.

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Succcess!

$6 worth of M10x1.0 brake unions and half a liter of fluid to bleed...and off to Harris Hill to check my work.

Result => huge improvement in braking. RF lockup now almost all gone. Can carry more speed and longer through T6 (see pic) and T9.

Combine that with a little less rear wing and more damper compression, and turned my best-ever single lap there on 200tw tires. 1:22.7

https://scontent-atl3-1.xx.fbcdn.net...f5&oe=5AC74C49

https://scontent-atl3-1.xx.fbcdn.net...f6&oe=5A93529B

I think the 7163 is a good match. It will spool fast on a k24a2 and since the bottom end will be forged, it also gives more room to grow if it's ever needed. 500-550whp should be a retune away.

forgive my ignorance... but what did you change?

Looks like he removed the factory proportioning valve

See post 367

missed that. thanks.

Horsepower and airflow correlate in lockstep. If you move 50lbs/min of airflow through the compressor, you will make about 450whp. Whether that 450whp happens at 1800rpm or 9000rpm is of pretty minimal consequence.

Not that simple, and we both know there's a whole lot more variables to it than just the compressor map, but I would certainly not mind being proven wrong with actual results vs speculation.
In a perfect world though, I would absolutely agree.

It's not speculation, it's an understanding of the physics behind engines and turbochargers. Head flow and RPM have virtually nothing to do with it.

e: To clarify, if the engine can't support 450whp worth of airflow, then you won't make the power, but there's absolutely no logical reason why a larger engine with a better-flowing head and a higher rev limit would need a larger turbo to make the same power as a different engine with less head flow and a lower rev limit. It would likely benefit from different compressor aero, since the larger engine will make that power at a lower pressure ratio, but making the turbo larger is just going to harm response needlessly.

Andy has to be competitive in autox as well as road course. That means running the smallest compressor that will meet his power goals. That's the 6758, with a little room to spare. The 7163 has headroom he'll never use and the heavier, slower spooling turbine to go with that.

When we built Deviate with a C30-74 Rotrex, we also made more than anyone expected. Gotta take what's published as mfr recommendations with a grain of salt and pay attention to what tuners are actually achieving with them. I'm just taking real world examples and applying it to Andy's stated goals. Bigger turbo is sexier but guaranteed to be worse in an autox environment.

His K series head probably flows around 330CFM, compared to 240CFM for Deviate (Rotrex) and only about 210CFM for Bullet (6258/355whp) and Rover (6758/450whp). Meaning he'll hit his power target at a lower pressure ration (less boost). That means he'll be a bit further down the peak efficiency island than Rover. With the 7163, at lower RPM, he'd be completely off that island for most of the powerband.

Rather than ramble on with a technical explanation of how turbos work, I'm going to reply to your criticism with your own post from earlier today, because I'm flattered when you think I'm wrong on anything, really.

Because science beats uninformed speculation every time there is a clock to answer to. Wanna race?

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He had a BORG ENGINEER pick the turbo for him. Maybe, MAYBE the dude might know a thing or two about EFR selection.
But hey let's keep arguing, and swinging e-peens.

:likecat:

We can have all the beers you want, dude, but I'm never going to stop calling you out when you post inaccurate or misleading things here. It's not personal., but I don't tolerate fools, and your posts demonstrate a consistent lack of technical competence in the topics you choose to discuss. The short version of the diatribe I don't have time to type out is that your consistently inaccurate postings make my job as a vendor harder. For the good of this forum, be better. You don't have to agree with me on everything, but you can't be blatantly inaccurate, and you are consistenly the latter.

It's classic engineering myopia, IMO. It's not that the Borg engineer doesn't know how to size the turbo, it's just that they are focused on the numbers instead of how it actually works in the real world. I'm sure a 7163 would be the more efficient option given the parameters of the application, there's no argument there. The difference between the BW engineer at his desk with a sizing calculator and me is that I have actual real-world experience with the 6758 at 450whp, and it works. It makes the power, it doesn't complain, it doesn't overspeed the turbo. Is it right at the bleeding edge of the compressor map? Yes. Does it make 450whp? Yes. Will it respond better in an autocross application than a 7163? Yes. Is it, in my educated, experienced opinion, the better turbo for the job? Absolutely yes.

I love me a 6758 at the bleeding edge. But even more I love the ability to crank it up, and have the headroom to do it :D
Anyways, back on topic (Andy if you want any of this rambling removed/deleted just let me know)

I recall engineers at Hoosier telling me their then new 275/35/15 would be best on a 9" wheel but ok on a 10".

In the real world that tire is best on an 11" wheel. Fact, supported by data. Not conjecture. Engineers are wrong every day.

Every single tire manufacturer's website recommends rim widths too narrow for their tires optimal performance. Whether that's written by a lawyer or sanctioned by an engineer is irrelevant. It's wrong.

Figured it was time for me to weigh back in...

1) I know a lot about tires, suspension, NA motors, etc...but my turbo knowledge is still on the steep part of the learning curve. So I am having to rely on others.
2) I have a short timeframe for development, since the car needs to have all of this working by early Spring.
3) While the car will indeed see some autocross action, it's primary purpose in life is the track. The car already has more than enough power to autocross in 2nd gear on street tires.
4) 450 hp is an arbitrary target. It came from doing some simple math. My McLaren is 600 @ 3000 lbs, and to accelerate the same, a 2000 lb Miata needs 450. If I end up with the capability for more than that, I'm not gonna complain. In fact, having some headroom is a desirable for future development.
5) Where it gets tricky is that the gearing as currently contemplated requires the turbo to spool as early as 3300 rpm to minimize use of 2nd gear. The models show this to be the case, but reality may be different. Worst case, I have to use 2nd gear more than I would otherwise. Just trying to avoid some shifting.
6) In other venues from other people I respect, I am getting the opposite input...go bigger...3076. This includes at least one person with direct turbo K24 experience. Hard to know who is right...if there is such a thing as "right".
7) The good news is that the 6758 and 7163 share the same housings/dimensions. So they can be easily swapped. The bad news is that I need to buy a turbo ASAP, since we are starting manifold fab right away.

Anyway, happy to get good, well-informed input...especially if it is educational (I can certainly use the education at this point). I really appreciate those that are taking the time to do so (especially my guy at BW, who has been holding my hand from the start).