5 Minute Turbo Tech Tips With Reed, Episode 2: Intercooler Pressure Drop

[@MikeyAntonakakis]

I've got a VGT turbo in a 1987 BMW 325 (HE351VE from 6.7L Cummins, 2.7L 2-valve low compression ratio engine). Goal for the car was to make really good power at moderate levels of boost (i.e. big turbo) while not giving up much in terms of spool. It's running Megasquirt 2 and I designed a controller/control algorithm for the VGT and wastegate. VGT control algorithm is coupled with dome pressure wastegate control (compressed air onboard). Since the VGT can increase drive pressure enough to unseat a soft/medium wastegate spring and defeat the purpose, they have to be pretty tightly coordinated - it keeps max pressure on the dome until I'm almost at the boost target, then lets closed loop dome pressure control take over. VGT runs at an rpm-based "effective" wide open state while cruising around (not fully wide open, to keep turbo shaft speed up without any increase in back pressure), closes up "just enough" (that's the hard part!) when boost target exceeds MAP, and ramps to fully open once MAP gets close enough to boost target. As part of the mapping of the VGT response, I added exhaust backpressure and at medium levels of boost (currently running 12-13psi with plans to eventually get closer to 20psi), with VGT wide open, drive pressure is a little higher than expected given the headroom I still have. Seeing about 1.4-1.6:1 throughout the rev range. This video made it occur to me that, hey, maybe it's my small water-air intercooler causing a decent pressure drop, and actually my drive pressure is a little better. It's a "350hp" capable unit which I believe I'm at or exceeding at current boost levels. So I guess it's time to add one more sensor to the mix and measure it!

[@workturbo]

Checking that intercooler pressure drop will help shed some light on the situation for sure! Glad this "may" have helped you out. VGT strategy on a wide rpm range spark ignited engine has to be "fun!!!"

[@MikeyAntonakakis]

@@workturbo definitely “fun” haha! Especially since I’m 6000ft above sea level. But I can get to full boost (slowly) at 3000rpm with a 7000rpm redline, and nearly instantly above 4000rpm.

[@terrygrossjr83]

Just found your channel, I bought a turbo from you a few years back. 😊

[@workturbo]

Thanks for checking it out Terry! Hope your Buick Turbo has been serving you well!-Reed

[@EricErnst]

Some if thr pressure drop can be attributed to the drop in temperature, not necessarily solely the restriction from the intercooler core.

[@workturbo]

Very good point that is on my list for a future video. Density vs temp vs psi should be easy enough to plot. Most tests that I have done validate the "2.5" psi ▲ makes up for the density recovery and is a good reason that number seems to be the acceptable amount. Explaining why this is the case will again be a great video topic. Thanks Eric for keeping that topic fresh on my mind. -Reed

[@mattmorrison6958]

It's fascinating how many effects, often counterintuitive at first, come down from things trickling down to the turbine side/work/drive pressure. It's easy to just look at intake pressure and temperature and completely forget what's going on at the other end

[@802Garage]

Do you have a rule of thumb for what is too much? For a given horsepower level perhaps?

[@PiterStreak]

From what I've learned, good cores stay at around 2-3psi

[@802Garage]

@@PiterStreak I've generally heard under 2-3 as well, but of course also depends on what boost level you're running.

[@workturbo]

Just like PiterStreak said, 2-3 psi max for good performance, but less the better. It is not uncommon to see that number rise as the boost pressure goes up.Watch for a new video coming soon that will help shed some insight on balancing pressure drop, efficiency and flow. Each one of these things directly impacts the other!

[@802Garage]

@@workturbo Look forward to it! 🙌

[@davidgriggs3967]

I have a 10psi wastegate spring and an ebay intercooler and see 9psi on my intake manifold after the throttle body.

[@jonathanenglish7109]

Cooler dense air should drop some pressure

[@JH-sc7cs]

Hi mate. I got a k04-064 mk6 gti if you're familiar. With iat at 20deg C I'm making 18psi peak and 15psi hold from the compressor nipple to factory n75, but at the IM I'm making 3psi more on both of those results of compressor. What could cause this ?

[@workturbo]

My first test would be using the same gauge reference for both points...most likely the data is skewed would be my 1st guess. Although I am still thinking this through, and no tests I have ever done have seen an increase in manifold pressure over compressor pressure. One possibility could be the intake "surging" and causing pressure spikes in the positive direction. The imap sensor could have a degree of latency or the point where it is reading has high amounts of reverberation causing incorrect data to be observed. Just brainstorming, but a very interesting phenomena if it is actually accurate data. Maybe some others could chime in? -Reed

[@finnroen2334]

I want to disagree with one claim. If testing the boost pressure from the turbocharger by connecting the boost gauge to the compressor cover in the beginning og the snail, far away from the outlet, the result will be WRONG as the pressure there is lower than the boost pressure out of the compressor outlet. :)

[@workturbo]

There is validity to your response, and it is measurable...but it also is a very small amount in most cases. Most people do not comprehend how a centrifugal compressor housing functions...most do not even know how a compressor wheel "works" as well. Knowing that the compressor tubing is an extension of the volute and has a purpose to a degree that is more than just connecting the compressor to the system will be a future topic. I will be going over this with real world data in the future...lets just say I have a lot of "ports" in a housing and piping for the data. On a traditional compressor cover with the port located outside the torus centerline in the "spigot", the pressure drop is minimal from what I have found "If" the compressor piping does not make a drastic volume change directly after that point. At least using my measurement hardware. I could spend quite a bit of time just studying this design attribute and testing variations... Thank you for bringing this up, it gives me motivation to learn more! -Reed

[@finnroen2334]

@@workturbo Having messed around with old Volvo and Ford T3 turbos there are som surpricing variances where the wastegate actuator signal hose is connected to the compressorhousing and also the length of the wastegate arm. I have concluded that for maksimum boost responce and maksium torque a large svingvalve, a short wastegatarm and the signal hose connected at the beginning of the snail on the compressorhousing is the right way to do this. The wastegate actuator spring should be long to have a fair amount of prelod at rest. A bleed valve / manual boost controller is of course a must. When Volvo turboconverted their stock NA carb engines they wanted the opposite results. The used smal swingvalves with longer wastegate arm, a wastegateactuator with a shorter stiffer spring and the signal hose was connected by the outlet of the compressor housing. Do NOT use a bleed valve here. All this to avoid detonation and clutch slippage. :)