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Advantages of OHC engines[850/1995] posted by thom on
Friday, 20 March 1998, at 7:47 p.m.
I was wondering, what are the relative advantages of the OHC type engine that volvo switched to with the b21, b23 et al. over the pushrod b20 and its ancestors? Just curious, I am sure there is some easy explination.
TIA
thom
Re: Advantages of OHC engines[850/1995] posted by Glenn Goodspeed on
Friday, 20 March 1998, at 9:31 p.m.
Overhead cam engines have fewer moving parts, i.e., no pushrods or rocker arms. This makes them easier to maintain and cheaper to build. Since the cams operate the valves directly, there is less slop in the linkage caused by the inertia of pushrods and rockers. This is especially helpful at high rpms, when a little inertia makes a big difference in the duration of valve opening.
These mechanical advantages of overhead cam engines give them more power than comparably-sized pushrod engines. -Glenn.
Goodspeed's Volvo 1800 Newsletter
Also less frictional losses[850/1995] posted by Juan on
Friday, 20 March 1998, at 11:00 p.m.
Re: Advantages of OHC engines[850/1995] posted by Peter on
Saturday, 21 March 1998, at 7:00 p.m.
Well... kinda.
Your comments about inertia and the HP-cost of moving valve train parts are spot on. However, I don't think OHC motors are by definition easier to maintain or cheaper to build.
In an inline motor, a single overhead camshaft engine will require at least some rockers. The most common setup is a camshaft running down the middle of the head with rockers (one for each valve) actuating the valves on either side. A double overhead camshaft engine probably won't require rockers, but does require another camshaft and drive system.
V-6 and V-8 motors require a cam or set of cams (and attendant drive systems) for each head making for more expense and complexity. I suspect only a SOHC inline motor could be - but proably won't be - cheaper to build than its equivalent pushrod motor. A DOHC inline or any V motor would be more expensive than its pushrod cousin.
As well, while bucket shims don't allow much valve clearance change, when you have to change them it can be a royal PITA. A rocker system is a rocker system. Whether it's actuated by a pushrod or a cam affects not at all the ease with which one can adjust it. A generic hydraulic lifter pushrod motor's is pretty easy to deal with, even if it doesn't give one the high RPM precision that an OHC engine will.
I grant you that the process of adjusting hydraulic lifters does tend to ruin a set of clothes and require a couple of cans of Gunk... You did say easier, NOT less messy.
Re: Advantages of OHC engines[850/1995] posted by Glenn Goodspeed on
Saturday, 21 March 1998, at 7:28 p.m.
Thanks for the primer. I've never actually opened an overhead cam motor, so I'm not surprised that my response needed some guidance. In fact, shortly after I sent it, I began to wonder if OHC engines commonly have rockers or not. Oh well, at least I got the theory right. -Glenn.
Goodspeed's Volvo 1800 Newsletter
Re: Advantages of OHC engines[850/1995] posted by Topi on
Saturday, 21 March 1998, at 11:32 p.m.
....and Peter has never opened a Volvo OHC valve cover neither.....
Re: Advantages of OHC engines[850/1995] posted by Peter on
Sunday, 22 March 1998, at 12:07 p.m.
Well that's true. But I have faith that Volvo designers haven't come up with anything TOO new.
Although... wasn't it you Topi that told me there are little trolls under the valve cover using hammers to open the valves? I bow to your greater knowledge of all things Volvo. ;)
Re: Advantages of OHC engines[850/1995] posted by Steve Seekins on
Sunday, 22 March 1998, at 9:11 p.m.
Just to clarify, the Volvo B21/23/230 series engines with single overhead cam do not have rockers. The cams bear directly on the valve stem buckets, so that, yes ther are many fewer moving parts. Valve lash adjustment requires a few tools and is not quite as easy as adjusting a rocker arm setup, but with the correct tool, a micrometer, a set of shims, and the ability to add and subtract decimal fractions, the job is not difficult (except on old Jaguar engines).
Volvo Club of America