Author Topic: More Pushrod 2 Valve Engineering: Hot Rodding  (Read 384 times)

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nicholastanguma

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on: March 29, 2021, 04:43:40 am
Further discussion based upon my last thread about pushrod engines still being fun in today's world of high tech overhead cam engine design.

It is true that being able to operate the intake valve and exhaust valve seperately is a huge advantage, no arguing against this, and I'm not trying. I'm a happy luddite, but not an unreasonable one.

Okay, but just how huge an advantage is "huge?"

For example: the humble Honda CG single cylinder, the most reliable motorcycle engine ever produced, having a single cam lobe that operates both the intake valve and the exhaust valve.



Several years ago a hot rodded Chinese CG clone engine was setting land speed records in the hands of a racing team from Cleveland Cyclewerks. With a displacement of 229cc and head porting and a Mikuni VM32 carb it was dynoing about 23 hp at 9200rpm...which is about the same as a hot rodded SOHC Honda CRF230 engine the R & D of which is about 30 years younger. How is this even possible? Perhaps the sohc CRF230 engine is able to produce good low and mid range torque along with the top end hp while the CG230 engine is all top end?

Cleveland Cyclewerks is known for making junk Chinese bikes with junk Chinese engines, nonetheless they were able to set records in two different vintage air cooled classes.

https://www.youtube.com/watch?v=bMpQgrQtODk

Low tech? Yes. Utterly obsolete for a performance mill? Evidently not.

In an interview the man who built the CG land speed record engine wrote, "Having a single cam lobe requires that both the intake and exhaust valves have the same lift and duration, not what one would expect as the ideal design for a performance engine! As development would reveal, this was not necessarily a deficit since most racing/performance engines breathe best with a very nearly symmetrical valve timing."

Source: RideApart
https://www.rideapart.com/news/254553/how-to-set-a-land-speed-record-on-a-3200-motorcycle/


So someone more experienced than myself please explain how the land speed record CG engine was able to make so much power. Does it really come down to "since most racing/performance engines breathe best with a very nearly symmetrical valve timing?"

Perhaps this simple engine architecture is able to make good top end hp, but loses virtually all the low and mid range hp as a result, and that's where it's inferior to an engine design that allows for differentiation between intake and exhaust valve timing? 
« Last Edit: March 29, 2021, 05:07:20 am by nicholastanguma »


ace.cafe

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Reply #1 on: March 29, 2021, 01:49:58 pm
What you are really asking here is the difference between a "single pattern cam" and a "split pattern cam".

They can both be good, and much depends on the other aspects of the engine breathing tracts.

A typical use of the split pattern cam is to reduce lift on the exhaust valve, and extend the exhaust duration on the closing ramp by a few degrees, maybe even up to 10 degrees.
This meters the exhaust gas exit a bit, so that the exhaust still has some flow going on during the last part of overlap, which theoretically might help the scavenging "pull" on the intake mixture for a longer time. The idea is that it helps get the intake mixture flowing better until the piston downstroke takes over intake duty.

In truth, single pattern cams also do this if timed correctly, but it is sometimes the case that tuners prefer the longer exhaust cam.

It comes down to increased pumping loss on the exhaust stroke vs the effects on the overlap. Different tuners have different thoughts on using this technique. It is popular on V8 engines lately. However, exhaust behavior on a multi-cylinder vehicle is much different from a single.

My take on it is that both single pattern or split pattern can be successfully implemented if the entire engine package is set up to effectively work with it. Nearly every thing in an engine is inter-related with other things to make systems. How well it is all functioning as a system is the key.
« Last Edit: March 29, 2021, 01:54:10 pm by ace.cafe »
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nicholastanguma

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Reply #2 on: March 29, 2021, 08:10:50 pm
What you are really asking here is the difference between a "single pattern cam" and a "split pattern cam".

They can both be good, and much depends on the other aspects of the engine breathing tracts.

My take on it is that both single pattern or split pattern can be successfully implemented if the entire engine package is set up to effectively work with it. Nearly every thing in an engine is inter-related with other things to make systems. How well it is all functioning as a system is the key.


Thanks for such a detailed response, Tom.  Single pattern cam, first time I've ever heard that term.  Okay, I think I understand.  Is the basic takeaway then: even an engine with the CG's "primitive" architecture could be an Ace Fireball if every component from intake to exhaust was designed for such a hot rodding role, with a power curve from idle to redline and not just all top end hp? 
« Last Edit: March 29, 2021, 08:13:40 pm by nicholastanguma »


ace.cafe

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Reply #3 on: March 29, 2021, 10:05:32 pm

Thanks for such a detailed response, Tom.  Single pattern cam, first time I've ever heard that term.  Okay, I think I understand.  Is the basic takeaway then: even an engine with the CG's "primitive" architecture could be an Ace Fireball if every component from intake to exhaust was designed for such a hot rodding role, with a power curve from idle to redline and not just all top end hp?

Yes, sort of.

You must remember that in a normally-aspirated engine, torque is directly related to displacement, so that smaller engines will have a more difficult time at low rpms than bigger engines.
Tq x rpm/5252 = hp
That's why I recommend hot-rodding the 500 more than the 350.

But to your point, yes if the breathing tracts, the compression, and the cams are designed to give a wide power curve like that, then that's what you'll get.
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AzCal Retred

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Reply #4 on: March 30, 2021, 05:22:01 pm
Add a 12AH battery and bolt on one of these bad boys! Instant supercharging - boost makes up for many cam deficiencies. You'll just need a "passing" push button switch...   ;D ;D ;D

http://www.seaflo.com/en-us/product/detail/663.html
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Reply #5 on: March 30, 2021, 10:05:27 pm
Add a 12AH battery and bolt on one of these bad boys! Instant supercharging - boost makes up for many cam deficiencies. You'll just need a "passing" push button switch...   ;D ;D ;D

http://www.seaflo.com/en-us/product/detail/663.html

Now someone's going to try it and when it doesn't work you'll be the bad guy  ;D
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nicholastanguma

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Reply #6 on: April 05, 2021, 11:23:46 am
Yes, sort of.

You must remember that in a normally-aspirated engine, torque is directly related to displacement, so that smaller engines will have a more difficult time at low rpms than bigger engines.
Tq x rpm/5252 = hp
That's why I recommend hot-rodding the 500 more than the 350.

But to your point, yes if the breathing tracts, the compression, and the cams are designed to give a wide power curve like that, then that's what you'll get.


Hmmm, I keep looking at the diagram I posted, and can't help but wonder...does working two valves off the same cam lobe unequivocally mean that both valves must have the same lift?

The engine diagram shows a follower and a rocker for each valve. That means there are 2 places where a different ratio can result in more lift for one valve compared to the other, am I right or wrong?

Changing the ratio might require moving the pivot point of the rocker or follower so might mean significant changes to the engine. It may not be worth the trouble especially if what the landspeed engine builder says is true and best performance is usually near symmetrical valve actuation. It might be possible to change the ratios by leaving the pivot point, changing the rocker and follower, and enlarging the pushrod tunnel. But it's still possible to change the lift for one valve and not the other or to change them in different directions, right or wrong?

I guess what I really want to know is: if Honda or Royal Enfield had hired Tom Lyons to engineer a balls-to-the-wall hot rod 250cc-300cc air cooled thumper for a streetable dual sport instead of a dedicated landspeed racer using the CG engine architecture or even some pushrod architecture of his own design, would he end up being successful at the task or saying to his bosses up front, "Just go with something SOHC, there's no good way to do what you want using pushrods."

When I look at 247cc Honda single overhead cam, 2 valve CRF230 air cooled thumpers tuned to make 30whp while still retaining torque in the low and mid ranges I can't help but wonder why I let myself stay so in love with pushrod design.
« Last Edit: April 05, 2021, 11:39:37 am by nicholastanguma »


viczena

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Reply #7 on: April 05, 2021, 11:41:54 am
OHC is the way to go with modern engine design. It is much easier to achieve power or torque over different rpm ranges. And it is easier to manufacture.
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ace.cafe

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Reply #8 on: April 05, 2021, 01:29:20 pm

Hmmm, I keep looking at the diagram I posted, and can't help but wonder...does working two valves off the same cam lobe unequivocally mean that both valves must have the same lift?

The engine diagram shows a follower and a rocker for each valve. That means there are 2 places where a different ratio can result in more lift for one valve compared to the other, am I right or wrong?

Changing the ratio might require moving the pivot point of the rocker or follower so might mean significant changes to the engine. It may not be worth the trouble especially if what the landspeed engine builder says is true and best performance is usually near symmetrical valve actuation. It might be possible to change the ratios by leaving the pivot point, changing the rocker and follower, and enlarging the pushrod tunnel. But it's still possible to change the lift for one valve and not the other or to change them in different directions, right or wrong?

I guess what I really want to know is: if Honda or Royal Enfield had hired Tom Lyons to engineer a balls-to-the-wall hot rod 250cc-300cc air cooled thumper for a streetable dual sport instead of a dedicated landspeed racer using the CG engine architecture or even some pushrod architecture of his own design, would he end up being successful at the task or saying to his bosses up front, "Just go with something SOHC, there's no good way to do what you want using pushrods."

When I look at 247cc Honda single overhead cam, 2 valve CRF230 air cooled thumpers tuned to make 30whp while still retaining torque in the low and mid ranges I can't help but wonder why I let myself stay so in love with pushrod design.

Yes, the cam follower ratio and the rocker ratio can be changed if desired.

This idea about ohc being better than pushrods only comes into consideration when the intended design requires some operation that is outside the normal ability of one or the other type of valve train package. Such as the need to rev over 10,000 rpm. This is seen in short stroke sport engines trying to maximize hp from limited displacement.

Much of this ohc vs pushrod commentary comes from people who read hobbyist magazines or motorcycle marketing copy. Not from engine designers.

Regarding the Honda, it's not the OHC that gives the performance. It gets the performance from the state of tune that the factory engineered. The cam just turns in its bearings and controls the valves. Nothing magic about a cam being located in the block or head, for most applications, UNLESS you need to turn high rpms which cause pushrods to be unuseble due to weight or flex.

I see these kinds of technical banter a lot on motorcycle forums. It goes sort of like this, "RE is so behind-the-times. They still use pushrods when all the Moto GP, or boy racer bikes use OHC. Until they get OHC 4-valve engines, they'll never be any good."
I'm sure you have read that on forums, because I have. These people are magazine readers who memorize marketing ad features and conclude that "no engine could be any good without those features". In my opinion, it is best to avoid such conversations, and concentrate on engine design which is a scientific activity, not a marketing effort.
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Reply #9 on: April 05, 2021, 01:55:38 pm

Much of this ohc vs pushrod commentary comes from people who read hobbyist magazines or motorcycle marketing copy. Not from engine designers.

These people are magazine readers who memorize marketing ad features and conclude that "no engine could be any good without those features". In my opinion, it is best to avoid such conversations, and concentrate on engine design which is a scientific activity, not a marketing effort.

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viczena

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Reply #10 on: April 05, 2021, 02:05:33 pm
Dont forget valves with higher lift, which require stronger valve springs. They can become a hassle with pushrods.

if you go the other way and double the amount of valves, then a OHC is also much preferable.
« Last Edit: April 05, 2021, 02:21:02 pm by viczena »
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nicholastanguma

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Reply #11 on: April 05, 2021, 08:37:28 pm
Yes, the cam follower ratio and the rocker ratio can be changed if desired.

This idea about ohc being better than pushrods only comes into consideration when the intended design requires some operation that is outside the normal ability of one or the other type of valve train package. Such as the need to rev over 10,000 rpm. This is seen in short stroke sport engines trying to maximize hp from limited displacement.

Much of this ohc vs pushrod commentary comes from people who read hobbyist magazines or motorcycle marketing copy. Not from engine designers.

Regarding the Honda, it's not the OHC that gives the performance. It gets the performance from the state of tune that the factory engineered. The cam just turns in its bearings and controls the valves. Nothing magic about a cam being located in the block or head, for most applications, UNLESS you need to turn high rpms which cause pushrods to be unuseble due to weight or flex.

I see these kinds of technical banter a lot on motorcycle forums. It goes sort of like this, "RE is so behind-the-times. They still use pushrods when all the Moto GP, or boy racer bikes use OHC. Until they get OHC 4-valve engines, they'll never be any good."
I'm sure you have read that on forums, because I have. These people are magazine readers who memorize marketing ad features and conclude that "no engine could be any good without those features". In my opinion, it is best to avoid such conversations, and concentrate on engine design which is a scientific activity, not a marketing effort.



In your professional opinion, then, could even the CG architecture be turned into a genuine little firebreather, using a scratch designed hemispherical combustion chamber and changing the cam follower ratio and rocker ratio and everything else you know about Fireballing a thumper?  I mean, the landspeed article states with a 65mm stroke that particular mill had a 9200rpm redline.


ace.cafe

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Reply #12 on: April 05, 2021, 09:11:44 pm


In your professional opinion, then, could even the CG architecture be turned into a genuine little firebreather, using a scratch designed hemispherical combustion chamber and changing the cam follower ratio and rocker ratio and everything else you know about Fireballing a thumper?  I mean, the landspeed article states with a 65mm stroke that particular mill had a 9200rpm redline.

Very hard to call something with a 65mm stroke a thumper, but hey, if that works for you, then fine.

Just remember, a LSR racer only needs to last a few minutes of lifetime.
Nobody would ever seriously regularly ride one on the road.

The first thing you need to get is an education in this engineering discipline.
Then you can start looking at some metal objects.

Asking me what kinds of tricks I can pull out of my hat about unknown cheap Chinese engines really isn't on topic for this forum.
This is a Royal Enfield UCE forum.
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nicholastanguma

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Reply #13 on: April 06, 2021, 10:15:39 am

Asking me what kinds of tricks I can pull out of my hat about unknown cheap Chinese engines really isn't on topic for this forum.
This is a Royal Enfield UCE forum.


Good point.  Thanks for helping this long into the topic.