Aero or Carb problem

[av8rirl]

I have a problem that hopefully some of you (maybe enzo) has seen before. Car is a single seater with inboard carbs (top of trumpets are flush with opening in top of rear bodywork)

The car in front also has inboard carbs but they are completely enclosed under the rear bodywork (ie no opening in the rear bodywork)

When getting a tow from the car in front, it is not a problem to tuck in and pull in the distance. However, when you pull out to overtake, it is not possible to even pull alongside the other car.

When the roll is reversed and he tows up, he can make the move stick (ie he is able to pull out of the tow and pass)

I have learned from this:
1. that the aerodynamics from the car in front is not to blame (otherwise I would not be able to tow up)
2. it is not a problem with the aerodynamics of my car (as it was possible to make the move work with outboard carbs)

Is it possible that:
1. When I pull out of the clean/still air (ie from the tow), that the air that would normally be sitting directly above the carb trumpets is being blown past because I am not now in clean/still air? How would I measure this? It is worth investigating because it is impossible to get past on the straights.
2. He just has lots more top-end power?

If I put a small deflection ahead of the rear body opening, will the air velocity over the carb trumpets stay the same when in the tow and also out of the tow?

PS: I can drive around him into the corners, through the corners and out of the corners. I am not loosing time on corner exit. This is purely a problem on the (longish and important) straights. Attached is a photo of part of the rear bodywork. The carb trumpets are highlighted with the blue ellipse (don't mind the cover over the carb which is to prevent people from dropping stuff in).

All help greatly appreciated.

[Adam43]

The idea that he has more top end power is probably the simplest explanation:

Both of you experience a tow, but when you pull out and your car suddenly experiences the same drag as his and comes off worse. You can use the tow to get near him, but his can is inherently faster in a straight line. He can use the tow to aid catching you down the straight and his power advantage clinches the deal!

The air is likely to be cleaner when you are not in tow as his car will not dirty it! There is also likely to be a lot more of it and the carb should work better. I suppose it is possible to tune it better for one situation or the other. The little deflector may aid the situation, but the air flow over your car is always going to be affected in someway when following another car.

Could it also be possible that you had different gear ratios? Maybe his are better suited to that circuit?

Anyway these are just some quick thoughts...

[THR]

if ya running more wing than him youd be slower on the straight and quicker out the corner.
could be just that.
you could try not being in the tow. and seeing if you gain on him then. if he runs away with it, he has more power.
your carbs should be getting more air id have thought.. being not under the cover.
could be ya top gear drops the revs too much also.. like adam said.... are u on the limiter at the end of the longest straight?

[av8rirl]

More info:
No wings and fixed ratio's for all cars.

I am wondering if the velocity and quantity of the air passing over the trumpets is changing from being in tow and when pulling out of tow (picture what happens your head when in tow: it is not really affected by the air ahead as you are in tow. Now when you pull out of the tow: you head starts to get the shakes/buffetting because you are now facing into the wind/air).

I hope I'm making sense.

[enzo]

Looks like maybe an FV?

First thing I'd do if I were you would be to get a proper airbox, scoop, and filter on that thing. Use a K&N filter - the largest you can fit - to keep the restriction to a minimum.

A properly designed scoop will not increase drag at all - in fact, if you spend the time to really do it right, you most likely can reduce what I suspect is a lot of turbulance coming off of the engine cover.

[av8rirl]

enzo: Not far wrong on the car...

Do you mean to force/ram the air? I am interested in doing whatever needs to be done, but surely you cannot build a scoop without incurring a drag penalty?? If you could give me a pointer in this direction, I'd be greatful and I will go off and research and try to design/build a scoop.

Many Thanks.

[enzo]

The "secret" is to manage the airflow around the drivers head and shoulders, and then across the engine cover properly so as to create as little turbulance at the tailend of the engine cover as possible. The small increase in frontal area of a scoop is well worth it if you can get the overall drag down.

For a possible scoop location, look at the latest Riley & Scott sports car, where the scoop is over the drivers head, IN LINE with the roll hoop. Study also the treatment of the air across the cockpit and across the drivers shoulders of the F1, CART, IRL, etc, cars. Also, a good site to look at is www.mulsannescorner.com


Note also the treatment of air coming out from UNDER the car - that's actually your area of largest potential gain, with the sides being next. BUT, note especially that all of that is for naught if the front end treatment is not good.

You can easily study the airflow patterns across the bodywork by the old yarn tuft method. Temporary mockup of different shapes can easily be done with cadboard, styrofoam, and duct tape. Take video of the car at about 40-60 mph, or 35mm stills if you don't have a video camera. We used to just run cars around our building on Sundays (when the rest of the tennants weren't there to complain!), and found that we could learn a LOT in just a few hours for extremely little cost.

[av8rirl]

enzo: Just so that we are talking about the same thing, what do you mean by scoop? I originally took it to mean a way of getting the air to the carb, but now I think that you are talking about something different. Thx.

[enzo]

No - a scoop is just a duct that feeds air to the carb.

[Griff]

I think Adam is on the right line, the guy in front
just has more power ! looking at the photo the carb
couldn't be in a worse place'you haven't said if the problem apeared when you cut the hole in the rear cover,
because I feel it would probably be better if it was inside!!!! The carb is drawing its air from a curved
(low pressure) area, air accelerates over the curved surface
of the engine cover lowering it's pressure, IE Less pressure Less air Less power.
As Enzo sugests a well designed and positiond air scoop
should cure the problem,if your reg's don't allow external
scoop's use a well positioned and correctly shaped Naca duct
Hope this help's
GRIFF.

[av8rirl]

Welcome along Griff. Thanks for the good feedback. Hopefully, you'll stick around for a while.

[Griff]

Thank's I dont have a lot to say because my typing
is to slow !!!! but allways keep a eye on the tech and racers forum,s hope I was of some help.
GRIFF.