Here is the deal, I'm doing some mods and I'm trying to figure out if certain things will make a real work difference.
I know my car currently has a Cd of .42, frontal area is 17.6, weighs 1776lbs with driver and has 99whp.
So if I'm exiting a corner at 80mph and I have a 1800ft straight after that what kind of effects will say lowering the Cd down to .40 make or pulling 50lbs out of the car or getting a 10whp increase out of the motor?
I'm assuming there has to be a way to calculate this.
I don’t remember all the equations off the cuff, but there is a book called racers math that will help a ton.
I had or have a notebook with many of them back when helping a friend design an open road race high speed car, but we ended up building a nice hot rod power tour cruiser instead.
It seems like you have enough data to run some simulations. However, you'll be making alot of assumptions. I think the best you could get is roughly right.
Drag is .42? Is your car a brick?
Toebra said:Drag is .42? Is your car a brick?
Doesn’t have to be a 55 Chevy to be a draggy pos....
Toebra said:Drag is .42? Is your car a brick?
it's a 70's Datsun hatchback, so... yeah
Tom1200 said:Here is the deal, I'm doing some mods and I'm trying to figure out if certain things will make a real work difference.
I know my car currently has a Cd of .42, frontal area is 17.6, weighs 1776lbs with driver and has 99whp.
So if I'm exiting a corner at 80mph and I have a 1800ft straight after that what kind of effects will say lowering the Cd down to .40 make or pulling 50lbs out of the car or getting a 10whp increase out of the motor?
I'm assuming there has to be a way to calculate this.
yes, definitely... although, it's more about simulating it, because the engine torque delivery changes over the course of time down the straight.
do you have a dyno plot? gear ratios, wheel diameter and tire specs?
in a lot of ways, this is easier than a 1/4 mile drag pass simulation, because you don't have to worry about the initial traction launch thing
There are other factors that could go into which improvements will give you the most gain. If you're racing in a context where you need to refuel at some point (especially if it's more than 1 refuel) or if you generally finish a race fairly low on fuel, go for drag reduction and weight reduction first, as those will get you faster without increasing fuel burn. Adding power will increase fuel burn.
Focus on the drag. Calculate how much hp you need to maintain 80mph in a straight then make a curve in higher speeds Will give you a very very basic idea without terrible math
In a corner with that power level you are 100% tire dependent on speed and what your setup will scrub. You could full throttle in no issue anything sweeping.
well, according to this: http://www.tritrack.net/horsePower.html
If you start at 80 and want to accelerate to 100 at .1G, you need 91 HP. If you want to maintain 100, you need 50.5 HP.
If you reduce the cd to .40, then you only need 90 and 48, respectively.
Looks like +10 HP is going to do you much better than minus .02 cd.
and according to this: http://www.wallaceracing.com/et-hp-mph.php
1776 lbs and 99 hp gets you an ideal 1/4 mile of 15.25 at 88.6. +10 hp = 14.77 at 91.5. -50 lbs = 15.1 at 89.5
So that makes +10HP look better than losing the 50lbs too.
BUT - if you have 50 less lbs, you probably exit the corner slightly faster...
Robbie said:
...BUT - if you have 50 less lbs, you probably exit the corner slightly faster...
And that's why reducing weight is such a great mod. It helps with every aspect of performance. Cornering, braking and acceleration. As an added bonus, in the early stages of car modification it's often free.
@toebra it's a 72 Datsun 1200 coupe and this is pretty typical for most cars of this era.
@sleepyhead the motor makes 78whp at 5400 rpm, 88 at 6000, 95 at 6500 and 99 from 7000 through to 8600rpm. 80mph in 4th gear is 6000rpm, so the car is in 4th 1:1, the tires are 20.6 x 6 x 13 Hoosier Vintage TDRs (0 tread ware rating)
It's getting very difficult to get weight out of the car. Going from a steal hood to a fiberglass on only sheds 10lbs. The gutted doors weigh 21lbs. The trunk weighs 12lbs and going to fiberglass only saves 4lbs. Removing the doors, hood & trunk would only remove 75lbs. The tranny weighs 54lbs, the complete motor and I mean complete with fluids, weighs 192lbs. The radiator weighs 7lbs and the complete rear axle with brakes is 92lbs. Heck the front fenders are only 8lbs a piece. The glass rear window weighs 8lbs. I've still got the full size lead acid battery in the car, so I can shave 24lbs there and I've got about 12-15s of misc brackets that can come out.
As for aero I still have the standard side mirrors on the car and I'm allowed to modify the body work behind the grill so there is some I provemnt that can be had there. I've already fitted he old garden edge trim air dam to keep air out from under the car. The complete Plymouth Superbird treatment is worth 12-15 horsepower at 100mph. There is a company in Japan that makes a aero nose piece but I've got no figures on how much it drops the Cd.
There is a fresh motor going in with a slightly revamped exhaust so the car may get as much as 110whp, I may remove the side door mirrors (I have a wink mirror in the car) and go with cage mounted side mirrors (.40 Cd) and will take around 40 pounds out. I'm hoping these changes will make up around 1.5 seconds per lap. On my home track I have 2 long straights (1/4 mile+) and 2 shorter ones (1/8 mile) all of which are entered going 80 to 83 mph.
Tom1200 said:@sleepyhead the motor makes 78whp at 5400 rpm, 88 at 6000, 95 at 6500 and 99 from 7000 through to 8600rpm. 80mph in 4th gear is 6000rpm, so the car is in 4th 1:1, the tires are 20.6 x 6 x 13 Hoosier Vintage TDRs (0 tread ware rating)
It's getting very difficult to get weight out of the car. Going from a steal hood to a fiberglass on only sheds 10lbs. The gutted doors weigh 21lbs. The trunk weighs 12lbs and going to fiberglass only saves 4lbs. Removing the doors, hood & trunk would only remove 75lbs. The tranny weighs 54lbs, the complete motor and I mean complete with fluids, weighs 192lbs. The radiator weighs 7lbs and the complete rear axle with brakes is 92lbs. Heck the front fenders are only 8lbs a piece. The glass rear window weighs 8lbs. I've still got the full size lead acid battery in the car, so I can shave 24lbs there and I've got about 12-15s of misc brackets that can come out.
As for aero I still have the standard side mirrors on the car and I'm allowed to modify the body work behind the grill so there is some I provemnt that can be had there. I've already fitted he old garden edge trim air dam to keep air out from under the car. The complete Plymouth Superbird treatment is worth 12-15 horsepower at 100mph. There is a company in Japan that makes a aero nose piece but I've got no figures on how much it drops the Cd.
There is a fresh motor going in with a slightly revamped exhaust so the car may get as much as 110whp, I may remove the side door mirrors (I have a wink mirror in the car) and go with cage mounted side mirrors (.40 Cd) and will take around 40 pounds out. I'm hoping these changes will make up around 1.5 seconds per lap. On my home track I have 2 long straights (1/4 mile+) and 2 shorter ones (1/8 mile) all of which are entered going 80 to 83 mph.
so, from dropping some numbers from your car into a spreadsheet last year... I have 1846#'s for "curb weight", and 105hp... although I didn't list down if that was engine or wheel. Is it fair for me use to 20% for driveline loss? Did you drop 70#'s from the car this year (awesome!)?
I'm picking at this a little bit... should be helpful for me to get out a similar sheet which I'd start prepping for Dusterbd13... {cough} last year, and perhaps eventually 'release' one out to GRMer's with gdocs to play around with (and/or find my errors)... or point out where I'm doing things "sub-optimally"
Ranger50 said:Toebra said:Drag is .42? Is your car a brick?
Doesn’t have to be a 55 Chevy to be a draggy pos....
I was thinking 914, so yeah
Airdam coupled with side skirts will make a difference for you, and would be cheap at Home Depot autosupply.
Maybe tiny motorcycle mirrors, rather than none
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