Guys,

Was just looking at the graph of a GT35R compressor tonight and pondering compressor surge (which I think I could momentarily get if I set full boost past 20psi) and I came up with a potential solution.

Wire up a PWM solenoid (like one you run a wastegate with) and plumb it in line of the signal boost line to the BOV.

Now a BOV opens because the pressure on top of the spring combined with the spring pressure itself (which hold it down and shut) is less than the pressure in the intercooler piping under it.

So, if while you're engine is coming on boost you decide that the turbo needs to be able to push a little more air out of the system somewhere (and the engine can't swallow any more), you can switch the pressure on top of the spring from being equal to what is under it (both say 25psi boost), to atmosphere.

Thus 0psi boost pressure is less than the 25psi under it and the BOV will open and bleed some air out of the system, allowing the compressor to flow more air at a specific boost level and giving the engine time to open up.

I have heard tell of people doing this in the extreme case of holding the BOV open until the engine is at an appropriate RPM then slamming it shut. Mucho boost increase! I'm thinking more about controlling it gently.

If you could be arsed doing this though I suppose you'd have an aftermarket computer and if you have one of them you can probably just set a boost curve relative to RPM anyway. Which I think the autronic has.. anyway.. something for you to ponder.

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

What rpm does this surge occur at? Are you sure it is comp surge?

Using that method you're not exactly seting the engine up to handle 20+ and not surge, you're just keeping it out of the surge zone for a little while.

Zac,

My engine doesn't do it atm because its only running 15psi. But if you look at the GT35R comp flow map:

http://www.turbobygarrett.com/turbobyga ... comp_e.gif

You can see that when you get above 1.25 bar boost (2.25 PR) the surge line moves right ward a great deal. Thats where the L28 will go into surge as boost comes on around 3500-4000rpm.

The engine should handle 20psi for a while. How else would you recommend I get around the potential surge problem?

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

I see what you mean. must be pretty close to that line.

I dunno, port the head and put a bigger cam in it?

Hope you are not implying that the L does not breathe very well

M600 will sort it Dave.

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Richard... http://www.youtube.com/watch?v=rqM9_Wq-TKc

sorry 26ODET, but how does a computer fix problems with a compressor

Dave, wouldn't making the engine breather better (ie bigger cam / bigger ports) "move the surge line" to the right even more on your particular engine?

_________________
frankenstein projects R us.

The Republic of Western Australia -
Propping up the Nation's Economy since 1901.

larger ports, longer duration cam timing would mean the air could flow through the engine (even if it's flowing straight out the exhaust port), so less back pressure and less chance of compressor surge. wouldn't it?

_________________
1971 Datsun 1600 - Z18ET
1996 Suzuki RGV250 VJ22
1996 Suzuki RGV250 SP VJ23

Richard,

Dunna be dissin' da L buddy.

Pro,

Richard is just niggling me because I'm sure he thinks a M600 is a waste on an old L.

Zac/Guppy,

Better breathing via cam/porting is always going to help you are quite right. But if I have it better breathing I'll probably also have it so it can rev higher in which case I'll probably want a larger compressor and then we could end up with the same problem.

I wasn't speaking about my car in particular, just using it as an example to explore an idea.

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

What Dave was saying earlier about using the ECU to control boost according to RPM.

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Richard... http://www.youtube.com/watch?v=rqM9_Wq-TKc

Just checked the doco and the SMC has engine speed/temperature dependent boost control calibration tables anyway so I'll just use that if it becomes a problem.

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

using the formulae here:

http://www.automotivearticles.com/Turbo_Selection.shtml

dave's right, at 20psi (2.36 pressure ratio), assuming 85% efficiency, an L28 will flow 30.6 lb/min. putting it right on the edge of the surge line.
as far as i can see, dave's method will work because it's allowing the engine to artificially breathe better, until it is ACTUALLY able to breathe better.

at 4500 rpm it's flowing 38.6 lb/min which is almost at peak (79%) efficiency of the turbo.

_________________
1971 Datsun 1600 - Z18ET
1996 Suzuki RGV250 VJ22
1996 Suzuki RGV250 SP VJ23

have a bleed valve on the intercooler piping sized such that it would bleed off say 5psi at 3500rpm, then have a solenoid snap it shut at 4500rpm?

that way you'd still be running 15psi up till 4500rpm, and it wouldn't be an absolute dog.

_________________
1971 Datsun 1600 - Z18ET
1996 Suzuki RGV250 VJ22
1996 Suzuki RGV250 SP VJ23

ah okay - personal joke....

_________________
frankenstein projects R us.

The Republic of Western Australia -
Propping up the Nation's Economy since 1901.

dave, nah I was implying that your compressor is too damn big

I think your idea would probably work - all the excess volume would just go straight out of the BOV as wasted energy. however as it moves a little closer to the peak efficiency, it might even make more power.

Why do you think that if you open up the breathing you will need a bigger compressor again? Where is peak hp on the comp map now?

Love the L, in someone elses car No, actually a M600 would be a good move.

While we are on turbo's, what size for a VG33 single? GT35R a bit small?

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Richard... http://www.youtube.com/watch?v=rqM9_Wq-TKc