Okay, I have a TO4B 60-1 A/R 1,15 and I see full boost at 5000rpm! What could cause so late boost? The guy who built the car said that the turbo is just so big and laggy and thats why I see full boost so late. okay A/R is 1,15 but still itīs not THAT big.

do you 2" IC piping could be one that cause late boost?
If I go larger piping should I go 2,5" or would 3" be too large?
how much it matter if IC inlet and outlet is 2" and piping is larger?

I also have custom made manifold but donīt know does it matter? theres pics of it.
https://www.rx7club.com/single-turbo-rx-7s-23/how-important-manifold-get-turbo-work-251675/

could elect. boost controller help?

I really need help because tha car isnīt very practical to drive at the moment.

 

Is all this volume pushing through stock engine ports?

 

That is a pretty big A/R, even for a rotary. 5K RPM sounds about right. I also wonder what your timing looks like as you are spooling up. Timing has a lot to do with spool. What is your tuning like? Leaning out your cruise and mid range map can drastically improve spoolup response. 13.5 AFR range for idle & cruising and high 12s AFR for mid range seem to be where they like to run best. Good luck.

 

I would think the bigger your IC piping, the more lag, no?

 

What you have is a turbo that is suited to make top end power but unless you are ALWAYS shifting at the redline and keeping that turbo at a boil, you are going to have this problem. You are not creating enough exhaust gas volume and velocity to spin this turbo until 5000 rpm.

Unless you're building a car for drag racing or spinning mega high RPM's, you really want to size the turbo for drivabilty, not for max HP.

 

So you really think that the A/R is way too big for street car? Can I change the turbo to smaller A/R without modifying manifold and DP? If it can be done what turbo or A/R do you recommend? Iīd like to see full boost at 3500-4000?

 

No...a high 12 is far too lean on a turbo rotary. 11:1 is considered safe.

-Joe

 

I believe this is a T4 exhaust housing, so it SHOULD fit properly if you go to a smaller A/R. I'm not a turbo expert the way my partner is, but with maybe some minor adjustments to the downpipe you should be able to switch it.
I called my partner and he suggested that the most driveable would be dropping to a .70 and should still flow enough to get you the top end power you're looking for... 400 HP is about the limits of a 13BT anyways before you start having to do a lot of reinforcement to the housings.
Dropping to an .84 A/R would drop your spool about 1000 rpm... and would be a second choice, but I think that probably for the street the .70 would be your best option.

 

If you really want to improve spool up this is my take on what you have to do.

You have to take the restriction out of the intake path as well as the exhaust.

Remove ANY obstruction infront of the turbo inlet- this means biggest/best filter, inlet duct you can fit and ditching the Air Flow Meter (getting stand alone). This has been known to affect spool- up since the days of carbed turbos- blow through spools faster than draw through.

Use megaphone style piping off the turbo to the TB. The IC inlet pipe should start the same size as the turbo inlet and slowly expand in diameter all the way up to the throttle body. Steps in size
are easier to do and will work well.

Improve flow through the TB and intake manifold- any pressure drop affects spool up. Remove any obstructions you can.

Port the engine in a manner known to improve spool. I don't port, but from what I have seen it looks like only moderate overlap, but fairly aggressive duration on intake, early opening exhaust.

Run the largest/ most free flowing exhaust from the turbo back. 80, 90 and 100mm diameter turbo back exhausts are available commercially for our cars- or make your own.

Use a divided, long runner exhaust manifold to the turbo for good gas expansion.

On my set-up the ignition timming seems to like to be retarded quite a bit just as the turbo is spooling w/ close trailing split timming and the fuel just leaned out enough the engine doesn't stumble (Very very rich). Then in low boost it transitions to very aggressive ignition advance, more trailing split and leaner. Watch those EGTs carefully when using manifold fuel extension combustion to spool the turbo.

Or simply make an anti-lag system.

Use a suitably stiff wastegate spring or a quality boost controller (ball check valve manual over bleeder type- or electronic) to stop wastegate from creeping open.

I have tried to follow these guidlines as much as possible and get positive manifold pressure ~1,500rpm, 3-4psi by 2,000rpm, 8psi by 2,500rpm, 10psi by 3,000rpm and full wastegated 17psi by 3,500rpm.

Around town I drive from idle and shift ~1,800rpm giving it little gas (5-15% throttle) to prevent boost and I still pull ahead of all normal traffic.

Do as much of this stuff as possible on your budget/time and it will all help. If anyone has anything contradictory or further to add I would be happy to try that out as well!