56mm hole - how much horsepower?

Ron Earp

Admin
For those of you that race SCCA you may well know that the E36 BMWs are perceived to be overdogs in the ITS class. Recently, the SCCA comp board ruled that the BMWs would be required to run a 56mm restrictor plate.

I don't know if this is a problem or not. I mean, Ford used 55mm (and smaller) mass air flow sensors on 5.0L Mustangs back in the day and some of these would make 250 hp to the wheels with stock bits.

So, those SAE types on the board, got any quick calculations that will show how much hp this can suppport? Of course, this will be RPM dependant I think, so, max at various RPMs might be the question with a 2.5L motor (inline 6).

Doesn't NASCAR use some small restrictors but still produce some serious hp?

Thanks,
Ron
 
Seems to me I remember back in the days when Bill Elliott was cleaning everyone's clock with his Ford NASCAR put in the restrictor plate rule so that the GM cars would have a better shot at avoiding being lapped by him. Eventually they used the restrictor plate for other purposes, but I think it started when the Fords just became too fast for the Chevys. At least that's the way I remember it. Best regards, Orin Meyer
 
I have heard that the cylinder heads ported for restrictor plate racing are so weird that they don't work very well if you take off the restrictor. They also use sky high compression ratio's and cams that are only limited by the state of valve spring technology.

If you can internally modify the engine, it shouldn't hurt that much, but if it has to be stock, that's a handicap.

Also, you have to look at the rules for the restrictor. If you can make it essentially a velocity stack, it will flow a lot of air. If it is just a 56mm hole punched in a piece of sheet metal and inserted somewhere in your intake, it could be a major restriction.
 

Keith

Moderator
Blue Oval is correct. I was at Talledega in 1987 when the Fords cleaned up (as they did at all super speedways) which owed much to the Cleveland style heads and their huge flow capacities. The following year, they were all bunched up and strangled by the restrictors. However, I believe they make the same kind of hp with restrictors, it just takes much longer to get there! /ubbthreads/images/graemlins/frown.gif
 

Ron Earp

Admin
Anyone have any real flow calculations? There must be a limit on air that can be pulled through a 56mm hole with a 10:1 compression 2.5 liter motor, cam that pulls decent vaccuum (stock E36 cam from BMW), regardless of cylinder heads. I'm looking for a max amount of air, and thus, horsepower. My guess is the max amount of air is pretty darn high and supports a lot of hp and this restriction isn't a big deal. Plate is 0.06", no radiusing of any kind. Where are our engineer fellows on the board?
 

Fran Hall RCR

GT40s Sponsor
Depends where the restriction is placed,
If it is a restriction in the throttle body or into the engine period.
If its a restriction into the engine then the way around it is to use a huge plenum that the engine can draw from and hence restriction/airflow losses are minimised.
Have a look at any of the current GTS cars and you will see that they have a tiny restrictor (32mm) per bank.So 64 mm total.There is also a very low rev limit.
These engines still make approx.600hp from a low revving V8
.
So I dont think that a 56 mm restriction is that big of a deal if its approached correctly....
 

Adam C.

GT40s Sponsor
Supporter
[ QUOTE ]
Have a look at any of the current GTS cars and you will see that they have a tiny restrictor (32mm) per bank.So 64 mm total.

[/ QUOTE ]

You can't just add the diameters like that when talking about flow. The equivalent single orifice would actually be 45 mm.


[ QUOTE ]
Anyone have any real flow calculations?

[/ QUOTE ]

I can tell you that the MAXIMUM possible flow through
a 56mm orifice is 1025 SCFM. Calculations to follow around lunch time.
 

Fran Hall RCR

GT40s Sponsor
Adam.
I wasn't just adding numbers .
I was refering to two individual restrictors ,one per bank.
I mistated myself
I agree that the combined would be 45 mm if it were a single orifice but that was not what I meant to say. /ubbthreads/images/graemlins/blush.gif
 

Fran Hall RCR

GT40s Sponsor
Adam.
where did you get the 1025SCFM number.
Is this a theoretical number on a sharp edged orifice or idea entry and ideal exit and was this number at sonic ?

Sounds a little high....
In a real world scenario (an engine) there would obviously be some reversion and at which point the airflow will physical stop so big numbers look good but are unrealistic...

Just playing Devils advocate here /ubbthreads/images/graemlins/wink.gif /ubbthreads/images/graemlins/wink.gif
 

Adam C.

GT40s Sponsor
Supporter
Fran,

No worries, you are correct, this is based on choked flow at the nozzle throat. That is why I said it is the maximum possible. I'm not suggesting that the engine will generate the pressure drop necessary (1/2 an atmosphere) to get there. Ron just wanted a back of the envelope type calculation, and this easily sets the upper limit of what is possible in terms of airflow. HP scales linearly with airflow, so I thought it would be good info to have.

As for flow "reversion", if the orifice is attached to the inlet of the plenum and there are more than 4 cylinders there is negligible pressure (and therefore velocity) fluctuations at the orifice. If there are 4 cylinders or less, then there is a significant pressure oscillation in the plenum. This would mean (based on the choked flow assumption) that velocity would oscillate between sonic and something less than sonic, so the mean flow rate would be something less than 1025 SCFM, but we already know that number is not realistic.

So Ron,

How much HP is possible? That is a more difficult question as pumping losses become a significant factor with restricted engines. We can estimate it, but it would take a few more pages of calculations. We would need to assume some maximum manifold vacuum, estimate the losses of the orifice to find the flow rate, estimate the gross indicated power, and then subtract the pumping losses to get a net indicated number. It can be done but the efforts will be lost on most people. How far do you want to go with it?
 

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