Installing bigger valves?

Discussion in 'Discussion Forum' started by CnC79X19, Jan 13, 2020.

  1. CnC79X19

    CnC79X19 True Classic

    So there seems to be some debate on what size valves can be fitted in these SOHC heads. I have a 1500 euro version with 36mm intake and 33mm exhaust and some say that 37.5 are the largest intakes that can be installed on this head? Another member just showed me a picture of a SOHC with 40mm intake and 35mm exhaust. I guess I'm just asking what my options are if I want to increase the intakes only?
  2. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
  3. CnC79X19

    CnC79X19 True Classic

  4. Ulix

    Ulix True Classic

    Stuttgart, Germany
    Yes, 40/35 is popular.
    That is what Sime uses in his modified heads for instance.
    In Europe, a variant of our engine came with 39.5mm intake valves.
    So you can use these.

    PBS, back in the day used 40 mm intake valves in combination with the stock 31 mm exhaust valves of the 1300cc engine for their 1600s.
    Works well too.
  5. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    I heard a explanation about relative valve sizes from the engineer at one of the leading aftermarket performance valve makers while I was at SEMA. By relative size I mean the size of the intake vs the exhaust valves. Had to do with how the engine performs at different RPM's - torque vs HP. He was familiar with the stock valve sizes on these engines and said they were designed more for top end power. In his opinion the way to go would be increase the intake valve size as much as possible and leave the exhaust valve size as it is, to help balance out the characteristics of the engine. His explanation was more specific with a lot more detail about how and why, but frankly I was short on time and for my specific projects I don't plan to change the valve sizes so I had to cut the conversation short. Many years ago I read about this subject in a excellent book on race engine design and development, but I don't recall all of the scientific reasons for it. So I'm just commenting on it for general interest sake and not recommending anything specific. But if this is something anyone wants to look further into for their engine build then some research may be of interest.
    AKimball92 likes this.
  6. CnC79X19

    CnC79X19 True Classic

    Do you happen to remember the name of the book Doc?
  7. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    It is a very old reference now (below), but the basic science hasn't changed. Interesting story; when I was a boy our family wasn't well off so we did not spend much money on non-necessities. And since we lived in a rural area, we did not have a lot of access to big stores and their merchandise. On one trip to the 'big city' we stopped at a major bookstore and I headed straight for the automotive section. This book was the very first hardcover I was able purchase for myself. It was highly regarded as the standard of performance engine engineering at the time, and that intrigued me to want to learn what it had to offer. I still have it on my bookshelf.

    There are better, more current references for this knowledge now. Try searching online for related subjects.

    "The Sports Car Engine, Its Design and Modification" by Colin Campbell.
  8. CnC79X19

    CnC79X19 True Classic

    Thanks Doc
  9. Simon Oaten

    Simon Oaten Daily Driver

    Even the old PBS literature points out that the cylinder head is grossly under-valved. Fiat produced later versions of the SOHC with big valves from the factory, "Tipo" 1600 I think ? Ulix ?
  10. gene cooley

    gene cooley Autocrosser

    Henrico, Va
    Available on Amazon $18.98.
  11. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    Geez, I'm surprised it is still around. It's a very old book and I'm sure there are much better, newer references. That just happened to be the best one available when I was a kid. Completely British applications for its examples/illustrations. But that's what was being modified in those days. The very first car I bought (at 13 years old) with all of my own money, and completely modified and restored myself was British. It was also the very last British vehicle I ever owned. :p
  12. AKimball92

    AKimball92 True Classic

    I believe Colin Campbell has multiple automotive text books that he or the publisher have kept up to date with different reverences and possibly the science behind them.
    Dr.Jeff likes this.

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