he asked me if i thought some engines were more affected by hot weather than others. especially, big motors vs small blocks? he also thinks small block mopars are affected less than small chubbys? i told him i thought all motors breathe the same air and that it was more in tuning. i know boosted motors can add more boost to compensate, but we are talking na. opinions please.........

I've suspected that very thing. I do believe small engines
are more sensitive to shift points....the more high strung
they are, the more sensitive to shift points. Same thing
with vehicle weight.

To back up what Fred said, it is more difficult to be as consistent with smaller engined and/or smaller carbed combinations. I see it all the time in class racing. This is assuming that jetting is "in the ballpark" since jetting that is too lean or too rich will skew the numbers. The better an engine breathes - carb size, head flow, compression, cam, rpm's, etc., the more consistent it can be - IF it's a combination of parts that work together. A combination of parts that work "together" is the first rule of going faster in any case.

One can even go so far as to say that a faster, higher performance engine stays on the track for a shorter time so there is less time for weather changes, including wind, to affect it!

A customer had two 311" motors for the same race car. Both flogged on dyno and at track.
(several converters,gears,tires,collectors,etc. normal class racing tweaks)
"A" engine +20 peak HP over the "B" engine. (same dyno)
The B"" engine had a much flatter torque curve.
The "B" engine was a better race-day engine.
During the track testing, the "B" engine was less fussy about converter stall, or shift RPM.
It proved to be less fussy about the weather as well. About 1/2 the ET change with equal weather changes. This info was collected over 3 years of accurate record keeping.

A big engine, especially in bracket tune will almost always have a flatter/fatter torque curve than a smaller one. I think this plays into the "bigger is better" theory.

With all the flogging, the "B" engine was .02-.04 quicker in anything other than mindshaft conditions where they were equal. But, that's another story.

The first thing that came to mind reading through this was "torque"; the more TQ, the less sensitivity. I'm perfectly willing to be proven wrong, but it's logical. This of course assumes a package of complementary parts/tuning.

if we are comparing cubic inches big vrs small.
and a net change..
i look at it like this...
an 8 hp loss on a 300 hp motor will be seen on track much more dramatically than an 8hp loss on 600hp.. mathmatically the loss percentage is greater for the smaller engine..
if we are comparing engine brands..
id say ..intake port architecture will factor ie; velocities,deadspots,sonic choke thresholds
will change dramatically with water grains.
piston and head chamber shape may effect losses
differently..
and different compression ratio's will definately factor... as far as how tempermental one will be to climate change..
cheapst

I posted this before in boosted section and still true:
you will notice none of the scientitst has responded.cause while your effort is commendable, the calculations will be soooo farrr off real world its not worth the effort.
there are so many way out vairlables on top of variables it wont work exactly as the math will work out.
like calculating snow flake shape and velocity LOL
if it was purely controlled 80% of the inputs just maybe it would be 50% accurate.
like buying stocks.
1 Mb. atmosphere throws it off. unless you use all derivitives and limits then its only a range and not real.
sorry for the downer
build for worst case every time with triple redundacy like military engines and you wont be disappointed cept for the bill$$$

Another thing about it is if your bigger engine makes more power (and it should) then the car should be faster too and the same percentage of a fast ET and slow ET means the faster car will change less. Example would be a 10.00 second car loseing 10% of its ET going from minshaft air to denver on a hot day means it loses 1 second, however if you have a 12.00 second car because it has a smaller engine and it loses 10% of its ET means it will lose 1.2 seconds so conversly a slower car has more to gain from good air. So in my point of veiw from a bracket raceing perspective the bigger engine has an advantage (assumeing you have no problem hooking it up and all).

very interesting! . in my much younger years, and not knowing as much about "tuning", i would "jet it up". of course in the heat it wouldn't run, but as the night air came it started picking up. now i try to tune "in the middle" which is the only way to tune a bracket engine. last week we put my lm-2 on one of the cars i sponsor, after recording 4 runs, we noticed the afr stayed close to 13.0 all the way down the track. maybe slightly lean? BUT, this week the DA was worse, now the carb is a little closer? at any rate he is WAY out front in points and he took the win yesterday.