rhosch

Good thoughts guys, thanks.

Of course I'll take recommendations from a chosen engine builder under serious consideration, but I like to get a decent grasp on the issues involved in a project like this to (a) help choose a builder that seems to be in sync with my objectives and (b) speak intelligently to them about options, issues, tradeoffs, etc.

The magnacharger et al aren't off the table, I'm just not convinced that the torque curve is really what I want. It isn't that I'm looking for more low rpm torque (although might have a side benefit of needing less gear changes in daily driving), but rather I'd prefer the torque curve be predictable so that pedal/throttle response is easier to anticipate/learn/deal with. Pretty much any option would be able to swing the tail around with ease in most gears, but a flatter more predictable delivery of torque makes that a bit easier to judge. i.e., a huge turbo kicking in like a rocket booster at highish rpm in this car would be a nightmare to learn and control. A whipple or similar most closely approximates the torque delivery pattern of a naturally aspirated engine... centrifugal is somewhere in between. I don't need increased peak efficiency, or increased peak power, or anything like that. With big power in a light car, things that help enhance driveability are key. More low rpm torque might seem counter-intuitive for that goal, but I think the flatter torque trumps that. Packaging the whipple thankfully isn't a problem with the higher rear clam than most cars have under a front hood, but turbos and magnachargers have been done too.

Got me thinking seriously about just how large to go on displacement though. If valvetrain reliability isn't a huge issue, I would prefer 8K smaller displacement/stroke. Would still like to target pump gas... if only variable boost control like a turbo wasn't such a kludge on positive displacement chargers!

stevieturbo

Size the turbos right, get a proper control system in place and there should be no issues with hard driving.

But the centri will be more exciting for such a car.

JoeNova

A centri blower will be more predictable. Boost will be a direct correlation with RPM, and part throttle torque out of a corner wont get you killed. The boost curve will be a flat line. Youll have more intercooler options and hood clearance, IATs will be lower and more pump gas friendly.

Id stay away from turbos unless you are sure to get them sized properly, and a quality boost controller will be able to keep that 3500 RPM neck snap under control.

Any setup can be made to work, its how you want it to behave.

rhosch

Eh, pretty sure I meant procharger when I wrote magnacharger above. Thankfully you were able to look past my ignorance!

But I'll take your advice to heart and continue reading about behavior of positive displacement vs centrifugal.

JoeNova

Here is a good example of how each builds boost.
This is a boost graph and NOT a hp/torque graph. It simply shows how each of the different setups were able to build boost. Keep in mind, almost none of these were properly sized on this combo. While the general shape of the graphs might remain the same, the RPM at which they start producing boost can easily be changed.



The two at the top are a roots blower and a twin screw. Roots builds boost earlier, twin screw a little later. The one that hits boost pretty suddenly at 3500 RPM is a twin-turbo setup, and the graph that slowly builds boost with an almost linear line is a centri. In a light car, it will have the best traction and most predictable response out of a corner. Much less chance to spin-out.

You can easily increase low-speed boost of a centri blower with pulley sizes, or a wastegate/boost controller. It'll be more efficient and have a LOT more options for intercoolers than either a roots or twin screw.

rhosch

OK, lets let the discussion go where it wants. I'll give you my understanding of the physics of the situation and we can go from there.

Tire spin is goverened by coefficient of friction between tire and road when wet, and by shearing strength through cross section of tire when dry and tire is wide enough (sidebar... thus why wide tires are much more helpful when dry than when wet). Both are linear relationships to force generated between tire and road for a given car weight, and in the case of dry, contact patch area as well. Coefficients change from static (first break loose) to dynamic (keep spinning) but relationship for both are conceptually the same. Given a constant weight of car (ignore dynamic suspension loading, especially in an ultima), tire size, and for a single gear, the force at the road is directly proportional to engine torque.

So tire spin is directly proportional to engine torque. Duh! But the reason for going through the logic is to highlight that rpm isn't part of the equation. If say 300 ft-lb is sufficient to spin the tires at 20 mph and 2000 rpm, its also sufficient to spin them at 60 mph and 6000 rpm. And if it is borderline OK at 20 mph, its still just borderline OK at 60 mph.

Ideally WOT would give a perfectly flat torque curve just under breakaway level for every gear. But short of electronic wizardry we have to use our right foot to modulate torque. Next best is an idealized engine with perfectly flat torque, and linear throttle response such that xx% throttle position gives xx% Max torque throughout the rpm range. Throttle response comes pretty close, so we are left looking for the flattest torque curve possible.

NA can get pretty flat torque curves. Whipple with a flat boost comes pretty close as well. I've seen plenty of dyno runs where variation over broad useable rpm range might be 10%, or say 50 ft-lb variation for a 500 ft-lb motor. And I've seen plenty of centris that are 2x or 3x that. Which makes sense when looking at the boost curves.

Simplistically and hypothetically made up abstraction, with a Whipple, 1/2 pedal = 1/2 torque = OK throughout that gear. But with a centri 1/2 pedal = 1/3 torque low = OK but = 2/3 torque up high which is not. Which jives with the experience of many in that prochargers are good on Takeaway and come on gently but can get squirrelly at higher rpm.

Now I'll be the first to admit I'm no stud race driver. I've driven a few fast cars and had some fun on the Autocross, but certainly nothing as wild as a high hp ultima. And I know what should happen doesn't always match what does happen so feel free to point out the obvious and set me straight.

In the end, 1k in an ultima regardless of how will require learning how to be judicious with the right foot. I'd just prefer a solution where response to my right foot is linear and predictable.

stevieturbo

You're way over complicating, over thinking, and confusing matters.

And no, you're thoughts on pedal position relative to torque do not apply. That's why a lot of modern cars with DBW have very non linear throttle vs blade maps, to try and give the driver a more usable torque range under foot.

rhosch

You may be right. But pedal to throttle response doesn't need to be linear, just non dynamic. Old school was... if you didn't move the pedal, throttle position didn't change. Do new drive by wire systems vary throttle depending on rpm, speed, load or whatever regardless of pedal position? Something I'll have to look into. Would be of benefit in a modern automatic, or clutchless manual, but not sure that's something I would want in an Ultima.

Don't mean to overcomplicate things but desired torque curve is pretty fundamental... that has to be settled before things like block choice mean anything. And I thought it was settled. Always glad to have more info to chew know though... up to the day of purchase that is.

stevieturbo

blade opening can be varied based on many factors in order to achieve different outcomes.

Just because a driver nails the pedal 100%, doesnt always mean that the same opening of the blade will achieve maximum power/torque at any given instant.

Obviously a lot is going to boil down to the ecu's abilities and the tuning etc of the pedal vs blade relationship, and of course the blade vs power/torque relationship.

Proper DBW installs can offer the drive a lot of benefits....but to release them it would take a lot of tuning time, whether road, dyno, track to optimise it for that driver/application.

Of course on the other hand, some ecu's simply mirror the pedal movement so may as well just be a cable setup.

Unless you're doing high level competitive racing....I really wouldnt worry about the above too much. BUT, it can make the vehicle much safer and faster to drive....as can other things.

rhosch

Cool, thanks for the info. Something I need to look into more. I think most recent Ultima builds have used the LS7/Corvette DBW setup, and I think that is the arrangement endorsed by the factory. But I'm not sure of the specifics. Its on my list of things to study for other reasons, one being a valet mode.

02sleeperz28

iTrader: (26)

Good read.

chuntington101

Just be carful thinking that a Procharger will solve your intercooling issues! The Ultima isn't intercooler friendly. You don't have huge highly effective scoops on the carlike you do on some mid engine cars. Also the fuel tanks (assuming you are using an LS engine) are located in the side pods and will restrict the size of intercoolers you could run in the side pod area.

Some, have mounted the cooler over the transmission and ducted the air from the top scoop to the intercooler. This works but requires fans under the intercooler to get decent airflow. Also will require work to the clam to get it to work well.

The only other option is Air to water but then you still need to find areas to mounted heat exchangers. The EVO with the Whipple supercharged LS9 has one HE mounted in front of the rad. Not sure is that would be enough or not or if it will have any impact on engine cooling.

The great thing about the Whipple is the packaging works really well.

rhosch

Yeah, the packaging especially on the GTR/Evo coupe, seems to be an advantage for the whipple and similar options. The inverse might be true for many other cars where vertical space over the engine is at a premium but there is more room laterally for prochargers, turbos, etc. Of course, any can be made to work and have been done.

I would think A/W intercooler would allow for more placement options? I was wondering about something like the factory have done with their front mounted IC but with forced induction engine cooling doesn't need to take a big hit either. Cooling may become the practical limit regardless of what route is taken to produce power.