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

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.

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

-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.

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.

4.8
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.

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.

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

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

I would expect so.

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.

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.

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.

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.

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

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!!!!!!!!!!!!!

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.

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.

The later Vettes use a transaxle and a torque tube like the 944 does, I think. And they definitely bolt right up to the LS motor. Might be worth a look to see if that can reasonably be mounted? You know it's strong enough for at least a moderate LS motor, and it definitely has a tall top gear.

-soD

No worries - I didn't know what you were starting with, and where you were headed.

I did some quick research. I have NOT played with the 944 transaxles, but the 911 transaxles from that era you can swap the drive and driven gears for pretty easy overdrive.
It MIGHT be possible you swap the fifth gear sets into the fourth gear for 1:1 in fourth gear, then take the fourth gear pair, swap shafts, and have a pretty massive overdrive for fifth.

This DOES mean a pretty big jump from third gear to fourth gear. But My bet is for road racing you will only use first through third gear.

Another option is leave first through fourth alone, and turn fifth into a huge overdrive. On the 911 gear boxes, you usually can't change first gear which is fixed to the main shaft. Don't know if this is true of the trans you are using.

any ls engine with an aluminum block, 5.7 might be the easiest to find. whichever engine, if you use one out of a car you're more likely to be able to use more existing parts if you wanted to, thinking oil pan and intake for clearance. then just build for low end / torque.

- before you build, price out what gm has, standard engine in the corvette now is an all aluminum 6.2 making 430hp

I like that 944 body...alraedy has that flared look that the 924 SOOOO desperately needed.
I say (from a non-engine point of view) that is all in the tires.
I rised a 99 Mazda RX7 in showroom stock SCCA. I rode on the steel 15 rims with 70 series tires.
When OI raced I ran 50 series on the 14 alloy rims. Made a world of difference in the 0-40mph start.
On my current dd I run 245/50/18 instead of 245/45 on the stock rims. really helps keep the rpms down on the highway cruise

you can get there with the old 5.7, but once you play with a 5.3 or a 6.0, you will NEVER look back......

yes... referring to gen3 / LS engines, there were 5.7s in corvettes firebirds, camaros and gto's. i assumed the OP was refering to those 5.7s, not the old iron boat anchor

So was I. Even the ls1 is pretty crude compared to the 5.3.

First of all, thanks for all the replies and please keep them coming. It is amazingly helpful especially in the aspect that most v8 forums have no regard for mpg.

A fellow on another forum brought up a point I hadn't thought about. There is VVT and AFM on generation 4 GM engines which also can be retrofitted to most LS engines. That gives me a lot better chance at reaching my goal. Has anyone tinkered with what can be done with VVT and AFM?

Drmiller100, a few clarifications.
-Why 5.3 and the 6.0 over the 5.7? They're aluminum blocks with the same design just difference size bores or am I wrong?

-Also, I will check into the 911 gearbox but I'm pretty sure it won't work. There's been a lot of discussion on the 944 ls conversion forum about this and nothings turned upso far.

-Could you pass the hot rod article on to me please? I would love to take a look at it. Also, are you sure a set of ls3 heads can work on a 5.3 block?

-I think I will be rebuilding the internals since I have some of the parts and it'll make it more efficient. I have lighten rods that will fit a ground down crankshaft. So lighten all the internals and up the compression. Also, honing the engine and putting in new pistons and rings will clean up some of the tolerance allowing for a more efficient cycle. I also simply want to build up an engine for the experience.

mcrews
-You are absolutely right. I love the 944's looks especially compared to the 924. I wouldn't mind to get the turbo front nose on it but that can wait until I get the new engine in.

One question still stands. What would get better mpg, a 400hp 5.3L engine or 5.7L?

It was very possible to do with the 901 boxes, but those were only used from 1963 through 1971 in the 911, and through 1976 in the 914. The 1972-88 911s used the 915 gearbox, and I have not heard of people flipping the gears around on those.


The five-speed 901-based gearboxes had second gear fixed to the main shaft. Unless you get a 904 main shaft, which is serious cubic dollars, even in Porsche terms. The 915 did not have any gears fixed to the main shaft.


None of the 911 transaxles will work easily in a 924/944/968 or in a 928, for that matter. And none of them will hold up to 400+ HP (with a healthy big-V8 torque curve at least!) under heavy use. The 901s tend to detonate when you put more than about 250 lb-ft through them; the 915s are better but still not great. The 930 gearbox could be an option to hold up to the power, but it needs work to flip it around for use behind the engine. Machining of the diff housing so you can flip the ring and pinion, for example. Or run it upside down.

It may be worth looking into the later 928 transaxles. They are set up to run off a torque tube, and they can hold up to a reasonably healthy V8 motor. I don't know what, if any, alternate gear sets are available. The early 928 boxes are not well regarded as far as shifting and durability, so I'd probably avoid those.

The 968 might also prove a reasonable donor for the transaxle, but again I don't know what alternate gears are available.

Some Google searching will turn up info about the existing gearsets in different years of 924/944/968 cars. In general, the later ones will be stronger and the turbocharged cars will have stronger ones. And I'd be worried about the strength if the thing is hooked up to a healthy V8.

-soD

The AOR transmission in the 944 turbo S has been holding up for a lot of guys running v8's. There's a few running 500hp to the wheels and haven't had any problems. They say not to drop the clutch and you'll be fine. There's a reinforcement plate for the AOR that some guys make that does help the cause. There was one car I found that detonated his gearbox but he took a bet to try and wheelie. I think I'm going to be fine as far as the transmission goes. I'm more concerned about the engine setup for now.

Doing some scratchwork, I think I should be running 2600rpm at 70mph if I run 235/17/50's. That seems to be better than most ls1 engines but I'm going to see if anyone's fit taller wheels in the rear.

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

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

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.

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.

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.

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

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.

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.

I did some more looking at I can get the gear ration very near to what I want. I can take the 6 speed transmission from a 968 and get a custom ring and pin. That will set me somewhere around 1200 rpm for cruising at 65.

I mentioned earlier that I would not touch a turbo kit but I realized there might actually be room for a turbocharger in the car. How would I use a turbo charger to pull out good gas mileage?

A turbo increases gas mileage if you downsize the engine and then use the turbo to match the power. It's very hard to increase fuel economy by adding a turbo, unless you have "lossless throttling" (aka Valvetronic, VVEL, Valvematic, Multiair), where the boost the turbo builds will reduce compression work.

1200rpm is frighteningly low! How are you going to do that? I see that there are 3.250 final drives that will get you down to ~2000rpm or so, how are you going to get it to 1200? O_O

That was a number someone threw out on rennlist (a porsche forum). The custom ring and pinion can be between 3.0 and the 3.44. The audi 2wd diesel has a 5th gear of .729 and a 6th gear of .600 that will fit into the 968 box. That doesn't seem to work out to be the 1000 rpm but more around 1900 if my math is right which is probably not. Transmissions are something I haven't spent much time looking into, if you haven't figured that out yet.

Whoa. 3.0 and 0.600 works out to be ~1500 rpm at 60 I think. Somewhere around there, too lazy to look up the tire size. That's quite good.