Quote:
Originally Posted by 32valvesftw
One of the pros to the DOHC, is that one can use a more aggressive cam grind and still maintain a good idle.
|
I'm curious as to what you basis for this comment is?
Quote:
Originally Posted by Paul Workman
Really? Can you provide data of examples where a NA OHV motor of comparably equal technology and displacement demonstrates equal peak power and superior power under the curve while maintaining a smooth idle? (I referenced the Mercedes M156 6.2L NA as an example of like displacement with like technologies - i.e., cam phasing, etc., and there are others. Example? For example: can you support your claim of comparable OHV being superior to a 25 year old LT5 across the rpm spectrum with a smooth idle?) Point/Counterpoint.
You're trying to make too many comparisons with too many variables. You throw out the Mercedes M156, a small production engine and compare it to an assembly line chevy motor that they make probably 10 times as many of. No, there isn't a fair comparison out there. The LT5 was a small production, hand built engine with tight tolerances and materials that weren't cost effective to bring to base corvette.
*..."engines that are every bit the equal..."*
Showing us the data to support that claim (w/ comparable displacement) would help to provide some validity to that statement...
Closest comparison I could come up with is Ford DOHC 5.4 (lincoln navigator version) vs Chevy 5.3 (truck version). Both have a flat torque curve that makes peak torque that starts at about 2800 RPM. However, the Ford drops off at 4800 RPM while the Chevy holds max torque until 5600 RPM. Both obviously have smooth factory idles. Do I think this is a completely fair comparison? No. There is no data that allows me to say they were shooting for the same goals when they built the engine and that was the best they could do.
How do those very special application BLOWN motors (rebuilt every 1/4 mile run) have any relevance to the architecture discussion at hand?? 
It seemed like you were implying that the architecture was a limiting factor in some way. If that wasn't what you were saying, ignore that.
Granted, VVT is a modern asset to OHV. However, it is even more of an asset to DOHC architecture as the exhaust valve timing can be independent from intake cam lobe timing. Point/Counterpoint.
Could be done with purpose built hydraulic lifters. It just isn't at this point because it isn't cost effective.
Well, I refer (again) to the example of the DOHC Mercedes NA M156 which shares those same technological advances AND displacement with GM's current OHV NA LT1. I dunno, but most of legit true apples to apples comparisons of DOHC vs. OHV, proving superiority of the latter is pretty thin in the evidence side of the argument.
I'm not claiming one is ultimately superior to the other. They both have their advantages. People in here want to use the reliability of the LT5 as a a defense for DOHCs, but that is nonsensical. The LT5 was reliable because it was hand built to close tolerances using materials that were designed for ultimate reliability. That fact remains that a DOHC engine has far more parts than a pushrod engine. That inherently makes them more prone to failure. If I went out and built an LS1 with aluminum block, aluminum liners, nikasil plated them and the aluminum pistons..etc. You'd have a motor that was more reliable than even an LT5. Why doesn't Chevy build that? Not enough people are going to pay $15,000 more for a corvette because the engine lasts for 250,000 miles instead of 150,000.
Far as blown V6's goes, one only has to see one of the 1/2 mile or 1 mile shootouts to see the truth of "If you ain't blowin, you ain't goin!" But, those kick-*** Nissan V6s are also DOHC, by the way... But, as Mike already said, hybrids w/ electric FWD may be what really makes DOHC moot - to your Point/Counterpoint.
|
ROI
For the couple hundred or couple thousand people who want a flatter torque curve, with a dead smooth idle and 8000 RPM top end, Chevy isn't going to spend 500,000,000 dollars to retool a plant to build a DOHC V8. And they would piss off more Chevy diehards than they would win over.
You could put the average corvette driver behind the wheel of a 1996 Grand Sport and a 1995 ZR1 and they wouldn't be able to tell you how they felt different. They are selling to the people who are one standard deviation from the center of the bell curve. ZR1 owners are two standard deviations out and Chevy will likely never build an engine that we think would be comparable to the LT5.