In the May 2009 issue of PHR, we introduced you to our 408ci small-block Chevy that now resides in our '75 Laguna project car. Designed to maximize the limited real estate inside a standard 350 Chevy block, the magic 408-cube number was arrived at through careful selection of the right handpicked core block (of 1995 vintage), and rotating assembly (a 4-inch stroker kit from Scat with Mahle forged pistons, forged crank, and forged 6-inch rods with excellent bolt-head and rod-shoulder clearance). When combined with a .600-inch lift solid roller cam from COMP, AFR Comp Eliminator 210 heads (making 298 cfm of flow at .700-inch lift), Edelbrock Performer RPM Air Gap intake, and an AED-modified Holley 750 HP, the combo made an impressive 551 lb-ft of torque at 4,600 rpm and 560 hp at 5,800 rpm.
It runs on pump gas, thanks to a street-friendly 10.5:1 compression ratio, so what's not to love about that kind of power lurking beneath a flat hood? As we assembled the Laguna over the last few months, nesting the 408ci small-block underhood, and pairing it with a bomb-proof 700-R4 transmission from Phoenix Transmission Products, we could hardly wait to bury the accelerator into that new OPG carpet. When we finally got our low-restriction side-exit exhaust built by LP Racecars-assembled from Dr. Gas NASCAR components-we realized we were in sight of our goal.
Over the last month, we've had time to sample the 408's power on the street. The righteous solid-roller side-exit snarl notwithstanding, we suspected something wasn't quite up to snuff. For all appearances, everything sounded and looked in order, but it sure felt like a few oats were missing from the proverbial feed bag. We went through the mental checklist of possible causes. In particular, the massive gross vehicle weight of our Laguna seemed like a valid "black hole" for some of the grunt we were missing.
Ernie Mena of Westech Performance Group in Mira Loma, California, is an expert at tuning c
Beyond that, we recognized that we were running a full accessory drive with a mechanical clutch fan, mechanical water pump, alternator, mechanical fuel pump, and power steering pump, while our engine dyno test was performed with just a mechanical water pump. The automatic transmission was also a possible culprit; we had ordered a converter based on the dyno numbers from a similar, yet not identical, engine. However, there seems to be quite a bit of slippage, as the stall speed is about 300 to 500 rpm higher than the engine wants. It's common for DIY builders to overestimate their engine power when ordering a torque converter-basically it's the outgrowth of optimism. Some transmission specialists compensate with converters on the loose side in order to avert disaster. In most cases, it's the right call. But could our converter be a little too loose?
Lastly, a lot of other factors creep into the power equation when an engine is installed in a car. Is the electrical power up to snuff for the ignition? Is the timing optimized? Is the fuel system equal to that of the dyno's? Are the headers and exhaust more restrictive than the dyno headers? And last but certainly not least, is it possible that the original engine dyno was a tad on the optimistic side? At the end of the day, we had no idea if any of these factors could mitigate all the power we suspected was missing. Without a dyno test, we might as well consult an Internet message forum-the modern-day analog of a magic eight ball. Thus, we set up a chassis dyno test at Westech Performance Group in Mira Loma, California, on their in-ground SuperFlow dyno. Once at Westech, tuning guru Ernie Mena strapped the Laguna to the rollers, and we got busy with a baseline test.
Our baseline showed 390 lb-ft of torque being produced at just over 4,600 rpm, and 353 hp at 5,700 rpm. Said another way, we took a hit of 161 lb-ft of torque and 207 hp between when the engine was dyno'd initially, and when it was installed in the car. This might sound like a lot to lose, but our prior experience with our other project car, the Street Sweeper Chevelle, tells us we aren't that far off. Our 496-cube big-block '68 Chevelle made 626 hp on the pump, but only registered 448 hp on Westech's chassis dyno-and that was after tweaking the tune-up. Our '68 Chevelle was up against similar parasitic burdens for a total loss of 178 hp, yet it still managed to go 11.30/117 in the quarter-mile, uncorrected. In the case of our '68 Chevelle, the collective effort of good tuning and a swap to a tighter converter netted us 29 hp more at the rear wheels.
The mission on this day was to maximize our tune-up where we could, and try to find areas we could improve upon at a later date, specifically the torque converter. After the 353hp baseline pull and a backup run to verify everything, Mena saw that the wideband O2 sensor was indicating an extremely rich condition, with the air/fuel ratio peaking at 10.5:1 at cruise-way too fat for good economy, and certainly rich enough to cause plug fouling and even ring wash. Mena also discovered a plug had been substituted for a power valve on the Holley's primary side (which is roughly equal to pulling out six jet sizes). Since tip-in was rough, Mena decided to replace the plug with a 5.0 power valve. (The significance of the power valve's number is not the jet size, but the vacuum level below that the power valve opens to allow fuel to enrich the engine via the power valve restriction channel. Change the power valve's sensitivity to vacuum, and you change the point at which the added fuel hits the primary side.) With that change, Mena also took the primary jet size down to 68. A quick check of the timing also showed that the timing on our DUI distributor was over-advanced by some 15 degrees, so it was set back to 35 degrees total. (It's a long story, but the locking thumbwheel on our DUI's instant timing knob had gotten loose, allowing the distributor timing to creep over time.)
One of the first things Mena did was replace the plug in the power valve with a 5.0 power
With the timing set at 35, a 5.0 power valve, and 68 jets in the primary, Mena made another pull in the Laguna, this time netting 362 hp, for a 9hp bump. Although the change produced more power, Mena wasn't yet happy with the tip-in, and saw room for improvement, as indicated by the wideband air/fuel data. Off came the primary metering block again, and the 5.0 power valve was replaced with a 6.5, effectively bringing the fuel in sooner. Also, the primary jets were fattened up to a pair of 70s. The rear block came off as well, and the 85 jets were replaced with 81s, for a net reduction in fuel. The overall fuel curve would then have more coming in sooner, but less as the secondaries hit. For the third and final run of the day, the Laguna belted out 366 hp at 5,800 rpm.
While the dyno tuning session did improve our power and overall tune, it also helped diagnose one other glitch, which we already mentioned: the torque converter. The dyno chart clearly shows the converter flashing between 4,400 and 4,600 rpm. In a perfect world, we'd like to see the converter achieve lockup well before then. Since we've seen excellent results by tightening up the converter in the past, we'll be going that route again and reporting back to you with new dyno results in a few months. To the positive, the dyno test also showed that we weren't running out of fuel at the top end, which we had some reservation about.
We're getting close to working out the glitches, and once we do we'll hit the dragstrip, or any other venue that won't slam us with a ticket. (They probably won't let us on the high banks, so we'll have to do our racin' a quarter-mile at a time for now.) We're also contemplating some other tweaks to the car, like finishing the interior, putting mufflers on the exhaust, and toning down the appearance for trouble-free street driving. We'll let you know what we decide soon!
Normally, you wouldn't mix changes in jetting with changes in ignition timing, but the ign
Before the third and last dyno run, the secondary side was leaned out from 85 jets to 81 j
The dyno screen compares the power of our baseline pull (black trace) with our final pull
We've Been Down This Road Before
Before you start firing off angry letters about how we were victims of overly optimistic engine dyno numbers, just remember that we've seen boat loads of power evaporate on the chassis dyno before. In the July 2008 issue, we wrote how our 496-inch big-block went from 626 hp on the engine dyno to just 419 hp at the wheels after being installed in our '68 Chevelle. Ironically, that 207hp drop is identical to the one we experienced this month with our Laguna. Still, 419 rear-wheel horsepower was enough to go 11.70s in the quarter-mile. After some tuning, improving the accessory drive, and installing a tighter converter, rear-wheel output increased to 448 hp, and quarter-mile e.t. dropped to an 11.30. If nothing else, it illustrates that the SuperFlow chassis dyno is consistently conservative.