Garage99

There has been a fair amount of talk online about the HTA68 turbo from Forced Performance. There were claims that it “spooled earlier than a VF39″, yet made “more power than a 20G”. Well, sorry to disappoint, folks, but I think it’s fair to say that neither of these claims are true. To be fair, I don’t think it was Forced Performance that was making these claims. Still, the HTA68 is a very capable turbo… I just think it would be a better match to a 2.0L WRX motor than the STi’s 2.5L.

Regarding spool-up, the only way anyone could claim that this turbo spools earlier than a VF39 would be if you were comparing a tuned HTA68 car to a *stock* VF39 car. At typical tuned VF39 STi will be making ~280 ft-lbs of torque by 3200rpm in 4th gear, while this HTA68 STi was making ~240 ft-lbs under the same conditions. I certainly would not call the HTA68′s boost threshold “late”, but the VF39 spools up if you just look at it funny or sneeze in its general direction. The HTA68 was in full swing by ~3700rpm during the 4th gear pulls on the dyno, which is perfectly acceptable for a fun street car.

It was clear that this turbo wasn’t going to make peak power like a 20G when the boost started to fall off around 5000rpm. Most 20G-like setups do a pretty good job of keeping the boost up almost all the way to redline.

I’m not saying that the HTA68 is a bad turbo… it is a very good bang for the buck and it offers a wide powerband with relatively quick spool. I’d say it fits between a VF39 and a Dominator 1.0 turbo, leaning closer to the VF39.

This particular car arrived with a tune from another tuner in Texas. During the baseline dyno pull, it was clear that something wasn’t quite right. The ECU was pulling a LOT of timing right where the torque peak should have been… you can see this in the chart at 3900-4000rpm. A quick compression test showed that the 85k mile motor was a bit tired, but compression was very consistent across all cylinders. Once we were sure that the motor wasn’t hurt, I reflashed the ECU with a good “starting point” calibration and got to work. From that point forward, everything fell into place quite nicely.


The parts:

• Forced Performance HTA68 turbo
• TurboXS front-mount intercooler
• TurboXS downpipe
• TurboXS midpipe
• Prodrive 2.5″ muffler (restriction?)
• TGV deletes
• K&N Typhoon intake
• DeatschWerks 740cc injectors
• GrimmSpeed 3-port boost control solenoid

The results:
Old tune – 254.1 hp @ 5450rpm and 271.1 ft-lbs @ 4450rpm
My tune – 287.9 hp @ 5700rpm and 301.4 ft-lbs @ 4450rpm … at the same boost level (within 1psi).

Before and after comparison:



Compared to our baseline stock 2006 STi: