Feasibility of pushrod/cam-in-block engines in Formula One [OHV/overhead-valve]

[Jolle]

Surely you are missing the main point ?
On a pushrod actuated valve the spring has to control the inertia of the valve plus everything up to the cam lobe. That's easily double the inertia of an equivalent direct acting valvetrain.

For the same cam profile, direct acting = smaller spring force = more power. Or alternatively, direct acting = higher allowable valve acceleration for the same spring force = more power, whichever works best.

Not necessarily.

A direct acting, cam on bucket valve train is inherently stiffer, but it is velocity limited by the diameter of the bucket. Also, it's a 1:1 lobe to valve profile, and as such, the extra acceleration at the lobe due to the system stiffness is needed because the lobe isn't being multiplied by a rocker, and needs the acceleration to get the same valve lift profile over one with a rocker multiplying the lobe profile.

Finger followers provide the best of both.

Again, the engine doesn't care how the valve is lifted. It only sees the valve lift profile. With the push rod system, there is some disconnect between the lobe profile and the valve profile through the rpm range, due to system stiffness, this is all measurable and compensated for, and in NASCAR's case, it is used to their advantage through controlled loft that gains open valve area with increasing rpm. Regardless, top pushrod racing engines are valve spring limited, not anything else.

We're talking pushrod actuated cam systems here. Pushrod does not have to mean 2 valve per cylinder.

A SOHC like Honda's CRF450, provides a narrow / compact top end like a pushrod actuated system does over DOHC, but with the system stiffness of a OHC platform as well as rocker multiplication through finger followers:
https://motocrossactionmag.com/wp-conte ... m-sohc.jpg

The engine doesn't care how the valve is lifted, but the cam and spring feel the difference between pushing 10grams or 100 grams of steel around at 12.000 rpm. Even a system like the CRF Honda (which I think is an old 70's design, I had it on one of my Honda XL's when I just starting to ride bikes in the 80's) doesn't make the cut for racing applications. It's too heavy and can't provide the hot cam profiles like, for instance the BMW S models with a dragger.

Pushrods have no place in modern racing, they are as obsolete as carburetors and solid rear axles.

[63l8qrrfy6]

Surely you are missing the main point ?
On a pushrod actuated valve the spring has to control the inertia of the valve plus everything up to the cam lobe. That's easily double the inertia of an equivalent direct acting valvetrain.

For the same cam profile, direct acting = smaller spring force = more power. Or alternatively, direct acting = higher allowable valve acceleration for the same spring force = more power, whichever works best.

Not necessarily.

A direct acting, cam on bucket valve train is inherently stiffer, but it is velocity limited by the diameter of the bucket. Also, it's a 1:1 lobe to valve profile, and as such, the extra acceleration at the lobe due to the system stiffness is needed because the lobe isn't being multiplied by a rocker, and needs the acceleration to get the same valve lift profile over one with a rocker multiplying the lobe profile.

Finger followers provide the best of both.

Again, the engine doesn't care how the valve is lifted. It only sees the valve lift profile. With the push rod system, there is some disconnect between the lobe profile and the valve profile through the rpm range, due to system stiffness, this is all measurable and compensated for, and in NASCAR's case, it is used to their advantage through controlled loft that gains open valve area with increasing rpm. Regardless, top pushrod racing engines are valve spring limited, not anything else.

Nothing you have said really addresses the fact that with a pushrod system the inertia will be much higher which in turn will drive higher spring load requirements. Keep in mind that for a system with rocker ratios different than 1:1, the equivalent inertia of the valve side components increases with the square of the rocker ratio - no free lunch there either.

The advantage with a finger follower is that the follower allows non-constant radius profiles that can be optimized to give a radius of curvature that is small enough to allow aggressive camshaft profiles without undercutting yet large enough not to experience large hertz stresses. That and the very high stiffness is why they have been used in all F1 engines I know of in the last 20 years or so.

A direct acting system (which I've used as a simple example rather than claim it is the absolute best set-up) is limited by the fact that the tappet dome radius has to be smaller than the smallest radius of curvature on the cam which somewhat precludes complex cam profiles for practical camshaft dimensions.


The only advantage of a pushrods is in stationary/marine engines where the cylinder heads must be serviced in-situ.
They really don't have a place in modern racing engines.

[coaster]

Jade Gurrs wrote a good book on the ilmor indy 2 valve V8, they built a valvetrain dyno rig and tested until destruction, once they reached indy 500 race distance they locked in the design.

[J.A.W.]

Since the tightly rules constrained current formula mandates matters so stringently,
(effectively banning any other 4T valve system, such as sleeve valve, or rotary valve),
& that also applies to other racing formulas still using pushrod 2V such as Top Fuel,
& NASCAR, its a moot point, really.

Having noted that, it is a fact that Mercedes-Benz & Nissan attempted to compete in
the Australian 'V8 Supercars' series using DOHC 4V engines (against Ford/GM NASCAR
derived, & highly developed 2V pushrod V8s) & found the arbitrary 7,000rpm race limit
(& I have a nearly 30 year old, 400,000km+ M-B roadcar still running fine, with that redline)
also imposed on their 5 litre mills certainly cruelled power development, & so dropped out.

OK, for sure, it might be possibly feasible for a pushrod actuated 4V race engine* to
compete at the fairly low rpm limits of the current F1 regs, but IMO, its unlikely to occur,
on the grounds of:

1,Durability/TBO requirements.

2, Perceived 'backwards thinking' engineering, (on corporate/marketing grounds).

*British company Singer, back nearly a century ago, raced a 4V pushrod machine,
while aero-engine maker Napier, justly preferred DOHC 4V for its uses back then, naturally...

[bill shoe]

Having noted that, it is a fact that Mercedes-Benz & Nissan attempted to compete in
the Australian 'V8 Supercars' series using DOHC 4V engines & found the arbitrary 7,000rpm race limit
also imposed on their 5 litre mills certainly cruelled power development, & so dropped out.

The 5 liter displacement limit is similarly "arbitrary". Rev limits, displacement limits, and many other limits are all somewhat arbitrary ways to limit power. The closest thing to non-arbitrary limits for a racecar engine would be a fuel-flow limit by itself (no other powertrain rules). This would probably lead to large-displacement but low-rpm engines. Despite the large displacement they would still need to be very compact on the outside. This makes it feasible for pushrods to be a reasonable choice. Airplane piston engines are also a great example of engines w/o arbitrary limits that use pushrods.

[Ringleheim]

Surely you are missing the main point ?
On a pushrod actuated valve the spring has to control the inertia of the valve plus everything up to the cam lobe. That's easily double the inertia of an equivalent direct acting valvetrain.

For the same cam profile, direct acting = smaller spring force = more power. Or alternatively, direct acting = higher allowable valve acceleration for the same spring force = more power, whichever works best.

Not necessarily.

A direct acting, cam on bucket valve train is inherently stiffer, but it is velocity limited by the diameter of the bucket. Also, it's a 1:1 lobe to valve profile, and as such, the extra acceleration at the lobe due to the system stiffness is needed because the lobe isn't being multiplied by a rocker, and needs the acceleration to get the same valve lift profile over one with a rocker multiplying the lobe profile.

Finger followers provide the best of both.

Again, the engine doesn't care how the valve is lifted. It only sees the valve lift profile. With the push rod system, there is some disconnect between the lobe profile and the valve profile through the rpm range, due to system stiffness, this is all measurable and compensated for, and in NASCAR's case, it is used to their advantage through controlled loft that gains open valve area with increasing rpm. Regardless, top pushrod racing engines are valve spring limited, not anything else.

We're talking pushrod actuated cam systems here. Pushrod does not have to mean 2 valve per cylinder.

A SOHC like Honda's CRF450, provides a narrow / compact top end like a pushrod actuated system does over DOHC, but with the system stiffness of a OHC platform as well as rocker multiplication through finger followers:
https://motocrossactionmag.com/wp-conte ... m-sohc.jpg

The engine doesn't care how the valve is lifted, but the cam and spring feel the difference between pushing 10grams or 100 grams of steel around at 12.000 rpm. Even a system like the CRF Honda (which I think is an old 70's design, I had it on one of my Honda XL's when I just starting to ride bikes in the 80's) doesn't make the cut for racing applications. It's too heavy and can't provide the hot cam profiles like, for instance the BMW S models with a dragger.

Pushrods have no place in modern racing, they are as obsolete as carburetors and solid rear axles.

You can say the same about any type of internal combustion engine whatsoever.

[J.A.W.]

Having noted that, it is a fact that Mercedes-Benz & Nissan attempted to compete in
the Australian 'V8 Supercars' series using DOHC 4V engines & found the arbitrary 7,000rpm race limit
also imposed on their 5 litre mills certainly cruelled power development, & so dropped out.

The 5 liter displacement limit is similarly "arbitrary". Rev limits, displacement limits, and many other limits are all somewhat arbitrary ways to limit power. The closest thing to non-arbitrary limits for a racecar engine would be a fuel-flow limit by itself (no other powertrain rules). This would probably lead to large-displacement but low-rpm engines. Despite the large displacement they would still need to be very compact on the outside. This makes it feasible for pushrods to be a reasonable choice. Airplane piston engines are also a great example of engines w/o arbitrary limits that use pushrods.

Bill, I think you'll find that the continuation of 1940's era pushrod-type light aircraft engines 'til today
is also a result of 'insurance liability' constraints, rather than any fundamental technical advantage,
(albeit they have been 'developed to a standstill' - in terms of everything - but basic engine architecture).

[Tommy Cookers]

the pushrod HO engine is .....
narrower (so has less drag and gives a better view)
easier to maintain (eg a cylinder that's down on compression is individually pulled and quickly fixed)
canted valve versions are widely used now
these are low rpm direct drive engines (a higher rpm ohc engine will import problems in having a reduction gear)
run very lean in cruise and very rich at full power

Porsche's 911-derived (and Honda's ?) aero-engines didn't make the grade

btw
9.2 litre CanAm Chevrolet V8s were smaller, lighter and more powerful than any other NA engines eg 7 litre Ferrari V12s

[Smokes]

Note pushrod aero engines tend to have a fairly long stroke and peak rpm of 3750rpm.
They tend to be designed for opreating at cruise around 2000 rpm.
The biggest issue with pushrod is the inertial the valve train slows the reponse of the engine i.e time to get to 5k to 10k underload.
This is one of critical things people forget about f1 is how quickly the engine responds to throttle demand.
In oval racing you dont really go from 3k rpm to 10krpm in in a .5 second like you would in circuit course. You are probably going from 7krpm to 10k rpm in .5 seconds.
I

[63l8qrrfy6]

Don't really think the overall engine inertia is such an issue with either pushrod or OHV setups.
The inertia "seen" by the crankshaft is only a quarter of the total camshaft+valve train total inertia.

[Jolle]

Don't really think the overall engine inertia is such an issue with either pushrod or OHV setups.
The inertia "seen" by the crankshaft is only a quarter of the total camshaft+valve train total inertia.

It has nothing to do with the crank, or the forces on the crank. It has to do with the cam profiles, RPM and the amount of stress/weight that has to be pushed up and down. At just 12,000 rpm, the whole valve, rod, etc has to come up and down 100 times a second. If it floats, the engine is kaputt. A lot of the gains of high rev power comes from a as sharp and extreme camshaft profile as possible, and with a DOHC and it's lack of rods, rockers, etc having to "vibrate" 100 times a second, and therefore less heavy springs, a DOHC setup is preferred.

Moden "free" engines have a very short throw, with the valves following the pistons so precise, that they almost hit the pistons. Which is shown on the Honda RCV's, where there are scratches in the residue after use of the valves. These kinds of profiles are impossible with those RPM's with pushrods.

[63l8qrrfy6]

Don't really think the overall engine inertia is such an issue with either pushrod or OHV setups.
The inertia "seen" by the crankshaft is only a quarter of the total camshaft+valve train total inertia.

It has nothing to do with the crank, or the forces on the crank. It has to do with the cam profiles, RPM and the amount of stress/weight that has to be pushed up and down. At just 12,000 rpm, the whole valve, rod, etc has to come up and down 100 times a second. If it floats, the engine is kaputt. A lot of the gains of high rev power comes from a as sharp and extreme camshaft profile as possible, and with a DOHC and it's lack of rods, rockers, etc having to "vibrate" 100 times a second, and therefore less heavy springs, a DOHC setup is preferred.

I agree, my point was that the inertia of the valve train has very little effect on the overall inertia of the engine in reply to Smokes' post.
As I have said in my previous posts however, the inertia of a pushrod system has a very large negative impact on engine performance due to the spring force requirement.

[coaster]

If all the joints were locked together it could overcome float, desmo style, but 2 valves a cylinder is to outdated, chuck the idea in the bin i say.

[J.A.W.]

the pushrod HO engine is .....
narrower (so has less drag and gives a better view)
easier to maintain (eg a cylinder that's down on compression is individually pulled and quickly fixed)
canted valve versions are widely used now
these are low rpm direct drive engines (a higher rpm ohc engine will import problems in having a reduction gear)
run very lean in cruise and very rich at full power

Porsche's 911-derived (and Honda's ?) aero-engines didn't make the grade

btw
9.2 litre CanAm Chevrolet V8s were smaller, lighter and more powerful than any other NA engines eg 7 litre Ferrari V12s

Well TC, I dont think a Napier Sabre mill would be "narrower", if it was a pushrod OHV unit,
& AFAIR, the ex-car derived Honda & Porsche aero-engines also fell foul of 'liability issues',
rather than any strictly technical problems with say, cam drives.

Maybe if Lamborghini had commissioned the big 8.2 litre iteration of their V12 - as so successfully
raced in boats - in time for late 1960's CanAm use, the McLaren-Chevs would've had a real NA
'run for their money' to contend with?

[J.A.W.]

If all the joints were locked together it could overcome float, desmo style, but 2 valves a cylinder is to outdated, chuck the idea in the bin i say.

Why limit yourself to 2V?

4V pushrod OHV engines are not all that uncommon, even Honda mass produced them,
(including via hi-tech 'turbo' form), & as it happens, Harley-Davidson still does..