Unicorn: a 4 Valve Head Pushrod Engine

[nicholastanguma]

Obviously today GM's LS range of engines proves that sticking with refinement of 2 valve pushrod architecture is both commercially and technologically viable--the LS V8 is the world's most popular crate engine for anything from old Jeeps to old Nissans to brand new boutique supercars like the GTM and the GTR.

In the moto world the now-defunct Motus proved that a pushrod V4 bike engine could provide similar performance--the Motus V4, even with a 2 valve head, could put out 180 hp and 123 ft lbs in high tune.

Harley-Davidson's new M8 v-twin remains pushrodded, although the heads sport 4 valves each. In comparison to the TC engine it displaces the M8 offers higher hp and torque at milder cam profiles.

Why GM and Motus chose to keep 2 valves I simply don't know--through some technological voodoo their engines somehow don't seem to need 4 valves. HD has gone to 4 valves and its new performance numbers are better, so clearly there are plenty of engineering nuances I don't yet understand.

In racing an engine's ability to rev high makes great sense, so SOHC and especially DOHC seem perfect for such applications. But for a street engine, wouldn't a pushrod 4 valve head mill be ideal? You keep the compact dimensions, mechanical simplicity, and lower weight of a pushrod design, but get the high breathing abilities of a 4 valve cylinder head at the low and mid range rpms where street mills typically operate.

Right, or am I missing something?

 

[85_Ranger4x4]

OHC engines are not really that heavy, just big.

I almost think OHC is simpler. Your valve train is all on top the engine if anything needs repair or if you want to swap cams. Either put timing chains on it or make it non-interferance with a belt and you are good to go IMO. It is like open heart surgery to swap the cam in my Ranger...

The downside for us mix/matchers is that even SOHC (let alone DOHC) doesn't really fit in anything that didn't have a big block option. It is a personal irritation of mine that for my Ranger I am stuck with either old Windsors or GM because a OHC mod motor will not fit without deleting my heater and moving the steering box outside of the frame.

Powerband wise for a truck I don't know if 2v vs 4v is a huge deal.

I know at least some of the Ford Powerstrokes are pushrod 4 valve engines and they don't seem to have much for problems with that part of it though.

 

[javajoe79]

It's just not necessary IMO. Like you said, look at the LS platform. Why complicate something that can already be built to suit pretty much any purpose you can come up with. LS engines don't need 4 valves.

 

[85_Ranger4x4]

It's just not necessary IMO. Like you said, look at the LS platform. Why complicate something that can already be built to suit pretty much any purpose you can come up with. LS engines don't need 4 valves.

They do take more displacement to match a 4V engine though.

 

[CampStewart]

A 4 valve will out breathe a 2 valve but with variable cam and ignition timing it mitigates some of that ultimate advantage. 4 valves shine more and more as rpm's increase. Overhead cams shine more as the rpm increases. If this was a sports car or drag racing forum I would think the discussion would have a lot more merit than it does in a forum where people are mostly interested in smooth low end power and torque. OP what advantage do you think you are going to see in an overlanding rig with a 4 valve pushrod head? How is it going to make your rig better?

 

[85_Ranger4x4]

A 4 valve will out breathe a 2 valve but with variable cam and ignition timing it mitigates some of that ultimate advantage. 4 valves shine more and more as rpm's increase. Overhead cams shine more as the rpm increases. If this was a sports car or drag racing forum I would think the discussion would have a lot more merit than it does in a forum where people are mostly interested in smooth low end power and torque. OP what advantage do you think you are going to see in an overlanding rig with a 4 valve pushrod head? How is it going to make your rig better?

All the big turbo diesels offered by the big three are pushrod 4V engines...

 

[CampStewart]

All the big turbo diesels offered by the big three are pushrod 4V engines...

Im sorry I thought this thread was about NA gas engines,

 

[85_Ranger4x4]

Im sorry I thought this thread was about NA gas engines,

I am sorry, I saw neither fuel type or aspiration mentioned in the original post.

For that matter both the 2v and 3v gas 5.4 had excellent low end torque despite being SOHC, the 3v even had two exhaust valves per hole.

 

[b dkw1]

The biggest issue with 2V is plug placement. Why Dodge went with the twin plugs. In my view, a SOHC 3V with twin plugs would be the best cumbustion chamber for a torque truck motor.

 

[NevadaLover]

If OHC is sooo great then why is ford going away from it and going OHV in the new godzilla motor? They spent so much time and money developing the triton series only to abandon it for OHV..... kinda tells the story doesn't it!

 

[CampStewart]

I am sorry, I saw neither fuel type or aspiration mentioned in the original post.

For that matter both the 2v and 3v gas 5.4 had excellent low end torque despite being SOHC, the 3v even had two exhaust valves per hole.

All the engines mentioned in the thread were gas NA until you brought turbo diesels into the conversation. The original post also mentioned the more compact nature of a pushrod engine as why he was promoting a 4 valve pushrod gas engine. Nevertheless I am curious why you imply that SOHC engines make good low end despite being SOHC as I was unaware that SOHC engines were known for poor low end power due to its design.

As Ford and GM were moving away from their older v8 engine designs and consolidating engine families they chose 2 very different paths. It seems to me like Ford has gone through a bit more experimentation and growing pains with their modular design than has GM with their LS. It is interesting that when Ford decided on a clean sheet truck engine not to be shared with their car line that they decided on 2 valve pushrod rather than OHC. I believe that the engineers in those 2 companies know far more about engine design than any of us do.

The new corvette would be a perfect showcase for a 4 valve pushrod engine if Chevy wants to push the envelope and finds enough of a performance advantage in the design.

Going back in time there were aftermarket 3 and 4 valve kits for Model T Fords in the early 1900's so the idea and application is not exactly new

 

[85_Ranger4x4]

All the engines mentioned in the thread were gas NA until you brought turbo diesels into the conversation. The original post also mentioned the more compact nature of a pushrod engine as why he was promoting a 4 valve pushrod gas engine. Nevertheless I am curious why you imply that SOHC engines make good low end despite being SOHC as I was unaware that SOHC engines were known for poor low end power due to its design.

It had more to do with valve count. The 2v had very respectable torque, with more valves the 3v was better.

As Ford and GM were moving away from their older v8 engine designs and consolidating engine families they chose 2 very different paths. It seems to me like Ford has gone through a bit more experimentation and growing pains with their modular design than has GM with their LS. It is interesting that when Ford decided on a clean sheet truck engine not to be shared with their car line that they decided on 2 valve pushrod rather than OHC. I believe that the engineers in those 2 companies know far more about engine design than any of us do.

Packaging had a lot to do with it imo. A OHC 7.3 would be massive.

It sounds good on paper but I wonder what is keeping the 7.3 from being a brand new 250hp 8mpg 460 since it has about the same tech. No variable cam, 2v... no tricks at all going for it.

The only thing replacing Tritons/Coyotes is twin turbo V6’s. The 7.3 is filling the void left by the untimely dismissal of the 6.8 V10 In the lighter Super Dutys.

 

[b dkw1]

Pushrod 4V is great for diesels where they don't rev that high. In something performance oriented like the Vette, all that valve train weight becomes a liability.

 

[NevadaLover]

The only thing replacing Tritons/Coyotes is twin turbo V6’s. The 7.3 is filling the void left by the untimely dismissal of the 6.8 V10 In the lighter Super Dutys.

Not really untimely, have you ever driven a triton personally, V10 or V8? They have to be revved to the hilt to get power out of them and the changes in CAFE and CARB regs made them obsolete, can't meet those requirements at the rpm triton series need to make power, that's why ford went ecoboost and godzilla.

 

[nicholastanguma]

OP what advantage do you think you are going to see in an overlanding rig with a 4 valve pushrod head? How is it going to make your rig better?

Yes, of course 2 valves will do well at low and mid rpm, and they'll do well at high rpm, too, but the physically greater area of 4 valves will do better at all of those rpm ranges. It's just physics. I'm not sure I any longer believe that "4 valves are only good for higher rpm," the auto and moto worlds both have too many empirical examples that display otherwise.

Everything I've been able to read about the '80s Krauser 4 valve heads for BMW flat twins, the '90s Feuling 4 valve heads for HD Evo Big Twins, the '90s Dominion 4 valve heads for small block Chevy V8s , today's Thunder Power 4 valve heads for big block Chevy V8s, and today's Hot Dock Racing (Japan) 4 valve heads for HD Evo Sportsters shows dyno graphs of improved torque and hp from idle to redline.

I'm beginning to suspect that OEM manufacturers didn't all pursue pushrod 4 valve designs because, being mass market companies, their business model depends on being able to sell enormous numbers of units every fiscal cycle. To make that happen requires marketing, and a largely non-mechanically inclined public will easily believe marketing hype about "This is now obsolete, so buy our new thing instead."