No boost cut???

hackish · Post by **hackish** » Wed Oct 19, 2005 4:11 pm

Having put some miles on my new clutch I attached the wastegate last night. I'm using an STi v4 turbo VF24. First run I got on the throttle and it jumped up to 15 PSI. Vacuum hose was cracked. So I replaced the hoses and running directly off the wastegate it runs 10-12 PSI. A little high for stock engine management and fuel I think. I've got to stick the wideband on and double check that.

I would have expected to see a boost cut before it hit 15PSI. How is the legacy turbo's boost cutoff implemented? Is it a MAP sensor or calculated from the MAF reading? Maybe something is unplugged or unhooked (no CEL).

-Michael

vrg3 · Post by **vrg3** » Wed Oct 19, 2005 4:19 pm

Overboost fuel cut only happens if you overboost for more than a second or two.

The stock VF24's wastegate actuator is set up to open at 10 to 12 psi? Wow.

hackish · Post by **hackish** » Wed Oct 19, 2005 6:17 pm

I'll hook up a compressor and gauge to it this evening when there is a spot in the shop for my car. My recollection of the STi was that it ran 12 on the WG and 16 stock.

It may have been a "modded in japan" turbo as Lachute Subaru who have done many swaps say that stock STi v4 should only run 14. 2 PSI over the WG doesn't sound right to me.

-Michael

andys2 · Post by **andys2** » Tue Oct 25, 2005 11:50 pm

The legacy AF system is MAF based so why are you so worried about fuel at 10+ psi?

91White-T · Post by **91White-T** » Wed Oct 26, 2005 12:04 am

andys2 wrote:The legacy AF system is MAF based so why are you so worried about fuel at 10+ psi?

10+ psi from a VF24 is much more air than from a vf11

dzx · Post by **dzx** » Wed Oct 26, 2005 3:23 am

I'm not sure i understand this. Shouldn't 10psi be 10psi no matter what turbo its from? Maybe the charge would be a little cooler depending on the efficiency of the compressor. So the upside to upgrading to bigger turbo would be you could run higher levels of boost whereas a smaller turbo would be out of its efficiency range and overboosting. Could someone clear this up for me?

vrg3 · Post by **vrg3** » Wed Oct 26, 2005 3:45 am

The efficiency of the compressor, the cooling effect of the intercooler, and the increase in volumetric efficiency due to the exhaust housing all mean that our engines can flow a lot more with an aftermarket turbo at 10 psi than with the stock one.

legacy92ej22t · Post by **legacy92ej22t** » Wed Oct 26, 2005 3:48 am

I think it's that the larger compressor will compress a larger volume of air to make said boost.

So if you put it in term of intake flow (these numbers are totaly fictional):

VF11@10psi = 223 cfm

VF24@10psi = 385 cfm

legacy92ej22t · Post by **legacy92ej22t** » Wed Oct 26, 2005 3:48 am

V strikes again!

BAC5.2 · Post by **BAC5.2** » Wed Oct 26, 2005 5:04 am

Lachute Subaru is a bunch of liars.

hackish · Post by **hackish** » Fri Oct 28, 2005 4:35 am

Well Lachute Subaru was doing STi swaps long before the WRX was even available in North America. I'd like to think they know a thing or two about these engines and turbos.

-Michael

hackish · Post by **hackish** » Fri Oct 28, 2005 4:36 am

Something to add about the boost cut. Indeed it is implemented via the MAF sensor and it occurs when the MAF signal hits almost 5V. Interesting news as the higher the RPMs the lower in PSI you will find the boost cut.

-Michael

vrg3 · Post by **vrg3** » Fri Oct 28, 2005 6:02 am

The fuel cut you're talking about is different from the standard overboost fuel cut that we usually refer to.

I don't know what exactly triggers the fuel cut you're referring to, but it does involve the MAF hitting its rail. It seems to have something to do with multiple sensors giving invalid (or unusual) readings simultaneously.

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No boost cut???

No boost cut???