blueprinting v8 engine (for Porsche 944 - goal: 400 rwhp but still maintain 35mpg)

[mcrews]

complelely off topic.
saw an 2012 Vet Grand Sport at the dealership
25mpg hwy epa.
325/30/19 tires.......BEASTS!!!!!!!! lol

[drmiller100]

I'm calling any 5.3 an LS3.

the 5.3 head is different then the 5.7 head like the 5.7 head is different then a ford 302 head.

note they made 450 horsepower NA.

Stock GM LS Engine - Big Bang Theory - Hot Rod Magazine

and another article - NA , stock everything with cam swap

5.3L LS Small Block Build - Here Comes Modern Mouse! - Super Chevy Magazine

[serialk11r]

A note on the cams, the reason low end torque drops off with a high rpm optimized cam is because of 2 reasons, the first being lower volumetric efficiency (duration is too long, charge gets pushed back out), and the second being poorer combustion efficiency from higher lift and a lower intake speed. If the second is not a problem, then the high rpm cams should improve fuel efficiency when puttering around town because they will reduce pumping loss.

Usually race cams have higher lift than stock, but I've always wondered what a "medium lift" + long duration cam would do. Higher lift definitely affects charge motion and thus fuel mixing and stuff, but I think the actual flow doesn't really increase after some point because the port itself becomes the main restriction.

[drmiller100]

multi port fuel injection changed somewhat the idea of having intake velocity.

Once the air is past the intake valve, what happens inside the combustion chamber is pretty much a different subject.

I'm most familiar with the progression of the chevy v8's but other series are similar. With the new chevies, they have a huge squish area creating large turbulence inside the combustion chamber.
They also have HUGE intake runners to let lots of air into the chamber. And they have large lift, but traditional short OEM duration. For a given duration, if you add lift you get more rpm range without sacrificing anything. Obviously there is a limit in camshaft profile, which can be added to by going to roller rockers.

For aftermarket, add more duration for even more RPM. With more duration, you can get even more lift keeping ramp accelerations within reason.

But this will only work if you have the port flows to support the cam.

lets talk MPG for a second.
What do you do for max MPG?

crank the compression WAAAAAAAY up, . Then run too much duration especially letting the intake run long, so at low RPM's you don't get detonation as you get reversion lowering.

then watch your igniton timing curve, reducing timing at peak torque to prevent pre ignition.

all of which is pretty turbo friendly.

[serialk11r]

You could be right, perhaps the typical race cam anecdotes apply to cars which run too much overlap or have other issues affecting combustion quality. As far as charge motion goes, I think it's still an important topic, although it's particularly important with direct injection because of the small amount of time the fuel has to mix. Increasing lift will affect tumble flow, from what I've read.

But yea, max mpg, you want a long duration cam (to squeeze the maximum amount of power out of the engine too), sacrificing low end VE, very high compression ratio, assuming you don't change low end combustion stability, which is likely a big if.

I saw some talk of the gearbox, and I'm not familiar with the speeds encountered on the track but I imagine you want 4-5 close ratio gears for track use, and a ridiculously long 6th for cruising. With such a powerful engine it's probably better to have more like 7 but 7 speeds are a new thing. With that much displacement and the 944s gearbox, your mpg prospects aren't looking so good, and the ratios have quite wide spacing which isn't ideal for racing, of course. With say a 968 gearbox (which has nice close ratios) and longer final drive (since your engine is relatively low rev, high torque), you could end up with a pretty healthy cruising rpm. That was just a random example, I have no idea what transmissions bolt up easily and whatever.

[some_other_dave]

Originally Posted by bp944

I think I'm going to be fine as far as the transmission goes. I'm more concerned about the engine setup for now.

I'm thinking the transmission is where you're going to make your biggest MPG gains. You want the motor turning rather slowly at cruise to get good MPG. Sounds like it will be hard to do with the 944 box without a lot of work.

I don't remember if the 968s had a six-speed; I'm thinking they did not but I am not sure. The 944s and 928s only had five forward gears (in the manual transmissions).

-soD

[bp944]

So I did some math and with a 944 5th gear in the turbo trans I will be running 60 mph at 2000rpm. So I think I should be okay with gear ratios. It's not amazing but it's better than the 2600 rpm I was told.

[serialk11r]

Originally Posted by bp944

So I did some math and with a 944 5th gear in the turbo trans I will be running 60 mph at 2000rpm. So I think I should be okay with gear ratios. It's not amazing but it's better than the 2600 rpm I was told.

2000 is alright, but could be better, considering the engine size. 35mpg might not be possible with that big of an engine, since (I think) the Corvette struggles to get 35mpg at relatively low speeds with even less highway rpm. Small 2L engines turning high 2000s at 60mph get low-mid 40 mpg, with a giant V8 you're effectively cutting an already low load in half, which will really kill your efficiency.

If you're willing to cut some displacement with a smaller crank and longer rods (which might let it rev higher but I don't know anything about how rev happy you can make a pushrod motor), then it would certainly help. I still think it's best to grab a 6 speed gearbox for the smaller spread between ratios (better acceleration, more choices of low speed cruising gear around town), and increase the final drive so you get better fuel economy and so you don't spin out in a cloud of smoke when you try to gun it in 1st. With a powerful large displacement engine like that a longer final drive will still keep your low speed acceleration at pretty insane levels, since there's just so much power.

[mcrews]

the key to the vette is certainly the tranny. i believe the last two gears or overdrive gears

[serialk11r]

Originally Posted by mcrews

the key to the vette is certainly the tranny. i believe the last two gears or overdrive gears

The numerical ratio doesn't really mean much since you have a final drive ratio at the axle to worry about too, but yes, the tranny has a very large gap in gear ratios going from 4th to 5th to 6th (I think it is like, 1, 0.71, 0.5? ZR1s are 1, 0.8 0.67 or something), whereas say the Honda s2000 has a completely useless 6th gear which is barely longer than the 5th gear. Combined with a somewhat long final drive, you get your ridiculously low highway rpm. A fuel efficiency conscious driver could use that 5th gear to handle low speed cruising duties since the 6th gear probably doesn't go far below 40mph (not sure what the actual rpms look like though).

Of course the motor is so big that in the end it's still less efficient than a smaller engine, but considering how it has ~3 times the displacement of these 4 cylinder engines, not a bad result at all.

One problem with manual transmission specs is that since they don't shift themselves, manufacturers have emissions (enrichment) to worry about when the EPA test decides to floor the gas on the highway in top gear (which apparently does happen), and drivers are too lazy to shift and will complain about lack of power... :/ The Corvette having so much power doesn't encounter this issue as much as smaller engines, which could be insanely fuel efficient with more fuel efficiency minded gearing, if only people didn't require 6th gear to have passing power.