philip77o

Hi, I just bought a 86 944 turbo, it is chipped and it has vac/boost gauge and air/fuel ratio gauge. On first gear the boost gauge goes to 0 PSI and when I change gear 2-3-4-5 it boost at 13-14 PSI

Techno Duck

You will see very little boost in 1st gear; low load on the engine and the engine revs too quickly.

Dougs951S

Yup. 1st gear is short and the problem is compounded if you are not running the stock turbo. I'm lucky to see a few lbs in 1st unless charging up a huge hill.

philip77o

Alright, thank you !

NM'87 951

I have a similar "issue". The car, in first, doesn't kick in boost until 4k or so, and then stops at 5600 or so. Second I get better kick, and then the transmission issues kick it 3rd an 4th. Good to know its pretty normal.

1987Porsche944WithRealLongName

I've never really understood how load correlates with boost pressure

Can someone simplify it for me?

KaiKai951S

From what I learned...Through experience is... Get used to rowing the shift and Listen to Hans Stuck on driving a Turbo. My 88 951S with K&N, MAF, Chippie, Arnold S. Injectors, K27/8, ported head, 3" exhaust, no Cat (Sorry Sabrina) dogs it in the low RPMssss... Like an e30 M3 with a lot more return, its all about the Revs dude...

Dougs951S

Its pretty simple. A turbocharger contains two halves. One can think of the compressor side as the "work output" of the turbo, and the turbine as the "driver" side. Hot exhaust gas spins the turbine, which rotates the compressor to produce mass flow. The 3 main parameters that determine how much thrust the turbine gets from the exhaust flowing over it are heat, velocity, and volume. As any combination of these 3 increase, so does the angular velocity of the turbine shaft. The turbo is a purely mechanical device and has no way to self regulate its rotational speed. If not for the wastegate venting pressure to regulate the volume of exhaust hitting the turbine, it would set up a snow ball effect that would quickly destroy the turbo from excessively high rpm. As the rate of speed increased, it would flow more air to the engine which in turn would produce a larger volume of hotter exhaust, feeding more energy into the turbine till it spat a wheel out. The reason boost correlates to load is that under low load conditions (flooring the car at 2k rpm in 1st, flooring it downhill at 2k rpm, ect ) there simply is not enough of the three previously mentioned exhaust parameters to supply enough energy to the turbine to get the compressor moving fast enough to generate useful boost. This of course also depends on the particular turbo since smaller ones require less energy to get moving quickly. All of this is also why there is so much more to picking a turbo than just the compressor wheel. Turbine wheel, hot housing, exhaust setup, and back pressure all play a huge part in determining at what point you start making positive pressure ( your boost threshold ) and how quickly the turbo responds once past that threshold ( lagginess ). The boost threshold will change in each gear due to simple physics. The lower gears provide the motor with more leverage, so it doesnt have to work as hard to accelerate the car in lower gears. This means less load on the engine. Going up a hill, being in a higher gear, or dragging the brakes = more load on the engine, which lowers the boost threshold but will NOT generally change the response time of the turbo appreciably. ( ie, if the turbo takes 1 second to spool up from atmospheric to 1 bar once its well past its threshold, it will generally take 1 second whether you are in 3rd gear or 5th gear )