Re: [DML] Turbo PRV.

[Martin Gutkowski <martin@xxxxxxxxxxxxxx>]

Marc Levy wrote:

>As far as specific answers to your questions (sourcing
>parts, and such), My plan would be to rebuild your PRV
>using parts from a Alpine, or maybe even a Venturi. 
>Those pistons/rings/rods/head gaskets all worked well
>with forced induction..  Legend had planned on a
>electronic boost control (I think 502 has this), but
>1860 has a vacuum based control with a threaded rod to
>adjust the waste gate (My guess is this is how they
>dyno-tested the engines).
>

Basically, Legend were ahead of Renault in this endeavour. Renault went 
on to develop the PRV in a turbo application for the Renault 25 Turbo, 
Alpine GTA V6 Turbo (replaced later by the A610) and the Renault 
Safrane, also available as Bi-turbo, but only in France.

These engines are all single turbo, even fire, and EFI with distributed 
electronic ignition (ie it has a distributor but only to distribute the 
spark, nothing else). They use a strange 66/3 trigger wheel that's part 
of the flywheel and is read by a sensor mounted in the bellhousing - 
making the UN1 bellhousings off any of the later Renaults quite sought 
after because they are otherwise identical. The mains and smalls are 
bigger on the crank and the even-fire is achieved by an ofset on each 
small end. The engine does run less smoothly as a result, but some later 
PRV had a balance shaft on the right-hand head. I've never seen one though.

I can only really speak for the 25 turbo engine as I've got one and have 
done some research on it. Marc, the wastegate is always set by a 
threaded rod. Its function is to pre-tension the spring which is 
compressed by a piston fed by air from the intake manifold. When the 
boost reaches a high enough pressure to compress the spring, the 
wastegate opens and exhaust bypasses the turbo. The electronic control 
works by bleeding off air from the wastegate control so that that piston 
"sees" a lower pressure (proportinately) than is in the intake manifold. 
So, basically, a turbo wastegate is always set to its -lowest- boost, 
and then the boost is increased by bleeding air off the wastegate line. 
A PWM valve is used to achieve this, and it works in exactly the same 
way as the frequency valve in the lambda system. The R25 ECU includes a 
speed sense which is used to limit the boost at low speeds because 
otherwise the car, which is front wheel drive, would wheelspin crazily. 
Obviously I'm not doing this on the DeLorean!

I think you may be referrng to a dump-valve (I think you guys call them 
blow-off valves) which is vacuum actuated and will allow the turbo(s) to 
dump their pressure when the throttles close. The Renault lumps all use 
a recirculating dump valve which passes back to the intake side of the 
turbo.

Hope this makes sense!

Martin





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