GrantG

That would be an interesting swap project! One of my favorite cars had a BMW V12 inserted into a Euro Mid-Engined chassis (McLaren F1).

1analguy

While a bulky Euro V8 might not physically fit into a Cayman (unless its displacement was truly tiny), a larger-displacement pushrod V8 just might fit because the cylinder heads are so compact...and that's part of my point. While all those cams/valves/rpms/etc. sound really sexy, what good are they if they keep the engine out of your car? After all, these engines have to haul themselves around (mass) and so, need to fit into their vehicles (bulk). Pushrods really shine on both these issues.

Also, don't sell pushrods too short on the rpm issue. An LS7 is good for 7000 rpm, but I do agree that the sound it makes at that speed, while very "NASCAR", just isn't as melodious as a flat six at the same speed...subjectively speaking.

As to rpm for the sake of rpm, we've become accustomed to those elevated Porsche engine speeds mainly because they're necessary for good power production from limited displacement. Horsepower is only a calculated number based on torque and rpm, and if there isn't a lot of torque, then there had better be a lot of rpm or a small horsepower number will be the result.

In terms of torque production, without resorting to forced induction, there is no tuning trick that can fully compensate for a lack of displacement. Porsche's Turbo is the result of this reality. It has a thick, almost lag-free, torque-rich power band...but at what physical and economic cost? They've had to add the bulk/mass of two turbos, two intercoolers, and all of the associated controls and plumbing required, to an already large, heavy, and expensive NA engine. And while the result is a wonderful and amazingly fast car, how much more amazing (and affordable!) would it have been with an equally-powerful large-displacement pushrod V8 that would have completely eliminated both turbo lag and at least 200 lbs from the back of the car?

I guess now I'm the one who's guilty of bench racing...because of course, such a car could never exist in our universe. Even without constraining European government regulations, Porsche's current clientele would just spit it out, as the car would be different from what they're used to. Would there be enough potential "different" clients to make up for that? Who can say?



And let's not forget Pagani...an "F1" with Bugatti-level fit and finish. Wow!

jimbo1111

You are 100% correct except you are missing a couple of key notes. The 6 cylinder motor that Porsche uses is designed around the chassis in that the weight is at the lowest possible position. Remember that it's a road machine and not a 1/4 mile car. Some compromises have to be made in order to have a well rounded sports car. Sure a V8 would do a lot in the strait line performance but at the other spectrum of road handling it would exacerbate a flaw to the chassis dynamics.

chuck911

1analguy and Jimbo, two great posts. I wonder if one of you can explain what (aside from regulators and customers) is stopping Porsche from building a pushrod flat 6?

jimbo1111

I would guess oiling issues and the way counter weights react. Isn't Ford building a flat plane 8 cylinder for the up and coming GT 350. Don't know if it's push-rod though.

chuck911

In this case flat plane refers to the crank, not the cylinders, which are a normal V-8 configuration. Flat plane cranks are inherently less smooth running than the normal cross plane cranks everyone uses, but the design is also inherently smaller, lighter and higher-revving. Here's a good article with a great animation showing why its flat not cross- http://jalopnik.com/what-is-a-flat-p...-ex-1659688239

The flat six is an inherently smooth running design. Going pushrod would to my ignorant mind seem to be merely swapping pushrods for the overhead cam. My guess is this means lower revs, which means you need more displacement- which the regulators have ruled out.

1analguy

The weight is at the lowest possible position...which is still quite high for reasons that I outlined earlier:

The crankshaft in a C7 Corvette sits several inches closer to the road than the crankshaft in a 991. While the V8 does have a higher cg than the flat six, when they're each mounted as low as possible in their cars, the lower position of the V8's crankshaft means that the whole engine can be pulled down far enough that its vertical mass centroid is probably very close to that of the higher-mounted flat six. Therefore, its comparative contribution to the car's vertical cg is functionally a wash. At your next opportunity, take a careful look under the hood of a C7 Corvette. Those V8s are really slammed low in the cars...especially the dry sump LT V8 in the C7. When I look under the hood of a 911, I'm always surprised at how high the engine sits in the car. Even with its low cg, it sits so high that I can't imagine it's contributing substantially to lowering the car's vertical cg.

As far as "well rounded sports cars", have you tried keeping up with a "top-heavy-V8-powered" C7 on a road course? While I can't get past its looks, I do realize that its mechanical package makes for an undeniably effective sports car.



It'll never happen. The Germans (in general) don't believe in pushrods. They think of them as last century old-fashioned. Even though pushrods still make perfect sense from a mass/bulk/cost standpoint, they look down their noses at them. I'm speaking from first hand knowledge here. I worked in a U.S. OEM engine plant for decades, and visiting German engineers were always blowing their noses on our most successful and reliable pushrod design. Trust me, if they aren't using it, it's no good...period.

chuck911

1analguy, I can sympathize with you on the pushrods, but you've lost me on the engine height. Here are cutaways of a 991 and corvette. The entire 991 engine sits at roughly axle level. Crank, heads, all the same level. Even the intake, which is pretty much plastic, sits within wheel height. Not only that, but when you get into the details Porsche has all the ancillaries- alternator, pumps, etc- packed into that same plane right about axle level. Check it out.

Sure it looks like chevy got the crank down low. But looking at their engine this may only have been to enable the driver to see over it! The heads are way up at the top of the tires, and the intake sits a good deal higher than that. There's a great big alternator that appears to be perched about as high as they could find and still fit under the hood.

If you're surprised how high the engine sits in a 911, instead of looking at the fans and covers, try looking at the engine. If you do that I doubt if you'll be surprised how much lower it is than the vette. You'll be shocked.

008

I've built quite a few track cars and race cars with pushrod designs including an 8 liter V10, 5l and 5.8l V8s. I get the attraction and advantages but there's are reasons every new clean sheet design is going to overhead cams, that's not going to change. In the U.S. Europe or Asia. Some are better than others and I personally think BMW and Ferrari are the master engine builders. I'm actually not that big of a fan of porsche's engine building capabilities outside of the 8 in the 918.

77tony

https://rennlist.com/forums/991/842093-g-g.html

https://rennlist.com/forums/928-foru...b-project.html

http://jalopnik.com/the-10-best-engi...ars-1510676043

991 is approx 100 lbs heavier (3/4 tank) and is better in the curves, LS3/525 928 (1/2 tank) is faster in the straights. Both a blast to drive. T

jimbo1111

Can't beat the chassis in the 991. Best thing since sliced bread.

1analguy

I'm not disputing the flat six's lower cg compared to the V8. My point was that the V8's cg in the car, when compared to the flat six's cg in the car, with both mounted as low as they'll go, is probably not that much higher. Don't forget to take into account the V8's substantially lower mass. There's about 25% less engine present...think about that for a minute. If you have 25% less mass that happens to be centered, say, a couple of inches higher, how much is it going to affect handling? Probably not much.

Your pic of the Z06 illustrates nicely the Chevrolet did indeed get the engine mounted low...remember, those front tires are about 1.25" shorter than the rears on the 991. That LT4 has a supercharger on top of it that is probably not even visible from the driver's seat, even with the hood open. Those Corvette development guys can never be accused of not thinking about their car's dynamics. That's why Corvettes always produce such great numbers. Now, if they would just show more concern for the car's refinement...



This is strictly an "apples-to-pomegranates" comparison. Is it really so surprising that a car designed in the mid 70s is out-handled by a car that was designed when the 991 was? Though, that breathed-on LS3 is a really tough act to follow. 525 hp (and lots of torque) from a 415 lb street-legal engine? Not to bad, for pushrods...