Car Talk

[TheReal_ND]

That's because the low end torque curve of a boosted qr is too massive for fwd cars. They are tuned for peak to come on after 5.5k. If you're only making about 150 hp off almost 20 lbs that's only 7.5 hp/lb which is sub par at best. What's your elevation?

[Speaker to Animals]

I'm leaving this here just in case.

[adwinistrator]

That's because the low end torque curve of a boosted qr is too massive for fwd cars.

I agree, which is why a Subaru doesn't have that problem. I'm much rather have 350 ft/lbs @ 3,750 rpms than gain an extra 100 hp @ 6,000 rpms. It's a daily driver, and this turbo is designed to give the boost profile to achieve this result.

They are tuned for peak to come on after 5.5k.

They are tuned to not be turbocharged. Depending on the Nissan, you're starting between 9.1:1 up to 10.5:1 in the Sentra Spec-V. Unless you're putting in new rods and pistons to lower the CR, good luck having your engine last.

Here, you'll need this when your engine bay is on fire:

Thanks StA, that came in handy.

If you're only making about 150 hp off almost 20 lbs that's only 7.5 hp/lb which is sub par at best.

Not sure where you've pulled these numbers from, and I used to have the same misconceptions about turbochargers and boost pressure as you do. Time for a thought experiment:

Two identical engines, one with a small TH A/R, the other with a large TH A/R. They are both tuned for the best performance on that engine. Both of the turbo systems reach 18 psi at 6,000 rpms. Would they have the same horsepower and torque at that point?

The answer is no. 1 PSI = 1 lbs force per square inch (not pounds of air per square inch). PSI does not measure air flow, it measures pressure. A larger turbo housing A/R will move more air, but will require more power to turn from the engine. It will not cross the boost threshold as quickly as a smaller turbo housing A/R, which is much quicker and easier to spool, but cannot move as much air through it.

Engine output with a turbo is not just a PSI thing, it's PSI and the airflow of the turbo. More airflow = more peak power, worse low-mid performance. PSI is only the pressure from the turbo. My smaller TH A/R turbo can reach max boost at 3,750 rpm, whereas a larger TH A/R would reach max boost much slower, like 5,000 rpm.

I'm not running this on the drag strip with a close ratio gearset, and I'm not willing to redline it to access the turbo's potential. It's an AWD car that I drive in the snow, I have fast throttle response with very little turbo lag. I would never give that up for extra hp, which is what a larger TH A/R turbo would do.

[TheReal_ND]

Yes generally QR owners use low boost that spools up top unless they change pistons and arms. Your numbers are too low for that amount psi though. What's your altitude? What's the setup? Is it using the stock intake, exhaust and intercooler?

[adwinistrator]

Yes generally QR owners use low boost that spools up top unless they change pistons and arms. Your numbers are too low for that amount psi though.

Please read my previous post carefully, specifically the thought experiment.

Its all about volume of 20 psi, a bigger turbo is gonna put out more 20 psi versus a small turbo. It will take longer to get to 20 psi, but it will move more air into the engine, which is where power comes from.

This might help you understand what I'm trying to explain:


I would prefer the red line instead of the blue line.

What's your altitude? What's the setup? Is it using the stock intake, exhaust and intercooler?

Low altitude.

Modification list:
IHI VF34 Turbo
Cobb Accessport v3
PTP Lava Turbo Blanket
Intake Manifold from 2008 WRX STI
Intercooler from 2008 WRX STI
K&N Typhoon Intake
GMS Silicone Turbo Inlet
Grimmspeed TGV Deletes (ported and polished)
Grimmspeed Crosspipe
Grimmspeed reworked stock header (ported, polished, ceramic coating)
Xo2 mid/y pipe
Nameless Axleback Exhaust
Grimmspeed Uppipe (ceramic coated)
Cobb catted Downpipe
Grimmspeed Electronic Boost Control Solenoid
DeatschWerks 740cc Side Feed Injectors
Walbro 255lph Fuel Pump
Phenolic Spacer for Intake Manifold (3mm)
Optima Red Top Battery
Cobb Short Throw Shifter and Bushings
Exedy Stage 1 Clutch
Subaru WRX Single Mass Flywheel
Tein S-tech springs/H&R Springs
Perrin Sway Bars
Progress Rear Sway Bar Reinforcement Brackets
RallyTek Endlinks

[TheReal_ND]

Oh wait now I get it. You got a small turbo. I'm used to looking at dynos of fwd cars with larger trim turbos. Yeah looking at your hp curve that wouldn't be very productive on a fwd car. It would just roast tires.

[adwinistrator]

Oh wait now I get it. You got a small turbo. I'm used to looking at dynos of fwd cars with larger trim turbos. Yeah looking at your hp curve that wouldn't be very productive on a fwd car. It would just roast tires.

There we go! I used to think the same way, thinking psi = power, it's an easy mistake since most turbo setups with higher boost psi end up with higher power.

Since I spend 99% of my time driving below 5,000 rpm, I'll gladly take the extra torque and reduced turbo lag, and try to keep my engine reliable, instead of a bigger turbo I have to wind to 6,000 rpm to utilize.

[de officiis]

[TheReal_ND]

Yeah but on the other hand, when you're at the track you don't spend a lot of time below 5k. I never boosted any of my cars but I had a few I wanted to. I don't know if I have any pics of all the cars I had but I'll list the fun ones I've owned

93 prelude with a JDM h22a

86 turbo coupe that the prior owner swapped a mildy built 302 in with hot cams, high comp pistons, MSD ignition, 650 holly and a Ford 8.8" locker with tall gears (can't remember size but it was a rare one from a Mustang.) The turbo coupe five speed mated up perfectly. Dual pipes with an h pipe crossover. Fucking loped like a beast.

82 Mustang ghia that I was going to swap the 302 in along with the locker. Very clean condition miss it. Good gas mileage

96 Miata after a guy at autoX sold me on Miatas for autoX

03 ser spec-v pretty much fully bolted except cams.

That's pretty much it except a couple daily driver fords and my E250 van can.

[Penner]

Money no object, Bentley blower





second choice, VW Schwimmwagen

Those kinds of antique cars looked like Mr. Burn's cars from the Simpsons: