Muscle Mustangs & Fast FordsProject Vehicles
Stock-Block Stroke Swap, Part 3 - Tech
Our Home-Built 331 Stroker Small-Block Gets A Carburetor.
A few issues ago (May '09) we brought you Part 1 of our home-built stroker small-block Ford engine, which chronicled the new Fast As Cast 190cc Trick Flow cylinder heads that we bolted down on top of our Competition Products 331-inch, stroked 5.0L.
To bring you up to date, we used a stock 5.0L block and stuffed it with a forged rotating assembly from Competition Products that allowed us to drop in a bored and stroked combination without the need for block modifications, something most stroker assemblies require. Trick Flow Specialties sent us brand-new Fast As Cast 190cc aluminum cylinder heads, along with a Trick Flow R-Series intake manifold and a Trick Flow Stage II hydraulic-roller camshaft to finish things off.
We followed the engine assembly up with Part 2, where we strapped the engine to the Horsepower By Hedrick engine dynamometer and turned out some very good numbers. Unfortunately we ran into some misfire issues on the dyno that took some time to figure out.
This month, we work to solve the misfire issue, and throw on a carburetor and appropriate intake manifold to get a carb-to-EFI comparison, as well as rule out an issue with the EFI system that we are using.
Our first dyno session at Mark Hedrick's Horsepower By Hedrick engine facility in Jacksonville, Florida, resulted in some stout numbers from the electronic fuel-injection setup. After about five dyno pulls, though, engine performance dropped off at 5,600 rpm, and subsequent pulls after that turned into a solid misfire at a much lower 4,000-4,500 rpm. This isn't the sort of thing you want to happen with a brand-new engine, but occasionally it does. We've decided to bring you all of the drama in an effort to educate our readers should they encounter a similar issue. That along with the fact that we intend to use this engine in other performance testing down the road meant we had to get this thing fixed.
We began diagnosing the issue by inspecting the spark plugs, eventually changing them out for a new set. This was followed by a new coil, distributor, module, and computer. None of these parts showed any reason to cause the misfire, so we performed compression and leakdown tests next. Both checked out just fine. Our SnEEC datalogger would have shown an ill-performing mass air meter or throttle position sensor, so those were ruled out as well.
As we mentioned last month, Mark Hedrick proposed the possibility of the cam walking in the block, and as we found out, it indeed was doing just that. The cam movement back and forth in the block affects how the distributor gear and the cam gear mesh, which can alter the timing. Hedrick made some modifications to the brass bushing behind the cam gear, as well as the camshaft retaining plate, and was able to achieve the needed camshaft endplay. With that, we decided to proceed with the carburetor test, just to bring you some more data on the combination and see how it works with a carb and with EFI.
Horsepower By Hedrick's Jimmy Hartley provided the induction setup for this test. The carburetor is a Holley 650-cfm four-barrel with vacuum secondaries, and the intake manifold is a Professional Products Typhoon dual-plane, air-gap-style intake. We also needed to swap out the distributor for a vacuum advance unit from an '85 Mustang GT. It features the steel cam gear needed to work with our hydraulic roller camshaft.
The 331 came to life as Mark Hedrick fired the engine up in the dyno cell. So far, so good. After getting it up to temperature, Hedrick let the dyno lever fly, and the motor ran up to 6,000 rpm without a hitch--no misfiring, just a smooth pull to redline. The numbers turned out pretty good as well. Peak horsepower on pull 1 came in at 397.2 with peak torque checking in at 388.5. With no changes other than extending redline to 6,100, the 331 turned out 401.2 hp and 391.1 lb-ft of torque.
Compared with the EFI setup, peak power and torque weren't remarkably different. Horsepower was within 5 ponies and torque was better with the carburetor by about 15 lb-ft at peak output. Where the carbureted setup made the biggest difference is average torque. At 3,000 rpm, the carburetor was ahead of the EFI by 14.5 lb-ft and it carried that advantage until about 5,200 rpm. The numbers about evened out between 5,200 and 6,000 rpm.
The most important result of the test, however, is that we found out that there isn't anything mechanically wrong with the engine. Our previous misfire must then be caused by an errant fuel injector or a bad wiring harness. Since the harness is new and the injectors are used, we're leaning towards the squirters. We sent them off to Trick Flow for evaluation and are currently awaiting the results.
Having spent three solid days on Horsepower By Hedrick's engine dyno, we think it's time to pull the Competition Products/Trick Flow 331 small-block off the rig, and get busy stuffing it in a car and putting it on the chassis dyno. In our next installment, we should have the misfire sorted out, and hopefully will have some track times to go with all of the dyno numbers. See you then.