Diesel World - - Q & A -

Can you ex­plain how a hood’s cowl in­duc­tion, not di­rectly con­nected to air in­take, can af­fect the air pres­sure in­side en­gine bay, which ac­cord­ing to what I’ve read could af­fect en­gine cool­ing? I’ve also heard that cowl in­duc­tion could slow the flow of air en­ter­ing through the grille area. I would think that if the air en­ters through the grille and ex­its through the hood and/or through the open bot­tom of the en­gine bay it would not cause heat­ing is­sues, com­pared to just ex­it­ing through the bot­tom of the en­gine bay with­out cowl in­duc­tion?

James North Via Email

Good ques­tion. Here’s a the­ory. Air­flow through the cool­ing stack (ATF cooler, A/C con­denser, in­ter­cooler, ra­di­a­tor) is a re­sult of a pres­sure dif­fer­en­tial—higher pres­sure ahead of the stack and lower pres­sure be­hind, ei­ther due to road speed or fan op­er­a­tion.

Also, at speed, there’s a high-pres­sure area lo­cated at the lower edge of the wind­shield due the hor­i­zon­tal air­flow across the hood stack­ing up against the wind­shield. This is why cabin

ven­ti­la­tion in­let duct­ing ex­ists at that lo­ca­tion— it’s forced-air ven­ti­la­tion. This area of higher pres­sure will force air into the en­gine com­part­ment via a cowl in­duc­tion hood un­less the cowl is sealed and is used to sup­ply air only to the en­gine’s in­take sys­tem.

So, any forced air en­ter­ing the en­gine com­part­ment, other than the cool­ing stack, can and will change the pres­sure dif­fer­en­tial and could likely re­duce the amount of air­flow through the air­flow-re­sis­tant cool­ing stack. Be­cause of the large ar­eas sur­round­ing the en­gine that al­low for hot un­der-hood air to exit, no hot un­der­hood air ex­its through the cowl hood be­cause of the pres­sure dif­fer­en­tial that ex­ists when the ve­hi­cle is mov­ing at nor­mal road speeds.

With­out the cool­ing stack (again, ATF cooler, A/C con­denser, in­ter­cooler, ra­di­a­tor), air­flow through the grille would over­whelm the cowl in­duc­tion hood in terms of air forced into the en­gine com­part­ment, but the stack makes it a lot harder for air to flow through even at high­way speeds… whereas open-cowl in­duc­tion is pretty much a straight shot into the en­gine bay.

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