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Restrictor plate for Time Trials? Experience?

PatientZero

TMO Advanced
115
53
Kansas City, Missouri
Autocross
10-20 Years
Does anyone have experience using a restrictor plate to kill power? My goal over the next year is to start running SCCA Time Trials in Midiv. The way my car sits I'd be well into Prod E territory. 3240lbs with driver/fuel, 480whp. I'd like to get back down into Prod D. With the allowed ballast of 150lbs I'd still need to get down to 440whp. This is probably a little bit more than I can kill in the tune. Ideally I don't want to add any weight and get the car down to 415-420whp. I'm thinking a restrictor plate would be the best way to do it. I haven't been able to find a great deal of information outside of using them on carbureted motors. I'm curious about placement and tuning.

On the Corvette C5.R the restrictor plate was in the intake tubing right in front of the intake plenum for the ITB's. I'm thinking I could probably fab something up to go in this area. The other question is tuning. Should you tune before or after? The only info I found said to tune the motor uncorked first and then add the appropriate sized restrictor plate to kill horsepower. Does anyone have experience with them and can confirm? Also, could the throttle body be adjusted so it doesn't open completely?

I would ask my tuner or engine builder but in both cases that is also me, so...?
 
Last edited:

blacksheep-1

TMO Addict
2,357
1,865
The mustangs use a similar style restrictor, you need to tune to the restrictor. I'm not sure this applies to your application, but back in the day, you could build a "switch" to run either tune.
 

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
1,830
297
Northeast
Blacksheep has a good memory about Mustang Boss 302 race prepped cars using restrictor plates back in the day.

IIRC that day was back in 2012 during the Pirelli World Challenge season when at some point after the season started the SCCA mandated restrictor plates on all Boss U code engines.

Mustangs ran away from competition the year before (2011) & did the same in the first 2 races of the 2012 season when the restrictor plate edict came down.

Of course the Mustang teams tuned for the TB mounted Restrictor plate, but after winning the first 2 races of the season, Mustang Teams did not win again in 2012.

There were 12 races in this series and by mid season the restrictor plates were taking a toll on the Mustang Teams resulting in too many DNF’s due to overheating issues.

In fact, in August of 2012 at the Mid-Ohio race, every single Mustang Team staged a Restrictor Plate Protest by completing the first few laps then all Mustangs returned to pit row.

So be careful when using restrictor plates to limit HP.

Look at your engine dyno HP output by rev band, decide your balance of Peak HP & Torque, then adjust rev limiters, timing & fuel maps. That would be my first thoughts.

Good luck.
 

bob

TMO Intermediate
94
59
sfo
In scca we use them all the time as "flat plane restrictor". In fact being lazy you can Rob's buddies at pheonix performance and they will send one to you in any size you want. You can ghetto them at home with sheet aluminum a hole saw, file, and patience but at 100 bucks it isn't worth my time when someone else has the patterns and can whip them out.

If you have to use one the race organization will tell you where to put it and what size to be legal. They are not an issue you just tune to them. A smart guy can do the math and predict horsepower thyen you have to tweek up or down to nail your number. In SCCA other guys have done the work and we are just order what we must use to be legal. We can have multiple tunes matching different restrictors to plug into our ecu if we switch classes say SCCA T2 and SCCA T1-LP and want to run 1 car in multiple classes.
 

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
1,830
297
Northeast
Why would the restrictor plates cause them to overheat?

I'd have to set the limiter at 4800rpm, I don't think that's a viable option.
View attachment 56629
Here is the history of the SCCA GTS World Challenge Series that effectively ended the Boss 302 GTS program as they were rendered uncompetitive in class.

Also documents engine overheating resultant from the mandated restrictor plate.

Today in SCCA World Challenge GTS competition, the Ford Mustang Boss 302 contingency staged a protest against the series by parking their cars in pit lane after the initial formation lap. The reason for the protest was actually two-fold. The primary reason for the protest was driven by high engine temperatures caused by the very small restrictor the Boss 302 GTS Mustang is required to run to reduce the horsepower of the Mustang. This causes unusually high engine temperatures and all Mustangs were experiencing high engine coolant and oil temperatures. Two Mustangs failed to finish due to engine related issues. Some engine oil temperatures were in excess of 310°F which is quite a bit higher than normal. Engine coolant temperatures ranged from 240°F to 280°F. This is significantly higher than normal operating temperatures and puts the engine’s computer into protect mode. This demonstrates that the restrictor has a compounding effect on the Mustang’s ability to make horsepower and be competitive. The competitors collectively chose not to race their cars rather than risk further damage to the engines in their cars.

Source: http://www.mustangcars.info/2012/08...ainst-scca-world-challenge-rules-at-mid-ohio/
 

captdistraction

GrumpyRacer
1,650
1,121
Phoenix, Az
Here is the history of the SCCA GTS World Challenge Series that effectively ended the Boss 302 GTS program as they were rendered uncompetitive in class.

Also documents engine overheating resultant from the mandated restrictor plate.

Today in SCCA World Challenge GTS competition, the Ford Mustang Boss 302 contingency staged a protest against the series by parking their cars in pit lane after the initial formation lap. The reason for the protest was actually two-fold. The primary reason for the protest was driven by high engine temperatures caused by the very small restrictor the Boss 302 GTS Mustang is required to run to reduce the horsepower of the Mustang. This causes unusually high engine temperatures and all Mustangs were experiencing high engine coolant and oil temperatures. Two Mustangs failed to finish due to engine related issues. Some engine oil temperatures were in excess of 310°F which is quite a bit higher than normal. Engine coolant temperatures ranged from 240°F to 280°F. This is significantly higher than normal operating temperatures and puts the engine’s computer into protect mode. This demonstrates that the restrictor has a compounding effect on the Mustang’s ability to make horsepower and be competitive. The competitors collectively chose not to race their cars rather than risk further damage to the engines in their cars.

Source: http://www.mustangcars.info/2012/08...ainst-scca-world-challenge-rules-at-mid-ohio/
Was there to see it unfold during my stint with a Audi team. Some mustang teams walked out of the driver's briefing at COTA. Between BoP and feelings that they had to run the R car to be competitive when the 302S was promised to be the baseline car for the series, there was a lot of frustration.

Takeaway for the folks here is that the coyote engine doesn't like being corked up. I'd rather try to tune restriction into it than physically place a restrictor. I don't like running sound-limited tracks as that also is very detrimental to the car.
 

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
1,830
297
Northeast
I just don’t understand how the restrictor plate caused overheating. NASCAR has used them for years.
Honestly? I don’t totally understand it either but there are a few things to consider.

From the gist of the article, the restrictor plates were exceptionally small and as race teams would tune for Max HP. If the current rules kept the existing fuel systems in place for the sealed race engines they were most likely running lean at high rpms. Which can send cylinder temps thru the roof, so to speak.

We see they mentioned Oil Temps at 310 degrees & Coolant temps at 240-260 degrees. Which answers many questions I’ve seen here as to what are safe oil & coolant temps for our cars on track. Coolant 205-220 / Oil 240-260 max was always my go to range / max target temps.

More about overheating from the article.

“The teams agree the use of inlet restrictors are the direct cause of the extensive engine overheating issues leading to eventual failures.”

“In consideration of the high replacement costs of a new Boss 302 engine and the replacement of failed components due to the high heat stresses being applied to these engines, and the resultant safety issues stemming from exhaust fumes entering the car, it is agreed SCCA must stop its parity by restricting the Fords.”

When you think about the what if SCCA didn’t limit the Boss 302S after the second race in the 2012 season, I would venture to say that Ford’s Boss 302 Racing Program was on course to be one of the most successful race programs in Ford Racing history. (Excluding the 60’s GT40 Race Program).

But I might be a bit bias cause I kinda really like my ole 2012 Mustang Boss 302 with its most unique ‘U’ Code Road Runner Race Engine. Could you tell? Haha, enjoy.
 

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
1,830
297
Northeast
Between BoP and feelings that they had to run the R car to be competitive when the 302S was promised to be the baseline car for the series, there was a lot of frustration.
Captian:

You are so right the Boss 302S was the Baseline of the GTS World Challenge Series because it was an affordable ready to Race Car offered by Ford Racing. It had a part number for the VIN. And the older FR500(S) was even less expensive and was still competitive in the series, taking a few podium finishes.

Reverting back to the Boss 302 R (the endurance car) was really not a cost viable option as MultiMatic charged a small fortune for one. (Even in Canadian Dollars!). Plus under the SCCA edict, it was just as corked up as the Boss 302 engine. The rules change was Mustang specific which is why Mustangs went from dominating the Podiums to a best finish of 10th place post the rules change. Hummm.. just imagine what should have been.
 

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
1,830
297
Northeast
Did the addition of the restrictor allow for tuning? Were the teams stuck with the turn they started the season with?
I never found the answer to that question, but I do know the series ran ‘Sealed’ race prepped engines. It’s is entirely possible SCCA did not allow engine control systems (PCM) tampering i.e. tuning.

As for the OP’s question: “Restrictor Plate Tuning Experience”. This is the only occasion I could think of where a restrictor plate was employed.

Personally? I’d take the weight penalty & shave weight if possible, Tune her for unrestricted max HP & move up a class or 2. I just never got better at anything competitive playing with those I could beat, on or off track.

What are your thoughts on de-Tuning?
 

bob

TMO Intermediate
94
59
sfo
I bet the problem was illuded to above. Lean mixture for max power increases heat. Trying to get max HP due to the rules change put motor longevity at risk and it still was not enough so that car dissappeared from the series. IMO you can always tune for motor longevity.
 

captdistraction

GrumpyRacer
1,650
1,121
Phoenix, Az
I never found the answer to that question, but I do know the series ran ‘Sealed’ race prepped engines. It’s is entirely possible SCCA did not allow engine control systems (PCM) tampering i.e. tuning.
In our Audi, we could adjust the tune, but boost values were restricted to stock. For the 302's, Ford Performance (then FR) provided updated calibrations to go with the restrictor plates (though wasn't always an immediate deal), but adding the plates themselves with no tuning changes resulted in the header/manifold and valve failures that led to all the uppity (not to mention some of the engines were the very spendy R/Y units that some teams ran over the boss 302R crates - they did make a bit more power from the blueprinting work and were homologated). I have a copy of the baseline rules, but not the VTS sheets for mustang, which is what PWC/SCCA used to show the specific to car restrictions.

I think you can make a plate tune work, but I'd recommend an approach that maximized torque/horsepower under the limit. With NASA, before we did an averaged HP calculation, "flat plate tunes" that limited the throttle body and timing to create a flat profile on a dynosheet were real popular.
 

bob

TMO Intermediate
94
59
sfo
adding the plates themselves with no tuning changes resulted in the header/manifold and valve failures
How does this happen with rich fuel mixture? Lean misfire, knocking, and excessive heat come with lean mistures. All a fat mixture can do is carbon build-up but that does not cause engine failure just dirty motors to de-carbon. What am I missing?
 

PatientZero

TMO Advanced
115
53
Kansas City, Missouri
Autocross
10-20 Years
I never found the answer to that question, but I do know the series ran ‘Sealed’ race prepped engines. It’s is entirely possible SCCA did not allow engine control systems (PCM) tampering i.e. tuning.

As for the OP’s question: “Restrictor Plate Tuning Experience”. This is the only occasion I could think of where a restrictor plate was employed.

Personally? I’d take the weight penalty & shave weight if possible, Tune her for unrestricted max HP & move up a class or 2. I just never got better at anything competitive playing with those I could beat, on or off track.

What are your thoughts on de-Tuning?
For the class I want to run only 150lbs of ballast is allowed. I would still need to lose 50hp. That's a little bit more than I can do in the tune alone. I'm considering going to a smaller camshaft, I just don't know how well that will play with the ported heads. Most likely it would be fine and it's probably the best option. Right now I'd be in unlimited class, I want to go down at least one class until I get more experience. Plus, alot more people are in that class to keep it competitive. On the flip side of this, I just talked to a guy in the Camaro Mustang Challenge series and he said all the LS1 Camaros run restrictor plates with no issues.
 

bob

TMO Intermediate
94
59
sfo
For the class I want to run only 150lbs of ballast is allowed. I would still need to lose 50hp. That's a little bit more than I can do in the tune alone.
I do not know how to tune but just started with HPTuners for the s550. I think if you need to prove HP on dyno like with NASA ST classes you can tweek throttle position and limit power. One of the NASA ST2 guys did that with ST2 to ST1 cross classing in the C5 vette and it worked fine. SCCA does not require dyno proof so put everyone in the same boat with listed engine mods and restrictor plates. So this same guy swapped plates for T1 and GT2 and used his HPT to download the corresponding tunes per restrictor plate.

That's what I'm doing next week getting 2 dyno tunes one for T1 no plate and T2 with plate and using my HPT to swap tunes as needed. Smarter guys double their costs doubling the number of tunes for 93 pumpgas for when the competition is light and 100 octane for the extra edge when $8/gal race fuel makes sense.
 

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