prodigy.

if i read your dyno chart correctly for the auto 3.8 at the rpms i cruise at on the highway whixh is about 2.5 to 2.7k rpm your turbo is making 50 f/lbs less than stock? at this rpm my sprintex is making 250 f/lbs or 100 more than stock.

am i reading this right? you may have built a drag racer but probably not a jeep. i think i would find your power curve completely unmanageable on the trail. must be quick though.

 

Lots of incorrect statements there.

*in general*...

A centrifugal supercharger produces BOOST proportional to the square of the engine speed. This produces an approximately QUADRATIC curve (not exponential).

The TORQUE GAINS from a centrifugal supercharger approximately/generally INCREASE LINEARLY with respect to engine speed. Because of the relationship between torque and power, this means that POWER GAINS will approximately/generally be QUADRATIC in nature with respect to engine speed (not exponential). It makes more sense to focus on the changes/shape of the torque curve, though, because that directly relates to the acceleration you will feel.

In relative comparison to centrifugal superchargers, other types of superchargers generally produce BOOST that INCREASES LINEARLY with respect to engine speed, and produce TORQUE GAINS that are somewhat CONSTANT (more of a "flat" gain across the rpm range). Because of the relationship between torque and power, this means that POWER GAINS will approximately/generally INCREASE LINEARLY throughout the rpm range. Again, it's the torque that you feel as acceleration. Power is a much more abstract idea that doesn't directly relate to anything you experience. So pay more attention to the shape of the torque curve through the rpm range to understand how it will feel/accelerate.


Regardless of the type of supercharger/turbo, the torque/power delivery definitely will not be "unpredictable". With each type, torque/power comes in differently as you press the throttle and move through the RPM range. That's all. Each one behaves differently, but predictably. If you don't have experience with a particular type of forced induction, it may behave unexpectedly to you at first (especially if you're expecting it to behave like something else). So sure, during your initial experience it may feel unpredictable to you because it's not what you expected, but you quickly adapt to how it behaves and it becomes completely predictable.


A lot of "in general" and "approximately" in there. Lots of factors in the engine design and compressor design can cause variations from the very general relationships I mentioned. Don't jump all over me with examples about how some real world examples doesn't fit the described relationships very well.


doc5339 interpreted your question completely different than what I thought you were asking. Your question is actually ambiguous on multiple levels:

1) Who are you directing this question to?

2) What is "the lower bands"? It would help for you to specify an RPM range.

3) What do you mean by "you don't show power gains"? Are you referring to an RPM range at which there is no data on the dyno chart? Are you referring to an RPM range where the gains are not as big as you would like?

~Jeff

 

^^^Yeah, what he said

 

Sounds like you are reading the chart incorrectly, mixing up power and torque curves.

Here's the chart again:



It has 3 torque curves and 3 corresponding power curves (stock, stage 2, and stage 3).

The torque curves are the 3 more faint lines. The stock torque curve is easy to identify because it's a fairly flat line around the 150 ft-lb mark. Look for the 2 similarly faint lines ABOVE the stock torque curve to find the turbo torque curves. We do not show losses at any engine speed. The 3 darker lines are the power curves. I recommend ignoring the power curves, because power is fairly abstract and not directly related to anything you experience. Torque is what more directly relates to what you'll feel as acceleration.

I would usually try to shift the focus to percent gains, rather than talk in terms of absolute number of ft-lb gains because different manufacturers may use different dynos, different procedures, etc., which can produce different baseline results. However, the numbers you cite for your Sprintex imply an agreement that stock makes about 150 ft-lbs torque, so I think it may be safe to talk in absolute terms here.

The Stage 2 turbo shows about 40-60 ft-lb gain (ramping up) in the the 2500-2700 rpm range you mention; not a 50 ft-lb loss. Stage 3 shows about about 50-70 ft-lbs gain in the same range.

Yes, this is less than the gains of a positive displacement supercharger in this RPM range.

BUT... you speak of freeway cruising. This dyno chart is not representative of torque available under higher engine loads at freeway speeds in lower transmission gears. At the lower RPMs, it takes more time for the turbo to spool up. A dyno pull is a fairly quick acceleration (more like a 2nd gear acceleration on the street with a 6-speed manual), so the turbo doesn't get a chance to spool up as much as it could at lower RPM. When on the freeway cruising around 2500 RPM, the turbo will be able to spool up more if necessary and produce larger gains because the engine is under higher load and you can't accelerate as quickly through the RPM range. Yes, it will take some time, a.k.a. turbo lag, but for freeway cruising it's usually not noticeable at all. Normal cruising behavior involves slower/smoother throttle changes to maintain speed up/down hills, gently adjust speed to move around traffic, etc. In these situations, the turbo spooling can "keep up" with your more slowly changing demands. If you suddenly stomp on the throttle in this situation, this is when you'll notice it takes up to 1 second or so for the turbo to fully spool up at lower RPM. But really, if you're suddenly stomping on the throttle on the freeway, the auto transmission will downshift, put you at a higher RPM, and the turbo will spool very quickly. If you have a manual transmission, you really shouldn't be stomping on the throttle on the freeway at 2500 RPM either - downshift first.


The huge strength of the turbo is the mid-to-upper RPM range.

Stage 2 makes 100+ ft-lbs gain from about 3200-5400 rpm, 150+ ft-lbs gain (about 2+ times stock torque) from about 3500-4700 rpm, with a peak of about 190 ft-lb gain around 4000 rpm.

Stage 3 makes 100+ ft-lbs gain from about 3000 rpm through redline, 150+ ft-lbs gain (about 2+ times stock torque) from about 3300-5000 rpm, 200+ ft-lb gain from about 3600-5100 rpm, and nearly 250 ft-lb gain from about 3800-4400 rpm.

Those are huge gains in the RPM range that is most beneficial for accelerating on the street/highway/freeway. Proper gearing, use of 4LO, and low transmission gears gives you the torque/power you need for slow off-road driving. It's on the street accelerating onto freeways, passing, etc., where the stock 3.8 really shows its weakness, and that's where the turbo more than makes up for the weakness

~Jeff

 

Exactly. More turbo like in delivery. The twin screw/roots/lysholm style charges the air right from tip off of the throttle making more torque down low where you need it. Drag racing, yep the centrifugal is the answer, daily driving....nope. Case in point, my buddy's civic SiR with vortech charger vs my integra with Jackson racing. By the time he was building boost, I was already maxed and gone. never to be caught. once he built boost, we stayed roughly the same distance apart. making close to the same boost....but mine would come on right from the get go, and just pull him like he was stopped.

 

Incorrect. Turbo will reach max boost/gains in the mid RPM range and carry big gains through to redline. See the torque chart in my previous post. None of the forced induction types are really "like" the others. Trying to describe them that way just causes confusion. There are some similarities in certain specific aspects, but to generally say a centrifugal supercharger is "More turbo like in delivery" is such a broad and incorrect statement.

In terms of the general torque curve shape, a turbo is like a hybrid of centrifugal supercharger at the beginning, positive displacement supercharger in mid/upper RPM range, and bigger total gains in the mid/upper RPM range than both types of superchargers. But then there's plenty of other aspects in which the comparison falls apart.

~Jeff

 

Well explained Jeff. I'd also like to point out that if people are considering 1/2 to 1 second lag when stomping the throttle - doesn't that happen now anyway as you downshift to a lower gear ( in an auto like mine) ?

Even in a stick, is that the deciding factor on dismissing a centrifugal SC or turbo? Not for anyone with common sense.

Moreover, these ARE NOT HIGH POWERED STREET CARS! SC'S or turbos are for adding power to a lacking engine in the case of a JK. If these guys are strapping on a SC and thinking it's gonna feel like an SRT8 then delusional from the get go and will never be satisfied.

I am still teetering between the Precision, RIPP and Sprintex.

Sprintex because of the instant jump and let's not forget that wicked sound the twin screws make.

But for highway driving where I need it most with a 4.5" lift and 37's the ProdigyPerformance or RIPP would suit me best I'd think.

Carry on Jeff...lol