Dan Baldwin
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Posts posted by Dan Baldwin
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The longer stroke crank (allelsebeingequal) WILL have a higher polar moment of inertia. BUT, this is WAY MORE than offset by the greater LEVERAGE offered by the increased stroke. So under a given load, the longer-stroke engine WILL rev up more quickly. The rotational inertia of the crank is all but negligible, anyway, as you pointed out. The ONLY rev issue regarding stroker engines is the slightly reduced redline (assuming equivalent-strength pistons). But this is also MORE than offset by the greater displacement.Originally posted by Michael:the long-stroke setup will have a higher moment of inertia, implying that it would take longer to spool up. However, there is a straightforward (but expensive) remedy: use lightened components; a lighter flywheel, lightened crank, compact harmonic damper.
Anyway, surely by now we can put to bed the myth that shorter stroke engines rev up more quickly. I hardly consider my street L31 to be "rev-challenged".
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The "correct" and far more practical way to measure backspacing is from the absolute backside of the wheel, outside the bead. Did a quick search to confirm and found: http://www.usacomp.com/Offset.htm
Doesn't prove anything, of course, other than that these guys reference backspacing to the absolute backside, not the bead-seat area. Note they refer to overall widths 1" greater than the inside-bead to inside-bead width, which seems to be standard, as far as I've seen.
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The "50" is not the tire width, it's the aspect ratio (lower number = shorter sidewall height).
225 is as wide as you want to put on a 7" wide wheel.
My 14X7 Enkeis and 15X7 Panasports are 8" wide total, 7" inside to inside. 4" backspace IS zero offset on a 7" wide wheel, I'm almost certain. Anyway, zero offset is what you want, and no flares or cutting are required to run 225/50-15s, but any higher aspect ratio at that width would likely rub. 205/50, 205/55, or 205/60 should also fit well without rubbing.
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On a dished-piston motor, yes. Flat-piston motor with N42 is 9.8:1, with P90 it's 8.5. For N/A, you definitely want a flat-top piston bottom end.
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Dude, an L28 with dished pistons and a P90 head would have you at 7.4:1 CR. I can't imagine even the most boosted engine running less CR than that, but if you did want to you could use a 2mm head gasket and go down to 6.9:1. Or you could bore 2mm over and use Z22 pistons for CR in the 6.0 to 7.0 range (ridiculously low). CR is VERY sensitive to combustion chamber volume, you don't have to reduce stroke at all in order to get REALLY low compression ratios. If you're running enough boost to justify CR that low, I'm sure you'd be running forged pistons anyway, which you should be able to get with a much smaller dimension from wrist pin to the top of the piston than L28 pistons or with serious dish, and again could get a CR as low as you want.
Your desire to use the L24 crank is bordering on fetishistic (izzat a word?). Even with a turbo, displacement is directly related to potential power output, so don't just throw it away for no reason.
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I thought crossovers were to help the out-of-sequence-cylinder problems inherent in 90 deg. crank V-8s. Flat-crank V-8s and GT40 style exhaust systems (with one pipe on each bank crossing over and being collected on the other side) don't require a crossover. Our L6s are have even exhaust pulses for the front and rear 3-cylinder groups, so no crossover should be necessary, right? I too would be very interested in any comparison results.
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Why so fixated on the L24 crank? You can get pretty much whatever CR you want with an L28 crank (or with an LD28 crank even). Why give up all that displacement?
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NO LIDS?! Well, I'd recommend getting some lids from the "help" section of the local auto parts store. And a few bottles of brake fluid. And a cheapo (~$6.00) one-man brake bleeding kit (small bottle with magnet on lid to hold it near the bleed screw, and tubes). Ever bled the brakes before? Jack up the rear of the car and remove the rear wheels. The bleed screws are on the little cylinders behind and at the bottom of the brake drums. Hopefully they'll loosen. Anyway, one at a time, loosen the bleed screw and attach the tube from the brake bleeding kit. Pump pump pump until the bottle is full, repeat until the MC reservoir is nearly empty. Then top off with fresh fluid. Repeat until fresh fluid is coming out of the wheel cylinder. Tighten bleed screw, and go on to the other side of the car and repeat. Now would be a good time to check the condition of the brake shoes, so remove the drums and have a look. Then repeat the whole process at the front of the car. Bleed screws are at the top/backside of the calipers. Check the pads, too.Originally posted by Nick_570Z:The biggest thing I've noticed is the people who owned it before me never put lids on the fluid reservoir. They're both pretty nasty, but the rear is pure black gunk. Anyway, I just want to know the cheapest way to help these brakes out. I am selling it as soon as possible, so I don't want to spend much money at all. Would a flush help? How is that done?
Brake fluid is very hygroscopic (readily absorbs moisture), so it's no wonder your brakes suck. Also, you've probably got corrosion in the lines, so repeating the process after a few hundred miles of driving is a good idea.
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Damping DOES reduce the natural frequency/resonant frequency of a spring/mass system.Originally posted by BayAreaZ8:Doesn't natural frequancy = resonant frequancy.
natural freq. = sqrt of k/m
I think damping does not change the natural frequancy, it changes the system response.
Go here: http://ccrma-www.stanford.edu/courses/150/vibrating_systems.html
From that site: "The natural frequency [omega d] is lower than that of the mass-spring system [omega o]."
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Yeah, I know you're talking about a turbo. But I *think* (me no turbo expert) you can run as much as 20psi at 8.5:1, intercooled. And a lot more than that (I'd guess) at 7.6:1. I just don't hear of ANY serious turbocharged street engines below that CR level, I think you'd be losing a lot more than you'd gain with more boost (30+psi?!). I think you should figure out what CR you want, based on what boost you want to run, then try to get there with the L28 crank (or LD28 crank?). The point is, you can get there without resorting to a shorter-stroke crank.
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I think you should try to reduce your accelerator pump volume before worrying with main jets. When you step on the gas, the accelerator pump dumps raw fuel into the runners, if you're bogging when you step on the gas, I think you're getting too much additional fuel. The accelerator pumps are, I believe, on the bottom of the carbs, and should be adjustable. Fiddle fiddle fiddle. Get the idle screws and accelerator pump adjusted so the car runs OK, then go to the dyno and see what your A/F ratio is throughout the rev range, and change jets based on that. Ultimately you will want air horns for them. Not having them makes the carbs' throats' effective diameter a lot less than 40mm. Would be good to have the air horns before going to the dyno.
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If your aim is to lose peak power and peak torque (relative to using an L28 crank), then you're on the right track. The higher rev capability would be more than offset by the lost displacement. Peak piston acceleration for an L28 at 7000rpm is the same as for an L24 at 7320rpm. That means you could rev 4.6% higher, but the displacement of the L24 is 13% less. So you'd have ~13% less torque and ~9% less peak power potential. There are L28 setups that will get you in the CR range you want, anywhere from 7.5:1 to 10:1.
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John,
That's the first I've heard that the original dampers used had a piston up sliding inside the wall of the strut tube, I thought they used inserts from day one, with oil between insert and strut wall. Did they really come originally with the strut housing as the damper housing?
The shock insert can be considered as structural, as the male slider has to carry some side load, which is transferred to the strut housing at the gland nut. Ever driven around with the gland nut loose? It feels like that corner of the car has no damping.
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Tomahawk, I wouldn't have any reservations about running 185 #/in springs all around with the same dampers.
Tim, if you acknowledged the much greater leverage ratio issue regarding Camaro front suspension, I guess I missed it.
Knowing the spring rate and the mass supported isn't enough to calculate the natural frequency. You need to know the equations of motion of the wheel related to spring compression. Or just the instantaneous leverage ratio at static ride height, which should give sufficient accuracy for us. Anyway, the leverage ratio should be pretty close to the same front and rear on our Zs, and the weight distribution is known to be pretty near 50/50. So, assuming a basic knowledge of Z suspension, just talking spring rates is adequate for this discussion.Originally posted by TimZ:And for the THIRD time, it doesn't do any good to talk about spring rates if you don't know what mass they are supporting.
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The dyno would be a much better place than the drag strip to figure out what's going on with your mixture, and how that affects torque/power.
Does the engine bog when you step on the gas? You may have too much accelerator pump volume. I'd encourage you to take one of your carbs apart and play around with it to see how the accelerator pump works. My OER Racing carbs have a screw on top that limits accelerator pump travel, allowing adjustment of how much fuel goes in through the pump circuit. The idle screws at the base of the carbs control the fuel going in at idle, and to a lesser degree during part-throttle operation, so there's another place to adjust. The MAIN jets are underneath the removable cover on top of the carbs, along with the smaller idle jet(s)/holder. You didn't say what kind of carbs you have (Weber, Dellorto, Mikuni, SK). There are differences, but there's always a MAIN AIR jet on top of the jet holder/emulsion tube, and a MAIN FUEL jet at the bottom. These determine what the mixture will be at more open throttle positions at mid to higher rpm. Bigger fuel jet => richer mixture. Bigger air jet => leaner mixture, particularly at higher rpm.
There's lots of info on the web on these carbs. Search around and read up a bit. You don't have to be a genius to get them working properly, but it would help if you knew about the major systems (idle, accelerator pump, main) and how to adjust them. Disassembling a carb with an exploded diagram like this: http://www.teglerizer.com/dcoe/dcoe45_tour.htm handy would be a big help.
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Damping DOES have a large effect on resonance frequency (not natural frequency, my goof (I think)). Only if the system is significantly underdamped is the effect of damping on resonance frequency negligible. The resonance frequency is what we're concerned with (I'm pretty sure).
I wasn't exactly chain-pulling with the caddy reference, that was sorta MY point. That in some systems that are underdamped, the "underlying" natural frequency is pretty much the resonance frequency of the system, but for a 240Z in track tune, you should be pretty near critical damping, I'd guess. Anyway, enough damping for it to have a profound effect on the resonance frequency.
As far as trying to "calculate" appropriate spring and damping rates, as I said before I don't think any of us is qualified to do a good enough job, taking into account the distributed masses of the car and suspension components, tire stiffness and damping, chassis stiffness and damping, bushings, etc (highly nonlinear and complex interacting systems). Time and effort better spent at the track anyway.
Regarding the "how come the Camaros can run these spring rates when we can't" question, for the THIRD time, the front suspensions on those cars are HIGHLY leveraged, and the wheel rate is nowhere NEAR the 600 lb/in spring rate mentioned previously. Their front and rear WHEEL rates are much closer than their front and rear SPRING rates.
Mudge,
I don't *think* I have too much front sway bar, I'm getting too much roll as it is, and the car handles pretty neutral. I might experiment with softening it at Lime Rock in a couple of weeks, though.
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On Zs the stiffer springs usually go in back. Stock 240Z rates are ~80 front/100 rear. My mid-90s ST springs are 160f/200r. Don't know which were longer. If you can figure out which are stiffer, they probably should go in the rear.
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Tim, you're leaving out DAMPING. It is just as much a determining factor of natural frequency as the spring and mass, and you can get dampers that are externally adjustable. Repeat after me: spring, mass, DAMPER. With a forcing function (da road acting on the mass and polar moment of da car).
And again, Camaros have WAY stiffer front springs because they have a lot of leverage acting on them. The wheel rates are a LOT closer to the same front/rear than the 600 front/140-160 rear spring rates would SEEM to indicate. F/R weight distribution is ~55/45. Wheel rates are probably on the order of 60/40.
As far as "oscillating", none of our cars' suspensions SHOULD oscillate. If they do, new dampers are in order. Leave the oscillating to yer great uncle Fred's '76 Caddy.
John,
I don't get the East Coast/West Coast setup discrepancy, but I'd bet money on the East Coast team. Unfortunately they've taken the year off from Zs. I think Chet's a hired gun driving a Miata now. Anyway, I arrived at my current setup somewhat independently, sort of a street/track compromise with aspects of East and West coast race setups. Works well enough for me!
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I'm making 235 rwhp with a 3.1 liter. N42 head shaved, ported and minor chamber reshaping by Sunbelt (40.6cc chambers), 302 deg./.550" cam, also by Sunbelt, 45mm 3x2 OER Racing carbs (similar to Weber/Mikuni/SK), 1 3/4" Nismo header, 3" exhaust, MSD 6AL ignition, stock dist. w/ pertronix and modified (reduced) mechanical advance (vac adv not used).
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I still don't believe it should be a problem for 90% of all track-driven Zcars, though. Motions from track irregularities should be damped out in short order on our cars, and shouldn't lead to any excessive pitch oscillations. Cars with high downforce (on the order of car weight or more at speed) might have to worry about this as they seem to be underdamped due to having to run very stiff springs.
Picking up tires seems to be pretty normal for cars with high grip, high center of gravity, and narrow track. I've seen pics of my competition's Shelby Omni picking up BOTH inside tires at Turn 3 at NHIS! I have pics of mine picking up the inside front (not much) there as well. Wheee! I know it's best to have all four on the ground, but given the above conditions, it is possible that tires will leave the ground no matter what the damping is. (Not saying mine is optimized, far from it I'm sure)
Anyway, again I don't think there's anything magical or special about the spring rates specifically, though I'd agree that in some cases having differing natural frequencies of the front and rear spring/mass/damper systems may be a good idea. Then again in other cases they may want to be close to the same. I run slightly stiffer springs and damper settings in the back of my car, don't know what the natural frequency difference is front/rear, but they're probably not all that far apart (stiffer spring => higher freq., stiffer damper => lower freq.).
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Nothing magical about having the same spring rates at all four corners. Hopefully (assuming there's not a TON of aero load), you'll be pretty neutrally damped (not underdamped bouncy or overdamped skatey), so bucking shouldn't be much of an issue. I would have no reservations at all about trying it, and I might next year. I'm currently at 160F/200R spring rates, 1" front bar, and no rear bar (disconnected). The faster ITS guys run more like 350F/300R (something like that, anyway), big front bar, and no rear bar. I might go to something like 225/225.
The reason a lot of cars (like my Z28) have MUCH stiffer front springs is that they have highly leveraged front springs relative to the rears. Wheel rates aren't nearly as drastically different front/rear on those cars.
As far as modeling suspension behavior on a Z, accounting for variable damping with speed an position, tire stiffness/damping, and chassis twisting, I think one would do better in terms of time, money, and effort spent just going to the track with different springs and adjustable dampers. He'd have a lot more fun, too!
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Damn, I KNEW I shouldn't have clicked here. I see the same misconceptions here that have run rampant in L6 land, and have been around for as long as I've been alive.
Misconception #1
Shorter stroke engines rev up quicker, but longer stroke engines have more torque.
Given engines of similar displacement, cam, CR, etc, they will rev up about the same speed under a given load, and will have similar peak torque (largely a function of displacement and CR). The benefits of having a shorter stroke FOR A FIXED DISPLACEMENT is that you can rev to a higher rpm limit for the same peak piston acceleration, AND you would have the opportunity to have bigger valves, which would improve high-rpm breathing. This does NOT mean you can get more power by destroking an engine. You will lose more than you gain, since you've only increased your redline by the ratio of the square roots of the two strokes, while you've lost the straight ratio (i.e., redline may go up 5%, but you've lost 10% displacement, so potential power is reduced by about 5%). Unless you're in a displacement-limited class, you want as much displacement as your engine configuration will allow. Assuming you want to be as fast as possible. And, believe it or not, a longer-throw crank WILL rev up quicker under a given load than a shorter-throw crank. Greater leverage. Displacement increase through longer stroke or greater bore both have the same net result: more torque to spin the crank more quickly.
Misconception #2
V8s are torquey low rev engines, 6s are revvier, and 4s are even revvier and have no torque.
This one must be due to the fact that most 4s are less than half the size of V8s, and to make acceptable power must be tuned for higher rpm operation.
The fact is that for a given displacement, more cylinders = more rev potential, allowing for more peak power. Of course tuning for high rpm torque (maximizing power) typically reduces low-rpm torque. So for a given displacement, and assuming tuning for peak power, 4s would be the low-rpm torque monsters, 6s would be less low-end torquey and more high-end revvy, and V8s would be even more revvy and less low-end torquey. The F1 guys have found that the optimum setup for their application is a V10. Lots of revs for peak power. Adding cylinders beyond 10 apparently introduces enough additional friction from greater total cylinder circumference (less additional power, more fuel consumption) to offset the additional rev potential.
Stuff to think about, anyway.
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Oops, forgot the Illumina settings. I've used 2 front/3 rear and 3 front/4 rear. I've heard that even 1 is too stiff for stock springs, 5 might be good for 250 to 300 lb/in springs.
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Mine are Suspension Techniques. 160 lb/in front, 200 rear. I got them in '93, and I've heard their springs are softer now. Anway, they lowered the car an inch or so, and as I said they're a good street/track compromise. That is, they're too stiff for the street and too soft for the track:) I don't remember but I *might* have ordered them from Arizona Z, or MSA.
Piston Speed Question/Quiz
in Miscellaneous Tech
Posted
You can't convert an acceleration to a speed. Peak acceleration occurs at zero piston speed, and peak speed occurs at zero piston acceleration. In setups with different rod lengths, rpm for a given piston speed will result in differing maximum piston acceleration (greater rod length => reduced peak acceleration). Here are the equations of motion I came up with:
r = stroke/2
L = rod length
O = theta = crank angle (0 at TDC)
w = omega = crank rotational speed (radians/sec)
yp (piston position) = r*cos(O) + L*cos[(r/L)*sin(O)]
y'p (piston speed) = -r*w*sin(O) - r*w*sin[(r/L)*sin(O)]*cos(O)
y''p (piston acceleration) = r*w^2*{-cos(O) + sin[(r/L)*sin(O)]*sin(O) - (r/L)*cos^2(O)*cos[(r/L)*sin(O)]}
y''p simplifies to -r*w^2*[1 + (r/L)] at TDC, where peak acceleration occurs. I made a simple spreadsheet to find peak piston speed and where it occurs. Should work out to be the same as your GMax equation, but for a 3.1 liter stroker I get a peak piston acceleration of 96,000 ft/s^2 (almost 3000 gs!) at 7000 rpm, with a max piston speed of around 104 ft/s (71 mph) at around 75 deg. ATDC. GMax equation gives 131,750 ft/s^2 though.
Somebody lemme know if I screwed up anywhere. My brain hurts.