Disclaimer: This a 5.4 Lightning related question but I do own 2 f-bodies and I trust your technical knowledge more than most at f150online.

That being said, at that forum we are engaged in an ongoing discussion as to why ported heads, cams, and headers don't show much gain on the dyno or track. Please read this post by another member and tell me if his logic seems correct:

"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.


Okay guys, is his logic correct? I'm building a very strong motor for the truck this winter and don't want to waste money on things that will show no power gains. Thanks guys!

-Spencer

 

Yes and I discussed this the other day in the Lounge when discussing a supercharged C5 vs ZO6. Fixed displacement blowers will put X amount of air in there regardless where as centrifugal are completely dependent on the downstream restriction/pressure plus intake restriction .

The only real concerns are:
1) Getting the air to the blower
2) having a stepup ratio so that the blower is not over spinning at peak rpm.

So the M112 is the limiting factor on our Lightnings (I have '02 True Blue one) and the Cobras.

For example. the Lingenfelter C5 runs 126mph and Z06 126 with same blower(m112) The current Corvette Fever has a bunch of superchargers w/Zo6 info.

PS, a fixed displacement compressor will increase the boost level with a less restrictive downstream setup but the mass of air is about the same. So what you get is about the same amount of air but just at a higher temp.

 

yeah, you have found out how the displacement type blowers really don't care about cams/heads etc......

It's also seen the in the blown C5 ZO6 discussion. A engine with 350hp vs one with 405hp and the same blower end up going the same speed. It's because the blower takes care of that. It's almost crazy what happens with that system, but the biggest thing to do is set up the engine to make more power via, more boost. Fix the CR and the rotating assembly to handle the boost. Then throw a bigger blower on it and put more pressure into it and that's the best way to get more power.

My thought on why the 4 valve engine vs the 2 valve from ford has 10hp difference is real. The difference is probably linked to the greater low lift flow area that the 4 valve has over the 2 valve. That seems to be a place that actually helps the positive displacement blower out. Vizzard has a great write up on valve seats, low lift flow and 2 vs 4 valve engines. Basically he says that 4 valves make more power because of the increased curtain area at low lift flow. With a blower that makes total sense.

Bret

 

I pretty much wrote that off as a marketing decision. Kinda like the LS1/LT1 vette vs Z/TransAm.

Or even so simple as a slightly faster pulley ratio but I haven't taken the time to measure and compare.

 

Thanks for the input. I guess I'll leave the heads and cams alone and spend the $$ on building a bulletproof shortblock. A stroker kit will soon be availible for 5.4 so maybe I'll do that. Thanks again guys.

 

Huh? I thought 03 Cobra guys were picking up 30-40 rwhp from exhaust mods?

Anyways as I see it even though the blower is forcing X amount of air in per revolution its not doing anything to get it out.

Look at whats going on when the exhaust valve opens. First pressure inside the cylinder is much greater than the pressure in the exhaust. Since gasses move from high to low pressure area's the exhaust flows out of the cylinder into the exhaust system as the exhaust valve(s) open. Now the piston moves up to push out any gasses left over after the initial blowby. Then the exhaust valve closes, intake opens, etc....

Now if the exhaust is all corked up (say due to an ineffecient exhaust system) there will be less of a pressure differential. This means there will be more pressure in the cylinder so the piston has to push harder to expel the burnt gasses. The harder it has to push, the less power you make (pumping losses).

Headers = less backpressure in exhaust

less backpressure = less energy consumed by pumping losses

less energy wasted = more power

I'm sure thats waaaay oversimplifed (hell it may not even be correct) but it seems like headers = power regardless if the motor is blown or not.

My $.02

 

Just FYI, a $2K Lightning exhaust...long tubes, cat pipe, and cat back have shown at most a 5rwhp gain, and that's on trucks running a lot of extra boost (15psi+). I know, it doesn't make sense though.

 

That sucks, all I can figure is the stock exhaust must be pretty darn good or my analysis missing something.

More than likely its missing quite a few somethings

 

I kinda agree with Jason. Gasses move from high pressure to low pressure. Air will move through the combustion chamber based on pressure differences. If the chamber is at 100 psi when the exhaust valve opens, and exhaust header is at 10 psi, you will blow down to the 10 psi in the exhaust. You'll still have 10 psi left in the chamber when the intake valve opens. If you've got 5 psi boost, you'll actually have air moving out of the chamber into the intake until the piston starts travelling down. This is one of the reasons older turbo cams had severely delayed intake opening timing on cams (to the point of negative overlap). Your exhaust pressure only affects you at 2 times - the exhaust cycle, with your pumping losses, and by diluting the intake w/ residual exhaust gas (remember that 10 psi?) What are the cam specs for the lighting? If the stock exhaust is not very restrictive (you'd hope it was designed properly), your HP gain isn't much since the boost forces air into the chambers during the 90% of the intake cycle (where in NA, the 10 psi residual pressure would be a huge deal). I don't know if this helps clarify it.

Andris

 

Even more

With a fixed displacement blower no more air can get into the cylinder than has been 'shoved' in by the rotors. There is very little leakage.

So if single revolution moves x amount of air, you will get x amount of air in there (hence the term fixed displacement). You may lose power trying to shove it in there because of the extra force it takes to turn the rotor from residual pressure or exhaust restriction, but it will still get in there.

So for the most part, it makes little difference to spend the money on the exhaust as long as exhaust flow is close to that which is needed. Yeah, some gain and difference but not like you would think assuming everything is sized within reason of needs. It will lower the boost but power is similar due to a fairly equal mass of air/fuel mixture.

With a centrifugal where flow is dependent on pressure, it can make a big difference. Outside of obvious power drive mechanics, Turbos and centrifugal supercharges operate the same way.

 

I would like someone to discuss the effect that uncapping an exhaust system has on the torque curve of a charged/turbo engine. I know several people who have such systems and uncapped, the low-end torque seems to almost dissapear, and overall performance seems to be hurt. Please explain...