Restrictor plate math help

AustinSN

Well-Known Member
Established Member
Beer Money Bros.
Joined
Jan 29, 2014
Messages
6,408
Location
the plains
Long story short, my road racing car makes too much power for it's class. I need to lose anywhere from 11-15whp.

I asked another guy for some help on restrictor plate sizing, he gave me info on his LT1, which has a twin 48mm TB, and putting a 40mm restrictor plate knocked it down 15hp. A surface area decrease of around 30.6%.

My car (98 Cobra) has twin 57s, following the same reduction in surface area puts the restrictor at 47.5mm at the TB. If my math is off on that, someone let me know.

Here is the problem, I want to put the restrictor further down in the intake where it is a circle instead of an oval (leading into the twin ports). I measured the ID of the intake after the MAF and it's 91mm. If I put a 63mm donut in the pipe (30.6% decrease), could that potentially put me in a similar position? I did crunch the numbers and it looks like twin 47.5mm restrictors put the single large restrictor at 67mm by area.

I'm going to have my wife's parents who have a 3D printer make up probably 5 different sizes that I can swap out to get the power where I want it but the longer it spends making pulls on the dyno, the more it will cost me. So getting it close on the first shot is preferred. I'm not even sure where to start, or if I'm even close.

Thanks.
 

SecondhandSnake

Well-Known Member
Established Member
Joined
Jan 29, 2011
Messages
1,739
Location
Columbus, IN
I think the logic is right when it comes to creating a restrictor upstream of the throttle plates- figuring out the necessary cross sectional area based on the total area of the twin throttle bodies multiplied by the correction factor you're looking for.

However i don't know if I'd run with the assumption that his percentage decrease will work for you. Do you have data for power and airflow for both of your cars? I would use that (and implicitly VE) to calculate it for your car. Airflow numbers would be king, but if not, you could probably make some approximations with VE.

In an ideal world, you would know LT1 airflow start and LT1 airflow finish, and that represents a 30.6% reduction in cross sectional area. You could take your actual airflow start, figure out what airflow you need to be at (linear interpolation based on target power/current power should be close enough), then interpolate/extrapolate that to his numbers and get your approximate change.

Ex: (LT1 delta airflow/30.6) = (Your delta airflow/X)

Lots of incorrect and over simplified assumptions, but it would be a start. If you had to, you could approximate airflow numbers based on some assumed VE values.

And if you're not confident in getting it on the first try, you could try a few different approaches. You could try bracketing with one that will overshoot and one that will undershoot. Then select something that is proportionally in between where you want it. Or just overshoot and interpolate based on that.

I feel like there's some much more elegant solutions/equations buried somewhere in one of my old fluid dynamics books. But I'll leave that to snover.
 

AustinSN

Well-Known Member
Established Member
Beer Money Bros.
Joined
Jan 29, 2014
Messages
6,408
Location
the plains
I think the 63mm further down the intake will do just fine.

Sent from my Potato

I think that may be my baseline, and make a 59, 61, 65, and 67 as well to test on the dyno.

I think the logic is right when it comes to creating a restrictor upstream of the throttle plates- figuring out the necessary cross sectional area based on the total area of the twin throttle bodies multiplied by the correction factor you're looking for.

However i don't know if I'd run with the assumption that his percentage decrease will work for you. Do you have data for power and airflow for both of your cars? I would use that (and implicitly VE) to calculate it for your car. Airflow numbers would be king, but if not, you could probably make some approximations with VE.

In an ideal world, you would know LT1 airflow start and LT1 airflow finish, and that represents a 30.6% reduction in cross sectional area. You could take your actual airflow start, figure out what airflow you need to be at (linear interpolation based on target power/current power should be close enough), then interpolate/extrapolate that to his numbers and get your approximate change.

Ex: (LT1 delta airflow/30.6) = (Your delta airflow/X)

Lots of incorrect and over simplified assumptions, but it would be a start. If you had to, you could approximate airflow numbers based on some assumed VE values.

And if you're not confident in getting it on the first try, you could try a few different approaches. You could try bracketing with one that will overshoot and one that will undershoot. Then select something that is proportionally in between where you want it. Or just overshoot and interpolate based on that.

I feel like there's some much more elegant solutions/equations buried somewhere in one of my old fluid dynamics books. But I'll leave that to snover.

I was kind of thinking about this as well, it seemed odd to me that the TB is that much bigger on the cobras vs the lt1s. I reached out to Todd Warren to ask if the TB outflows the engine, and he told me it did. So my numbers may not even be close.

He also gave me other avenues to try, I think over winter I may try degreeing the cams within factory spec as the rules allow, but to bring down the HP number and maybe bring torque up a little. I have tons of room on torque but I'm quite high on HP.
 

13COBRA

Resident Ford Dealer
Established Member
Premium Member
Single Barrel Sirs
Joined
Jun 4, 2012
Messages
22,317
Location
Missouri
Not allowed per class rules.

I can run a timing map from another year car, we looked at that as a possibility and decided against it.

Why not play it risky and just hope the track dyno reads low? lol
 

AustinSN

Well-Known Member
Established Member
Beer Money Bros.
Joined
Jan 29, 2014
Messages
6,408
Location
the plains
Why not play it risky and just hope the track dyno reads low? lol
Lol if I place well and someone says the car is too strong, they can seal the hood and OBDII until it gets to the dyno.

If it ends up being high, I lose points and contingency bonus. Which depending on how I placed, and the amount of cars, could be $500-$1000.

It's worth it to make sure it's legal lol.
 

13COBRA

Resident Ford Dealer
Established Member
Premium Member
Single Barrel Sirs
Joined
Jun 4, 2012
Messages
22,317
Location
Missouri
Lol if I place well and someone says the car is too strong, they can seal the hood and OBDII until it gets to the dyno.

If it ends up being high, I lose points and contingency bonus. Which depending on how I placed, and the amount of cars, could be $500-$1000.

It's worth it to make sure it's legal lol.
Until you make it legal by a couple horsepower....then the dyno reads 5hp high and you're still screwed lol

Sent from my Potato
 

black4vcobra

Well-Known Member
Established Member
Premium Member
Party Liquor Posse
Joined
Feb 22, 2009
Messages
4,460
Location
Cottage Grove, WI
Not allowed per class rules.

I can run a timing map from another year car, we looked at that as a possibility and decided against it.

How about just a smaller or more restrictive air filter?

They don't use a TT 4V but Motortrend has their Engine Masters series that looks at the effects of changing many variables of an engine and what it does to the output. One of the videos was comparing air filters.
 

AustinSN

Well-Known Member
Established Member
Beer Money Bros.
Joined
Jan 29, 2014
Messages
6,408
Location
the plains
Until you make it legal by a couple horsepower....then the dyno reads 5hp high and you're still screwed lol

Sent from my Potato

Yeah that is a risk as well, everyone says 260hp is what you should shoot for, that's the "fast" number because you can run lighter.

Problem is my car is high on weight by 100lbs at 267hp (class max), I can cut 40 from myself but taking 60 out of the already gutted car would require a good bit of cutting.

How about just a smaller or more restrictive air filter?

They don't use a TT 4V but Motortrend has their Engine Masters series that looks at the effects of changing many variables of an engine and what it does to the output. One of the videos was comparing air filters.

I think it has an OEM filter in it already. Not sure if I could cut that much filter, and that costs more than 3D printing.

I built a restrictor "donut" last night at my wife's parents house, I'm going to go back down tomorrow and grab it to see if it fits the OD. If it does, I'll make 4-5 more with narrower IDs. It was painfully easy lol.

They only had PLA, once I get the right number on the dyno I'll pick up some PLA+ and run the printer slower and a bit more tidy to make a cleaner one.
 

Users who are viewing this thread



Top