This question is prompted by by Dad’s old 1999 Cadillac (Northstar V-8) going off the junkyard-the engine blew at 150,000 miles (head gasket). The Northstar engine was a very advanced all-aluminum V-8-it was a DOHC engine, and could rev to 7000 RPM. Unfortunately, the early ones had a problem-the head bolts wold work loose and cause failure of the head gaskets. My question: now that 6, and even 8 speed automatic transmissions are common, why do you need a high-revving engine? A pushrod V-8 or I-6 will produce acceptable torque at low RPMs, and with the multispeed transmission, why have all the complications of OHCs, variable valve timing,etc.? A pushrod engine has a much sturdier valve actuation system, and such engines regularly give 200-300,000 mile service.
In the case of my Dad’s car, the failre was suddent-the engine overheated and when I pulled off the road, the oil was cooked. The repair for this is $3200 (and not guaranteed)-yo have to drill out the clock bolts and insert helicoil supports for the headbolts. Why not se the old slow revving pushrod engines?
Because that’s how you develop high horsepower. With variable valve timing the engine can develop good torque over a wide range of RPM (including very low-RPM torque for launching), but if you want high horsepower, the equation is still torque X RPM, so you’ll need the ability to achieve high RPM if you want a high-horsepower engine.
:dubious: “Sturdier?” In what sense? It adds a bunch of parts (the pushrod and the rocker assembly).
Re: your dad’s Northstar engine, I don’t see how its failure was connected to the fact that it was DOHC.
The long timing chains/belts that are needed to actuate the valves (in a DOHC engine) are a weak link. The pushrod V-8’s use a short, wide timing chain that rarely breaks-some Chevrolet 350 V-8s last for years without valve timing problems. As I said, if you can just rev the engine to 4000 RPM and shift through 6-8 speeds, why the need to rev to 7500?
All aluminum block engines will have corrosion problems, because the head bolts are steel and the aluminum block will cause galvanic corrosion. The early Nothstars were particularly vulnerable, because the headbolt staking wasn’t the best.
Energy density (lbs/hp) is a reason for both high revs and aluminum blocks.
The DOHC 4.6 Ford engine revs to the moon and is strong. They can support 1200+hp in race cars. At stock power levels, they last forever. I assume the new 5.0 DOHC is just as nice, but makes a lot more power stock. High revving, high powered V8s are fun.
At the extreme, consider F1 engines - there have been various rule changes, but typically they’re 1.5 liters, rev to ca. 20k RPM, and produce around 600 horsepower. The future of high-mileage, adequate-power, near-zero emission engines is likely 1 liter with a powerband between 3k and 10k, and a redline of 10-12k.
High revs are fun, but I have an iron-block FE that’s a blast between 3 and 5k.
it’s not that “high-revving V8s” aren’t needed anymore, it’s that they’re being replaced by V6s and forced-induction 4-cylinders. and in order to meet fuel economy standards, those engines are going to be peakier and with narrower powerbands. The 8- and 9- speed transmissions (along with CVTs) give more forward gear ratios in order to keep the engine within that narrower powerband as much as possible.
Why were they needed in the first place? Lighter overall weight and higher fuel economy, of course. Cast iron/steel gasoline blocks will last for many, many miles. Diesel blocks are even heavier but that’s due to the required higher compression ratios. However, diesel engines aren’t very popular in the U.S. passenger car market.
I’m just asking a general question but if it’s known that “the early ones had a problem-the head bolts wold work loose”, does Cadillac recommend that the head bolts be retorqued as part of the routine maintainence scheduale? It would be expensive, time-consuming labor but if it’s needed, it’s needed.
I was always told growing up that you should always get at least a V6, and a V8 for a large or full-sized vehicle. Four cylinders were downright anemic.
While I think this was true even into the mid-nineties, I don’t think it is anymore. The most vivid point for me is the 1999 Ford Explorer I bought a few months ago. It is (to me) a rather large vehicle, but only has a V6. The performance blows my mind. Acceleration is simply unreal.
Compared to my presumably similar (weight-wise) 1991 Buick Park Avenue also with a V6, the difference is indeed night and day.
I’m curious what effect the newer engines will have on the power boat market. IANA Mechanical Engineer, but I suspect the narrower powerbands mentioned by **jz78817 *won’t be a good thing. Boats rarely have multi-geared transmissions (they have only one ratio forward and reverse) and I suspect are better off with torque available in a wide RPM range. My boats have all had fairly large (5.7 - 7.4 L) engines that spin faster than a similar engine in a car (>4000 rpm). But the low-end torque is absolutely necessary to get on plane. I’m assuming that boats will continue to use marinized auto engines as in the past.
My WAG is that they’ll solve this by using variable pitch propellers, rather than shift-able transmissions.
*Is that the right term? I mean a transmission with multiple forward gears like a car.
Because a 5.7L V8 that makes a peak 300 lb-ft of torque at 3500RPM will not produce as much power as one that makes a peak 300 lb-ft of torque at 6500 RPM. Want power? You need RPM’s in addition to torque. Put the two engines side by side at the drag strip, and the high-revving V8 will win (assuming the low-RPM torque isn’t ridiculously compromised in pursuit of high-end torque). You can have all the gear ratios you want so as to keep the engine in its powerband, but if one engine is making nearly twice as much power, it’s going to come out ahead.
FWIW, I’m not sure of the relevance of your dad’s Northstar engine failure. The fact that his head bolts backed out doesn’t support your claim that the timing chain is a weak point for DOHC V8 engines. Moreover, it need not be an inherent weak point: if testing shows that it fails too frequently, it can (and should) be redesigned to reduce the failure rate to an acceptable level. The technical challenges are no different than the long timing chain/belt on a DOHC V6 or inline 4.
Not all engines have aluminum blocks, but these days virtually all of them have aluminum heads. Galvanic corrosion isn’t particularly a problem for head bolts on these engines.
marine engines aren’t subject to the same regulations as car engines (for example, you can still buy two-stroke outboard motors) so the marine builders will likely still set up the engine suitably for their application.
even then, it’s not like marine use requires the latest engines anyway; Mercury Marine is still selling the old Gen II small-block chevy for inboard/sterndrive use.
Engines can be tuned to produce narrow or wide power bands. Valve timing, valve opening duration, air intake volume and length, and exhaust header size can all be altered to produce the wide power band required for marine use (plus a more effective water jacket).
Prop size and pitch is decided by the engine’s HP and power band available, hull type, and overall weight. The object is to allow a particular engine to reach maximum RPM under normal conditions. Changing the pitch of a water ski boat from 21" to 19" would allow the driver to pull more skiers (ie more weight) but the driver would have to be careful not to exceed the engines RPM limit when running without a load. The same engine configuration might be limited to 10" prop if it’s expected to move a boat that is 3x heavier.
Interesting topic, I just retired from the trucking industry as a mechanic leadman. The trend over the past 30 years has been lower RPM’s with a wider power band concentraring more on torque using higher 3rd member gears and less gears in the transmissions. We whave gone from 16 speeds to 13 speeds to 10 then 9 and in some cases 7 gears. Our rpms have gone from a max of about 2500 to 1800. The trucks use to run down the road at about 1900 rpms and now they are running down the road at 1400. Horsepower is up because torque has gone up so much. Fuel mileage in many cases has closed to doubled. Fuel prices have gone up 10 fold.
Smoothly delivered high-revving power and bunches of torque? In what world is that NOT needed? I’m a practical man and so am all about owning a 4-cylinder. But purely from an aesthetics standpoint, given similar torque & HP, I’d much prefer the confident roar of a strong V8 to the intimidating screech of a supercharged 4-banger.
It would be interesting to see how a V-8 mated to a CV transmission would work. As I understand it, the engine stays at maximum torque/rpms, while that transmission ratio increases. That would sort of argue that a low revving V-8 wopuld work just as well.
It would be a heavier engine to produce the same horsepower. Not a one for one relationship though because the constant speed engine could be tuned for maximum efficiency.
the cylinder layout doesn’t really tell you much about what kind of torque/power curve the engine has. A V8 isn’t inherently any more “torquey” than any other configuration. Now, certain configurations are more conducive to particular characteristics; e.g. it’s “easier” to get a long-stroke “lugger” with a straight-six due to its layout and balance characteristics.
Don’t 8 combustions per rotation tend to provide more torque? I think I knew the answer to that once, but don’t recall. I might be thinking of something else.
All other things being equal, engine output torque is proportional to total engine displacement. So a 5.7-liter V8 will put out about the same torque as a 5.7-liter four-banger (neglecting the difference in torque-versus-RPM curves for the two engines).
A 5.7-liter V8 will be smoother than a 5.7-liter four-banger, owing to the greater number of (and smaller magnitude) torque pulses per engine cycle.
Except it won’t make as much power as the high-revving V8 will. Are you not understanding that a high-revving V8 makes more power than a low-revving V8 of identical displacement?