I replied to another post with this, but figured I should start my own thread.
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I'm kinda new around here so nobody knows me. But I am an engineer, and I work with engines everyday. I work in the Engine Research and Design Department at SwRI. So maybe that short background will help give my thoughts some legitimacy.

Have any of you ever owned naturally aspirated cars in the past? Have you tried modding them? Have you noticed that no matter how bad the factory exhaust manifolds were, when you put shortys on, nothing significant happens? Then you put on longtubes, and holy moly, look at the power gains. Sometimes on 5 liter 'stangs it was 25 rwhp with a full exhaust with longtubes.

Now the lightning owners see that, and they think the same applies to them. But there is one huge difference between the two. And I'm sure everybody can guess that that difference is the supercharger. There are different forms of superchargers, but the fact that the L's is a constant volume device makes it special.

Since the supercharger is a constant volume device, each turn of the rotor moves "X" volume of air. Therfore, the mass of air depends on the density of the incoming charge and the rotational speed of the blower. NOTHING ELSE. Take a look at the blower specs on Eaton's webpage. (The link was given on here a week or so ago, and probably several times before that). If you look at the first plot, the show flow in CFM at two different delta pressures. Notice how the lines are pretty much on top of each other.

Ever wonder why your L with a 2-valve PI head is rated at 380, and a new Cobra with a 4-valve head is rated 390. Only 10 hp different. While a mustang GT with 2-valve PI head is rated 260 and old Cobra with 4-valve head is rated 320. A huge 60 hp difference. The reason is no matter what you do downstream of the blower, the airflow is the same. 2V or 4V, port or no port, exhaust or no exh, cats no cats, the airflow is pretty much the same. Now when you make mods that increase the density of the air at the blower inlet (filteres, throttle bodies, larger MAF's), you can see that there will be more air mass flow through the engine. That's why these mods always show pretty good power increases.

When measureing power, there are several things you loot at. The IMEP give the indicated power looking at the thermodynamics in cylinder. The BMEP is gives measured brake power. The FMEP is the friction and is the difference of the two. All of the downstream modifications will show no difference in IMEP (and I used the term NO somewhat loosly as there is the possibility of 1 or 2 hp here or there). The BMEP may show some small increases due to a reduction in FMEP (and I'm lumping accessory drag in with FMEP). If you look at the next two plots on Eaton's page, they show that as delta pressure goes up (more boost) then the outlet temperature goes up and power consumed by the blower goes way up. If you change things downstream that allow the air to flow more freely through the engine, then what happens is the mass flow stays the same, but the pressures are slightly lower due to decreased resistance to flow. In other words, the boost actually goes down. Since the boost is now lower, the delta pressure is lower, and the power consumed and heat generated by the blower go down. This is what contributes to the slightly higher power. The IMEP is the same, its just that you reduced the FMEP, therfore increasing BMEP.

The problem is these downstream mods are very expensive for the marginal change in power that you get. Its also the reason the L is relatively inexpensive to get to go several tenths faster, and then all of the sudden, you hit a brick wall, and it costs a lot more money to go faster.

Now when the M166 comes out!!!!....the airflow can go up again, and the power can come up.

On another thread about the new mustang, people were questioning why it was "only" 390 hp. A response was that the exhaust was corked up to meet noise restrictions and that when you put an exhaust on there, it should go a lot faster. I replied that even if you uncork it, nothings gonna happen. A poster on here informed me that engines are a system, and that when you make changes in a system, it affects the overall system. I agree 100%, its just that you have to understand the system to get an idea of how the system might react to a change.

That's my book about mods. Any other questions...feel free to ask. I work w/ engines, combustion bombs, fuels, lubes, you know, the fun stuff. I've magaged to learn about combustion, knock, emissions, etc. Unfortunately most of my work is emissions related, but I do have 1 project now that only deals with power. Most of the time, I'm the one asking the questions. It's certainly refreshing to actually understand something for a change!!

 

I would have loved to read your post but being an Engineer myself I hate reading......especially THAT much

--Joe

 

Do you publish a glossary ?

The acronyms are giving me a headache

Doug

 

Yeah the post is long, but once I get going, its hard to stop!

I guess the motivation for writing it is I hat to see people waste money on stuff that't not really going to work. Especially expensive stuff like heads. But then again spending money is what makes the world go around. Maybe I shouldn't bother ;-).

 

Uhm... one inconsistancy that I noticed...

Lighting 2V 5.4L 380 HP, Corbra 4V 4.6L 390HP... so... not going against you... but that kind throws a wrench in some of your conclusions...at the very least, that comparison.

 

Your right, there is a differece. But look at the torque difference as well. The smaller displacement is being allowed to turn more RPM's, bringing the power up. Peak power is at 6000 (estimated) from the march issue of Motor Trend. Power is still related to airflow, and the Cobra has to spin to a higher rpm to get the same airflow and power. Peak power on the L is around 5000 rpm.

Sorry, I should have mentioned that too.

 

ah, touche'


yah,,, L torque is 450, (stock) corbra is 390...

BIG difference...

 

Is it just me, or does it look like this guy really knows his ****?

DUCK

 

I'll second that, this guy knows his chit.

PS I'm a new guy too, I think I have that knowledge somewhere!!

 

If the stuff I read in English was as entertaining as that, I would be makeing A's rather than bairly passing...

After thinking a bit about that, you are so right it's scary. It's all logical sence, but no one had put it all together like that befor. You have indeed earned my respect, and with a record breaking 14 posts .

Now im off to go do some thinking... after some thing to eat.

 

Doesn't the new Cobra have a rear entry M90 blower vs. the L M112 top loader? Another variable.

 

Yes it is a rear inlet......just not sure if it is a 90 or 112.


--Joe

 

From MM&FF...

Most of this power comes from a Gen-IV Eaton supercharger, which makes 8 psi boost max. It's the same M112 used on the Lightning, but it is fed from the rear for a lower hoodline. That's not all, though. There is also a new design 4-valve cylinder head, with better flow for improved torque and the cams were also designed with more torque in mind. The lift is the same, but the duration has been altered. A 90mm MAF and twin 57mm throttle body deliver the air to the blower.

 

There are some obvious differences between the mutang and L that make direct comparisons a little bit of a stretch. It's just the best I can go on.

If the 4V head has better knock resistance, it would be easy to lean out the fuel mixture or advance the timing to make some more power. The thottle arrangement and intake plumbing is different. Even the compession could be different. There's just too many variables to say that 380 = 380 just because it has the same blower.

The real point of my rant is just to provide some understanding of what modifacations have a pronounced effect on power. It should be easy to see why a 200 dollar filter modification and a 800 pulley and chip combo make so much power, and a 1000 dollar full exhaust won't do anything.

I have to give a lot of the credit for the above info to my friend and co-worker who was reading the corral when the 96-98 Mustang guys were using the SVO blower. There was a peak they were able to get, and no amount of extra money got them any faster. Some even spent the 2000+ on SVO heads, and hardly got a performance gain.

Now dual stage compression like I saw on here a couple weeks ago is a really cool idea. Run a centrifigal blower into the eaton, and slow the eaton down. It would be really cool if you had an intercooler between the blowers. Then you could operate both blowers in the meat of their efficiency curve and get higher airflow and more power. As my friend pointed out, there are many applications in chemical and processing plants where dual stage compression is used. Then you need less power to turn each compressor, and the combination nets a more efficient package.

I need to start writing less!!

 

Very interesting post and very well explained.

Remember boys and girls torque times rpm = hp

So hp is in fact just a calculation, nothing more.

There's 2 ways to make more hp.
1)increase torque at a given rpm
2)increase rpm while mantaining torque.

I remember when Honda tried to race their 4 stroke gp bike against 2 strokes. Well 2 strokes create twice the power pulses as 4 strokes at a given rpm, so 4 strokes need twice the amount of cylinders, but that was against the rules. So Honda came out with a v4 8 valves per cylinder (this looks like a v8 in power strokes) and dual connecting rods. Now get this the 2 strokes where redlining at 9000 rpms, but the Honda having half the power pulses had to red line at 20,000 rpms. Now get this the 30 ci engine made 150hp.

If we made a v40 with 320 valves then we could have a 1500hp na engine Of course we would have to spin it up to 15,000 rpm to leave a stop light.

Anyway, if you want to learn more about how na engines work then look up the Honda nr-500 project. Now if someone would just give me a link so I can understand sc the way this guy does.