Landracing Forum
Tech Information => Technical Discussion => Topic started by: RebekahsZ on October 03, 2015, 06:58:54 PM
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I'm sure this is a complicated question, but I'm looking to start simple: I'm at 174mph and want to start working toward 200mph in the standing mile at Ohio. I have found that f-body.org/gears/ is very accurate to my car when it comes to modeling tire size, rearend ratio, trans ratios and rpm. I understand that I will need a lot more power to make the jump, and that I will need to make the safety mods required in the rule book. What I'm looking to work out at the moment is rear gear and transmission ratios. The help I need from you smart and experienced guys is: what rpm do I need to gear for? In other words, do I want to go they the timing lights at the rpm peak on my dyno curve, or some other rpm? Currently my car goes thru the beams at 174mph at 6200rpm. My peak torque is at 4150rpm and my peak hp is at 5450. With current gears, Im at my peak hp rpm at the 1/2-mile point. So, for the second 1/2-mile, I'm racing with progressively less hp as I approach top speed and the timing lights. Please offer advice.
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Think in terms of Tractive Effort for the speeds you are trying to run.
Torque x Transmission Ratio x Rear Ratio x Tire Correction
BASE LINE---Use your existing rear tire RPM ( revs per mile) as a value of 1 and compensate from there
needless to say dyno sheets help
RPM is your biggest friend
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It's a waste of time spinning your engine past the point it noses over, 5500 in your case. You either need to up the usable rpm or run enough gear to get the job done. It takes less power to turn a tall tire than a tall gear so go to the tallest tire you can fit under there then go back to the calculator & see what axle ratio you need to get the job done in direct drive not overdrive if you have one. If you don't have enough HP to pull it you'll have to find some more. :|
Sid.
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Thanks to you both for your replies. Kiwi belly tank-you and I are jiving. Exactly what Im talking about: over the next couple of years Im hoping to work down the chassis and setup issues, then in the last year (I'm hoping), the final issue will be a power race to build a powerplant to get the job done. I've clearanced the rear fenders to allow a 28" tire-no way I can get taller than that on this car. Next is available rearend ratios: I ran to 174 with a 3.54 gear, now I have acquired a 3.36 rear gear (the longest legged complete diff available from the production line without going into the performance parts catalog and trying to setup a diff from scratch). If I follow the gearing chart that I have proven (at least to 174mph), with this gear I should be able to go 201mph, but I have to make sufficient power to 7000rpm in 5th gear (a 0.84 overdrive). I guess my follow on question is: do I then build an engine to make power peak at 7000rpm or some higher or lower value? I will run the numbers on that 3.18 R&P before I ask any more questions.
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I know you wanted a simple answer---but the formula I posted will let you work through that ---you are going to have to have enough TE to accelerate to 200+ to
We are on all the same page
do you have some torque figures and lets work the formula so you can get a feel for the issues
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The simple answer is yes you need a lot more HP and yes it needs to happen an a higher RPM. There are several calculators around that will tell you how much HP you will need.
:cheers:
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Just for kicks.... We run a 91 Camaro in the Ohio Mile. 424.24 hp 402.39 torque. @ 6250rpm, 350auto, factory 10 bolt with factory 3.08 w/mini spool, 28" tire. Amy shifts at 6500, Best speed in June 180.577. What have you done in the areo department?
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Ran the numbers for 3.18, which is an R&P ratio that I can get from Nissan Motorsports. It looks like this drops my 200mph point down to 6400 engine rpm.
SPARKY-I want to crunch the numbers on that formula, but I need some clarification first (BTW-thanks for helping me). The max engine torque my car made on the dyno is 435 at 4150rpm. The engine torque available at higher rpms is much less (at 6200rpm it has dropped to about 300 if I read the dyno chart correctly). So do I crunch numbers with max torque or the torque at my goal timing light velocity and associated rpm? Secondly, do I calculate the tire correction by figuring corcumpherence and dividing that into a mile? And I will have to figure in my .84 overdrive.
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Sparky replied
Associated rpm
I put the formula on my XL dream chart per 100 RPM steps
RPM Torque x Transmission Ratio x Rear Ratio x Tire Correction = TE
6200 l 300 l 1.00 l 3.54 l 1.00 = 1062 # ft. TE
now lets assume you shifted at 6200 you will fall back to 5200 rpm guessing your torque will be some where around 375
5200l 375 l .84 l 3.54 l 1.00 = 1115 # ft. TE
6200 l 300 l .84 l 3.54 l 1.00 = 894 # ft. TE
This is the red neck way of being able to compare Apples and Oranges
Dave Dahlgren has a trick software programs as well as others but I print mine on a spread sheet so that I can figure and change gears with tires I have 24 25 26 28 and which will cover the 10% steps in GM rear axels
now lets us assume when you shifted it at
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Captthundarr-thanks for offering your specs-that helps a lot. I think we would have similar power if I just used a cam to raise my peak. Perhaps I will see you at the Sep 2016 meet? That's my next anticipated participation. I expect to still be a 170-ish car depending on wind speed and direction. I had stability trouble when lifting after the timers this past June, and trying to correct this will be the big project for the winter. My only aero so far has been an airdam (which also functions as a 1/2-belly pan), but I hope to return with an adjustable splitter and a rear spoiler this year and spend the entire 3 days, not trying to go faster, but trying to get the car to go to top speed (whatever that is) and back to staging without me having to go to the port-a-potty to rinse out my fire suit. If I get to it, I also hope to install a chute, and to start learning about how that affects things. Also, if I get to it, I may try the 3.36 rearend. I guess I'm hoping for a head wind in order to stay out of trouble for exceeding my safety classification. I hope to have that sorted to the 200mph specs for 2017 season. I guess I'm in this for the foreseeable future.
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Part of the gearing conundrum is whether you will make more than enough power to do 200, in which case running above peak RPM is helpful. If not, winding it past where it noses over doesn't work (as kiwi said above)
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I believe we met in June, first while admiring you car, second trying to bum a front tire/wheel as we had one blow at 160 good luck with the areo.
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Depends on how fast it falls off
thats why you use the chart--- it will tell you where the cross over point is
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Worrying about gearing is the least of your problems and a waste of time at this point. That will fall out once you have produced a power curve with sufficient power to go 200 mph.
Interpolating from the data you have cited, it would appear you utilized about 325 hp to go 174mph.
Until you generate 325hp*(200/174)^3 = 495 HP, and the rpm at which that might occur, you don’t know where to start on the gearing.
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OR if you know what gears and tires will get you there --Then you will know what you are going to have to have for an engine other wise you are just trying to optimize what you can do with your eng.
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Yeah, that was me trying to loan you a tire. I'm gonna be chatting you up about chute issues in the spring. I had so much fun at the May and June meets just trying to sort out handling, and going 13mph faster as reward for the trouble. Really excited about the new 3-day format to allow more testing (and more hanging out getting to know folks). I don't expect to make another record attempt for a couple of years. Looks like its gonna require that 3.18 R&P. I'll be spending my time looking for head/intake/cam packages that will provide power gains with the peak power moved up significantly.
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TR X RA= FDR
.84 x 3.54= 2.97
In my opinion You are geared almost perfect now--- you just need a bunch more RPM and proabably heads and cam and valve springs just need to be able to pull the OD and 28" tires That final step of 16% should let you keep a good eng in it's power band. You need an eng you can shift at 8000-8500
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So to be in the safe range... build an engine with 650 HP that revs to 8000.... or just buy on old nascar motor.... :-D
have fun with your project... :cheers:
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I set my shift light for peak HP, and gear for desired speed at peak HP rpm, or slightly over. I think peak torque has nothing to do with this equasion. :-D
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SS1 said
"So to be in the safe range... build an engine with 650 HP that revs to 8000.... or just buy on old nascar motor.... grin have fun with your project... cheers "
that should get it done !!!!!!!!!!!
Sparky > Fred by definition: RPM X Torque/5252= HP
Its all about RPM and torque---- HP is just what we call the relationship!
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I'm researching cams. But, while I'm searching things: where do I start looking for these "old NASCAR motors?"
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Google and E-bay !
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Thanks-found a few used NASCAR engines ($-ouch!). Likely to go forced induction before I go that route.
fredvance-thanks for your clear and concise advice. That's kinda the bottom line I was looking for, though I learned something from everybody.
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Glad I could be of some help.
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I set my shift light for peak HP, and gear for desired speed at peak HP rpm, or slightly over. I think peak torque has nothing to do with this equasion. :-D
given that the "socially accepted slippage" on salt is 6%. Is there an allowance made for this slippage :?, ie at 200mph 6% equates to 12mph, in this situation do we gear for 212mph at peak HP rpm so as to achieve our desired 200mph speed :?
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I gear for the speed I want to run, no adjustment for slippage
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I gear for the speed I want to run that gives me the most TE based on my engs. dyno chart and the speed I wan to run.
But I also try to design the eng to what available gear/tire combos that are available
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We did a lot of dyno work to make the power band as broad as possible, and Mark designed a bullet-proof valvetrain so it could pull well past the power band in the lower gears and it would fall back to where we needed it once we were in top gear.
I've got a timing slip and a WOS certificate that shows this approach works pretty well.
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Make a spreadsheet (and graph) of drive wheel thrust available, for each gear in turn. This will give you a set of overlapping curves, one for each gear ratio. As you go up through the gears, you will notice that there is less thrust is available, the reason for the top out limit of a vehicle. Selecting effective shift points is very obvious with this type of graph.
When rear wheel thrust available = total drag power required, you are done accelerating. A diff/trans/overdrive ratio change MAY change your top speed, slightly, but without more bhp, you are done.
Most vehicles accelerate best using the GREATEST AREA UNDER THE POWER CURVE. This may or may not include peak torque. It is going to depend on the shape of your power curve. Ie, wide and relatively flat Vs. narrow and peaky. You want to make an informed choice here.
The formula required is available in several places, I think Sumner has it somewhere on his pages, it is in several vehicle dynamics text books and in Carroll Smith's tome as well. You might be rewarded by a Google search.
This is a pointless exercise without an ACCURATE dyno sheet with torque and bhp curves. Your current engine's bhp, tq & gearing tells you where you are. Some sort of plan for future changes is an excellent idea.
To quote one of my mentors:
"Information based decision making is ALWAYS better than Voodoo and killing chickens at midnight." It has always worked well for me, thank you HB2!
My advice is: Start keeping a notebook, get a laptop and some racing specific software, or freeware. MOST of the time, you get what you pay for. You can keep good records in a notebook by hand, for next to nothing. Well, except for the discipline to actually do it. BTW, if you are computer savvy, you can do a lot of analysis with Excel, especially if you can write your own formulas.
:cheers:
Fordboy
P.S. I know at least one guy on this board who got pretty good results with the information based approach. He posted just prior to me.
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what I call TE is what FB calls tire thrust
as the old add used to say "where the rubber meets the road"
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Further to Fordboy’s response #27, instead of plotting thrust, plot the horsepower (from the dyno curve) versus vehicle speed as determined by the rpm, driveline ratios and tires for each gear. This will also give a pattern of curves which make determining shift points a trivial exercise. Adding the “horsepower required” curve, from rolling drag and aero drag will then intersect the the power available curve, generally on the tallest gear ratio and that will be the terminal speed possible with that set-up. Done on a worksheet such as Excel, gear ratios, rear end ratios, tire rollout etc can then be easily varied to optimize acceleration and top speed. Because, as Fordboy stated, “Most vehicles accelerate best using the GREATEST AREA UNDER THE POWER CURVE” and with the above, you have a picture of that.
As for Sparky’s obsession with tractive effort:
I gear for the speed I want to run that gives me the most TE based on my engs. dyno chart and the speed I wan to run.
If he is really doing what he says here, he is hurting himself, since the maximum tractive effort will occur at torque max, which is always down the power curve from the peak power, and POWER is what it is all about.
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And yet Sparky has a "blue" hat, to go along with his red & Merlot.........
He must be doing something right........
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IO
In my opinion you have to gear for the speed you need with the best TE combo for that speed
I ran 305 out the back with 28" tires at 7800+ 2000 rpm above peak torque. Back up run I pulled the OD 1/3 mile form the trap--- it would not pull OD --it nosed over and hence my 299 red hat
I went back home and played with the charts and figured out I could get 17ft # more TE by going town to 24" tires in od # went back next month and it ran 307 and 309 out the back for a 305 avg at about 6850.
In my opinion you have to gear for the speed you need with the best TE combo for that speed. I now have some 23" tires that I will try to bump Seths record. the puts me slightly down the torque curve but up in TE because of the smaller tire.
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Thanks, SPARKY. I'm new to this...specifically what does "out the back" mean? I'm serious, I'm still learning the lingo. 300-damn that's fast...fortunately, I'm gonna be a concrete/asphalt landspeed racer since I'm way out here on the eastern side of the nation and the past two salt events that I have tried to attend have been weathered-out. My lifestyle/marriage isn't compatible with something so unpredictable when it comes to budget and schedule. So, I'm looking at Wilmington and Loring for my experiences. So, I have 1 to 1.5 miles to get it all done. My one toy car will be serving me for landspeed, autocross, drag racing and even some commuting when it isn't broken. The only other car I own or have time and space for is my truck. So there will be constant set-up changes and a lot of compromises.
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So---Sparky (reply #31) unwittingly demonstrates that power cleans torque’s clock.
He runs 305 at 2000 rpm above torque peak, which would put it near the power peak.
He then invokes overdrive to bring the revs back down to the torque peak and GOES SLOWER.
He then changes gearing and runs 305 at 6850 rpm, 1000 rpm above torque peak but again climbing up the power curve. Note how the best speeds were achieved at revs well beyond the torque peak, and consequently, the tractive effort peak.
Q.E.D.
Imagine what he could do if he just geared for peak power!
Next try is with smaller tires, which will put him yet further up the rev range, further from peak TE but closer to where the power presumably is.
Sparky, join the rest of the world!! It’s simpler and more effective.
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So---Sparky (reply #31) unwittingly demonstrates that power cleans torque’s clock.
He runs 305 at 2000 rpm above torque peak, which would put it near the power peak.
He then invokes overdrive to bring the revs back down to the torque peak and GOES SLOWER.
He then changes gearing and runs 305 at 6850 rpm, 1000 rpm above torque peak but again climbing up the power curve. Note how the best speeds were achieved at revs well beyond the torque peak, and consequently, the tractive effort peak.
Q.E.D.
Imagine what he could do if he just geared for peak power!
Next try is with smaller tires, which will put him yet further up the rev range, further from peak TE but closer to where the power presumably is.
Sparky, join the rest of the world!! It’s simpler and more effective.
I agree gear for peak Power.
Torque gets you going off the line HP give you the MPH
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---that is why RPM has equal value in the HP formula
When I work with a eng. builder I am willing to give up a little HP if I can move the torque curve up more or keep it from looking like falling of a cliff.
R-Z
At Bville there are 3 timed miles at the beginning of the 1st timed mile the 2 mile mark, there is an entry speed 1/4 mile long timing trap---we call it "entry" or "1/4 speed" at the end of the 3rd time mile which is at the 5 mile mark there is the standard 132' timing trap that gives you your terminal velocity speed as opposed to your avg speed for the last timed mile. We call this "out the back door" . When B'ville racers are talking to fellow racers lots of times the leave the hundreds number off. Ie he did 9 out the back or something like he was running in the mid 70s and it is up to you to know the class and guess if they meant 170 270 or 370
The entry and exit speeds are to help the racer try to figure out what he and his car or getting done!!!!!!
FB said it all ---read, reread and learn and use what he is putting down---I learned to use TE selling trucks it let me compare one drive train to another in working with a customer ---FB has been using it at the race track for years-but the dyno sheet with a good Excell "dream Sheet" will help you understand what you need to do to get there--------THE FUN IS IN THE JOURNEY
LSR is a wonderful addiction that leads to a sickness--- SALT FEVER for which there is no know cure!!!! and becomes a lifestyle!