In the last installment we left off installing and dyno testing Pypes hi-flow exhaust system on our buddy Tim Cairone's '87 Buick Grand National. We realized gains of 38 hp and 35 lb-ft by ditching the stock, restrictive exhaust. Issues with the tired, stock turbo (leaking oil, smoking intermittently at idle and while driving) prevented us from risking a strip test on the Pypes exhaust. We called the folks at Precision Turbo for their advice-should we go with a stock-rebuild or upgrade to a stock-appearing type turbo? Precision offers a large assortment of different size, direct bolt-on turbos for the Turbo Buick, and for our intended build-up Precision recommended we upgrade to its PTB-5758 turbocharger. It's the latest midsized unit for the GN, using the most recent in turbo technology (compressor and turbine housing shape, billet compressor wheel), which will bolt right in place of the original, tired-out turbo.
It was easy to remove and replace (R&R) the tired turbo for the new Precision Turbo, and we chose to bolt on the stock wastegate/exhaust elbow housing and downpipe to the new turbo for a fair test (stock 34mm vs. Precision's 58mm compressor wheel). A testdrive told us the larger turbo was slower to spool, but hitting harder at full boost (19 psi). To find out how much more power the new turbo developed, we visited our friends at Tune Time Performance for dyno testing. After Matt Hauffe (Tune Time's tune-master) put the hammer down on TT's Mustang chassis dyno we realized 25 more hp (from 278 to 303 rwhp) with the same torque output (440 lb-ft). While the larger turbo made more power, its added turbo lag showed both torque and horsepower peak at 500 rpm higher. This is a natural occurrence-the larger the turbo, the longer it takes to spool.
We hoped the added turbo lag wouldn't hurt the e.t.'s as much as the added power should help, but at the track we could only launch at 0-1 psi of boost. Our 60-foot times worsened from the previous 2.1 to 2.3 (full boost wasn't until 100 feet down track). Also working against us was a bad-air summer day (85 degrees, 55 percent humidity, 29.74 barometer, 3,000 feet altitude corrected). Even under these conditions we saw gains in trap speed of over 4 mph, but only dropping a tenth (best e.t. of 13.50 at 104.68 mph) due to the excessive turbo lag.
However, we heard RJC Racing's new hybrid boost controller would help the boost come in quicker, and we were equally as quick to dial them up to give it a try. Meanwhile we also wanted to test out a couple bolt-ons to better take advantage of our new turbo, which might also aid spool-up. A 3-inch downpipe from G Body Parts was first on the list, replacing the stock 2.5-inch piece should in theory reduce backpressure and (hopefully) turbo lag. A testdrive after first installing the RJC Racing boost controller confirmed its effectiveness, as did the G Body Parts 3-inch downpipe. The bigger downpipe added a tangible increase in power as well as responsiveness, and after installing these two products we felt there should be a big reduction in e.t.'s (hopefully into the 12s).
In the meantime, of course, we had to quantify these improvements on Tune Time's chassis dyno, and to see just where peak power occurred. Our seat-of-the-pants feel was confirmed as the the power curve did in fact come in sooner and last longer. Peak torque (450 lb-ft, 10 lb-ft gain) was now 500 rpm lower and horsepower (303 hp, 0 hp gain) occurred at the same 4,500 rpm. With the power curve broadened by 500 rpm (previous 3500 to 4500 rpm, now 3,000 to 4,500 rpm), it helped make Tim's GN feel faster and deliver better all-around driveability.
Here's the power brute from...
Here's the power brute from Precision Turbo (PN PTB305-5857). It's a true bolt-on to replace the worn-out, oil-leaking original.
It features larger housings...
It features larger housings and wheels (compressor and turbine). It's actually a mid-sized unit (58mm billet compressor wheel) when compared to the typical 10-second TB that usually runs a size 67- to 76mm turbo.
The stock turbo performed...
The stock turbo performed well for over 20 years and over 100K until we turned up the boost. Notice the stock 2.5-inch downpipe and the stock exhaust elbow / internal wastegate housing-both turned out to be a major restriction for the new, larger turbo.
It's a simple task for anybody...
It's a simple task for anybody with the right handtools to remove the stock turbo. The turbo's bracket has two bolts holding it to the passenger side cylinder head. The three bolts to the exhaust inlet are a little difficult to get at. A couple of universal sockets (14- and 15mm), extensions (6, 8, and 12 inch), a good ratchet, and a few wrenches make the job easy.
After removing the stocker...
After removing the stocker and placing it next to the new unit, there's a noticeable size difference. Here's looking at the compressor housings and wheels of the new and old turbo. Larger turbos might have more lag (take longer to spool), but make more power. Using today's technology we'll try a few fixes to help reduce turbo lag.
The old and the new turbo...
The old and the new turbo both have 3-inch inlet bells. On the stocker the inlet bell tapers down with sharp edges (causing turbulence) leading to a 34mm compressor wheel.