Mustang MonthlyHow To Engine
How to Build a 390FE Stroker - 500 Pound-Feet!
Check this out—real street power from FE iron heads, factory induction, and an Eagle stroker kit
How do you build brute street power into a 390-based Ford FE big-block yet keep the engine stealthy? We've learned you stick with the fundamentals of where real street power comes from. It isn't that Madison Avenue "horsepower" thing, but instead peak torque, which is where street engines live most of the time. When we were planning this engine, our focus wasn't horsepower, but instead reliable midrange torque for Main Street and the freeway. We also wanted some measure of fuel economy, which comes from having a lot of torque and a light throttle.
When you consider what we have for cylinder heads—vintage C8AE-H 390 castings void of any port work mated to the aluminum 428 Police Interceptor dual-plane intake manifold, our dyno numbers are remarkable—405 hp and 505 lb-ft of torque and without any tuning. We wanted stealthy FE big-block power and we got it. On the surface, it appears stock. However, we fitted it with Eagle's 431ci stroker kit (PN 15810) with a 4.125-inch stroke and 6.700-inch forged I-beam rods mated to 26cc dished Mahle forged aluminum coated pistons, which puts compression around 10.5:1. We're doing this with a cast steel crankshaft. The 428 Police Interceptor dual-plane intake manifold employs long runners with high ceilings, which give the 390's 4.080-inch oversized bores every advantage when you increase stroke. We're taking the 390's 3.780-inch stroke and stretching it to 4.125 inches for a whopping 0.345-inch increase. Torque grows because we're giving the 390 mechanical advantage coupled with greater cylinder volume, which makes the Eagle stroker kit an excellent value considering what is gained for the money.
The message here is affordable performance from Ford parts and the advantage that comes from increased displacement via stroke. What we get from stock 390 heads is velocity at low rpm. We're huffing 431 ci through those modest ports and the big surprise is stunning torque, which is what spanks the other guy at a traffic light.
When Jim Grubbs was shopping for a good hydraulic roller camshaft for this engine, he reviewed the lineup of good street roller cams from COMP Cams and chose PN 33-422-9 with 110-degree lobe separation, 0.521/0.521 intake/exhaust valve lift with 218/224 degrees at 0.050-inch duration. The result has been a smooth idle and crisp right off-idle torque, which is exactly what Grubbs wanted from this 390 stroker. Torque begins to come on strong at 2,000 rpm and peaks at 3,200 with an earth-moving 504 lb-ft of torque. And this is how you want power to roll in from a street engine. Horsepower is meaningless unless you're going racing and plan to keep the throttle at 6,000 rpm.
As in the past with other FE engine builds, Grubbs went to Crane Cams for fully adjustable 1.76 ratio roller rockers, pedestals, and shafts for our 431ci stroker. Some pedestal modification is required depending upon cylinder head and fastener type. Grubbs confirmed appropriate pushrod length using a checker from COMP Cams.
The Polygraph Room
This has undoubtedly been one of the most unusual FE engine builds we've ever experienced because this engine didn't respond as expected. Jim was confident it would make a lot of torque, but not 505 lb-ft at 3,200 rpm. He also expected peak horsepower (405) to show up around 6,000 rpm. It showed up early at 5,400 rpm and fell off the cliff rapidly thereafter. What this means for the street is brute holeshot power down low when the light changes color. With 505 lb-ft of torque at 3,200 rpm, you have a rocket ship if your 431 happens to be installed in a Mustang. And to get these numbers with 390 GT heads with Police Interceptor/Holley induction is remarkable. For an engine to be finished making power at 5,400 rpm means it's going to last and it will deliver some measure of fuel economy with a light throttle.
01. The foundation for our 500 lb-ft 390 stroker is a ’68 390 block with 4.050-inch bores. JGM Performance Engineering will bore and hone to 4.080 inches and give it 4.125 inches of stroke. These are run-of-the-mill ’68 390 C8AE-H head castings with 67-70cc wedge chambers and 2.02/1.55-inch intake/exhaust valve sizing. Port sizing is 2.34x1.34 inches intake and 1.84x1.34 inches exhaust. The bonus here is torque.
02. The 390 block is bored and honed to 4.080 inches to accommodate 26cc negative dish Mahle forged pistons with ductile iron rings. JGM Performance Engineering opted for the 26cc dish to keep compression tolerable.
03. Oil passages are enlarged right off the pump for improved volume. This modification is about greater volume at the main and cam bearings. Restrictors installed in the cylinder heads will keep oil down low for improved bottom end lubrication.
04. Main caps have been reconditioned and line bore honed for greater main bearing security via good crush. Note the crisp machining pattern, which enhances cap security. It is suggested you use ARP main bearing cap studs instead of bolts for greater stability.
05. Eagle’s cast steel crank with forged I-beam cap-screw rods (PN 15810 ) delivers 4.125 inches of stroke, which gets us mechanical advantage and increased cylinder volume. Both add up to a substantial increase in torque. Coated 4.080-inch diameter Mahle forged pistons deliver just the right amount of squeeze at top dead center thanks to the 26cc dish. JGM Performance Engineering has dynamic balanced this assembly, which is ready for detailed inspection and blueprinting.
06. Jim Grubbs of JGM Performance Engineering tells us FE engines tend to get too much oil up top and not enough down below. He installs restrictor plugs in the cylinder head oil galleries as shown to reduce flow to the valvetrain where it isn’t as important and keep it at the main and rod journals where it is needed most. Increased oil flow at the main and rod journals helps carry heat away from the journals.
07. These are box-stock ’68 390 GT cast iron heads with 67-70cc chambers and 2.02/1.55-inch valves. We’re running them without port work just to see what out-of-the-box heads can do. And with basic port and bowl work, power gains can be even more significant. JGM is performing basic cylinder head improvement work—fresh valveguides and hardened exhaust valve seats for use with today’s fuels.
08. Guides have been cut to accommodate Viton valve seals, which wear better than the more common Teflon seals.
09. Hardened exhaust valve seats are being installed for improved durability. Original iron seats wear quickly, especially with unleaded fuels. This is standard protocol any time you’re rebuilding an older iron head engine.
10. Head decks are milled as have the block decks for perfect fit. Remember—when you mill heads and block deck surfaces, you make the chamber smaller, increasing compression. What’s more, the angle between heads and intake manifold can change with the potential for coolant leaks. Check these dimensions closely prior to assembly.
11. JGM is going with new Speed Pro stainless steel valves in the interest of durability. These valves will be measured, dressed, and checked for proper fit.
12. This dimension indicates piston deck height, meaning how far down or out of the bore a piston is. This 0.0005-inch marking means the piston is 0.0005 inches below the deck. Rare is the short-block where all eight pistons would be the same distance in or out of the bore. However, you want this dimension as close as possible to keep compression ratio as close as possible across the board. This is why you should do a pre-assembly mockup to ascertain clearance prior to final assembly.
13. Crane Cams set us up with adjustable aluminum 1.76 ratio roller rockers complete with shafts and pedestals. The shafts can sometimes be troublesome to get depending upon demand. However, they’re also available from Speed Pro. Jim Grubbs uses a pushrod checker from COMP Cams to determine proper pushrod length. The key is getting the roller centered on the valve stem with the valve open. You do not want side loads, which will eat up guides and stems.
14. True top dead center is checked using a dial indicator as shown. The crank is slowly turned back and forth until the connecting rod is at 12 o’clock on the crank journal and the piston is at top dead center where there is no travel.
15. The cam is degreed and compared with the provided cam card. Expect there to be some variations with nearly any camshaft. Double check your numbers.
16. Crankshaft endplay is checked and should be 0.004 to 0.010 inch. Endplay should be closer to 0.010 inch if you’re going to do some weekend racing.
17. A good used timing cover from Mustangs Etc. is fitted and installed using the crank spacer, which gets the crank centered on the seal. If the seal has a garter spring, pack the spring pocket with engine assembly lube to ensure security. If the spring falls out you will have a leak.
18. We’re going with a Canton road race pan and windage tray from Mustangs Plus, which is optimum for a street-driven 390 because it keeps oil around the pickup in hard cornering. There are two gaskets used here. One between the windage tray and pan, and another between the windage tray and block rails.
19. We hit pay dirt when we unearthed this pristine 428 Police Interceptor dual-plane aluminum intake manifold. This manifold is virtually identical to the 428 Cobra Jet cast-iron high-rise with the exception being weight. The Police Interceptor intake transfers heat better. The long intake runners give us great low to midrange torque. This manifold helps a 390 stroker make torque right off idle.
20. Holley’s Street HP series 750-cfm carburetor delivers excellent performance in all driving conditions. The key to street performance you can live with is proper carburetor and jet sizing for the application and enine size. You can have too much carburetor at 850 cfm and not enough at 650 cfm. The HP was chosen for its delicate balance of low-rpm performance and as an atomizer that comes on strong at high rpm.
21. Talk about affordable pumping power! This is Summit Racing’s own M6905 Carter Super Mechanical fuel pump with 120-gph capacity, which makes it optimum for the 390- and 428ci FE engines. The Carter pump has ¼-inch female NPT inlet and outlet sizes and offers a factory appearance.
22. At a glance, this Summit Racing iron high-volume water pump, PN SUM-312421, passes for a factory stocker and gets the job done for under $70 plus any shipping and taxes.
23. Ford Powertrain Applications (FPA) was created just to make perfect fit long- and short-tube headers for Ford vehicles. This is the ceramic-coated Tri-Y header for the FE series big-block in a Mustang and Cougar plus Ford/Mercury intermediates. Flanges are laser cut and designed to fit full-size, intermediate, and compact Fords. What’s more, you don’t have to sweat out conventional collectors because these are ball and socket.
24. The oiling system is primed as shown with an air drill via the distributor bore. Priming removes most of the air, confirms pressure, and saturates all moving parts prior to fire-up.
25. PerTronix’s billet distributor with the new Ignitor III programmable module makes light work out of dyno tuning. Jeff Latimer of JGM Performance Engineering set the rev limiter at 6,400 rpm but we never came near it. Peak horsepower arrived at 5,400 rpm with peak torque at 3,200 rpm. This engine never had to work hard for its bacon.