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Originally Posted by CAP'N B
The pushrod is a weak link, the lifters are a weak link and the valve train instability probably cause more total engine failures than probably any other failure. The OHC engine lends itself to better valve train stability and higher performance, higher revving engines. OHC design is a bit more expensive. But you spend money anyway to make your engines more powerful.
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Sure, they are capable of much higher revs, but what's the point? LS small blocks are already revving to 6500 to 7000 RPM reliably in stock form. Going beyond that to the stratospheric RPM levels that would absolutely necessitate OHC is not practical, and produces driveability problems for daily driver road cars. An engine that runs really well at 8000-9000 RPM is going to be a pig in the RPM range street engines spend the vast majority of their time in.
And what makes a pushrod engine inherently less reliable. I could give you a long list of engines from GM, Ford, and Chrysler, current and historical, that are damn near bullet proof.
Quote:
Originally Posted by CAP'N B
Think of this, with direct injection there is no potential for detonation, a diesel engine has no spark plugs, it uses high compression to produce heat high enough to burn diesel fuel. You can throw a match in a bucket of diesel and it will go out. Gas explodes. So a high compression gas engine with direct injection could possibly do away with spark plugs.
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Quote:
Originally Posted by CAP'N B
What about the possibility of doing away with spark plugs. Nothing at all was commented on that.
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Exactly how would you do it? Gas and diesel have unique characteristics that make each not suitable to running like the other. Direct injection engines still have to deal with the potential of detonation. They can just run a little higher compression ratio without it happening. Both fuel and air are in the cylinder during the compression stroke. The timing is still determined by a spark plug, unlike a diesel, where only air is in the cylinder during compression, and the injection of the fuel is the timing. Setting up a gas engine to detonate at precisely the right time every time regardless of variables like temperature, load, fuel quality, etc, would be about impossible.
Alternatively, running gas off detonation by timing it like a diesel would have problems, too. Gasoline requires a fairly narrow fuel/air ratio range, while diesel does not. Hence, a diesel runs without a throttle, providing the necessary heat and pressure to run off detonation even at an idle. I think a throttled gas engine idling or under a very light load (basically with a lot of vacuum) would have a difficult time producing enough compression to cause detonation.