I'm building an FFIM. What would be the ultimate design?

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buckshotglass

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Oct 21, 2006
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If you could make the ideal FFIM for the 7M, any shape, what would it look like? Would it have risers inside or not, and why.
Is making a FFIM totally based on the fact that it's a pressure situation, therefore less about flow? My feeling is it's about flow in the lower RPM/boost levels, and not so much as the pressure builds.
I am making a one off piece, and I am looking for either good links to Plenum Theory or any useable input you might have.
Thanks.
 
I

IJ.

Grumpy Old Man
Mar 30, 2005
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RPM range?

Which Turbo?

Auto/Manual?

How do you want the power to hit?

These are some of the variables that you can tailor to make your FFIM right for YOU ;)

Short runners kill off low end long runners improve it

Small plenum volume makes the boost hit harder Large is softer
 
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IJ.

Grumpy Old Man
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Ok this is just what I've found in testing parts with no real hard science ;)

Long runners = cutting a stock plenum off the upper runners and welding the new one on.

Short runners = 40mm long from the head to the Plenum mounting flange
(any shorter and it makes it really hard to get to the intake bolts)

Plenum size Small = 1 x engine capacity

Large = 2+ x engine

I ran medium/long runners and 1.5 x plenum with a 3540r and it was UNDRIVABLE at 3500 rpm with uncontrollable wheelspin and even on Traction control the MoTeC had a hard time keeping it sane so it wouldn't blow through soft cut and into hard cut.

I changed to short runners and a large plenum and this improved things out of sight but killed off ALL the low end torque making the car a slug off the line and needed a 2800 rpm hi stall to get it back to "quick"

I'll be going middle of the road with the new FFIM and have welded the new baseplate to the full length lower runners and will shoot for 1.8 x plenum

 
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buckshotglass

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That is good news. Everything seems to fit easier if I just use the lower factory part.
I was trying to keep some of the upper runners to maintain the bottom end, but the air flow must make a harder turn to get into them, so it's probably about equal when you factor the restriction.
Thanks for the info. I have watched lots of Banks turbo videos, and it's hard to find actual hard facts on plenum design.
 
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IJ.

Grumpy Old Man
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"Risers" as in velocity stacks?

As far as I can gather it's not a great idea for forced induction as it upsets flow when positive pressure but does help out off boost.

Agreed there's not a lot of hard fact to be found on plenum design that's why I ended up making 1 at each end of the range and trying them in the real world to see what change they made.
 
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IJ.

Grumpy Old Man
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Without spending a shitload of hours on a flow bench it's all guesswork taking the basics I've outlined into account.

Problem with the flowbench is it's NA and the FFIM will behave very differently under positive pressure.

Lost wax is a nice way to build as it allows for a LOT of detail work, I'd look at very short risers just to bell the start of the runner as this may help and not upset flow when +.

Another detail I'll be adding this time will be some rolled beads in the roof of the plenum opposite each port ala RB Nissan plenums.
(should help turn the air towards the baseplate and will remove the flex from the plenum making it quieter at idle)
 
tte

tte

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IJ. said:
Ok this is just what I've found in testing parts with no real hard science ;)

Long runners = cutting a stock plenum off the upper runners and welding the new one on.

Short runners = 40mm long from the head to the Plenum mounting flange
(any shorter and it makes it really hard to get to the intake bolts)

Plenum size Small = 1 x engine capacity

Large = 2+ x engine

I ran medium/long runners and 1.5 x plenum with a 3540r and it was UNDRIVABLE at 3500 rpm with uncontrollable wheelspin and even on Traction control the MoTeC had a hard time keeping it sane so it wouldn't blow through soft cut and into hard cut.

I changed to short runners and a large plenum and this improved things out of sight but killed off ALL the low end torque making the car a slug off the line and needed a 2800 rpm hi stall to get it back to "quick"

I'll be going middle of the road with the new FFIM and have welded the new baseplate to the full length lower runners and will shoot for 1.8 x plenum

Click to expand...

Good info Ian...

Cheers,
Roy
 
D

dbsupra90

toonar
Apr 1, 2005
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dave-

with the 66mm turbo, you would probably want medium length runners. that turbo doesnt just scream low end torque, and short runners would make it even moreso. as far as volume, id shoot for 5L.

what is interesting tho, the large volume/short runner feels more responsive. i noticed this and so did IJ.

the best comparison i have would be comparing it to cams. size vs hp vs powerband.

small volume/long runner: stock cams

medium volume/mid runner: 264 cams

large volume/short runner: 272 cams
 
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IJ.

Grumpy Old Man
Mar 30, 2005
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BHG: As I said earlier there's no real hard science available, mine has been trial and error so now I feel fairly confident I can tailor a FFIM to suit my combination ;)
(this is #4 for me)
 
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