The 1965 Mustang Convertible and 289 “Hi-Po” on the left and the same engine in cross section on the right

The Hi Po 289 CID V-8 and Shelby GT 350 and 350 GT-R V-8s

Mustang

The Mustang 289 CID High-Performance V-8 (“Hi-Po”) developed 271 horsepower. It’s weight per horsepower was less than two pounds, an excellent figure, as lighter, more powerful engines helped not only straight line performance but handling as well. The horsepower per cubic inch of displacement of 0. 95  was close to the the 1960s benchmark of 1.0 set earlier by the original 1957 283 CID fuel injected Corvette V-8.

Ford’s approach to the Hi-Po was to not simply “hop-up” the basic 289 V-8. It was conceived as a special engine that would have reliability and durability built in to match its performance. All the engine parts were carefully designed and tested to withstand the extra stresses of
high-performance usage.

Shelby GT 350

The Shelby GT 350 engines start life as the production line Hi-Po V-8 and they were received at the Shelby shop in stock form. Shelby Competition engine builders Cecil Bowman and Jack Hoare looked upon them as a set of rough castings to be worked up into a racing engine, regardless of whether they would be installed in a GT 350 or GT 350R.

Perhaps the most surprising thing about the engines is the number of stock parts that were used and also how little the finished product really varies from the original Ford unit. In fact, it was the extremely precise re-assembly and attention to detail which resulted in the 35 extra ponies in the GT 350 and 85 in the GT-R, rather than any very radical modifications. It was evident that detail work was the secret to the Shelby’s power bands.

The first procedure was to strip the engine down to the last nut and bolt and then reassemble it with modification The standard bores are honed out yet, the stock pistons and rings are used, with a ring gap of 0.018 in. A considerable amount of time is spent on the heads with precision attached to the valves, springs and valve seats. On the GT-R, the major modification was the fitting of 0.0625″ larger valves. This increases the diameter of the intakes to 1.875″ and the exhausts to 1.625″. To take advantage of the larger valves, the ports are opened up, and finally the combustion chambers are machine-polished. The next step is to mill the heads to obtain the maximum allowable compression ratio.

As far as compression is concerned, on the GT, 10.5:1 compression ratio is achieved by milling the heads to ensure accuracy. On the GT-R, 11.6:1 is used when race gas is available – although engines have been raced with ratios as high as 12.5. In order to obtain 11.6. it is necessary to mill about 0.040″ from the heads, mostly because a lot of metal has to be removed from the combustion chambers to obtain the desired finish. To insure that the compression ratio is correct and that the capacity of each combustion chamber is the same, each chamber blueprinted in the normal fashion.

Courtesy of Virginia Mustang https://www.virginiaclassicmustang.com , here is a fully-dressed Shelby 350 GT V-8.

Although the stock crankshaft and connecting rods are retained, Shelby felt that Ford’s balance is not fine enough for the 350 GT and GT-R, so the shaft is re-balanced with the damper and clutch assembly attached as a unit. On the race engine, the oil feed holes to the journals are relieved slightly and the journals themselves are polished with crocus cloth and finished with jeweler’s rouge. The race engine bearings are Clevite bronze T-77.

In the race engines, the oil capacity is increased to 8 qts. The GT 350 receives its own special oil pan to give it 7.5 qts capacity. An oil cooler is available and fitted to the race engines.

The stock oil pump is retained but the clearances are checked carefully and on the race engines, the spring tension of the pressure relief valve is increased. In that case, because of the resulting increase in oil pressure, the oil supply to the rockers is reduced. This prevents an over-accumulation of oil in the rocker covers when the engines are run constantly in the 4000-7000 rpm range.

The stock 271 HP camshaft of the 289 engine has a wide torque curve that makes the car reliable and relatively easy to drive but at the sacrifice of maximum power output. Therefore, a variety of different grinds were tried, although they were fairly conservative in order to keep the torque curve within reasonable limits. Shelby learned that the valve train in the 289 engine is the limiting factor although 7200 RPM is a safe rev limit.  A Cobra Hi-Rise intake manifold was used. A Holley four-barrel 715-cfm carburetor was used, as were lightweight tubular Tri-Y exhaust headers.

Shelby GT 350-R Specs:

Overhead valves. Cast iron block. Displacement: 289 cubic inches. Bore and stroke: 4.00 x 2.87 inches. Compression ratio: 11.6:1. Horsepower: 350 @ 5400 RPM. Torque: 350 @ 4200 RPM Five main bearings. Mechanical lifters: Camshaft Duration: Intake: 310 degrees, Exhaust: 310 degrees, Lift: 0.4600″ @ .020″ lash. Overlap 82 degrees. Carburetor: Holley 715 CFM 4-barrel Model 4150-C Number 4118.

In GT-R trim, the Mustang was a terror. In street trim, the GT 350 far exceeded the performance of the standard Mustang. It’s acceleration allowed a quarter-mile time of 14.7 @ 92 MPH. The GT 350-R could reach 13.9 and over 100 MPH!