Ok, while talking with some friends, the discussion of the LS1 various other motors came up. The biggest question was: why didn't they make the LS1 OHC? They'd already developed the OHC design for the ZR-1, so why not carry this technology over into the new model? OHC's seem to offer much HP per cube than pushrod designs (Mustang Cobra) without a problem. It can't really be cost because Ford GM both have OHC V8's in less expensive cars. Why not pull this into your flagship car?
The only thing I can come up with is that it won't fit under the hood. Can somebody clear this up for me?

 

Actaully it is cost. Its still cheaper to use LS1 based engines or vise versa (engines the ls1 was based on) to power all of there trucks. Compair the price of a northstar to an LS1. Plus it is cheaper to design something using old proven technology, and GM has tons of old tech based V8 technology. The only way that I see the OHC or DOHC would be worse is the compleate inability for GM to use any type of variable valve timing, so no super economy low power at low rpm, high power low economy at high rpm. Also OHC has more moving parts or at least a more complex set of moving parts which would cost more to produce.

 

Cost, cost of maintanence, and they wanted to be able to close the hood :D

 

You could make it fit under the hood by using a dry sump system--but GM won't due to cost.

It takes more power/torque to turn 4 cams (2 per head) operating a smaller number of valves each than to turn 1 cam with move valves including the friction loads of the pushrods and rockers. This is because a single cam turning 16 lobe has a pretty constant load, while a single cam operating only 4 inlet valves has times where nothing is happening interspersed with times where 1 valve is opening or closing. At no time is the closing of one valve helping to open another valve. All this unsymetric loading shows up as extra pressure on the drive mechanism (gears) and ultimately in Friction.

This kind of unbalanced load increases with the square of the RPMs, so while a 4 cam V8 could be made to rev 10% (FORD) to 40% (Ferrari) faster/higher, the energy to run the valves could be significantly higher--leading to lower gas mileage wih the increased performance.

Contrary to belief, a 2 valve head can flow a lot of gas at low to moderate RPMs. Its only the RPM band above 5000 where 4 valve heads show significant gains over 2 valvers. These same engines also show lower performance in the lower RPM band (unless even more equiptment is added to the engine (VTEC, VVTi,...)

Bottom line: If you can fit more cube under the hood with a single cam 2 valver, most of the time you will be better off than a smaller multi-cam multi-valver. Especially when fuel economy is on your checklist of desireable engine parameters.

 

Corvettes have always had engines (except the ZR1 experiment) with the profile of low reving and high torque.

I believe it's and issue of tradition.

A switch to the multi-overhead cam engine would greatly change the nature of the beast! Just buy a BMW is that's what you want.


[Modified by wwashing, 3:55 PM 3/6/2002]

 

I’d rather get big block :rolleyes:

 

Aren't you glad they didn't? Look what we can accomplish with our old fashioned pushrod motor. Upwards of 450 streetable HP and GREAT gas mileage. Why mess with that? Besides, us with mod fever would have to dip VERY DEEP into the wallet for a head and cam job. Pretty much everything would cost more. Many of us would be too intimidated to take apart a multi valve OHC motor, while taking one of these apart is "cake".

 

I used to wonder the same thing untill I drove a DOHC car and drove a Corvette. It is all about TORQUE! IMHO Just because it is new doesn't make it better. They both have advantages but I love that torque :D

Jerry

 

Willi and Mitch said it, Probably the number one reason is..................... The low drag coefficient resulting from the low engine height . The front hood is soooo loooooooow!


[Modified by BRADS C5, 9:30 PM 3/6/2002]

 

go to http://www.idavette.net/hib/ls1c.html for the full story

Pushrods and Why.

By now, you’ve probably exclaimed several times, "Heck, guys, your ah....‘new’ engine has got friggin’ pushrods. How in blazes can ya call that ‘revolutionary technology’?"

For ultimate performance, it’s tough to beat DOHC and 32-valves; nevertheless, GM Powertrain decided to use pushrod-operated valve gear for the LS1. Why? To quote John Juriga, "The LS1 is the only engine in the Corvette for 1997. We think a base engine at 345 net horsepower is plenty of power. If that can be done with one cam, 16 pushrods and two valves in each hole; we can live with that."

There has to more to this issue than that, and we intended to ask Ed Koerner, Gen III Chief Engineer, to comment further. Unfortunately, Chevrolet denied our request for an interview with him. We later sent Chevrolet a question about the valve issue to forward to Mr. Koerner, but that went unanswered as well, so heck; we had to guess.

First, the obvious: money. It costs less to build a pushrod engine. There is one cam, not four; one cam drive chain, not three, 16 valves and associated parts, not 32 and a less complex head design. To have a reasonably flat torque curve, the DOHC LT5 needed a complicated, expensive, computer-controlled, secondary throttle system. The LS1’s advanced, two-valve head eliminates the need for that. Lastly, the C5 version of the Gen III is derived from a cast iron passenger car/light-truck powerplant to be built in the tens of millions, so the cost of developing the LS1 can be distributed over a much larger sale of similar engines.

Second and less obvious: attitude. It is unlikely General Motors will ever again see the brash thinking that spawned the LT5. Development costs were excessive, purchase price was high and sales numbers were too low. In 1992, GM was almost broke and at one point, literally, only days away from closing its doors. These were sobering thoughts to the high-level execs who wrote the checks and answered to stockholders. The aftermath, the downsized, "Dilbert Era" of the mid-90s, was traumatic for the General’s bad-boy, car-guys. LS1’s technology is cutting-edge, but it had to come from a different side of the blade than did LT5. This new engine’s existence was contingent on cost-effectiveness, as well as performance and that meant 16, pushrod-operated valves.

Want more? Well, how ’bout marketing? Gen III’s main, target market is going to be trucks. About 25,000 a year will go in Corvettes but hundreds of thousands a year will go in trucks. Many people don’t see overhead camshafts as a positive selling point for truck engines. Truckers want cheap, simple, reliable and durable powerplants and that means a single cam and pushrods. A final consideration may be packaging. The C5 engine compartment was not designed for the width a four-cam engine.

Our reaction to the LS1’s valve gear? Well, frankly, we don’t think any Corvette needs technology for its own sake. We have too much of that already. The LS1's superior cylinder head allows near-LT5-level performance right now with only two valves per cylinder. Like John Juriga, we can live with that. If you think the LS1 can’t touch the LT5; development engines are running on GM Powertrain dynos at the 400hp level with little modification. Will there be a "super-LS1" in the C5? Our fearless forecast is: yes, perhaps by 2000; however, you'll see a 400hp LS1 even sooner from Corvette tuners like Doug Rippie and John Lingenfelter.

:yesnod:

 

Tell your friends it doesn't matter how the power is made as long as there is enough of it. So far GM has proven they can make enough of it without the fancy tricks. Technology is only good if you get something for it.
Bill

 

Dave Hill and others have told me that it is a combo of the things mentioned above (cost, etc.), but the main reason was height; too tall for the Vette's hood line. :cool:

 

Hmmm- some good points. However, I still have some questions:

1) If a DOHC engine is so difficult to get low-RPM torque, how does the Cobra engine do it? And in a $35k car? We're paying between $45 and $55k and it's not used because of the added cost? Hmmm, seems pretty silly to me. If the blue oval guys can put it in a car that sells fewer cars than the C5 for $10k less, GM is doing something wrong. Now, I'm under the impression that the Cobra engine produced pretty good low-end torque. Of course, feel free to correct me if I'm wrong. Additionally, I'd like to see some curves for the engine if you guys know of any...

2) Cost. While the cost to develop the LT5 was so high, WHY NOT CARRY THIS COST OVER TO THE NEW MODEL? Spread your development cost over the new C5 and recover some of that R&D expense... It doesn't make any sense to spend huge amounts of money to develop a product, release it, then cancel it because it cost too much. If you can use it, do so!

3) History. Corvette engines have always been low RPM torque monsters. They wanted to keep with tradition.
Now this is just plain silly. They've always had non-runflat tires too, but the C5 went with the EMT's. They've always had the transmission in front of the driver, yet this model put it behind us for better weight distribution. They've always been the leader in GM's technology chain, yet they don't get something as obviously superior as DOHC? Heck, we get a HUD and an antiquated engine? Sure, this is a state-of-the-art antiquated engine, but it's still ancient.

The fuel economy arguement probably holds water, as does the hood height. Making the engine dry-sump would probably be cost prohibative- so I'll buy that.
Now, don't get me wrong- I love this engine. It's got plenty of power, gobs of torque, and it's got mind-altering abilities, especially when you consider the fuel economy! But imagine what it could be if they'd finessed the LT5 a little more and dropped it into our C5? Wow.

 

Haven't paid too much attention to the latest Cobra performance but the cammer didn't do to well in the power department when it first came out. LS1 Camaro's were eating it up.
Bill

 

Ok comparing the mustang to the corvette is different from comparing the mustang engine to the corvette engine but they are related. Think of it this way, ford takes a very very old car design changes it to IRC, but not a good IRC because we all know they get weel hop, and puts a low output DOHC engine in it that is based off there SOHC engine. The engine displaces 4.6l and actually made more like 300hp then 320. Lets compair that to the 4.5l Q45 engine which makes 340hp, or better yet the 4.2L lexus engine with 310hp, never mind the 5.0L 400hp BMW M5 engine or the 3.2L 333hp (wow) M3 engine. The ford engine isnt really that high output when compaired to thoes, yet the car weighs 3400lbs. What does the weight matter? Well if GM can spend money keeping the corvette weight down to 3100lbs yet still have an ultra rigid body structure and an IRS with no wheel hop, what does it matter that they use a remanufactured aluminum truck engine with 350hp? What does it matter that its fule economy is better then fords DOHC set up and that It dosent really weigh any more then the DOHC ford engine? It obviously saves GM money, and if it dosent affect performance who cares if its a 5.7l with 350 hp. Think about a real comparison. Think about the 3.2L M3 engine with 333hp at the other end of the spectrum. It is no cheaper then the vette engine (not even talking about the entire car) and it is out performed compleatly by the old fashoned vette engine. GM could make a DOHC set up. In fact they will make a high output one. But it will cost at least 20Grand more and will be in the cadilac XLR. The secret to GM technology and success, is simplification.

 

...and if you don't think the "low tech" LS-1/LS-6 engine family is a brilliant performer...re-read "All Corvettes are Red." :chevy All that power AND all that economy...make mine Chevy! :yesnod: :cheers: :D


[Modified by silversport, 3:22 AM 3/8/2002]

 

I think people forget that OHC setups were around before pushrod setups. So actually the pushrod technology is the "new" technology.

 

New technology or old, the LS1 / LS6 definitely is a cut above the earlier small block in power and torque output without all of the headaches of OHC.

From the perspective of technology, and pure output per liter, I prefer the cammers, expecially the Ford 4.6. I even measured up my C4 for a Q45 motor (its way too wide).

However, when it comes to getting power on the road, the LS1 is one heck of a compromise between power size and weight. Who needs cams on the top anyway when this car leaves just about everything else in the dust?