Topic: Installing aftermarket air intake on 90 YJ

[C.Redbeardd]

Ive tried looking for them, but I dont think anyone sells them. Any reason why i can find one for the pre and post 90 yj's but not the 1990? And if there is a way to make an aftermarket work? Any suggestions? Thanks!

[vprsrul]

Ive tried looking for them, but I dont think anyone sells them. Any reason why i can find one for the pre and post 90 yj's but not the 1990? And if there is a way to make an aftermarket work? Any suggestions? Thanks!

I tried to find one myself, only ones I can find are for 91 and up....

[C.Redbeardd]

I believe I've found em for the pre 90 yj's but they just skip 90. Anyway, Ive heard there is just something different with how its setup and people just don't make em for this year. 

[chardrc]

can you link to the pre 1990 one? seems weird that 1990 would be different from 89 just to totally change for mpfi in 91

[vprsrul]

can you link to the pre 1990 one? seems weird that 1990 would be different from 89 just to totally change for mpfi in 91

ditto....

[C.Redbeardd]

well im not totally sure whats different, but ill have to research it again. bahh

[sharpxmen]

well im not totally sure whats different, but ill have to research it again. bahh

buy a k&n filter and put it in your stock airbox. increase the opening in the front sheetmetal below the headlight and replace the  narrow square tube with at least a 2.5'' tube. remove the restrictor in the back of the airbox - that will give you same airflow as an aftermarket CAI. You can go even further by getting a larger airbox from pick and pull (junkyard) and get a KnN for that one and for best results replace the plastic/rubber hose with an aluminum pipe 2.5'' in diameter and use couplers and a reducer to the airbox if that opening is larger - this setup should be good for at least 350cfm if not 400, more than you need for the 4 banger.

[C.Redbeardd]

awesome! Thanks man. Much appreciated!

[C.Redbeardd]

buy a k&n filter and put it in your stock airbox. increase the opening in the front sheetmetal below the headlight and replace the  narrow square tube with at least a 2.5'' tube. remove the restrictor in the back of the airbox - that will give you same airflow as an aftermarket CAI. You can go even further by getting a larger airbox from pick and pull (junkyard) and get a KnN for that one and for best results replace the plastic/rubber hose with an aluminum pipe 2.5'' in diameter and use couplers and a reducer to the airbox if that opening is larger - this setup should be good for at least 350cfm if not 400, more than you need for the 4 banger.

Whats the cfm stock?

[sharpxmen]

Whats the cfm stock?

260cfm at 6000 rpm ideal (that means if you have no losses which is 100% VE and that never happens) - the way to calculate is
2.5L or 150 cubic inches total engine displacement => since is a 4 stroke/4 cyl it means it will require 1/2 of that for 1 rotation (2 rotations for a full cycle). So first translate the cu.in. displacement in cubic feet by dividing with 12^3 (1'x1'x1'=12''x12''x12'') = 1728 and multiply the result by the top rpm (i used 6000) - divide the result by 2 (2 full rotations for a full engine cycle).

so it's something like this (displacement_in_cu_ft/2)*rpm=260.4cfm

EDIT: you can approximate the VE for the Jeep engine with about 75% which means the actual requirement of the engine at 6000rpm is 260x0.75=195cfm, the more restrictive the intake the less VE, so by improving that you are actually increasing the VE which in turns means more air per cycle and ability to make more power.

[chardrc]

as far as i could tell the tbi air box doesn't have a restrictor in the back.. but it does have the restrictive tubing in front of the box to the grill. while your at it you may want to look at the hot air tube from the exhaust manifold to the air box.. mine was falling apart and being sucked into my air filter so you may want to see if yours is about to do the same.

[sharpxmen]

as far as i could tell the tbi air box doesn't have a restrictor in the back.. but it does have the restrictive tubing in front of the box to the grill. while your at it you may want to look at the hot air tube from the exhaust manifold to the air box.. mine was falling apart and being sucked into my air filter so you may want to see if yours is about to do the same.

that's a good tip, you definitely don't want hot air getting into the intake once your engine is warm. does that have some thermostat actuated gate? my 95 YJ does not have that at all, not sure why the tbi has it, could be that in the winter when the engine is cold might form ice below the butterfly - if you live somewhere with mild winters i would think you could get rid of that (or at least fix the gate or the thermostat if it has one).

[chardrc]

ya it has a vacuum actuator door thing to open and close the hot air source. the pipe is just a fabric tube mesh material and mine rotted in half basically so i completely removed it. since i did that ive been wanting to remove that actuator door assembly but haven't figured out how to yet in a non destructive way. didn't really think about "fixing" the hot air system since it seems like my jeeps cold weather starting issues are more related to it not wanting to turn over at -20 degrees

[sharpxmen]

260cfm at 6000 rpm ideal (that means if you have no losses which is 100% VE and that never happens) - the way to calculate is
2.5L or 150 cubic inches total engine displacement => since is a 4 stroke/4 cyl it means it will require 1/2 of that for 1 rotation (2 rotations for a full cycle). So first translate the cu.in. displacement in cubic feet by dividing with 12^3 (1'x1'x1'=12''x12''x12'') = 1728 and multiply the result by the top rpm (i used 6000) - divide the result by 2 (2 full rotations for a full engine cycle).

so it's something like this (displacement_in_cu_ft/2)*rpm=260.4cfm

EDIT: you can approximate the VE for the Jeep engine with about 75% which means the actual requirement of the engine at 6000rpm is 260x0.75=195cfm, the more restrictive the intake the less VE, so by improving that you are actually increasing the VE which in turns means more air per cycle and ability to make more power.

just a bit more explanation here: your goal is to have minimal pressure drop at the required cfm - the pressure drop is related to pipe diameter, roughness coefficient (how smooth is the pipe inside) and the length of the pipe, so for longer pipes you want larger diameter, the smoother they are the better (mandrel bent aluminum, stainless or steel are best - cast plastic is rougher so it will have higher loss but you can get away with larger diameter). all the high perf filters have a cfm rating but the higher the number the better as it will result in less pressure loss which directly affects your VE - for example the difference between a 2'' pipe and a 2.5'' pipe for 20'' of length the pressure loss at 200cfm is 0.27psi, between 2.5 and 3'' is  0.05 psi - so the benefit of going to 3'' in this case is minimal, from 2 to 2.5 is more significant - of course there are other factors like couplers and bends - the smoother the whole assembly the less pressure loss and better airflow to the engine.
EDIT: using same parameters, the 1.5'' pipe vs 2.5'' pipe press drop difference is 0.9psi, going to 1.25'' gets the pressure drop difference compared to 2.5'' to 2.5psi for 200cfm - so you get the idea. The 2.5'' diam for Jeep seems the right compromise between the size and pressure drop, going to 3'' will not result in significant gains but at 2'' will be considered restrictive.

[C.Redbeardd]

as far as i could tell the tbi air box doesn't have a restrictor in the back.. but it does have the restrictive tubing in front of the box to the grill. while your at it you may want to look at the hot air tube from the exhaust manifold to the air box.. mine was falling apart and being sucked into my air filter so you may want to see if yours is about to do the same.

Mine is getting all ripped apart. I just saw it a little while ago.