why pushrod?

[ebola]

In sweden last year one respectable race team owner said to me - "for drag race use pushrod engine only".
Since then Im searching for suitable block - my project is small cc alcohol engine, and this claim makes me wonder - why? Now I have to choose some old block, and actauly 100%remanufacture it rather than use OHC better design, some german engine trouble - free gadget.
If I understand right, OHC heads has less friction load, needs less lubrication, big valves and parts have more heat sink - but on the end it doesnt sound like something important.Any sugesstions on that claim? Thanks.

 

[patprimmer]

It may be due to the origin of the sport and the tradition of pushrod V8 engines the USA.

Pushrods offer no advantage over OHC engines except for simplicity of cam drive and no need to remove cam if you remove heads between rounds. Engine performance and durability of valve train both favour OHC.

With OHC and bucket type followers, you cannot run inverse flank cam grinds, but you can if you use a rocker arm and a roller follower.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[turbocohen]

pushrods act as shock absorbers and offer distinct durability advantages when rapidly opening a valve against high cylinder pressures.

 

[thundair]

Pushrod engines allows for big rocker ratio changes like 1.7:1 for intake and 1.5:1 for exhaust.

The flex in the pushrod is a bad thing and the trend is to stiffen all the valve train components.

So in the end you can have a much more aggressive valve action with it being amplified by the ratio.

I guess you could design a overhead cam with larger rocker ratios but you are definitely limited with a bucket over cam..

Cheers

I don't know anything but the people that do.

 

[FoMoCoMoFo]

I love pushrods,long ones,own about a hundred of them,OHC zero. but I would not like my chances racing with them in any class that allowed OHC. if you see them in a race car it's probably by rule.

 

[SomptingGuy]

This is a very interesting thread. I'd always assumed pushrods were generally mandated by (American) race series organisers to preserve a bit of history/nostalgia (and to perhaps prevent modern, non-American engines joining in).

If there are real positive performance benefits from pushrod engines I'm all ears.

 

[BrianPetersen]

Pushrod engines can be more compact than an OHC engine of comparable displacement because the cam mechanism and pushrods use space that is otherwise not utilized.

If you want an example ... take a look at a traditional Ford 5.0 V8, and compare it to the OHC 4.6 V8 "mod" motor that replaced it. The 4.6 is physically larger.

But, in general, pushrod engines have been on the way out for automotive applications. OHC has other advantages besides space that are becoming more important. Easier to implement variable valve timing, for one thing.

Pushrod engine layouts are mandated in NASCAR and drag racing because that's the way it has always been ...

 

[IceStationZebra]

Pushrod engines are mandated in most of the pro drag racing classes in the USA. They claim it "keeps costs down." And it is usually easier and cheaper to find parts since they are much more common - chicken and egg thing.

One advantage of a pushrod engines is easy head removal if necessary. Many years ago (around '90 I think) there was a pit crew race for the Top Fuel teams. (Top fuel - long skinny cars with supercharged 500 cid engines) They had to:
- start the engine then shut it off
- take off supercharger, heads/headers, cam, clutch, pistons
- reinstall and restart
I think the winning time was about 8 minutes!

ISZ

 

[ebola]

well, I'm building "outlaw" style car - I can use any electronics at ignitions,any fuel, but I definately go natural aspiration high CR on 4 cylinder engine, carburetor and manual gear box. I have choice from various modern and old style engine basis I can use for project.My personal experience with modern engine heads at drag racing is not very good.Multivalve heads suffer from high load/heat, tiny valves melting in moderate boost/nitrous applications and so on.Heads are made precisely with thin wals between water/duct/chambers.Intakes do not distribute fuel equaly.So there is no BIG difference if I use modern engine or old style, because there almost everything I have to modify.My problem on the end will definately be most about durability in high load I guess.

 

[patprimmer]

Modern stress analysis means a part can be designed with less guesswork, so there will be less over design to cover what used to be unknown factors.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BrianPetersen]

And that implies that the lack of enormous safety margins in modern engines is not the fault of being OHC instead of pushrod ...

I've heard of folks in the import drag racing scene who are getting 600+ horses out of Honda DOHC 4-banger car engines, and you can get 500+ horses out of a 1300cc Suzuki Hayabusa DOHC 4-cylinder motorcycle engine. Yes, you have to change everything in order to do that.

 

[IceStationZebra]

ebola - "...OHC heads has less friction load, needs less lubrication, big valves and parts have more heat sink.."

To say that OHC engines have less friction is a gross generalization. It depends greatly on the type of cam drive, number of cams, number of valves, and valve actuation scheme being used. Same for "lower lubrication requirement". A gear driven cam with solid lifters will need less oil than a chain driven OHC w/hydraulic lifters.

As far as big valves - if both motors are 2 valve/cylinder then it is cam independent. If it is a 4 valve/cyl engine you will have smaller valves -- but the total airflow will probably be more than the 2 valve engine.

I have also read that the air flow advantage of 4 valve heads goes down some as the compression ratio goes up. If you have a hemispherical chamber you need a high dome piston to raise the compression ratio - and this has negative affects on flame travel and air flow. The other choice is to flatten the valve angle and leave the piston flat. This puts a sharper bend in the runners and increases valve shrouding - which reduces airflow.

If you want suggestions post where in the world you are. Of course opinions are like belly buttons ... everyone has one.

ISZ

 

[tbuelna]

The five main reasons pushrod engines are still in use for production automotive engines:

1. Cost
2. Cost
3. Cost
4. engine packaging
5. Cost

 

[BrianPetersen]

IceStation, the trend in motorcycle engines has been towards a very low included valve angle and either flat-top or slightly dished pistons. The bend in the intake port is reduced by having the intake runners at approx 45 degree inclined angle relative to the head. It's easier to get decent squish-bands in the chamber with a low included valve angle without shrouding the valves as much. The slight theoretical reduction in valve area by doing this doesn't seem to be an issue ... Yamaha just recently gave up on 5 valve per cylinder heads because 4 valves per cylinder works better. (First they gave up 5 valves on the MotoGP bike, and now on the 2007 R1.)

I think the new Chrysler Hemi engine also does this, although with only 2 valves per cylinder and pushrods. Less included valve angle than the old Hemi, relatively flat-top pistons instead of the big dome that was on the old ones. It goes to show that the combustion chamber shape doesn't have to depend too much on the way the valves are driven, although certainly cost would favor having only 2 valves per cylinder and all of them in a straight line ...

 

[Majik]

I would think it would be difficult to get a cam/bucket system adapted to large lift long duration cams assuming we are modifying a stock component for drag race duty. If we're just making brand new stuff then I'm sure it wouldn't be a problem. Getting the lift most drag race big cube engines make would be extremely difficult on some OHC engines with out any rocker arm setups. It'd be bump sticks from hell.

 

[thruthefence]

Anyone remember the big Ford 427 cid SOHC engines run by Connie Kalitta years ago in Top Fuel? I don't think they were world beaters, but maybe they just lacked development?

 

[FoMoCoMoFo]

I remember

 

[Lou Scannon]

The breathing & rpm potential was definitely there, but the 426 Hemi was practically a mainstream engine compared to the 427 SOHC - cheaper, more availability, greater knowledge base...
Compare the 351 Cleveland to the small block chevy - the Cleveland had way more potential, but the SBC dominated most of the time for the same reasons as above.

 

[FoMoCoMoFo]

OHC is capable of the same lift and profile as pushrod. the advantage to ohc comes mostly from less reciprocating parts/mass. higher rpm's and or lower spring pressures. so no increase in torque but more horsepower potential. ford may have decided that 425HP from the cheapo 427 was enough ( chevy also toyed with ohc around the same time) as was stated above it came down to cost/benefit how much trouble and expense to make the engine in that grocery getter turn 8,000 rpm

 

[ebola]

but mentioned 427 sohc was 2 valve per cylinder, right? Not multivalve engine? I understand that oxygen rich fuel doesnt require high flow head, because oxygen is stuffed in with fuel and air is more needed to get compressible/combustible substance rather as oxygen source.Nitro engine is capable to rew 10 000 rpm with any type of valvetrain because of fast combustion.Once drag race is mostly 60ft race, torque is base figure and late rpm hp peak is not big thing.Simple.
Multivalve variable timing stuff is all common fuel related thing I believe.