Carpokes Porsche Forum

Yes Jason, and I have a brand new GT35 1.01 TS V-band housing purchased maybe 5 years ago that I will end up not using because I found out that trying to make it fit with the steering shaft was going to be too much work. If you are interested, let me know, but someone with a RHD car may have far less trouble to make it fit.

333pg333 wrote: Mon Apr 08, 2024 3:30 pmJust for curiosity, I wonder what the next size frame up would look like? Say the G40-900.

Well, after studying a little more how the match bot works I might be able to answer this question.
Let's start with the turbine map of the G40-900 :



Let's consider the smallest 0.85 A/R housing, which supports a peak flow of 35 lbs/min.
With engine data such as 3L and 30 psi @7000 rpm it takes a turbine expansion ratio of 2.98 to reach the "Turbine Corrected Flow" of 35 lbs/min of our G40-900 0.85. On the cold side the corresponding "Air Flow Rate" resulting is 84 lbs/min, which the G40-900 barely manages with a peak flow of ~81 lbs/min :



We can however decide to be content with a max engine speed of 6700 rpm where peak flow of 81 lbs/min is reached, with a corresponding slightly lower flow of 33.7 lbs/min for the turbine.

So basically a G40-900 0.81 will support 30 psi of boost up to 6700 rpm on a 3L engine with a "stock-ish" VE, BSFC and other non-trivial values entered by default in the matchbot that we have not touched, all of this for what it's worth obviously.

Lag will be the elephant in the room. Since the default values for VE correspond, I assume, to engines running conservative, street cams, the conclusion to this pseudo evaluation is entirely questionable. Other interesting results from the matchbot are a peak torque of 631 lbs.ft at 5000 rpm (where I assume that peak boost of 30 psi is well reached already (I'm sure the stock girdle would love that much torque (if something else doesn't break before))) and a peak power of 755 hp at 6700 rpm.

Here is some more musing, trying to corner the performance of my GTX3576R 0.82.
Peak compressor flow = 64 lbs/min
Peak turbine flow = 27 lbs/min
Engine size = 3.019 L

The boost and turbine expansion ratio are adjusted from 4000 to 6300 rpm to reach the peak flow values of the compressor and turbine in the output data calculated by the match bot.
I first noticed that the turbo could support 32 psi at 5000 rpm but the calculated torque figure was a whopping 690 lbs.ft so I adjusted the boost down to a more reasonable 26 psi.


Now if we use the available BW compressor maps in the match bot, here is how the plotting goes with the EFR7670, often compared with the GTX3576R as it flows the same peak of 64 lbs/min :


We are right at the right edge of the compressor map but contrary to the GTX3576R we do not have a somewhat "vertical" right edge I mentioned previously and which we could "lean on" as to reach the same-ish peak flow over a wide range of pressure ratio when using a slightly undersized compressor and avoid stalling it and suddenly lose power by stepping out of the map.

Another observation is that plotting the GT35 0.82 on the matchbot suggests that the corresponding BW hotside would be some "74mm 0.83a/r", a larger wheel than the GT35's 68mm wheel.


We could reduce the values for the expansion ratio and reach the same peak power with larger hotsides flowing more than 27 lbs/min. Let's try a GT35 1.06 turbine housing that flows a peak of 31 lbs/min :


Now if we compare between the two housings another key figure such as exhaust manifold pressure which we may equate to back pressure, we can see that the back pressure to boost ratio steadily increases with engine speed and peaks for both at 6300 rpm :
GT35 0.82 : 29.9/23 = 1.3
GT35 1.06 : 26.8/23 = 1.16
This is a rather insignificant difference but this should give a clue on response/boost build up through the rev range, and goes in line with what I have experienced first hand and that the GTX3576R has a terrific response yet flows pretty well up top despite stalling above 6300 rpm @23 psi.
This sort of concludes that it may be difficult for me to find a better turbo for my engine, all other things being equal and assuming the match bot is of any relevance, but just for fun I will assess some other turbos with the match bot and certainly adjust my boost profile to what the match bot suggests to experience how its calculations translate in real life.

All excellent reasons for a much larger ie. More efficient intercooler! Great research guys

Look at the tq jump from 3-4000rpm!

I was going to ask the same thing re a Twin Scroll, however it's a bit of a hassle to fab up all the extra pipe work.

Why is it in these spreadsheets for the two turbo trims it’s showing 10% wastegate open at 2000rpm and 18% @ 3k ? Shouldn’t we be fully closed until desired pressure is reached then roll open wastegate?

It seems that 100% is for a wastegate fully open, 0% for fully closed.
But yes, studying closely this parameter along with exhaust manifold pressure and how they vary depending on the expansion ratio (turbine A/R) may give a better idea about response down low than only assuming based on back pressure at peak rpm.

It could be tempting to try a GTX3582R as even if it seems 500 rpm laggier than the GTX3576R on 2L engines it should flow more at the same boost pressures over the mid range, should provide the missing flow up top and might give away perhaps only 300 rpm of response down low on a 3L engine with a lower tendency to boost spike than the GTX3576R and its significant jump in torque at 4000 rpm that I experience and that shows in the spreadsheet as Patrick noticed. However since my low CR engine seems to love lots of boost I may get closer to the limits of reliability with boost pressures between 25 and 30 psi because of the extra torque and power.

According to the matchbot, a GTX3582R reusing the current Tial 0.82 turbine housing would support 26 psi up to 7000 rpm with an exhaust manifold pressure of 36.8 psi, giving a ratio of 1.4, which I think is not too bad. Would certainly run out of traction earlier with the torque figure expected - over 520 lbs.ft between 4000 and 7000 rpm should make for an interesting experience.

Here is the corresponding spreadsheet, here limiting boost to 26 psi to keep the torque down :


However, if we compare the power and torque progression starting from 4000 rpm the matchbot tells me the GTX3576R offers the most area under the curve, despite its lack up top... Food for thought.

We're resuming working on my car now. All this makes me want to see what even 20psi will give us. There is a concern from the shop that why bother pushing it to create failure and have to spend more money on it. I feel like 20psi should be well within the realms of safety with these massive and highly torqued studs (tune dependent of course). We shall see if sanity or curiosity wins out at the dyno.

With the CR and cam specs you mentioned it sounds to me there may be room for quite a bit more than 20 psi... feels a bit odd to me to suggest now pushing it a little after suggesting in the past a conservative approach in view to your recurring misfortune... Trust your tuner is the bottom line, but if the sleeves end up moving I'll be glad to send you a set of used S2 studs of unknown mileage for real cheap... lol...

I'd also be a bit wary of revving such a long stroke engine that high with our typically big and heavy pistons, if the specs are still as in the email you sent me 4 years ago. I'd keep the boost down the higher the rpm...

I guess you are still turbo shopping for asking about the G40-900?

Thom wrote: Tue Apr 09, 2024 8:41 am Yes Jason, and I have a brand new GT35 1.01 TS V-band housing purchased maybe 5 years ago that I will end up not using because I found out that trying to make it fit with the steering shaft was going to be too much work. If you are interested, let me know, but someone with a RHD car may have far less trouble to make it fit.

When I finally get there I will reach out to you.

Things have been on the back burner lately. Partially my difficulty in finding available glyco mains.
Also saving up for a 2024 Ducati DesertX as my wife and I go on lots of motorcycle adventures and we want better off-road capabilities than our current motorcycle. Motorcycle adventures equals happy wife and then I can spend money on 951’s.

Just for fun if you have time could you plug in the 3594RS we have mentioned in your thread. Very curious how that obscure turbo stacks up.