Building an inline 6 Chevy 250 engine

The Chevy inline 6 is a design that has been around in one version or another since 1929 until 1990 in North American-market vehicles. During the fall of 1961, a modernized version of the six was introduced (with the Chevy II) which has 7 main bearings and a short deck displacing 194 c.i.d. - the rear block face was redesigned with a bell-housing pattern which matched the Chevrolet V8's in production (both the small block and W-series) - transmission bell-housings for the V8 (both manual and automatic) were shared. The Chevy straight 6 250 c.i.d. engine was brought out in 1966. It has a 0.280" longer stroke (3.53") and the same bore (3.875") as the 230 c.i.d. straight 6. It boasts a 7 main bearing bottom end- quite an improvement over the previous generation inline's 4 main bearings. Although production ceased in 1990 for automotive use, the inline six (including its four cylinder variant still in production as the Vortec 3000 marine/industrial motor) was produced overseas in Latin America for Brazilian-market GM products until the 2001 model year. 250 production ended in 1984 (North America only) but the 292 tall deck engine remained in production in Mexico until 2001. Besides Latin America (e.g. Brasil, Argentina) where the six was locally manufactured, it was also manufactured by GM's Port Elizabeth, South Africa assembly plant until the company divested from South Africa in the mid-1980's during the Apartheid regime.

The integrated intake head used with the 250 (except the 292 which retained the separate intake) between 1975-1984 isn't well suited for performance duty. Some pundits claimed that the integrated cylinder head resulted in increased fuel economy and NVH (noise, vibration, and harshness), its another relic of the malaise era which lasted until late 1984 when the 250 was last optioned with the Chevrolet/GMC truck/van lineup until the 4.3L replaced it. About all that can be done for a head with this intake, is the 1 barrel can be swapped for a 2 barrel by using an adaptor. Integrated intake cylinder heads had one-barrel carburetors until the 1979 model year (the light trucks with the 250 used a Rochester Varajet 2 barrel from 1979-84 which is not performance oriented. Not the hot ticket. The lump port mods along with porting, larger valves, etc. can be done to ANY head, but without the ability to use a high performance intake, these mods would be largely wasted.

Using the earlier head (or one from a 1975-1990 tall deck 292) with the detachable intake from a 194, 230 or 250 cid inline Chevy 6 offers a much better platform. Because of the current lack of aftermarket support for inline Chevy 6 cylinder heads, the hotrodder is left with the removable intake head from the early/mid '70s as the best choice for performance use, unless the cost of a custom head can be justified.

A relatively easy to find head to swap on the 250 inline Chevy is casting number 3864883 from a 194 cid Chevy inline 6 engine. If it's milled 0.060", it will give about 10:1 compression on a Chevy 250 using stock type pistons.  

[http://sissellautomotive.com/ Kay Sissell] has also done some custom heads for the inline Chevy six. He now offers ported cast iron heads and other parts.

Throughout its production lifecycle, the inline 6 Chevrolet came with a one barrel carburetor. Carter YF carburetors were used with the early engines until some point in the mid-1960s when the Rochester Monojet (1MV) was phased into production. The 1MV was used until the end of the 1979 model year (with passenger cars) but continued production with the 292 until its 1990 phaseout in North America and marine applications. Rochester 2SE (Varajet) was introduced in 1978 - only known vehicles using the Varajet were the C/K light duty truck (half ton) and G-series vans including the P series box vans. One drawback is the uneven fuel mixture with the stock one barrel carburetor where the front and rear cylinders have an uneven fuel distribution.  

Beginning in the mid-'70s, the inline 6 Chevy (with the exception of the tall deck 292) got an integrated intake cylinder head similar in design to the Ford Falcon Six. This was a step in the wrong direction for a performance engine- the intake being cast as part of the head means it would take too much work to remove it to be able to use any kind of 4-barrel carb. There are three different designs for the integrated head - casting number #355795, #370696 was used with the mass market passenger cars e.g. X and F platform automobiles (Nova (includes its rebadged clones), Camaro) which has a wide footprint valve cover. A mid-cycle refresh in 1977 with casting number 377131 with the one barrel carburetor base continued in production until 1979 but used a valve cover sized similarly to the conventional head with a shorter height, smooth face, and the PCV tube behind the oil fill hole (a known visual cue which IDs an integrated cylinder head is the position of the PVC breather tube; the truck/van integrated head 250s after 1978 had a taller valve cover with internal baffles). Light duty trucks c. 1978 - 1984 will have casting 14015499 (this one has the Varajet carburetor base - integrated intake runners are curved and incorporate a dual outlet exhaust manifold).

But the detachable intake head (3895052, 3895054, 3927763, 83413880) can be fitted with a large variety of intake manifolds from the cast iron stocker to custom EFI (including multipoint fuel injection setups e.g. the use of Jeep 4.0 fuel injectors and the use of the Jeep throttle body requiring the use of a custom fuel rail and modifications to the intake manifold), IR, 4-barrels, staged 2 barrels, multiple carbs from Stromberg 94/97s to Weber's, to name a few.

(GM do Brasil did manufacture multipoint fuel injection (with revised cylinder heads using the separate intake manifold) for the 250 - Brasilian market Chevrolets (Brazil-market Chevrolet Omega and its local produced GMT400 trucks had the 250 (4.1L); production ended in 2001.)  

*[http://www.iskycams.com/onlinecatalog.html Isky catalog], see pages 126-127  

The Chevy inline 6 uses a pair of gears and no chain to turn the camshaft. There are high performance gear sets available from Comp and others. The cam gear on aftermarket performance timing sets is often aluminum. The undesirable gear set with the micarta (plastic) cam gear is best avoided - these were used during the mid-1970s - mid 1980s with the 250 and 292 (including the Pontiac Iron Duke which shared its design with the 153 with the exception of a crossflow cylinder head and repositioned distributor while using some components from the concurrent Pontiac 301) in both passenger cars and light trucks.