density altitude per hundredth

[Maverick]

I am trying to find out if my altitude numbers I use to find out if my car is going to pick up and by how much are close, It seems like the people I talk to, say around 200 feet per hundredth of a second. My numbers are less than that,
Does the size of the motor affect the feet per hundreths factor, less if the motor is real big? If you had a real small motor, like a 283, whould the altitude in feet be less for each .01 seconds in performance?

[RacingRicki]

Post or PM me some numbers and I will tell exactly what your ratio is. All I need is the Density Altitude and your ET for a hand full of what you considered good full runs (no brake runs).

[Maverick]

I have three runs with the same set up. 9.925 @DA of 2535, 10.05@DA of 3208 and 9.886@ DAof 2338.
I have two same settings but the two are the same setup, 9.951@DA 3409 and 9.844 @DA 3052.
I thought I come up with 50 feet per hundredth, which seems like a lot of time per foot. Just trying to figure out why the feet is so low per hundreth

[RacingRicki]

My prediction software comes up with .019 sec/100 ft, which is pretty close to your 50 ft/ .01 sec number. The software shows a really good prediction line with the data your provided so I wouldn't worry about it too much. See the attached image.

The other two runs yield a ratio of .030 sec/100 ft.

I could see a smaller engine being more air sensitive. A relatively small change is going to mean more with a smaller displacement engine than with a larger displacement engine.

Again, if you seem to be coming up with the same ratio all the time, don't worry about what other guys have. Based on the numbers you provided, it looks to me like you have a pretty predictable combo.

[Maverick]

Thanks, that info makes me feel better. I have a strange combination. Thanks again Steve Williams S/G 351N

[leeseracing]

I know this is an old topic - but it piqued my interest. I'm wondering why a smaller displacement engine would be more affected by weather change. If I understand DA correctly, the concern is that the effect of pressure and humidity in the air take the place of where there could be more oxygen (which is good for combustion). When DA is a high number, that means that the air is crappy - could be a low barometer, but it also could be high humidity (again, high DA = low oxygen) If that is the case, wouldn't smaller engines feel an equal effect as the large engines - because they both are working with an equal percentage of oxygen in the air? If anything I would think that a larger displacement motor would be able to do MORE with the increase in oxygen in "better" air - because it can pull more into the chamber on the intake stroke (although, again, it would be the same percentage as the smaller motor, so how would smaller be more affected?)

I'm curious - I'm tracking DA for myself but we haven't changed motor size in 20 years so I've never really been exposed to how smaller/larger displacement reacts to changes in DA. Thanks for any responses!

[SuperCompDiv3]

Smaller engines are much more sensitive especially when barometer is added into the equation.

The only correct way to figure ratios is like Don Higgins said:
Same day, same lane. 60 foot must be within .01. That is the only way to verify ratios of air quality to ET differences.

I see many stockers that are 50 - 80 feet per .01 on the Crew Chief Software. While guys with 632 inch motors can be upwards of 400 feet per .01. The reason I have been told is the carburetor size. Smaller carbs bottle up the efficiency and are more sensitive.