What Are The Advantages of Pushrod Engines Over OHC?
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by bigpoppapreston » Sun Jan 14, 10:20 pm
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What Are The Advantages of Pushrod Engines Over OHC whether SOHC or DOHC?
With Run What You Brung classes and race teams with budgets that can write a check and the bank bounce...if DOHC was the ticket, wouldn't someone manufacture a set of killer heads that would be highly competitive for such teams?
-Poppy
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by Keith Morganstein » Sun Jan 14, 10:30 pm
Main adavantages of pushrod engines (as I see it) are a smaller package that is simpler and less expensive to manufacture.
Many/most factory 4 valve DOHC heads are "killer" especially with professional modification.
Automotive Machining, cylinder head rebuilding, engine building. Can't seem to quit
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by hoffman900 » Sun Jan 14, 10:33 pm
Most rule sets either explicitly forbid DOHC / 4 valves per cylinder or severely restrict them compared to a 2-valve head.
Where the rules are open, you'll always see a 4 valve chamber - whether that's opened by DOHC, SOHC, or pushrods is a different story.
OHC engines typically can run much more aggressive acceleration and jerk rates than a push rod actuated valvetrains.
Pushrods do make for a more compact system, (think the LS series as opposed to the Ford Modular). That said, if the ACO / IMSA opened up the rules so that the 4-valve competed with the 2 valve on equal footing, IMO, it would force the hand of GM to build a 4 valve cylinder head for the Corvette.
So advantage to pushrods:
Compact
Easy to manufacture
Lower COG
...and that's about it.
-Bob
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by modok » Sun Jan 14, 10:52 pm
Racing classes and even tax laws are often limiting displacement per crank revolution.
If you limit displacement then it seems clear higher rpms and better breathing capacity is the way you go.
Eventually will end up with a cylinder head that is nearly as big and heavy as the lower end......and is that good? Maybe, maybe not.
If the rules specified a engine block and car dimensions, leaving the heads and induction open, could end up not being able to see over the hood, perhaps you can use a periscope to see where you are going.
if the engine was required to be a particular physical size and weight, I don't know that overhead cams would be an advantage at all. Maybe, but maybe not. It's been going back and forth for years.
Last edited by modok on Sun Jan 14, 10:56 pm, edited 1 time in total.
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by hoodeng » Sun Jan 14, 10:54 pm
fam Product Page
High cam with finger followers is the current top end of town in Formula 1 , mounting the cam lower in the head lowers the center of gravity of the engine ,finger followers allow far more radical valve events than cam on bucket,whose events wise is not much different to flat tappet on cam without the mass of the pushrod and rockers.
That said , some years ago McGee were building 3V quad OHC V8 top fuel engines ,but money for McGee ran out and persistence of the pushrod camp won out ,so the only offerings still today are the 2V pushrod engines, which are unlikely to be beaten for a time to come.
Cheers.
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by bigpoppapreston » Sun Jan 14, 11:03 pm
https://www.hagerty.com/articles-videos ... tte-coming
Packaging and cost primarily mainly. Ford's Vodoo is a nasty lil powerplant. 32 valves with a flat plane crankshaft. In fact I've read that one of the rumoured optional engines for the highly anticipated mid-engine corvette will be a flat plane 32 valve v8. rpm and over 700 naturally aspirated horsepower. $$$$$$
-Poppy
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by induction apprentice » Mon Jan 15, 6:00 am
I agree with most of the comments above. Interesting topic. Modok is hitting some key points. Mustang general shape all be it some are great looking cars. But are shoe box shaped primarily because the ohc heads are a foot thick. That usually leads to inept intake manifold designs trying to find clearance and causes engineering deficiencies that possibly out weigh any advantage of a 4 valve configuration. Also. I don't know for fact. I am assuming now. But I always assumed it was hard to match the acceleration rates of a rocker arm in a overhead valve config with large lifter wheels, large journal diameters and high rocker ratios. Finally. An overhead cam engine must have greater oiling requirements that may add to drain back and sump volume issues to solve. Especially in production wet sump applications. Maybe has something to do with the amount of restricting of oil volume to the top end they are designing into modern overhead cam engines. I see a lot of them coming in with seized cams. Yes usually lacking oil changes. But they are definitely sensitive and maybe not the greatest in a day to day passenger car application. But again. I am speculating. I don't have facts. just an imagination.
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by peejay » Mon Jan 15, 6:09 am
Vee engines have a smaller package for the same power output.
Tht's pretty much it.
Notice that even as GM eliminated all pushrod inline engines years ago, they still are married to pushrod V8 engines, and they only stopped doing pushrod V6s for marketing reasons. The 3.6 is a significantly larger engine than the 60 degree or Buick V6 engines.
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by ccsc » Mon Jan 15, 7:03 am
you probably know this but GM has not completely stopped the pushrod v6 - the LV3 is a 3/4 LS, and is afaik still standard equpt in MY18 full sized base model trucks. I believe because of best low to mid torque per pound of engine weight and mass of fuel consumed.
Re: best induction - it is (seems?) clear the finger follower 4 valve config is best for highest output, but it is really interesting that Honda for instance no longer produces, as far as I'm aware, a high output, high rpm capable finger follower PASSENGER car engine, similar to that produced in the
Acura TSX. This was a 2.4L version of the S engine, with finger followers, and dual cam lobes for low and high speed, in a 4 door sedan, among other body styles. It has been replaced with a version without the complex valvetrain, but with a turbo for better low and mid range (current CRV I believe)
Re: the comment about valve accelerations etc - there is an outstanding thread running right now about cam design with info about accelerations, overlap area, etc. I would appreciate some input on this thread about the actual values that are "manufactureable", with nothing more than current
hot rod practices, comparing valve motion of 4 valve to roller follower and rocker pushrod designs. Remember the Iilmor pushrod Indy engine that was outlawed?
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by hoffman900 » Mon Jan 15, 7:06 am
peejay wrote: 'Mon Jan 15, 6:09 am
Vee engines have a smaller package for the same power output.
Tht's pretty much it.
Notice that even as GM eliminated all pushrod inline engines years ago, they still are married to pushrod V8 engines, and they only stopped doing pushrod V6s for marketing reasons. The 3.6 is a significantly larger engine than the 60 degree or Buick V6 engines.
Like I said, if the 4 valves were given equal footing in rules as the 2-valve with the Corvette C7R, then GM would have no choice but to go to a 4 valve head. The difference really can be that much.
As far as Top Fuel goes - rules. A OHC would go a long way to mitigating the fact they have to deal with pushrod flexing - especially on the exhaust side.
The Illmor pushrod engine only had the advantage of a rules given increase in displacement and boost over 4 valve/ cyl engines (0.8L increase and 10 inches of boost) . Take that away and there is a reason we haven't seen another one.
Shoebox car designs are typically driven by safety standards, particularly the pedestrian safety standards. Also, side impact standards have played a role as well. The Camaro and Challenger are just as if not more boxy than the Mustang. Old chassis that have had Mod engine swaps don't need taller hoods. All of this is easily refutable. Single carburetor V engines are what lead to high hood heights.
The EMC in showed what 4 valve heads (and their increase in valve area for a given bore size) can do. The Mod motors that year used stock blocks, stock head castings, stock rocker arms, and a limited production OEM manifold. The pushrod engine that came in 4th behind them was a LSX (GM race block) with aftermarket heads and just about everything else.
Finger followers close and surpass the gap of a high rocker ratio pushrod. At some point you're limited by the valve acceleration (which at the highest levels are maxed out in both configurations). That said the inherent stiffness in a OHC package as opposed to a pushrod system is MUCH greater and that allows you to do other things. OHC is more reliable for high rpm running due less moving parts.
From a strict engine performance perspective, OHC / 4 valve per cylinder engines have higher performance potential than pushrods. Formula One and motorcycle builders are not deterred by the higher COG / space component because they're just that much better. If there is an application where the size would be a hindersnce it's there, and it just hasn't materialized.
OHC engines can do more with the port locations since the pushrod is not in the way.
As for rpm, no matter the displacment (from a lawn mower to a ci engine), if you're racing, you need to keep finding ways to find RPM. OHC simplifies that.
Some applications use a SOHC with finger followers to actuate 4 valve cylinders (like the Honda CRF450R) DOHC allows more tuning options - either on an engine stand or real time by the computer as the events can be changed independently.
-Bob
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by Truckedup » Mon Jan 15, 7:49 am
Other than for the best power for a given displacement, valve train is not important in non performance vehicles...Think of PU trucks, you got DOHC, SOHC,semi hemi push rod and wedge push rod...All the most powerful gas engines have about the same performance and fuel mileage ... the ohv's have a bit more displacement obviously..In the near future DOHC with all the tricks may be necessary to pass emissions...
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by bigpoppapreston » Mon Jan 15, 9:00 am
http://www.hotrod.com/articles/-ams ... allenge-2/
One, two and three top finishers in the EMC were 4 valve engines. While the Hemi didn't finish quite as well, it did make some good power. What was the explanation given as to give the mod motors their own platform to play in, as well as to give the Hemi their own platform besides their obvious superior design?
-Poppy
Re: What Are The Advantages of Pushrod Engines Over OHC?
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by pamotorman » Mon Jan 15, 9:14 am
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push rod engines are cheaper and easier to mod and that is why the LS is so popular with hot rodders