Tuning the Fiat

INFORMATION BASED ON THE RESEARCHES OF V. H. TUSON, RELATING TO THE SPORTS BALILLA, BALILLA “I0/I5,” AND TYPES 500, 509 AND 509a

THE 995 c.c. 65 x75 mm. Fiat sports “Balilla” came to this country in 1935, being the first Continental small sports-car to be offered for some years. It had proved its merit in the Mille Miglia and was soon putting up an excellent showing in British races, sprint events, trials and rallies. It was followed in 1938 by the “10/15” larger-engined saloon “Balilla” and, of course, by the now immortal Type 500. V. H. Tuson, at our request, has kindly placed at our disposal his information relating to tuning all these Fiat models, and also the vintage Type 509 and 509A. His 995 c.c. “Balilla” sports two-seater was developed to lap Brooklands at about 90 m.p.h. and to reach 98-100 m.p.h. when stripped for racing, achieving 88-90 m.p.h. in road trim. It possessed splendid road holding, and acceleration that gained for it many successes in sprint events during the time Tuson raced it, the standing start half mile taking about 30.6 secs. These results were obtained with a 10 to 1 compression-ratio, on special fuel.

In hotting up the 955 c.c. engine it is essential to make up a new three-branch exhaust pipe assembly to obviate the sharp bends and small off-take pipe of the exhaust portion of the existing manifolding. Use 1 inch back plate and adopt wide sweeping bends, or, better still, bring the pipes out horizontally through the bonnet and merge them into one, taking the off-take pipe through the wing and introducing a flexible section to counteract movement of the rubber-mounted engine. A new inlet manifold should he made, on the lines of the standard one, of V shape, to take the downdraught carburetter, but with rather easier bends, and brazed up where it crosses the centre exhaust pipe to form a hot-spot, otherwise acceleration will be impaired. The diameter of the manifolds will be determined by the port size. If the ports are merely cleaned up, the inlet manifold should be of the same diameter, except for the vertical portion carrying the carburetter, which should taper from 30 mm. diameter to the diameter of the main portion. The two end exhaust pipes should be about one thirty-second inch greater diameter than the exhaust ports, while the centre exhaust pipe should be port size for the first 3-4 inches and then taper to the same diameter as the end pipes. It the pipes extend about ⅛ inch behind the mounting back plate this will assist in lining-up the pipe assembly, as the standard manifold is located by short sleeves. If the ports are opened up the foregoing pipe size’s should be used in relation to the changed port size. However, it is desirable to open up the ports so that the taper from seat to throat is maintained, which taper is usually more pronounced ½ inch to ¾ inch from the seat, and, in any case, the gain is not more than 4 m.p.h. at speeds in the region of 95 m.p.h., and the advantage is definitely at the upper end of the range. A power-driven rotary tile should be used, and the inlet ports opened up 2 mm. in diameter and the exhaust ports 1½ mm. in diameter. If there is any doubt about maintaining the taper it is better only to clean up the ports, a good compromise being to ease internal corners and taper off the valve guides where they project into the ports, apart from surface smoothing. No benefit is gained either by fitting two carburetters or a large single carburetter. The inlet valve guides can be cut down ¼ inch, but on no account should the exhaust valve guides be shortened, although phosphor, bronze guides will improve heat dissipation. Stronger valve springs are only required for racing and then should not exceed the strength of the standard springs by more than 15-20 per cent.

The compression-ratio can be increased to as much as 11 to 1, but special pistons are required to raise it above 8.5 to 1. Pump ethyl petrols can be used with ratios up to 9 to 1, above which alcohol and benzole will have to be added. Acceleration is improved by using a ratio of over 8.5 to 1, but speed will not show material improvement. This ratio is generally satisfactory and can be obtained by machining 2.5 mm. from the head. As many “Balillas” have non-standard heads, it is desirable to determine what compression-ratio is in use before machining the head. The standard ratio is 7 to 1 and approximately 40 c.c. of light oil will fill the combustion space. The standard gasket should be retained and the head should not be machined by more than 2.5 mm. With the 8.5 to I compression-ratio, three-branch exhaust system and cleaned-up ports, a road-speeded of 84-86 m.p.h. should be realised in touring trim. On “Pool,” a 7½ to I ratio will be more satisfactory, to obtain which five-sixty-fourths of an inch should be machined from the head.

The concave recesses in the sides of the combustion chamber, which allow freer gas-flow when the valves are open, should be smoothed and slightly increased in depth, especially if the ports are opened up.

The clutch pressure plate is fairly heavy and cannot be lightened, so as much metal as possible should be taken from the flywheel, consistent with safety. The makers balance the assembly as a whole, so each item removed should be marked for correct re-assembly and, if the machining operations result in the removal or reduction in depth of any holes which existed for balancing purposes, these should be re-drilled to the original depth.

So far as general tuning is concerned the engine is extremely sensitive to ignition timing and over-advance results in power loss with no “pinking”. The correct advance is approximately 20º, of which 10º is automatically controlled, so that the initial setting should be only 10º. A little more may be possible, and the exact maximum advance to individual engines must be a matter of experiment. Tappet clearance is equally important and should be maintained at .004″ inlet and .008″ exhaust. Valve timing is: inlet opens 12º before T.D.C., closes 50º after B.D.C.; exhaust opens 60º before B.D.C., closes 10º after T.D.C. The standard carburetter settings are: Choke 22 mm., main jet 105, compensator 60. For road work these can be changed to: —Choke 23 mm., main jet 110 or 115, compensator 65. A 65 compensator, or even a 70, may be experimented with.

With compression-ratios up to 10 to 1 Champion LA10 sparking plugs are suitable for all except sustained high speed, for which Champion JA11 may be needed, although these are liable to oil up in ordinary use. The standard tyre size of 4.00″ x 17″ should never be exceeded, as the “Balilla” is slightly over-geared: an increase to only 4.50″ x17″ slows the car by 2-3 m.p.h. even in racing trim. The fabric universal joints should he regularly inspected and replaced every 6,000-7,000 miles. The connecting rods may, if desired, be lightened at the small ends, being reduced in wall thickness at this section to 3/16″ outside the little-end bush. The rods are matched for reciprocating weight in pairs and the shank of a rod should not be filed unless it is obviously larger than its fellows. If possible all rods should be brought to the same weight, and if sufficient time is available the shanks can be polished and surface cracks eliminated, although the “Balilla” is not prone to throwing rods. Oil escape holes are provided in the bearings for cylinder wall lubrication and these should be retained unless the bores are badly worn. The holes should not exceed one-thirty-second of an inch diameter. The bearing white metal is prone to crack under heavy loading, and the rods can be remetalled with high-grade material with advantage. The safe engine speed is 6,400 r.p.m., but 5,600 r.p.m. should not normally be exceeded. Above 5,600 r.p.m. tappets are liable to break and score the camshaft.

So far as the 1,100 c.c. “Balilla” saloon is concerned, it can be tuned to give a maximum approaching that of the sports 955 c.c. “Balilla”. The  increased weight will result in less brilliant acceleration. The design is virtually the same save for a bore and stroke 68 x15 mm. (1,089 c.c.) and an aluminium head. The exhaust manifold is of improved design and may be retained, after being cleaned up and correctly aligned with the ports. If desired, the inlet ports may be increased in diameter by 1½ mm. at the throat and the exhaust ports by 1 mm., in which case the manifold should be correspondingly opened up as far as possible. To open the ports up further than this introduces complications with the valve seat inserts, and if more drastic opening up is sought the existing seat inserts must be removed, when 2 mm. (inlet) and 1 mm. (exhaust) can be removed from the diameter of the recesses and a further I mm. taken from the diameter of the new inserts, both inlet and exhaust, to give an increase in port diameter of 3 mm. inlet and 2 mm. exhaust. If this is done, the new inserts must have an interference fit of 0.004″ and be pushed in after the head has been heated to about 250º C. Messrs. Wellworthy. Ltd., or a similar concern, might well be entrusted with the job. If the ports are opened up in this manner a new manifold will be necessary, on the lines of the one previously described, although it will be preferable to keep the pipes inside the bonnet. This engine is also very sensitive to ignition timing. The total advance is 36º, the automatic timing operating, over 20º, so the initial setting is 16º before T.D.C. Tappet clearances are as for the 995 c.c. engine and valve timing is:— inlet opens 10º before T.D.C., closes 50º after B.D.C., exhaust opens 53º before B.D.C., closes 6º after T.D.C. Solex carburetters are fitted and choke and jet sizes should be increased one size if the ports are cleaned up and two sizes if they are opened up, by altering the inserts, subject to experiment. Otherwise, the notes on hotting-up the 995 c.c. engine cover the 1,089 c.c. type. Tuson’s saloon, with 9 to 1 compression-ratio and opened up ports (normal inserts) was timed at 83 m.p.h. over to quarter of a mile, with only a short approach run, and his racing-bodied two-seater has lapped Brooklands at over 92½ m.p.h.

The popular little s.v. 52 x 67 mm. (570 c.c.) Fiat “500” will likewise respond to hotting-up, although we believe that the 1938 and later series are more durable and better suited for the purpose than the 1936 series; also that the transmission is heavily stressed at speeds much above 70 m.p.h. Ports should be cleaned up, flywheel lightened and the compression-ratio increased. The last-named is best put up to about 8 to 1. The speed should now be up to 62-65 m.p.h. and a further increase will result if a new manifold is made up and fitted, having easier curves than the standard one in respect of both inlet and exhaust tracks. The vertical inlet section may be made 26 mm. in bore to take a carburetter of this size. Carburetter settings will be slightly in excess of standard and 70 m.p.h. should he attained in this form, which is the highest safe speed permitted with standard gear ratios. An o.h.v. conversion head is available but does not greatly improve performance (the maximum being about 60 m.p.h.) unless an improved manifold is used and the compression-ratio raised to 8 to 1, when acceleration is slightly better but general performance about the same as with the s.v. head. In any case, engine speed imposes a limitation at this point.

Not malty of the old o.h.c. 57 x 97 mm. (950 c.c.) 8 h.p. Fiats remain in use (they were in production from 1926-30), and as special parts for these models are no longer available from Messrs. F.I.A.T. Ltd., much of the interest in hotting them up is lost. However, a sports model was listed and Tuson tuned one of these cars to do 83-84 m.p.h. and to lap Brooklands at 76 m.p.h., a light two-seater body being used. The earlier Type 509 has an inlet pipe going down through the cylinder block and is difficult to deal with, while the lubrication system will not take kindly to more than touring speeds. The Type 509A has a larger oil pump; and modified oil-delivery pipes, while a detachable, combined inlet and exhaust manifold is used. Special pistons used to be available from Messrs. F.I.A.T. Ltd., giving a compression-ratio of 8 to 1. Tuson cleaned up the ports and opened up the valve throats by about 1½ mm., but retained the standard manifold. With the 6 to 1 top gear ratio the engine will not stand more that 60-62 m.p.h. and a 5 to 1 axle will have to be made up. The main bearings should be put into 100 per cent. condition and the oilway in the front main should be increased in width to approximately 3/16″. The steel discs at each end of the crankshaft should be removed and all sludge and dirt cleaned out. Oil pipes should be clean and a sound fit to obviate leaks. If these precautions are not taken, big-end trouble will be experienced. The three-speed gearbox with rather low second gear is a drawback of this early model.