Intake Spacer causing cracks?

Strype · Feb 27, 2006

Mine and my buddy's car both have the black plastic intake spacers (his 3/8", mine 1"). The last 2 times he took his off, his Explorer intake has started cracking around the bottom. I've heard a few other people on the site complain of cracks... are it the Spacers causing this? Maybe flexing

$uperstang · Feb 27, 2006

dude that is not cool! I have one and sure hope that it does not happen.

Michael Yount · Feb 27, 2006

That likely has to do with someone overtightening the intake bolts -- nothing to do with the spacer. It happens regularly. People don't seem to appreciate just how fragile that aluminum casting is. You should use the proper torque wrench -- and there's no reason to tighten the upper to lower bolts past 120-140 inch-lbs. (10-12 ft-lbs.). Your local machine shop should be able to weld/repair the cracks for you.

vristang · Feb 27, 2006

When you are using a spacer that is not the same material as the rest of the intake it is VERY important to torque properly. The plastic and phenolic spacers that are so common, have different thermal expansion characteristics.
The end result is an increase in stress on the forward and aft feet on the upper manifold.

Also worth considering is the order in which the intake bolts are removed. If you pull the inboard bolts first, then any residual stresses left from the heat cycling will have to be taken up by the small forward and aft bolts. It's a good idea to un bolt the forward/aft bolts first.

I also discovered this issue the hard way. I busted the forward and aft flanges off of my Cobra manifold while using a phenolic spacer.

Weld them back together, and torque it carefully.

jason

Strype · Feb 27, 2006

Awesome- Thanks guys. My friend used a torque wrench... hmmm.

302RollinHard · Feb 27, 2006

HoodStrype--not to hijack, but will the 1" spacer fit under a stock hood????

Rick 91GT · Feb 27, 2006

Who makes a plastic intake spacer? Never seen one...

As far as the phenolic, Mike is right on...

Michael Yount · Feb 27, 2006

Rick - thanks for clarifying - my comments were with regard to phenolic spacers - they're about as non-compressable as the aluminum is.

hoodstrype - more problems occur when torque wrenches aren't used than when they are used; the problem's not the use of a torque wrench. It's overtorquing the fasteners. Definitely use a torque wrench -- otherwise you're guessing. And our 'snug-meters' aren't too accurate. 10-12 ft-lbs. is plenty.

vristang · Feb 27, 2006

HoodStrype said:
Awesome- Thanks guys. My friend used a torque wrench... hmmm.

I always used a torque wrench (and torqued it per the manual) with the cobra manifold and phenolic spacer too. It still snapped.

My feeling is that the problem comes from the phenolic swelling from heat at a different rate than the Al. Add on to that the small thermal mass of the forward and aft bolt bosses on the cobra manifold, and you are in for some trouble. The small feet cool much quicker than the rest of the manifold.

I am just guessing, but I will not run anymore phenolic spacers on my cars.

jason

Michael Yount · Feb 27, 2006

The phenolic is MUCH more thermally stable than the aluminum is Jason -- what did you torque it to? Sounds to me like the ears were simply overtorqued -- that's what causes the problems for most folks. They simply put too much torque on the ears.

troyGT03 · Feb 27, 2006

Michael Yount said:
The phenolic is MUCH more thermally stable than the aluminum is Jason -- what did you torque it to? Sounds to me like the ears were simply overtorqued -- that's what causes the problems for most folks. They simply put too much torque on the ears.

totally correct. bottom line. overtightened or mis-torqued.

vristang · Feb 27, 2006

I am sorry to disagree. Back in those days I was very meticulous about getting the torque to spec (although today I do use German torques on my comp cam intake; Gudentite ).
I know my torques were not off back then. I feel quite safe in ruling out torque as an issue, well, in my instance anyway.

Michael you are right, the phenolic should have a pretty low cte. That is the problem since Al has a fairly high cte (coefficient of thermal expansion). The cte mismatch is what causes residual stresses with heat. All of those residual stresses would be taken up by the phenolic, except where the Al is thin (at the fwd & aft feet). At these feet the Al would be able to cool slightly quicker(which creates some level of stress in itself), and leave the heated phenolic (because of its larger, and more insulated mass, in that region) to act upon the small feet. The residual thermal stress is cyclic with every use of the motor, and Al is absolutely horrible when fatigue cycled. (Adding heat to this region would add similar stresses to the same area)
Basically the problem exists because the central mass of the phenolic/Al does not cool very quickly, while the fwd/aft feet cool very quickly. This is acceptable when bolting the Al upper to an Al lower, but when you add in the thermally stable compund more residual stresses are left on the fwd/aft feet.
The more I think about it the worse the cobra manifold design looks.

Sorry for being long winded, but I usually am . Hope it made sense, as beer and engineering principles don't generally mix.
Feel free to disagree, as this is just a theory of mine. I have taken no steps to prove my hypothesis.

jason

Strype · Feb 27, 2006

302RollinHard said:
HoodStrype--not to hijack, but will the 1" spacer fit under a stock hood????

Hmmm... We're both running Explorer intakes, so not really a fair question. I doubt it with a stock- I'm for sure it won't fit with the explorer... that's where my Harwood 3" cowl comes in. I almost had to auction off an organ to afford it... $475

Yeah, I think he went to factory specs (110? whatever's in Chiltons). So I guess I need to untorque mine. Thanks!

Michael Yount · Feb 28, 2006

Vristang - your rationale isn't unreasonable to me -- it's just that all that only comes into play, I believe, if the manifold is overtorqued in the first place. Hence, it comes back to that. I've had the 1" phenolic spacer on mine for almost 4 years and 20K now; on and off multiple times and never a problem. I still think the key is not to over tighten them in the first place.

$uperstang · Feb 28, 2006

I torqued mine to 18ft/lbs I beleive so this is too much and I should back down to 12? The motor has only been runa for a few hours so far and has not been driven.

monte87 · Mar 1, 2012

I torque mine up w TQ wrench 12ft lbs, never had any probs ever.

jsw81 · Mar 1, 2012

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