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Discussion Starter #1
Just wondering what kind of increases/decreases you guys have seen in your ET's and MPH's from changing DA's (density altitude)?

Ran a 12.6 @ 110.8 mph in a DA of 2,242 ft in mid July. When I ran my other car back in mid April, the DA that day was only 727 ft (cooler, low humidity, etc). That's about a 1,500 ft difference in DA.

So if I hit the track again this fall in the cooler weather and get a DA that day around 725 ft or so (60 degrees, 30.00 pressure, low humidity, etc) what effect would that have on my ET & MPH? Would I run a 12.5 @ 112 mph all else being equal or is that too hopeful?

I'm thinking of doing one run in the fall without changing anything from my tune in July to establish a baseline for that day and then make changes to things like timing or carb. Otherwise I'll be comparing apples to oranges running in summer vs fall weather.
 

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Well, it will depend on how far off the car was setup when it ran. For example, my car is jetted for the cool air. So in the summer it will slow down a little more than if I leaned it out. But it is enitrely possible to see a tenth from hot to cool weather. My car will go mid to high 9 teens in the summer and then go 9.0's in the fall.
 

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Try this

This will get you an idea of what you can expect for given conditions:

http://www.wallaceracing.com/convert-et-to-msa.php


It works for me, I dropped about a 0.10 in the 1/8 when temp dropped about 20 degrees after waiting for the track to clear a wreck the other day. The calculation was pretty close.
 

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I run my cars lean so cooler weather doesn't speed them up so much and they are much easier to dial in all year long plus I also have highly modified carbs which also makes a difference. The biggest change I see is in 60 ft and 330 due to cooler tracks and better traction. So in my opinion how much change a car sees really depends on how good the tune up is. A engine with alot of jetting can pick up alot from weather changes. My brother took one of my cars to Ohio and ran his goofy fat tune up in it and ran 2/10s faster than I ever went with it. He was so proud of that. However I was winning races and he couldn't win a round......LOL
 

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Discussion Starter #7
Interesting stuff, thanks for the input! I'm pretty sure my setup is running on the rich side right now.

Interesting about the tune setup and running consistent in all types of weather. I haven't got quite that far along into racing yet, I'm just happy my car runs good and made it to the track and back in one piece, haha! But now that I know it'll hold up, maybe I'll try to get the carb dialed in a little better like you're saying.

I was just hoping that maybe I'd drop .1 or .2 and gain 1 or 2 mph next time I go in the fall. When I ran a few weeks ago it was 85 and hot. So if it was 60 or 65 I'm sure that'd help me out.

Sounds like I'm still at the stage of your brother where I'd be all happy about dropping time in the fall weather, haha! I see what you mean though, if there is that much change just from weather you need to dial in your tune a little better. Guess I'll have to work on that more this year and next year.

One thing is for sure, I've certainly learned a lot since joining this forum earlier this spring. I've had my Nova for almost 15 years now, but since I've joined this forum I've learned so much its amazing.

Thanks again for your input. Sounds like I may pick up .1 or .2 this fall if things work out right. Maybe I can get into the 12.40's before next season when I swap out converters!
 

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I'd say you'll drop a solid .10 & gain 1 MPH with a 13-1500ft DA drop, if your on the richer side, 1.3 & 1.5 MPH is possible, At one of the tracks i used to run at i would differ around 2.5 .3 & 3 MPH from summer too winter, even leaning it out & playing with timing in the summer netted not much of anything.

Are track was 940ft. above sea level, in the summer it would shoot as high as 4000ft. DA, in the best winter air it was good for 600ft. DA, My suspention was not the best, It really rear'd its ugly head in the winter when i needed it the most, I had a solid 4 & 4 in that car, never touched the jetting or timing, in the summer it would adverage 11.teens & high 11.0s @ 119-120 from day too night with 60fts around 1.62s, in the winter on 1.60s it would run 10.8s @ 123+, as you can see the 60fts. didn't change much, but the torque would play havick on my suspention in the winter.

On the norm its useally .1 tenth & 1 MPH for every 1000ft.
 

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Joe, what is your definition of lean, do you have a target AFR? My target is 13.0 +/- 0.5.
I don't use or have total faith in AFR or EGT but I do run a EGT probe in one cyl just to keep a check on the EGT. I am a firm beleiver in reading plugs. All the new gagets are tools to help you but reading a plug is the law. Sort of like the heart doctor will tell you. We can give you every test known to man but nothing is as good as a heart cath, you see what is actually happening. LOL
 

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I don't use or have total faith in AFR or EGT but I do run a EGT probe in one cyl just to keep a check on the EGT. I am a firm beleiver in reading plugs.
I hear ya Joe, and agree there is nothing like reading the plugs. But an AFR log can do something a plug read cannot, tell you what the AFR is doing at any point on a run down the track. It has been invaluable in tuning my accel pump nozzle and cams. Another advantage is time, I can see results immediately, make jet changes and get back to staging. In SoCal that can make a difference between making 5 runs in an evening or making 2. :(
 

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I hear ya Joe, and agree there is nothing like reading the plugs. But an AFR log can do something a plug read cannot, tell you what the AFR is doing at any point on a run down the track. It has been invaluable in tuning my accel pump nozzle and cams. Another advantage is time, I can see results immediately, make jet changes and get back to staging. In SoCal that can make a difference between making 5 runs in an evening or making 2. :(
Don't have a problem with people using whatever tools they find helpful but my friends and I don't feel they are that important or necessary for everyone. I never change jets. I do have a highly modified carb though. I launch at 6000 RPMs at WOT so shooters and carb cams aren't relevent to my situation. I won a race a few years ago where my junk ran 5.30 on a 5.30 index every pass over 9 hrs from 3 in the afternoon til midnight and 100 degrees to 70 degrees with a rain delay for 2 hrs. Qualified #1 with a 5.300 and ran dead on every pass but 1 I lifted on but the data showed it would have been a 5.30 also. I made the finals in 50% of the races I entered the past 3 yrs and I won 50% of those. My point is I don't think you have to have every bell and whistle to be consistant and win races and championships.
 

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Discussion Starter #12
Sounds like you can get your car dialed in good in more than one way using different methods.

I think it'd be cool to have the AFR stuff logged and have the data instantly so you can make changes right away. Maybe one day I'll fork out the $$$ and get that stuff all setup. But for now I'm just going to have to stick with how the plugs look and what my ET and MPH does on my timeslip.

I can see how when you get more serious into drag racing how all of this stuff becomes even more vital and you need to do whatever you can to read what the motor is doing.
 

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I'd say you'll drop a solid .10 & gain 1 MPH with a 13-1500ft DA
Thats pretty close to how mine reacts to DA changes. I ran my best et, 12.23 @ 109 with DA around 500 ft. This last race I was running between 12.30-12.34 at 108 with DA around 1700 ft.
 

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Discussion Starter #14
That's cool. Hopefully I'll drop .1 just from the cooler air, and then maybe a little more from some adjustments and stuff.

I just swapped in some lighter springs in my distributor last weekend and the car feels much better now, more low end power and feels better out of the hole, etc. Hopefully that's good for some additional time off my ET too.

Maybe I can get to a 12.40 or so before I do any major changes like swapping out the converter, etc.
 
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