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

[bp944]

I'm working on blueprinting a GM LS engine for my porsche 944 that will become my track car. I haven't been able to find something close to what I'm doing but please link me to a similiar build. I don't want to repeat questions and waste people's time. I'm building the engine from the ground up and would love to hit 400 rwhp but still maintain 35mpg. I do not want to use a turbo charger or NOS. As of right now, I want to build the engine as efficient as I can for this power and then see where the mpg ends up since there are many other factors that matter. I'm just going to go ahead and ask too many questions.
1-Does lightening internals actually better gas mileage (about 10 lbs lighter)?
2-Should I go with a 5.3, 5.7, or 6.0L engine?
3-I assume higher compression is a must?
4-Higher flow heads give one a better gas mileage or not?
5-Larger fuel injectors use more fuel so they must weaken mpg?
6-Does one want to shorten the stroke or lengthen it?
7-Does the piston shape (dome, flattop) have any effect on mpg?
8-What exactly is the best hp curve to aim for with a cam? Should it be a very smooth curve or peaky? Does one want a low hp at low rpm so it uses less fuel?

Thanks for looking!

P.S. If one is interested more in what my car plans are. I'm swapping a GM engine into a porsche 944 with a 944s transmission. I'm stripping the car down and building it back up, eventually. Fiberglass panels, new carpets and sound deadening, racing seats, roll bars, and rebuilt suspension. In the end, I'm hoping for a car that weighs in around 2600lbs since it weights 2900lbs currently. I'm aiming for the irrational but I'm hoping it'll happen. I'd love to have a daily driver that gets around 35mpg, 400 hp, and can still be an aggressive track car.

[TheEnemy]

1-Does lightening internals actually better gas mileage (about 10 lbs lighter)?
2-Should I go with a 5.3, 5.7, or 6.0L engine?
3-I assume higher compression is a must?
4-Higher flow heads give one a better gas mileage or not?
5-Larger fuel injectors use more fuel so they must weaken mpg?
6-Does one want to shorten the stroke or lengthen it?
7-Does the piston shape (dome, flattop) have any effect on mpg?
8-What exactly is the best hp curve to aim for with a cam? Should it be a very smooth curve or peaky? Does one want a low hp at low rpm so it uses less fuel?

1 Not sure, it would be more responsive
2
3 Very much so, for both performance and FE
5
6 I don't think that has much effect on overall FE, but a longer stroke will move your torque curve lower down in RPM.
7 From what I have read a flat top piston is better for higher compression as far as pinging and detonation.
8 My 2 cents on this one is one that comes up as soon as possible, while still getting your track performance at a higher RPM.

Gear it so that your cruise RPM is about 2000-2500 maybe lower for the smaller car with that engine. That also depends on your cam, if it doesn't come up untill 2200 RPM then gear it for 2300-2500 cruise rpm.

Improved aerodynamics will help both on the track and FE

[Iexpedite]

-Does lightening internals actually better gas mileage (about 10 lbs lighter)? Lighter components will reduce your engines rotating mass thereby freeing up power.

-Should I go with a 5.3, 5.7, or 6.0L engine?
I am not familiar with the 5.3, but the larger cubic inch motor should produce more low end torque. Properly geared you can use that to run the engine at a lower RPM. Gearing is going to be the key to this whole thing.

-I assume higher compression is a must?
You will increase the power produced in every cycle by adding compression.

-Higher flow heads give one a better gas mileage or not?
Maximizing flow helps but you still need to ensure you have good velocity. Otherwise your low end torque will suffer.

-Larger fuel injectors use more fuel so they must weaken mpg?
Not sure about this. If your computer is controlling pulse width, then it will adjust for low speed operation by shortening the pulse on a larger injector. I guess it depends on what computer you will be running. A tuned stock or aftermarket computer should be able to adapt. Factory unmodified hardware may increase fuel use.

-Does one want to shorten the stroke or lengthen it?
Long stroke is going to make more torque...not sure what the tradeoffs are.

-Does the piston shape (dome, flattop) have any effect on mpg?
A good hypereutectic piston matched to cylinders in an attempt to reduce quench height will make the most difference.

-What exactly is the best hp curve to aim for with a cam? Should it be a very smooth curve or peaky? Does one want a low hp at low rpm so it uses less fuel?
There are a lot of RV/towing cams out there that make a fair amount of power while producing strong torque numbers.

[big shafe]

Reducing rotational weight shows greater gains in engine output than just dead weight a car has to move.

Your injectors need to be sized according to max engine load needs. Detailed tuning/calibration may be able to help you recover on the small load end, but you have to size to keep from them running 100% duty cycle at max engine load.

[tortoise]

Originally Posted by bp944

-Should I go with a 5.3, 5.7, or 6.0L engine?

4.8

-Does one want to shorten the stroke or lengthen it?

For a given displacement, a longer bore gives a more compact cumbustion chamber, which you want for thermal efficiency.

If you can make it work, GM Active Fuel Management (google it), aka displacement on demand, would be helpful.

You'll want a monster, granny gear, which will be completely unused in track work.

As others are pointing out, you want low volume ports and a low duration and overlap cam, which are, of course, not the way high output engines are normally set up.

An interesting, challenging, project.

[smokey442]

I did a 6.0 for a customer awhile back. Went from 18 mpg to 28 mpg engine mods only. Too many to list here. PM me and I'll give you more details. Lower piston and rod weight allows you to remove material from crank counter weight. Instead of drilling to achieve balance shape leading edge of counter weight into an airfoil shape. Trailing edge can be a knife edge. Take a look at the high $$$$$$ aftermarket custom cranks for a visual. Mathematically compression and expansion ratio are the same. To achieve power and better FE it really the higher expansion ratio we are after. Both compression and expansion can be manipulated via valve timing events. Properly designed dished pistons are always the best. Smallest injector consistent with your power requirements produces better idle quality. (smaller fuel droplets) slighlty better mpg at low throttle settings for the same reason.

[some_other_dave]

Originally Posted by bp944

1-Does lightening internals actually better gas mileage (about 10 lbs lighter)?

It should, but it wouldn't surprise me if the difference is too small to notice.

2-Should I go with a 5.3, 5.7, or 6.0L engine?

Small for fuel efficiency, large for max power and/or torque.

3-I assume higher compression is a must?

I would expect so.

4-Higher flow heads give one a better gas mileage or not?

Heads that have good port velocity at your freeway engine speed will help fuel economy. Heads that flow tons at WOT and high RPMs usually don't have good velocity at low RPMs and throttle settings, which where you are on the freeway.

5-Larger fuel injectors use more fuel so they must weaken mpg?

Not really--to a point. If the injectors are so large that mixture control at low throttle and RPM is poor, the fuel economy will likely be worse. If, for instance, your injectors are at 40% duty cycle at WOT at the red line, they may produce uneven mixtures at cruise or idle.

6-Does one want to shorten the stroke or lengthen it?

Depends on the displacement you want and what shape you want the torque curve to be. In general, the longer stroke motors will make better low-end, while the shorter stroke motors will rev a little more easily. Short stroke and long rod is an interesting combination, as it will have lower side-loads on the pistons, so less friction. Might be worth a look, might be too small a difference to care about.

7-Does the piston shape (dome, flattop) have any effect on mpg?

Yes, it should have some effect. It will also allow other mods that also have an effect. The piston crown is effectively the bottom of the combustion chamber, so it has to work with chamber shape. Flame front movement, "squish", mixture quality, and lots of other factors all play off of each other here.

8-What exactly is the best hp curve to aim for with a cam? Should it be a very smooth curve or peaky? Does one want a low hp at low rpm so it uses less fuel?

For the highest power, spin the motor fast. In order for it to breathe well at high RPMs, it will usually lose low-end torque, so the curve will be fairly peaky. In general the lower-RPM cams will be less peaky.

If you want freeway MPG, then you want an engine that makes good torque at a fairly low RPM (<2000, possibly much less for a big V8) and a transmission geared so that freeway speed is right there. Let's see, what does the C6 Vette motor turn on the freeway? I think I read 1600 RPM, so that sounds like a good figure to shoot for. Even lower revs might be good.

That may be tough to do with the 944 trans. I'm not sure they made alternative gear sets for it like they did the 901-based boxes (e.g., early 911). Those have several dozen ratios to choose from, and the drive gears and driven gears can be swapped to invert the ratios in many cases! Not sure about the 944, though.

If you can't get the revs down on the freeway, your MPGs will suffer significantly. So check into gearing choices. You may also find that 400+ HP will be more than the 944 box can deal with for very long.

Sounds like a fun project, good luck with it!

-soD

[drmiller100]

You should get what you want pretty easily.

Consider strongly the 5.3 with aluminum heads and aluminum block - slightly less ultimate horsepower potential then the iron block but lighter by a bunch.

Hotrod magazine had a writeup where they took a bonestock junkyard 5.3, added dual exhaust, cold air intake, camshaft, custom tune from EFI live to richen WOT and valve springs developed over 400 horsepower. Granted, they were spinning it to 7000 rpm, but the 5.3's LOVE to spin.

Now combine that with your pretty small (narrow) 944, strip some weight, and you are pretty much there.

The next trick is to keep the mileage up. A bone stock 2005 corvette, with 6.0 gets 25mpg according to the epa. The corvette is a BIG car. 3300 pounds. CD .30 or so. big frontal area of 22.3 square feet.

944 is 2800 pounds, cd of .35, 19.5 square feet.

Should get better mileage.

Pretty cool conversion.

Camshaft is 100 bucks, valve springs is 120 bucks. In theory you don't have to pop the heads to change the cam, but I probably would - you have to pull the intake off anyway.

When you pull the intake, check out the intake ports. If you know much about porting or heads, the ls3 engines are MAGNIFICIANT!!!!!!!!!!!!!

[drmiller100]

Another cool conversion is the 240z datsun with a 5.3. I've built a bunch of 240's with 350 chevies - you can get down to 2200 pounds or so and have it still streetable with the 350. I think an all aluminum 5.3 could get down to 2000 pounds.

I'd want overdrive tranny though.

[bp944]

I failed to give a bit of information.

-I already posses the car and am set on using it. I will give it to you drmiller100, the 240z datsun is an gorgeous car.
-I am going with a LS engine since it has been done many times and is know to work well especially with the 944S transmission. Although I am going to have to figure out the gearing since the final gear is sitting at 3,000 rpm at 75 mph. Not the best cruising setup.
-I'm also doing this as a road racing car not a drag car. That might change a few things.