compressor surge?

[DSM "BISH"]

i was watching a best motoring video back from the olden days and they had old group b rally cars on there and a few old turbo jdm race cars and they all had compressor surge. the way they sounded. why would they build them like this if compressor surge is such a bad thing? is it cause it is better even though it hurts the bearings in the turbo because they can afford new ones or is it becasue there turbos arent ball bearing turbos? im just very confused. will someone please clear this up for me.

 

[DSM "BISH"]

would anyone like to contribute to this discussion. its technically related to the newbie forum. just tryin to learn a little about compressor surge.

 

[polarmoment]

They built them that way because it results in faster lap times. Bypass valves have never and will never improve the performance of a car.

Look up any of the old Group C, IMSA GTP, turbo era F1, etc cars and you'll find that not one of them used a blow off valve. When you shift quickly enough, the boost remains in the pipes and the turbo barely slows down, which results in almost instantaneous throttle response as you get back on the power. When the blow off valve vents pressure on a street car, I'm sure you've noticed that it takes a second for the boost to get back up where it was.

A blow off valve serves two purposes on a street-driven vehicle:

1. Reduces turbocharger-related warranty claims for cars running factory boost over ~10psi. A lot of turbocharged cars that ran less than that did not have OEM bypass valves. Some didn't even have intercoolers. Look at the turbocharger design on some of the cars that came with them from the factory and you'll see why they used them. The celica all-trac didn't have one, but the MR2 using the same 3SGTE did. The reason was the ceramic turbines used in some of the MR2 turbochargers. They were a lot more sensitive to sudden changes in wheel speed. IIRC, the celicas only ever had the inconel turbines, not the ceramics. the later ST205s, which ran more boost in OEM trim, did come with bypass valves.

2. NVH (noise/vibration/harshness) reductions. Most people don't want to hear compressor surge. Still others would hear it and think something was wrong with their new car and rush to the dealer and waste their time with the non-existent problem. The bypass valve contributes to the overall "smoothness" of the drive of the car.

One strategy that some of the turbo-era F1 cars used that DID improve turbo response was installing a secondary butterfly on the turbocharger inlet.

This is the BMW/Megatron engine used by Benetton, Brabham and Arrows:

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It was a production-based M3 block, destroked to 1.5L, ~6.5:1 static compression running over 70psi in qualifying trim and made ~1500hp (BMW's dynos only went to 1400HP)

They used ITB's as primary throttles, with the secondary throttle mimicking the primaries. The idea was to create a vacuum that the compressor could spin freely in to reduce pumping losses across the compressor, therefore maintaining impeller speed. Renault used a similar setup on their twin-turbo V6s.

 

[DSM "BISH"]

this is incredible! thank you so much this is exactly what i was looking for.