Air gap manifolds are highly recommended on this site. I've heard that they can cause problems in the winter. Is this true?
Vizard says that they're good for an extra 3/4 of a point of compression, but might not be good for a daily driver in the north. He recommends using a water heated set up, how does one do this?
When I was researching Dynamic Compression ratios, I read that 8.25:1 or so is a safe ratio to shoot for if expecting hot temperatures. Is this number for air-gapped or non-air-gapped engines? It would make sense if it was for engines with air-gapped intake manifolds, because Vizard puts it at 7.5:1 for street engines in his article "The Power Squeeze", but I would prefer to hear from someone who has done it.
Thanks

 

The difference in performance between an air-gap and a non-air-gap manifold (assuming otherwise equivalent design, like the Performer RPM vs. Performer RPM air-gap) is miniscule. I've run them back-to-back and there's no noticable difference under most conditions. A dyno might be able to tell them apart, but you won't.

The non-air-gap intake does make a noticable difference in cold engine drivability, though. That's assuming your heads even have functional exhaust crossover passages, like factory heads do. Otherwise, there is NO difference in cold weather drivability between the two intakes.

Forget water cooling/heating the manifold. There's no reasonably priced typical aftermarket performance intake that's set up for that.

There's a reason why the factory used exhaust heat to warm up the intake manifold and you can believe those crafty old buggers who designed the system knew what they were doing.

At times of light throttle the exhaust heat can easily warm up the intake area under the carb rather quickly. BUT... when you go wide open, the radical increase in fuel flow through the carb and into the intake can RAPIDLY suck the heat out of the intake. Those exhaust gasses, hot though they may be, are very LOW DENSITY. The total heat energy they put into the intake can easily be overwhelmed by the cold fuel and air passing over the plenum floor and other areas at WOT.

That's why there's so little performance difference between air-gap and non-air-gap intakes. Pretty crafy of those old buzzards, huh?

 

That is pretty crafty, and it does make sense. So I should probably not grab an air-gapped manifold for my Connecticut daily driver. Thanks!
That being said, I'm still not sure I understand completely. Is there really that dramatic an effect on usable compression? .75 of a ratio seems pretty significant, and I'd really prefer that my engine doesn't suffer from detonation.
What dynamic compression ratio do you have for your build, if you don't mind me asking?

 

Problem is, virtually EVERY relatively inexperienced hot-rodder, MIS-estimates the "compression" of their motor by AT LEAST that much.

The error of FAILING TO ACTUALLY MEASURE the parts AS INSTALLED, is the culprit.

99.99999% of the time, 1st (or even, 10th) time engine builders fail to realize how much the "assumptions" they make are WRONG, and how dramatically that affects their "calculated" "compression".

The one most common mistake is failing to account for deck clearance. That's the distance between the piston top at TDC and the top of the block. Stock, that number is .025" AT BEST, more normally is .030" on THE BEST cylinder, and may vary by as much as .015" MORE from there.

Then on top of that, virtually ALL cast pistons, MOST hypereutectics (such as SpeedPro / Sterling / whatever the box says this week) and QUITE A FEW forged (such as TRW / SpeedPro) add an extra .020" ON TOP OF the already excessive stock clearance.

If there is any ONE SINGLE THING you can do to make your engine run better, it is, to make the deck clearance consistent on all 8 cylinders, by careful control of crank grinding ("index" grinding instead of "on the wear"), rod length, piston "compression height" (distance from center of the pin bore to the top); and then, ZERO DECKING the block to the rotating assembly.

#2 is, avoid pistons with the "rebuilder" .020" of extra clearance, then pay whatever it takes to get your machinist to get it PERFECT instead of buying machine work on the low bid.

Here's a quick example to show you how much error your "calculations" might have, if you fail to take deck clearance into account. Let's build a .030" over 350 with flat-top pistons and 64cc heads. We'll use 4-eyebrow pistons, and we'll assume 6cc in the valve reliefs.

350 .030" over (355 CID), .039" gasket (FelPro 1014), 6cc valve reliefs, .000" deck clearance (piston comes up EXACTLY even with the deck): 10.3:1

Same motor, "nominal" stock deck clearance (.025"): 9.72:1

Same motor, as above, add .010" for the cyl w shortest stroke, longest rod, & tallest deck: 9.51:1

Same motor, as above, add "rebuilder" .020" deck clearance: 9.11:1

Same motor, sink the valves into the heads from acoupla valve jobs, increasing chamber ccs by 4: 8.77:1

Got the picture?

Which motor are you building?

How ACCURATE are all those "numbers" you're plugging into your "calculator"?

You just saw a variation from 10.3:1 to 8.77:1 produced by numbers you probably didn't even know existed, and probably haven't taken A SINGLE ONE OF into account in whatever you're building.

Whaddya think now about a 0.75:1 difference from some "calculator"?

I'll tell you what I see: Garbage In, Garbage Out.

 

Sofa,
I appreciate the reminder; really I do. But you have told me this many times so far, and it is so completely ingrained in my head that I have Cc'd everything at least 6 times. I measure like a daytrader checks his stocks. I know you mean well, so thank you, but let's assume for a minute I've taken the time to measure.
How does air-gapped vs non-air gapped effect chosen dynamic compression?

 

It doesn't. If it does, it's by a tiny little amount. Definitely not 3/4 of a point.

 

Thank you. That's what I needed to hear.