What Calvin said. I wouldn't call the engine 'fragile', but it does have an Achilles Heel in the cast pistons. It's not an engine you should over-boost, unless you're willing to accept that the fun ride might not last all that long. If you rebuild the engine, install a set of JE forged pistons. Then you can play with the boost a bit more.
The carb cars don't have a smart ECU/ O2 sensors/ knock sensors that can control lots of boost. The stock dump valve is set for 10 psi. If you bump the boost up, then setting the max for just under 10 psi will make a noticeable difference in performance without requiring re-engineering the intake and fuel systems.
An aftermarket boost controller is a good option. The stock wastegate has a boost signal hose that directly connects the diaphragm to the plenum. The wastegate opens gradually as boost builds in the plenum. That means when you launch from a no-boost, off-idle condition, the wastegate is progressively dumping more and more exhaust gasses overboard at the same time the turbo is trying to build boost. It's counter-productive.
A boost controller is a smart valve in the hose between the diaphragm and plenum. It remains fully closed until a preset boost level is reached, then opens all at once. That means boost and low-end power build much more quickly.
Even if you leave the max boost limit stock, the engine's low end performance will be significantly improved, but the max boost and horsepower will be the same. Better performancee without increasing the peak stress on the engine or drivetrain. Or, an adjustable/ programmable boost controller can also be set for a higher max boost without the need to modifiy the wastegate. Just turn the knob... keeping in mind the dump valve is still set to 10 psi.
A good companion accessory for the boost controller is a Blow Off Valve (BOV). It's different from the dump valve in that it has a diaphragm that's plumbed into the manifold on the downstream side of the throttle butterflies. Whenever the throttle is closed enough to create manifold vacuum (idle, lift off when shifting, over-run, etc), the BOV opens to vent excess boost so it doesn't act as back pressure on the turbine and cause it to decelerate quickly.
With a BOV, the turbine rpm stays up more during temporary lift-off, and with a boost controller boost builds more quickly when the throttle is re-opened. Between the two of them, turbo lag is minimized and the low-end boost curve rises more quickly to a fuller fatter shape without exceeding the pre-set boost limit. Off the line response and initial acceleration are better, but peak performance is pretty much unchanged.
That applies whether you keep the stock boost, or bump it up a couple of psi.
If you increase boost beyond 10 psi, then you will also have to do something about the stock dump valve ('dump' it, or shim it up to an appropriate higher vent pressure), and ensure that the fuel pump and fuel pressure regulator can continue to provide fuel pressure to the carb inlets at 4.5 psi above the boost level.
If you just rebuild the stock turbo, inquire about upgrading the bearing. A ceramic ball bearing can result in the turbine spooling up much faster, along with less turbo lag and a more rapid overall rise in boost.
Talk with John Welch at WC Engineering about turbo upgrade options he may have available for the carb Turbo Esprits. Expect better efficiency and less heat build-up with boost, rather than lots more boost... and maybe a little more boost. John has more Esprit-specific knowledge than most generic turbo shops.
Install a K&N filter element. It has less pressure drop across the filter media and will result in a small incremental jump in power. Not big, but the small upfront cost of the washable filter lasts a long time. The effect is enough that you should re-jet the carb a step or two richer.
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If you ever rebuild the engine, install the JE forged pistons mentioned above, and specify 8.5:1 compression instead of the stock 7.5:1. Then re-curve the distributor for a more aggressive advance curve and install the green-dot, 104 MOP pulleys on the cams. All together, that will build more cylinder pressure (power) without the need to exceed the dump valve's 10 psi vent setting.
Putting a 104 cam on the intake side only will produce more top end power, but at the expense of some low-end throttle response and torque. If you do it all at once, it will translate as an overall increase compared to stock. However, if you do all but the cam and experience that first; then adding the cam will be noticeable as a small step backwards at the low end of the rev range... but it will pull like a train above 4000 rpm. Stock cams for a mostly-street car, 104 intake cam for an occasional track day car. Choose, you can't have both at once.
If you need more performance than that, then you should consider selling the car and trading up to an SE or later Esprit with the stronger UN-1 transaxle and drivetrain, along with the more sophisticated engine management system.
Good luck,
Tim Engel
