Seeking Advice from MT.net brain-trust
 

Hello All,

So the time may have come for me to construct the next version of my mutant. Last winter I built the first prototype of a compound turbo-super. Through the summer I identified several areas of need and addressed them where possible. Where it was impractical to make changes, I added them to the "next build list".

As currently constructed, I am using a 1.6 stock bottom end block with a GT2871 48 trim chrurbo blowing into an Autorotor 2076 twinscrew. It is a riot to drive, but is also not without its warts. The next version will be based on a '94 block with a GTX2867R and Eaton MP62. From a practicality perspective, I know this setup isn't ideal for what is traditional around here, but my objectives aren't traditional either. I came to the conclusion quite some time ago that this car will never been well suited for the track. Its primary purpose is to get me from my house to work and then back home (~10 miles). I rarely exceed ~50mph, and don't particularly care to wrap it. I do love torques, and with my current tire/gearing/power setup traction is totally optional at (nearly) any RPM in 2nd gear. This will never be a high-hp build. I am OK with this, I don't really want to get into the drivetrain modifications associated with big power. Ultimate targets are ~240-250 lbft (rwt) everywhere with a 7200 RPM limit. The current configuration is capable of 200+ (>~2500 RPM) so I think this is reasonable. I have convenient access to e85 and have been running a flex setup for the last year. I don't really care to optimize power on gasoline, but I do want to retain the option of using it in the event that I find myself needing fuel where e85 isn't available or if I get a bad e85 blend. I know that there are other ways to achieve similar results, but I enjoy the challenge of doing something a bit differently and having something that is uncommon.

This is the first car I have really ever done anything with, so my limited experience prevents me from being able to make a decision on what static compression ratio to use in my 1.8. I am comfortable street tuning with MS.

I am and will continue to be limited to ~ 225kpa absolute manifold pressure.

Parts/options/plans (parts are all on-hand)

Car is well sorted from a suspension, stiffening, brake system, fueling system, cooling system, charge cooling, instrumentation, and electrical standpoint. I will probably go 3.63 or 3.9 with 6-speed in the future, but that is outside of scope for this winter season. Exhaust is in need of, and will get an upgrade from the current 2.25" to a 3".

Shell:
'91

Tranny/diff:
5-speed with 4.1 LSD.

Engine Management:
MS2PNP v1.2 (on-car)
J&S (on-car)
Greddy Profec bspec2 (secondary boost control, primary is handled by MS, on-car)

I will ultimately be going to the latest and greatest MS, but that is outside of the scope of this winter season build.

Ignition:
COPS (on car) or LS coils (on-hand)

Fuel:
DW200, ID850s

Head Options:
common - ST singles, SUBs, 1mm OS I/E, VICS

I have both a BP4W and a VVT on hand. Will most likely go VVT with VVTuner from DIY until MS3 is installed. Ex-intake and bp05 cam(s) on hand if advisable.

Hotside:
Begi T25 cast, divorced long-tube

Engine/Internals:
Carrillo A-beams
BE street strip
ARP
ACL
ATI

ST 10.5CR or WISECO 8.5CR

So all that to ask, what would people recommend for the head (pretty sold on VVT but will entertain advice) and compression ratio. I have poured over the internets and have my own thoughts, but would like to hear from the experts. It looks like there would be ~ 6% power difference between the two compression ratio options. If this is for free, I will go for it. If I am in for trouble at 10.5, I will stick with the 8.5 and be happy with it.

Thanks!

hmmm, this probably belongs in Engine Performance. Sorry.

Mods please move if you have time.

Thanks.

You might find this amusing, the parking garage obviously amplifies it, but it is pretty ridiculous.

With no lofty power goals, I don't think 10.5 is worth the hassle. For a full-time E85/E98 car, maybe, but for a street-driven car that will see pump gas, 8.5 will give you an easier time tuning and a lot more peace of mind.

Compounding is nutty. Turbo blowing into the super? Hotside or coldside? Sorry if it's a refundant question, feel free to point me at a build thread :)

I was just talking about compounding with a buddy. Such a great concept. But I don't want to deal with the headaches on track. Also interested in vids/build thread.

Thanks Andrew, I really respect your advice and will follow it. I was leaning that way anyway. The E85 around here has a way of disappearing. I was even considering doing a local facebook or something to try to drum up business for the suppiliers. "Hey dumb*****, we have E85, use it..."

Not redundant, no build thread. I haven't really posted much about my project.

Yes Turbo blowing into coldside SC. The SC is mounted where the alternator typically resides, the alternator is now located where the AC compressor used to be. The system runs SC at low RPM, compounds until the turbo is spooled, mostly TC through the mid-range, and then compounds again on the high side where VE's are dropping. The absolute manifold pressure limitation is a result of having the full manifold pressure present on the SC outlet. Rotor pack and shout seals have a tendency to fail at larger pressure differentials. Eaton is developing a new technology specifically for this type of application, but it is not available yet, and I suspect it will be cost prohibitive for awhile when it does become available.

I suspect your copy of Webster's defines "nutty" as "having or showing a great lack of intelligence or common sense." :)

I think the concept could be implemented in a way to be reliably trackable, but am not sure the benefits of my version of the compound would be of value on the track. I was shooting for low end with a decent high end. Turbos that satisfy the low end piece have a tendency to crap out on the high end. Superchargers that provide the high end are really pushing the technology limits and all sorts of other fun things start to happen. Even if you were to do this on the track with a "Jack-of-all" compound, my setup is not easy to work on. With the packaging and complexity involved, it takes me a full 4-6 hours just to remove the coldside.

I will probably post an update to my meet-and-greet post here in the next couple/few days. The car is already pulled apart for the new changes, and I didn't really do a very good job of taking videos of it last summer when it was well sorted. The youtube video I posted up there was in the first week(ish) after compounding. There were several changes made, including tuning after that video.

Intro Post
https://www.miataturbo.net/meet-gree...summary-85186/

There are a couple early pictures and some tech description starting here if interested:

starting at post #182

https://forum.miata.net/vb/showthrea...06#post8108106

Because the engine has to breathe through the turbo no matter what, IMO it makes the most sense to use the blower to spool a much larger turbo. Something like an intercooled Whipple combined with an EFR6758 and a large A/R turbine. The magic would come in combining the two to increase flow without breaking stuff. It would probably require a huge amount of wastegate flow as well.

A good project for someone with about 300hrs worth of spare time and $20k they'd like to dispose of :)

lol, who has a disposable 20K? I am super patient ... to a fault at times. I will jump on something I might use if the price is right, sometimes years in advance. The old Bell autorotor kit for instance cost me $400. The unused MP62 was $350. For a while there I had like 6 superchargers around here. I have since liquidated most of them, and am actually ahead.

I suspect I passed the 300hr mark a couple years ago. Ask my wife, she is probably keeping track. This is much more of a journey build for me, not so much of a destination thing.

Don't think for a minute that an 0.86 A/R housing for the BB GT28R is going to get by me if it is at a decent price :) Been on the hunt for over a year now.

Actually, now that I think about it...

The incremental dollar cost I have in the compound element is less that 1K. I bet a product that reduced spool RPM by 1000 on a 1.8 (closer to 2000 on a 1.6) would be pretty popular around here. Of course, development and support of said product would be quite an investment, and ASP would probably rocket to the 7-10K+ range. That is getting into LS swap territory.

I was just about to say you could probably be into a somewhat built LS for that amount of money, still have monster HP, and avoid having to deal with the kind of kludging you'd need to do with your compound boot setup.

Motor dropped off at builder today, so that is cool. This will be my first built motor. Went with the 8.5s as suggested.

I am going VVT, so the MS3 will happen this build season. I have a ton of stuff to do, hopefully the car is back on the road before July. I didn't get it on the road until July of last year after compounding last winter.

This sounds like the BS we tried in the high performance diesel world.
My first "small" twin turbo setup was a 64/71 over an S480...and that made 805 hp...I tried to make more and lunched 20K worth of expensive rods and junk.
Put the plain S480 on and made 1050hp...Lost some spool of course.
Then I moved up to a S500/91 based setup...same thing...at some point you hit that small turbo like brick wall. I think I made just shy of 1000 hp. I also added more fuel and heat was insane!
Another motor in the garbage!
I pulled the piping and crap off and made 1430hp....Spool was down a bit, but so were egts. Horsepower and torque went through the roof!
I know guys that have spent well into 6 figures trying to make a turbo supercharger work....Overall they failed....No thanks!
Ten pounds of Sh!t in a five pound bag!

With the EFR you can make enough power to break every part on the Miata, and it will spool better than any standard twin turbo.
Even with a very large EFR, drop a gear and hang on!

Twin turbo EFR?
There are guys tearing up thousand dollar bills as fast as they can pull them out of their wallet..... making that work in the diesel world right now!

Just saw this thread...

Sound like an interesting setup is about to go together. If you knew you had E85 all the time, I would have recommended the higher compression ratio, but if you ever gotta run pump gas then lower comp is better of course. The next time I build a bottom end I'm going to run high compression as I always have E85.

Looking forward to seeing how your new setup performs!

so, the 0.86A/R turbine housing magically appeared for me.

Dude accidentally posted it on ebay for BIN $1.00 with $7.40 shipping. Obviously I swiped it instantly.

I hooked him up, no need to punish his mistake.

side note... I can't believe how much better the casting on this VVT is compared to the B6. I have read about it being better, but seeing it first hand...

This may be stupid, but isn't this set up twincharging rather than compounding? I thought twin charging was supercharger and turbo where compound was two turbos, one feeding the other...

While I do not know if there are formal definitions for compound vs twincharge...

My definitions are as follows:

Compound: A forced induction system where the compression occurs in stages. In an ideal compound setup, the final outlet pressure ratio is equal to the product of the pressure ratios across each compressor stage. Compound setups may be comprised of any compressor technologies, such as the super-turbo, the turbo-super, the super-super, and the turbo-turbo (like Pat's)

twincharge: A forced induction system where each compressor outlet is into a common pressurized volume. The outlet flows into the common plenum are summative. Once again, the twincharge setup is not technology specific, but we commonly see the two turbo form, or twin-turbo.

Technically, my system is both a twincharge and a compound boost, depending on the operating conditions. Specifically it transitions from a compound at low RPMs to a partial twincharge at high RPMs.

Hope this helps, and that it has a relative degree of accuracy.

thank you for clearing up my confusion

By that definition all compound turbos are twin charge?

All turbochargers are technically superchargers too. Thankfully, we aren't a bunch of pedants.

Colloquially:

• "Twincharge" refers to any setup which uses both a supercharger AND a turbocharger in any configuration (series/compound or parallel)
• "Twin turbo" refers to any setup using two turbochargers in parallel
• "Compound" refers to any setup using two chargers in series