Mustang MonthlyHow To Engine
Stroker 390 Dyno Test
Trans Am Racing puts our stroker big-block to the test
Last month, we showed you how to take a sedate 390 FE big-block and give it stroke, which ultimately translates into torque and more than its share of horsepower when it's time to pin the butterflies. This time, we want to get into the more finite details on how to build common sense street power into a 390 High Performance V-8. Too many of us get caught up in that dreamy eyed horsepower thing without considering first how our engine will be used.
"Horsepower" is an oft-misused term coined by Madison Avenue advertising agencies because it sells automobiles. It also helps the aftermarket sell engines and performance parts. However, horsepower is a meaningless dynamic unless you are going racing. Even then, it's rather meaningless because horsepower is used only at high rpm and for a short time at wide-open throttle. What matters most is torque--the grunt factor that gets us going before it hands all the fame and glory off to horsepower. We infuse stroke into an engine to make more torque, not horsepower. Torque comes of the raw mechanical advantage of stroke--that long lever you learned about in high school physics. With a lever, you can move the world. The more stroke and connecting rod length (rod ratio) we can give an engine, the more torque we're going to make. And that is what this 442ci FE is all about.
"This 442ci stroker demonstrates how well this combination of parts works together," explains Trans Am Racing's Mark Jeffrey. "When you run the air hat on a dyno, it monitors how much air is being drawn into the engine. The dyno's programming calculates how much power is made. When the percentage of in and out have a huge differential, you know by the VE numbers that you made a poor decision with parts selection. It means your parts aren't working in harmony. To get torque, you need bore, stroke, and the right valve timing events as they relate to piston travel. Although this seems simple in theory, it isn't in reality because there are so many hurdles. You've got to be a great engine architect in order to outfit your FE with the right combination of parts and chase it with proper tuning.
Mark has an instinct for cam and head selection and isn't afraid to push an engine to its limits. In the heart of this 442ci FE stroker is an aggressive, yet streetable, Comp Cams hydraulic roller camshaft, which unleashes power not only from its profile but also from reduced internal friction. It gets better with Comp Cams' new shaft-mounted rocker arm assembly for FE big-blocks with Low-Riser cylinder heads, including the Edelbrock Performer RPM castings that Mark has fitted to this FE. These 1.76:1 ratio, 1.200-inch wide billet rocker arms pivot on precision needle bearings where nothing can disturb their geometry. What's more, they're fully adjustable. End stand kits are purchased separately.
Mustangs Plus set us up with a Milodon oil pan, windage tray, and pick-up for our Trans Am Racing 390/442. Mark needed to do some clearancing in order to clear the crankshaft counterweights and oil pump. The windage tray keeps oil out of the spinning crank and rod journals, which prevents foaming at high rpm. Even if you're building a stock 390 FE, a windage tray is still a good idea for those wide-open throttle experiences. Oil windage also consumes power because it creates internal resistance when oil wraps itself around the crankshaft and connecting rods at high rpm.
When you build an engine, plan an exhaust system to match. Tuning an exhaust system is as tricky as choosing a cam, selecting heads, and deciding on a manifold and carburetor. Your Mustang's exhaust system begins at the header flange and ends at the tailpipes. Every decision you make in between determines rear wheel power.
We've chosen Hooker Super Comps for our dyno session. However, street torque doesn't come from huge 1-3/4-inch equal length primary tubes and a large 3-inch collector. It comes from smaller primaries and a smaller collector into 2-1/2-inch pipes. Header tube and pipe sizing boils down to what you want the engine to do in the car. Be conservative about pipe sizing and you will get torque. When pipes are too large, you lose torque but gain horsepower.
Torque comes of the raw mechanical advantage of stroke--that long lever you learned about in high school physics
Block Prep Tip
We didn't have space to touch on this one last month, but block prep is key to durability. You will find a lot of FE blocks out there with what appear to be misdrilled main journal oil galleries. We're convinced this offset, or "misdrill," was intentional to control oil flow to main and cam journals. Jim Grubbs of JGM Performance Engineering believes FE blocks can use a little help, which is why he modifies the main journal oil galleries to improve oil flow to both mains and cams. Jim also distresses castings to minimize the risk of cracks at casting parting lines where he grinds these surfaces smooth.
It's All Gotta Fit
The difference between building a Mustang stocker and a restomod is fitment and compatibility of aftermarket performance parts. For example, our good-looking Ford Racing "Cobra LeMans" cast valve covers wouldn't clear the Comp Cams shaft-mounted rocker arm assemblies without minor internal grinding at each end of the valve covers in order to clear the end supports.
On The Dyno
Every engine testing experience is unique. This is only the beginning of dyno testing Trans Am Racing's 442ci FE big-block. First, we want to know what this engine will do right off the engine stand. Then we want to conduct a series of tests with both Edelbrock dual-plane and single-plane manifolds to see what happens to this engine's powerband. Because dual-plane manifolds have long intake runners for better low to mid-range torque, you can expect torque to come on strong around 2,500 rpm and pass the baton to horsepower around 5,800 rpm.
The single-plane manifold with its straight shot into the intake ports means horsepower on the high end and a loss in low to mid-range torque. Long intake runners deliver torque because they afford velocity through those runners as low to mid-range rpms.
In the months ahead, we're also going to test this 442ci FE stroker with Holley's new EFI throttle body injection system to see what can be expected from both carburetion and electronic fuel injection. We're also going to try different sized carburetor and jet swaps to see where power can be gained via small changes.
The numbers here demonstrate what you can do with a 390 FE block when you fill it full of displacement and mechanical advantage. By adding stroke and a pinch of bore to the grocery-getter 390 along with ported Edelbrock Performer RPM heads and induction, plus Comp Cams roller tappet technology, we wind up with 582 horsepower and 557 lb-ft of torque--both between 5,000 and 6,000 rpm.