationship between the mass of air and the HP output is proportional, everything else being constant. Do you know otherwise? There's a thing called brake-specific fuel consumption that is used in predicting fuel needs as well as evaluating properly running engines (a 4-stroke engine should burn .5 pounds of gas for every HP). I'm going 7.5" stepped no matter what. I was just interested in understanding the math and the theory and how accurately it translates to real life. I mean, there are several tuners out there that claim a more efficient IC doesn't even add any HP. That's crazy.

I'm no good with these maths, way over my head lol.

From what I've been told and understand, a better/more efficient FMIC doesn't itself add any Horsepower, what it does is allow you to run more boost and timing consistently, which in turn results in more power and torque, before timing corrections kick in and cut power.

[stewbets]

Originally Posted by Saif2018

I'm no good with these maths, way over my head lol.

From what I've been told and understand, a better/more efficient FMIC doesn't itself add any Horsepower, what it does is allow you to run more boost and timing consistently, which in turn results in more power and torque, before timing corrections kick in and cut power.

And those aspects are certainly true, but I would consider them to be fringe benefits, at least for this exercise. There is a significant density gain in cooler air vs. warmer air which leads directly to a significant increase in the amount of fuel that can be burned and subsequently a significant increase in power. And no tune is required to take advantage of this. And on top of that, you can also run more timing and/or boost like you're talking about for even more gains.

With that said, the bulk of the increased power will be realized on multi-gear pulls where there is a substantial difference between your before and after IAT.

[Saif2018]

Originally Posted by stewbets

And those aspects are certainly true, but I would consider them to be fringe benefits, at least for this exercise. There is a significant density gain in cooler air vs. warmer air which leads directly to a significant increase in the amount of fuel that can be burned and subsequently a significant increase in power. And no tune is required to take advantage of this. And on top of that, you can also run more timing and/or boost like you're talking about for even more gains.

With that said, the bulk of the increased power will be realized on multi-gear pulls where there is a substantial difference between your before and after IAT.

Yes that's true, colder the air coming in the better, if you want to minimise the charge air temp as low as possible, then you'd want Meth injection on top of FMIC upgrade. Open to correction, but I don't see any other feasible alternative.

I read abut some water to air charge air cooling similar to S55 system, but thats probably going to require fabrication and deep pockets, not sure if it's worth it.