Any possibility of introducing DOHC as against push rod technology in C6 ???

[Jacko]

Well, although I'm quite partial to the LT5 engine, there are couple things to take into consideration.

First of all the ZR-1's LT5 engine was a joint venture between GM Powertrain and Lotus in Hethel, England. At the time, GM owned Lotus. Because the construction of the engine was going to be mostly aluminum, GM didn't have a lot of experience building all aluminum engines. As a result, they looked at several outside sources for manufacturing and chose Mercury Marine; a very well known, reputable outboard manufacturing company located in Stillwater, OK. The idea/concept of the LT5 engine actually began in 1985 as the Corvette engineers had the insight to realize that a very serious threat was coming from overseas. Hence, the LT5/ZR-1 project began.

At the time, a DOHC was the way to go to create the high revs and power necessary to push the Corvette into exotic car performance territory.

As the ZR-1 matured from 1990 to 1995, so to did GM's ability to massage more power out of the pushrod motor. Hence, the birth of the LT1 engine in 1992. The LT1 engine put out 300 horsepower; 75 hp less than the 1992 LT5. Even though the engine lacked the 75 extra horses, the LT1 powered Corvette also lacked some of the ZR-1s extra weight. Over the years, the ZR-1's weight increased to just over 3500 lbs I believe (see http://corvetteactioncenter.com/specs/c4/zr1/zr1specs.html for exact details). The difference in performance grew smaller between the LT1 and ZR-1.

Rob, seems it's a broken link here?
http://corvetteactioncenter.com/specs/c4/zr1/zr1specs.html
Jacko

 

[militant_x]

Two circular valves don’t cover as much area inside the cylinder head as 4 valves would. This generally leads to lower flow numbers for the two valves. The 4 valve engines get more flow, and more control over their valve timing and overlap.

Rpm is usually limited by the valve train because of the “poppet” valve arrangement is dumb. In a pushrod engine there is friction between the lifter and the lifter bore, inefficiencies in transferring this movement through a pushrod etc. However, in this arrangement a valve spring has to control the weight of the valve (retainer, keeper etc), to some extent the rocker.. AND the weight of the lifter and pushrod (they must all stay in contact or the valve will float). This is generally a lot of weight. A 4 valve engine doesn’t have this, they have however to compress twice the amount of springs, it is a tradeoff.

To say one form of engine is more powerful while one is more torque-ish is pretty shortsighted. Either engine is more efficient at in a certain power band, this band gets chosen by the engineer to achieve best economy and power through their chosen transmission (gear spreads etc). A 4 cam engine can be tuned (cams) to get great low end just a pushrod engine does. Pushrod engines do this because to make them rev is hard (spring load mainly). I bet though that most jap cars that rev to forever are coupled to a nice set of 4.11 rear end gears or something like that.


I lost my direction with this post…

Ohh yeah, 4 cam engines can be tuned to have a wider power band. An engines power band is a pretty complex thing to put together, simply it is a balance of mechanical efficiency and volumetric efficiency, that is, friction and the amount of air taken in compared to what is needed. An l98 engine made such great torque because:

At the low end its small diameter tuned length runners allowed air to move fast (compared with the slow moving piston…. slowly increasing vacuum) and fill the chamber pretty well. At the high end this because a restriction because not enough air was moving and too much power was being lost to friction.

That was an example.

Next point.

Why power/liter is almost always dumb.

A small engine, like a 3 liter only has 3 liters of volume to sweep, that is, less area to drag its piston rings on. Even though they must rev higher to make power, the friction efficiency is staked in their favor. A small engine is almost always more efficient in this regard, also its parts are generally lighter etc. Power/liter is also dumb because it says nothing about the engines power curve, which is also dumb to argue because that needs to be argued only when considering the cars rear gears, transmission, weight, traction, wheel diameter, etc. Compare two 5.7L v8 engines, one makes 500hp (easy to do with a 350), the other only 250…. See the point? Which one is street able?… you don’t know, they both could be. Boost is just cheating in this type of comparison as under ideal conditions boosting one atmosphere doubles the volume of the engine (im not this naïve, just simply).

There is so much more I wanted to say in this post, but I keep getting lost in my explanation of my understanding of engine engineering, everything seems to build of each other.

My personal favorite however would be a variable length intake runner engine coupled with a sphererical rotary valve head, with maybe some boost, a close ration tranny…. And ohh yeah, 7.0 liter 427 turbo fire!!!!!

But I would love a 427 that spins them tires at will.. the numbers don’t even matter that much for me. It just so much fun to spin, bring the front end up, and throw your passenger so hard into the back seat =) hehehe. I know however, that should not be a production car, people could kill themselves really easily (like an old hemi, or ZL1 (or so I hear, wasn’t alive)).



-later, -chris



EDIT: text was too small?? It is now set to size 3. It gets weird when you paste your text into the window.

 

[Edmond]

I couldn't read that last post, could you enlarge the text?

 

[militant_x]

sorry, the size in the edit window and in the preview window dont match. Size two looks really small, and three is looking huge now.....

-chris

 

[WhalePirot]

I would love a 427 that spins tires at will

Ken's does and is what, 421cid?

Mine does, at 406cid. revs easily to fuel shutoff at 6300. Easy to work on; parts easy to find and not too pricey. Kinda what Chevy is about. I don't waste fuel on ANY ricers and few others, unless it is a fast bike.

Northstars are not supposed to reuse the heads, I was told. My pals Eldo has been a PITA since day one. Can you say engine #7? Caddy reused his heads on #6 and it didn't last.

What is the advantage of OHC? Seems our man in Goose Creek nailed it w/ valve train loss and rollers, etc. Solenoids MAY make all this mute with variable timing, etc.

The Viper I drove was impressive in acceleration...PERIOD!
I've not found a Japanese car that was all that much fun to drive, including the NSX. I have shopped Porsches and others many times and came home to the Vette. To each his own. Torque, baby! Repairs don't cost me my first born.

I've rebuilt OHC engines and pushers. For my money and more of what Chevy has been about, I'll stick with the push rollers, for now. I think the dealers are in a similar boat, as one poster said. Hell, many Chev dealers couldn't even fix the C-4 when new.

Then again, the Vette is a technology showcase and LT-5s and the like will pop up to remind the world that GM has world class engineers, too.

 

[taegdh]

Hey I go to sea for a month and the OHC arguement has raises it's head again. Figure.

But the idea of solenoids driving the valves is an idea which has been tried in labs, but doesn't have the reliability of a chain (how about a good gear drive setup from bowling green) and a good cam and roller lifters and rockers.

Yeah, Yeah, Yeah! Honda and all those high volume FWD cars are double OHC, with variable valve timing, operated with solenoids, but this is a two edged sword!

When it comes to a Vette we need an engine with good low end torque, and we need to keep the weight low. I don't have any figures in front of me, but I'd estimate that if GM were to do that to the venerable Chevy V8, it would add another 100 labs to the engine, plus enlarge the width of the engine to make it not fit in the already tight engine compartments of the C5 and follow on models.

Take a look at the (groan) F-word, and their 4.6 liter with ohc and dohc in the m-word. The dimensions of the engine are really wide and it makes working on it in the vehicle a real chore. Plus look at the power band on it, it needs higher rpm to produce the horses, and the torque doesn't even come close to the pushrod engine it replaced.

Plus, if you look at the power bands of all these OHC engines with all the fancy solendoids and variable valve geometry, they have no real useable torque down low, and have to really wind it up to make any real horsepower! Unless you're building a purely performance engine for your vette, (for instance say you're entered in the 24 hour race at LeMans, or Daytona, etc.) then it might be something to consider exotic.

I would prefer the KISS set-up, where I got lots of good torque, and reliability. I don't make enough to field one for a track and one to drive daily!

TTFN, Taegdh

 

[Brett]

The main two things I see people saying is wrong with DOHC is that "it's too difficult and complicated to work on" and "they don't produce as much torque."

I call B.S. on both accounts.

In response to "it's too complicated", nearly every single automaker across the globe uses OHC technology.The only reason us Americans think its too difficult is because GM hasn't fully adopted the platform yet. It's all relative. Once they do, and it becomes the standard, parts will become readily available, and technicians will become readily available.

In response to "DOHC has no torque", I can say look no farther than the new Nissan Titan full size pickup truck. It has a 5.6 liter DOHC V8 that produces gobs and gobs of torque. It produces more torque than the Ram and it's 5.7 liter Hemi, and the brand new F-150---even right off the line down low. And the full size truck line is all about torque.

Lastly, power per liter is a useless argument. The only reason you ever hear about power per liter is because across seas cars are taxed according to their engine size. The larger your engine, the more you have to pay. It is an irrelevant statistic here in the States.

 

[c5vetter]

Brett,

AMEN, on the Titan! have one, and I tell you what, pulling a 26-foot fully enclosed trailer with my 1963 Z06 onboard, I blow by people, even in the mountains, and still get around 13 mpg!

 

[Hib Halverson]

It's hard to speculate if GM will ever try to take on the exotics again, Bullwinkle. Some would argue that there is no need to, while others believe it to be imperative. (snip). --Bullitt

It is not hard to speculate that point. In fact, no speculation is necessary.

GM is not going to build a fourth model of the Corvette as some kind of hundred-plus grand exotic.

GM, through VLE David Hill, has said that time and time, again.

Those that really understand what the Corvette has been, is now and always will be will also understand why a move into the 100,000-350,000 price range would be the end of the brand and we've known it for the last 52 years and perhaps the end of it for good.

 

[rkreigh]

as an owner of 4 zr1s (still have 2) I am inherently VERY biased but let me add a few things to this discussion. the technical advantages of the DOHC engine, while very expensive and heavy to produce is substantial. the flow from the smaller multiple valves offer a big performance edge, ability to rev much higher for a wider power band, and inherent durability over pushrod engines. that's why most all "high end" race cars use this layout.

all that being said, weight, cost, and physical size hindered the LT5. there was already a "mild" LT5 with 475 HP that could pass emissions back in mid 90s, but it wouldn't fit the c5 and as Dave M. pointed out, they could achieve similar power at much lighter weight with the LS1. cutting weight and cost, @ the 350 HP level, was better overall than a 400 lb heavier car at 475 HP which would be harder to balance and keep good gas mileage and handling.

also keep in mind, the LT5 had a hand in getting GM "on the map" by producing a very exotic vette and the technology which led to the northstar, aurora (indy !!!) and dramatically raised the bar for high quality tolerance specs.

without sounding too arrogant, take a ride in a ZR1 at full song and then see what you think. lift the hood of a zr1 and a z06 and see where the folks will go to gawk. the LT5 is an amazing engine, just not feasible for the new lower hood lines and too heavy to produce a tossable light weight vette. very pricey too but if produced in volume, the LT5 would have gotten MUCH cheaper and mercury offered GM OUTSTANDING deals on the engines if they ramped up the production numbers. alas, it was not to be.

there are a million ways to make power, as for me, the 390 stroker LT5 at 624 HP (515 at the wheels) with no nitrous, blowers, and still getting 23 MPG with 4.10 gears is a very good combination. top speed pull is awesome on the car and it has more bottom end power than I can hope to hook up.

now back to the present, the new c6 brings remarkable "efficiency" and with 400 HP the dyno curve compares more than favorably with the LS6 and it has more bottom end grunt than the LT5. at 45k, the car is a super bargain and overall a very high quality advanced pushrod engine that is both lighter and more powerful than the previous vettes. and by staying affordable, it's a very good thing.

but to give some credit where it's due, the LT5 layed some of the foundations for the engine designs to come and improving packaging, cost, and weight considerations. the new LSx engines produce decent power from "old school" pushrod tech that more than compete with the LT5. but they will never possess the exotic nature or technical superiority to the LT5 which has proven to be ultra powerful, durable and just plain sexy. it just ended it's life long before the LT5 project could be fully developed. the variable cam LT5 would be way over 500 HP and much lighter than the early engines.

as far as working on it, it's no worse than any other DOHC engine and MUCH easier than the ferrari/porsche/lambo cousins. the real advantage is NOT having to work on it. with the 4 ZR1s I have owned the most complicated problem I have had is putting the MAP hose back on!! and this is on cars that see 7k plus on a daily basis.

drive the zr1 and remeber this vette raised the bar and set the legendary performance that has gotten the vettes to come to where they are. the car was way ahead of it's time and still competes favorably with the modern powerplants 10 years newer.

don't dismiss DOHC tech, it's good stuff, just not cheap or light, speed cost money, and the zr1 delivers!! nuf said.