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COMP Cams General Motors Engine Technical Specs and Information

GENERAL MOTORS TECHNICAL INFORMATION

Engine Types | General Tips

Small Block and Big Block Chevrolet engines have become legendary since they were introduced over 50 years ago. The people at COMP Cams®, along with a lot of top engine builders, have spent many years improving these and other Chevrolet powerplants. We know the basics, and the basics are important. We have also learned a few tricks along the way.

If you’re rebuilding an engine, it will be worth your while beforehand to either read our books or watch our videos on the subject. If you need more help or have any questions, call our CAM HELP® line at 1-800-999-0853.

Engine Types

V6 90º -uses “18”, “09” and “56” Prefixes
The 4.3L V6 90º engines produced since 1985 are equipped with two types of camshafts and three types of cam drive systems. Early 1985 and 1986 engines (Prefix “18”) were produced with flat tappet cams. Hydraulic roller cams were first introduced in 1987 (Prefix “09”). These two cam types require different timing chain sets, because the hydraulic roller cams have a stepped nose like the LT1 engine as described later in this section. In 1992, a balance shaft was incorporated in this engine, which necessitated a third timing chain setup to drive both the cam and the balance shaft. The cam (Prefix “56”) in this balance shaft engine is also shorter than the earlier model and does not have a fuel pump lobe.

Small Block V8
There are several varieties of small block engines currently in use in the aftermarket. Although they mostly use the same block, the cylinder heads are very different and require totally different camshafts. The valve arrangement in the heads is different, and therefore the lobe placement on the camshafts must coincide with the head you are using. The “54” is the first cam to completely stray from the original design. It is .300” larger in diameter.

The nose of the standard early model cam “12” is very different from the later model hydraulic roller “08”. The nose of the hydraulic roller “08” is necked down to accommodate the cam retention plate. The “07” is an LT1/LT4 shaft which has a longer dowel pin and a center hole for the distributor. Be sure to check the diameter and depth of the hole in the front, as well as the length of the dowel pin.

Standard Small Block Chevy -uses “12” Prefix
This is the basic engine configuration found on all small blocks from 1955 until 1987, when Chevrolet introduced the roller cam. It’s important to note that the 1955-1957 versions require an oiling groove to be machined into the rear cam journal, but the camshafts are interchangeable.

OE Hydraulic Roller Small Block -uses “08” Prefix
Beginning in 1987 small block V8 engines were equipped with hydraulic roller cams. There are differences in the block to accommodate a cam retention thrust plate and antirotation mechanism for the lifters. These blocks are identifiable by bolt holes for a cam retention plate behind the upper timing sprocket, as well as bosses and tapped holes in the lifter valley.

The camshafts on these engines have a step nose and smaller bolt pattern on the front of the cam. An earlier model camshaft may be used in these blocks by using the appropriate timing chain set and adding a thrust button when using a roller cam.

LT1 and LT4 Engine -uses “07” Prefix
These engines are, as far as the camshaft is concerned, essentially the same as the hydraulic roller engines, with one exception. The distributor is driven from the front of the cam, requiring some changes in the cam core. There is a deeper pilot hole in the front of the cam, as well as a longer dowel pin to locate this drive. These engines require a special timing chain set, and they utilize a self-aligning rocker arm. COMP Cams® LT1 and LT4 Magnum rocker arms can be found on page 268.

Buick Head, Splayed Valve GM and Dart Buick
Small Block, -uses “19” Prefix
Buick made a small block cylinder head in the early 1980's which fits a small block. This head was made by Dart and is still very common in drag racing and some oval track racing. The camshaft is different because of the different valve arrangement in the head. The valve arrangement of Splayed Valve GM and Dart Buick heads are identical to the Buick, so the camshaft is the same.

SB2 -uses “03” & “04” Prefixes
One of the latest versions of true race engines from GM is the SB2. It uses a totally different valve arrangement, and therefore a different core. One major item that changed is that there is a special SB2 block. The lifter bore spacing and lifter angle are different, specifically to work with a flat tappet for cup applications. When using a standard block and SB2 head, use prefix “03”. When using the SB2 block and the SB2 head, use prefix “04”.

GM Gen III/LS1/LS2/LS6 -uses “54” Prefix
This engine is the first real departure from GM's original small block design. Almost none of the parts carry over from previous engines. The camshaft is a steel roller cam, approximately 4” shorter and .300” larger in diameter, so there should be no problem with misapplications.

Big Block -uses “11” Prefix, Gen VI Big Block -uses “01” Prefix, 8.1L Big Block -uses “46” Prefix
This engine was introduced in 1965 and with the exception of some of the very early 1965-1966 models, which had an oiling groove in the rear journal, the camshafts are interchangeable. This includes all of the big blocks, including the Mark V, with just about any style cylinder head until the 454-502 Gen VI was introduced in 1996. This engine is similar to the earlier standard big block. The heads are interchangeable, but there is nonadjustable valve train. It is equipped with a hydraulic roller camshaft. There is a positive camshaft retaining plate on the front, and the nose of the cam is stepped down to accept this plate. The lifter bosses are taller to accommodate the lifter anti-rotation plates. This engine will require a special timing chain set.

Older Engines
The very first 1955-1957 265 c.i. small blocks had a unique oiling system. The same holds true for the first 1965 and 1966 396/427 big blocks. When one of these blocks is used, it is necessary to machine a small groove in the rear journal of the cam to allow oil flow to the top of the engine. COMP Cams® camshafts come without this groove, so it is important to check the vintage of your block prior to camshaft installation. COMP Cams® can perform this operation or supply the specs to you for local machining.

This chart shows the valve arrangement of typical Chevrolet cylinder heads. You can see each of these will require a different camshaft.

General Tips

Springs
By far, the most common problem encountered when installing a new high performance camshaft is the incompatibility of the existing valve springs to the new cam. Factory valve springs are designed to work with stock, low lift camshafts, and since most aftermarket cams have higher lift, the springs must be replaced with compatible components. It is highly recommended and a requirement of the warranty that the suggested springs be installed along with any COMP Cams® cam. Most big block engines come stock with a double spring consisting of small diameter wire and many coils. Some people think that because it has double springs it is already high performance. However, nothing could be farther from the truth. This particular spring is one of the worst for accepting extra lift. Almost all big block engines will require a spring change along with the cam. Whenever installing a Hi-Tech™ racing cam in any small or big block engine, the cylinder heads must be equipped with the correct valve springs, screw-in studs, guide plates and hardened pushrods. The increased loads and ultra high speeds of the racing engines make this a necessity for valve train stability.

Small Block Spring Pockets
When machining a small block head for larger diameter valve springs, be aware that the area around the spring pockets in the head is very thin, especially the end or outboard exhaust. Care must be taken not to machine through the head when increasing the diameter of the spring pocket. You can round the edge of the cutter used to machine the pocket to resemble the diameter of the wire in the valve spring. Another way is to insert a .030” standard 1.250” diameter spring shim in the pocket prior to machining and cut only down to that point. The safest way is to seek professional help before ruining the heads.

Studs
When you are using a high performance camshaft and have problems with the valves not staying properly adjusted, one of the first things to check is the rocker arm studs. Most early model small block heads utilize pressed in studs. When high spring loads and high engine speeds are used with these stock type studs, they tend to pull out of the heads. You can check for this by laying a straight edge across the top of the studs to see if any of the studs are too high and out of alignment. If so, the heads should be removed and machined for screw in studs.

Factory small blocks were equipped with 3/8” studs and rocker arms. One of the most common practices on these engines is to replace these with larger 7/16” versions similar to those found on the big block engines. This is a simple conversion but requires a roller trunion rocker arm. See page 269.

In 1991 Chevrolet introduced the Mark V Big Block, which comes from the factory with a non-adjustable valve train. When changing to a non stock camshaft, the valve train must be converted to adjustable. We developed a special stud (Part #4514-16, page 277) to convert the heads with no required machining. Also on page 275 is a series of Magnum rocker arm kits engineered specifically for these engines that include this stud.

Flat Tappet Break-In
All flat tappet cams will require special attention during the break-in process. Due to recent changes in motor oil formulas, a switch to a diesel or non-synthetic racing motor oil in combination with COMP Cams® #159 Camshaft Break In Lube is mandatory in order to avoid camshaft failure during break in. Cams requiring dual valve springs during normal operation will also require that the inner valve spring be removed during break in so that critical lifter rotation can be established. The appropriate COMP Cams® lifters, and correct valve springs, rocker arms, and pushrods are also absolutely essential to ensure long camshaft life. Please refer to the instructions in your cam box for complete procedures or page for our tech bulletin on the topic If ever in doubt, please call the COMP Cams® CAM HELP® line at 1-800-999-0853.

Roller Cams
Several points must be considered when installing a roller cam in an earlier block designed for a flat tappet cam. Flat tappet cams are ground with taper on the lobes to force the cam to the rear of the engine. Roller cam lobes are ground flat, so a thrust button must be used to keep the camshaft to the rear of the block. Most racing roller cams are steel billet cams, which require an upgraded distributor gear. Most street roller and hydraulic roller camshafts are made from an austempered material which is compatible with the standard gear; however, COMP Cams® composite distributor gear is the best choice.

Hydraulic Roller Cams
When installing a hydraulic roller cam in an early model block, it is necessary to use a special hydraulic roller lifter with a link bar assembly to keep the lifters from rotating in their bores. In addition, appropriate- length pushrods must also be used. A roller lifter, being physically longer, has a pushrod seat that sits closer to the rocker arm than a flat tappet lifter pushrod seat –necessitating a shorter pushrod. A thrust button is required to keep the cam from “walking” forward in the block. A wear plate is also a required (though inexpensive) part, which serves to prevent the rear of the camshaft gear of the timing set from excessively wearing the engine block as it works to keep roller cam walk under control.

When installing a flat tappet cam in a block originally equipped with a hydraulic roller, it is necessary to change the entire system. The cam, lifters, pushrods and timing chain set must all be changed in this case, as none of the old parts will interchange.

Self Aligning Rail Rocker Arms
Originally, the small block engine used a machined slot in the head to guide the rocker arm on the valve. It has been common to enlarge this hole and install a guide plate when switching to a high performance valve train.
In 1988 with many models and later, on all engines, Chevrolet utilized a small alignment slot in the valve tip end of the rocker where it contacts the valve. Although there may be an alignment guide on the head, it is not hardened and is used only to align the pushrod during assembly. This guide may not be used with a standard non-aligning rocker arm. When building a high performance engine, we recommend that the alignment guides, pushrods and rocker arms be replaced with the earlier style parts. When building a mild street engine, COMP Cams® developed the Magnum and Pro Magnum Rocker Arms™ designed specifically for the late model self-aligning design.

High Ratio Rocker Arms
A higher than standard ratio rocker arm moves the pushrod closer to the rocker arm stud. This makes it necessary to check the clearance between the pushrod and the head where the pushrod passes through. This is a very common problem and should be carefully checked whenever a rocker arm ratio change or pushrod diameter change is made. We offer a special tool (Part #4710) to machine this on page 339.

Rocker Arm Geometry
Proper rocker arm geometry is required to ensure the maximum benefit from any cam design. Camshaft base circle, block deck height, cylinder head design and lifter design all contribute to possible errors in valve train geometry. It is simple to make compensation with pushrod length. Usually, a longer than stock pushrod will be necessary in a high performance engine, but care must be taken to choose the correct length.

Rocker Arm Slots
One of the most frequent problems encountered when changing to a high lift camshaft is the slot in the rocker arm will contact the rocker arm stud, resulting in camshaft, lifter, rocker arm and/or stud failure. This is prevalent on both small block and big block engines with stock rocker arms. Always check this and change to either a roller trunion rocker or a long slot rocker arm when contact is evident. COMP Cams® Magnum Rocker Arms are a good solution to this problem.