Turbo Guys - What can be done to keep the turbo spooled between shifts?

[RarlyL8]

Hey there HP junkies!

The posermobile is running awsome! Boost is locked down at 0.7 bar, throttle is locked down to 3/4, all done to allow room to grow once I get used to the present power level. All CIS systems are working perfectly, A/F ratio is set at 13:1, CO is presently at 1.5% and will soon be set at 3% (the temp has dropped from 70F to 25F resulting in slight changes). I have been trying to film the first and second gear smoke fest but the weather just won't cooperate.

Now - on with the question.
When I change gears during a "run" the boost is lost momentarily. I know WHY this happens, what I'd like to know is HOW IT CAN BE REDUCED. The 915 tranny requires a full one second to shift between gears while off throttle and loosing spool. SPEED SHIFTING IS NOT AN OPTION. I've got WAAAYYYY to much time and money in this thing to blow it up. Besides that, I'm unemployed so a $10,000 rebuild would seriously shorten my vacation.

SO - any ideas? C2 Turbo type bypass valve maybe?

[avendlerdp]

Does it have a blow off valve? You need to vent the turbo pressure in the manifold off so the turbo can keep spinning freely when the throttle is closed. Otherwise the boost pressure will slow or even stop the turbo and you start over on the next shift. I will give you the part # of a cheap one off a Volvo I use on my Mazda rally car.

[Stephen B]

Don't 930's have a bypass valve?? It looks like mine runs up from the pressure side and dumps into the I/C.
If not, 993 bypass valves are about 30 bucks from the dealer.

[john walker's workshop]

the turbo cams suck. doggy as hell. an SC grind makes a world of difference. headers will spool it faster. a C2 turbo bypass valve setup helps. if you do that, block the stock bypass piston with a length of PVC pipe so it can't move. that way, the housing can stay in place and you don't have to use a long neck intercooler. on my 87, i just blocked the bypass piston, and if you shift reasonably quickly, the turbo stays spooled up. if you don't, it can stall. but i have a G50 in that car, so it does shift faster than a 915.

[John Rogers]

We did the following to our 1987 930: SC cams, head work, long neck intercooler (double size) and an HKS boost controller. The new intercooler replaced the pop-off valve so there is NO turbo lag and boost comes up at 1600 RPM or so. The HKS controller allows an over-boost of 1.25 bar for 5 seconds (adjustable) and it is like a huge kick in the pants! The boost stays up during shifts since we do not try to shift fast and when the next gear is reached the power is still on. Good luck.

[rattlsnak]

To go along with what JW says, mine is a C2T with GHL's, and have no lag, or spool down. (due to the C2 redesign and headers)

[ZAMIRZ]

It could be in your driving. When I drive a turbo car, I don't lift all the way off the gas between shifts (granted you're going for some hard acceleration) so that it keeps the turbo spooled up between shifts and then when i dump the clutch in the next gear i floor it real quick as the clutch is engaging to give it that extra oomph so that it doesn't bog under load. This way, you feel only a very slight hint of lag.......besides that, it looks way cool too if you do a downshift and fly-by because the engine runs rich between shifts and puffs some black smoke......damn, i need some boost .

later,

amir

[john walker's workshop]

yes, the boost coming up against the closed throttle plate is what stalls the turbo, so if you don't close the throttle all the way.........

[arrivederci]

here is NO turbo lag and boost comes up at 1600 RPM or so

Do you mean that you can make -some- boost at 1600 rpm? Or that you can hit full boost (ie, 0.7-0.8 bar) at 1600 rpm? I see people making all sorts of boost claims, but have yet to see any incredible low rpm boost myself. Let me define my understanding of LAG and hitting boost just so that everyone is using the same definition.

lag - The period of time between flooring the accelerator and achieving boost.

boost - I usually equate this to achieving FULL boost. If not full boost, then what's the use?

I can get full boost in 4th gear at 2500 rpm, but it takes a while (5+ seconds -- laggy) for it to reach full boost at that rpm. At 3500 rpm, there's very little lag, although its not as responsive as a n/a car...so there's still lag.

My car:
K27 7200
B&B Intercooler
HKS EVC-IV
GHL headers with dual outlet muffler
drilled airbox
stock cams

[beepbeep]

BOV are there to prevent compressed air hitting closed throttle, so those suggesting that one should crack open the throttle obviously don't understand how it works.

I'm yet to see non-diesel car engine achieving full boost at 1500 RPM. Even small-turbo equipped engines will achive full boost somewhere around 2000-3000 RPM. Boost-treshhold (point where RPM's are high enough for turbo to start making boost) for stock older 930 is somewhere around 2500 RPM. K27, headers and cams will lower this somewhat.

Rarely8:
There is a BOV in your car (altough of pretty complicated design) that will let turbo recirculate air when you close your throttle. Make sure it works correctly. You can also block it and use ordinary plastic Bosch recirculationg BOV. Stay clear from ricer-style HKS open-air BOV's (even if some like it's sound)


Better cams will certainly shorten turbo-lag but i doubt they will help turbine-spindown much.

Also, why run with only 3/4 of throttle travel??

[Bruce M.]

I'm not sure whether this is "ricer style", but I run a modified Lindsey Racing BOV they use on their big HP 944s, vented to the air. Before that, my '78 930 did not have a BOV, or at least it didn't have one I could identify as such, and if it existed it didn't work.

[RarlyL8]

Looks like I need to clarify things a bit.

Engine specs:
'78 3.3L 930 California engine w/all smog removed
Stock CIS
Stock BOV
SC Cams
Euro Exhaust
K27-7200 turbo
1.0 bar spring, now 0.7 bar
B&B large intercooler
Modified stock muffler

Boost comes at ~2800 and is full on before 3200. This is in 4th gear. The momentary loss of boost between gears is exagerated by my shifting technique (or lack of).

The 3/4 throttle is the result of incompatable SC throttle cable-to-930 motor. I have the part to fix this but decided to leave it be until spring.

I was wondering if an aftermarket BOV might improve this situation.

[dean]

Most turbo motors have some lag between shifts. My 930 has maybe 1/4 sec lag. My TDI Jetta is much worse, maybe 1/2 sec. I don't know if an aftermarket CPV will help. If I were you I would fix the throttle first. That would contribute to more lag. I have the 993 CPV in my car, and for $26 and some fabrication time it was well worth it. Plus I get that 935 sound between shifts.

[avendlerdp]

Sounds like a BOV is the key here. The one off a late 80's Volvo turbo made by Bosch is cheap and flows plenty. If you want you can use two. Just route them back into the intake somewhere before the turbo and after the air flow meter...

Then you can close the throttle and the turbo can spin free, free I tell you!

Alex

[avendlerdp]

Oh one last thing. You have to do a recirculating BOV with CIS or the vented air will cause a rich condition and shoot flames out the tailpipe on decel. Unless you want that of course...

[bell]

i've been using (don't flame me ) 1st generation mitsubishi eclipse bov's for some time now (on my turbo dodge), they can handle (and not leak) up to around 2 bar. they have the standard flange to mount and are aluminum. you can find them at bone yards for around $10 or used on ebay between $20-$50.
one thing to remember is that the springs in bov's are "preset" to handle different pressures, is it possible you have to strong of a bov for your boost application? if the spring is too strong it could not be opening all the way causing spool down.
i also make sure i use a filter on the bov, under higher boost levels the bov can open under the pressure and leaves the possibility of pulling in dirty air.
my .02

[ZAMIRZ]

Originally posted by avendlerdp
Oh one last thing. You have to do a recirculating BOV with CIS or the vented air will cause a rich condition and shoot flames out the tailpipe on decel. Unless you want that of course...

Of course you'd want that, it's the whole point of running straight through exhaust on a quick turbo car. It's gonna look SICK, as a matter of fact I hope he films it and posts it.

later,

amir

[Amac]

Recirculating BOV is the way to go!!
I'm running (2) and works great.
The flames are a cool by-product.
With BOV's in place your car will jump
after shifts. Headers will also help.
Wish I could get boost at 1600!!!!
With all my mod's 2500 to 2600 is best
I can do.
Majestic turbo is building me a K-30, maybe
it will drop some.
Good luck!

[RarlyL8]

Recirculating BOV, isn't that what the stock unit does?

Is there a "kit" that will adapt my intercooler into a long neck and using a smaller C2T type BOV? I'd like to junk that big nasty looking stock unit.

[dean]

yes the stock one does recirculate. I don't know if there is a kit. There must be. I built my own.