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Technical Curiosities:
Opel’s Cam-In-Head Engine

May 3, 2013 by Matt

Opel Cam In Head CIH Engine Motor GT Manta

As with single-point fuel injection, the design of Opel’s cam-in-head (CIH) engine was an attempt to bridge the old and the new, to incorporate some up-and-coming features while using as many existing parts as possible. It’s a transitional form, as it were, between ’60s and ’80s tech.

Fitted to the 1.9l, 4-cylinder blocks of the engines of Opel’s GT and Manta coupes, among others, the CIH head is a hybrid of overhead-cam and pushrod technology. The camshaft is located in the cylinder head, as in an OHC engine, but the valvetrain still uses a pushrod setup’s rocker arms and lifters. It’s as if someone had taken a pushrod design and simply moved the cam upward until the rods themselves were rendered irrelevant.

Opel Cam In Head CIH Engine Motor

Advantages? The CIH engine was obviously an easier sell to Opel’s corporate overlords at GM, reusing as it did much existing pushrod valve gear while still offering some of the benefits of a true OHC engine. The valvetrain is more compact than in a pushrod engine and its associated inertia is much less, allowing a redline north of 10,000 rpm for race-prepped CIH engines with roller rockers and suitable springs and cam profile. Hydraulic lifters can be easily used, and in case they aren’t, valve adjustments are much more straightforward than they would be if the cam operated directly on the lifters. And significantly for the Opel GT, with its low-profile hood, the location of the camshaft farther down meant the engine’s overall height is lower than if the camshaft were truly overhead.

Opel Cam In Head CIH Engine Motor Schematic Diagram Drawing Layout Timing Chain Gear Sprocket

Downsides of the CIH engine mainly revolve around the standard limitations of a non-crossflow, 2-valve design, including relatively poor airflow and necessary compromises in combustion chamber design. Also, the cylinder head casting is relatively complex, which introduces a risk of cracking, and the head was only ever made out of cast iron, incurring a weight penalty over an aluminum head. And whatever the valvetrain’s inertial advantages over a pushrod design, there still exists considerably more valve gear than a more direct OHC layout.

Opel’s cam-in-head engine was a stepping stone, but a unique and noteworthy one.

Image credits:

Editor’s note: This post is part of an ongoing series spotlighting obscure automotive engineering solutions. Read the other installments here:

Filed under: Opel, Technical, Technical Curiosities


  1. areopagitica says:

    Note to Spannerhead. You will note that I have cherry picked a dozen or so of your essays that fall into the scope of what I try to present on my own blog. I hope this does not meet with your disapproval. Please drop in and say hello if there is anything I have uploaded that merits your comments.

  2. areopagitica says:

    I see the name of my Facebook blog did not appear in my note to Spannerhead, so I reiterate it here. I am blogging as :Looking Back Racing, not Areopagitica, which is the handle I use for myself, or Areopagitica1@gmail that is my email address. Welcome to anyone who wishes to visit.

  3. areopagitica says:

    Regarding the CIH Opel engine, the valvetrain does not look greatly different than one side of a Ferrari Colombo, or a Bavaria or Toyota Crown six, or some Isuzu and Mitsubishi engines. The lack of laterally disposed valves for a hemi chamber and crossflow porting does seem an unnecessary compromise from those designs which mostly existed at the time this was launched.

  4. areopagitica says:

    Putting an overhead cam on the Chevy inline six to achieve the Pontiac OHC six was a similar exercise to this CIH, and was done in 1966. The servicing disadvantages of having to retime the cam when removing a head for a valve grind would seem to outweigh any real point in doing so, since the combustion chambers were not improved, nor the non crossflow porting on that Pontiac. Indeed, some of its ports were siamesed, in comparison to the full twelve ports found on the OHV Chevy it was based on. Actually, since it was a deep skirted block it was not truly a derivation of the Chevy at all.

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