Clearance above stacks

neilca · 06-29-2009, 10:19 AM

I have been working on this car for the past 11 years. Finally made it to the track. One issue I have is the engine feels flat. One source I am investigating is in the watershields.

The car has been lowered so much I had to raise the engine to get a better angle on the half shafts. This brought the carbs in very close proximity to the engine/trunk lid. I used the stacks and watershields from a 911 with AC. That is the filters are shorter and the stacks are shorter. The clearance between the stack and the watershield top is 1 inch. Is this enough clearance for a 2.0 engine? Can this be choking off my engine? I am running Zenith's with 32 mm chokes. Here is a pic

fredmeister · fredmeister's Garage

possibly, why not take a run with the shields off and see what happens, there is a cloud that forms from intake reversion above the velocity stacks, but hard to say for sure.

neilca · 06-29-2009, 12:39 PM

I was going to do that next time at the track. I was hoping some one would have a rule of thumb about this area.

wolffie911 · 06-29-2009, 05:59 PM

I did some research on this several years ago for my race car and as I recall a reputable source (and I can't remember who it was) said that the clearance above the air horn had to be 1.5 to 2 times the diameter of the air horn. I'm sure that the 1" you have isn't going to cut it.
BTW, nice car!

barney911rs · barney911rs's Garage

Do what we did on mine. Use the top of the water shields as a template and cut out two flat pieces of thin sheet metal or aluminum to use to cover just the top of the air cleaners.

neilca · 06-30-2009, 09:03 AM

The tops are the things that are 1 inch from the stack. The sides have no influence. I can see from your picture your air cleaners are about 3 inches high. Mine is about 1 1/2 inch high.

I haven't haed time but I am going to calculate the heght using Fred's formula. It sounds about right to enclose the reversion.

Thanks,

neilca · 06-30-2009, 01:55 PM

Seems I should be 2.4 to 3.2 inches above the stack.

I was thinking of going to taller stacks say 4 inch and running the screens over the individual stacks. Anyone try that?

barney911rs · barney911rs's Garage

What I was getting at was my solution opens up the sides to get better air flow. I don't think the room on the top will matter that much. With the shorter filters, the sides of the water shield is blocking the air flow.

Are you talking about the height of the trumpets inside the filter area being to close to the top? Maybe I mis-understood what your problem was? Would not be the first time.

Do you have room to run the ITG foam filters? They let air in from the top, which might help your situation.

neilca · 07-01-2009, 01:26 PM

John,

Yes the problem is the distance between the stack and the top. The ITG filters is a good idea. I am going to look into it.

I am taking the car to a chassis dyno on Friday. I willl try with and without the aircleaners and see if we see anything. I will let you know.

Thanks,
neilca

barney911rs · barney911rs's Garage

If you have different length stacks, try that as well, it will make a difference. Get the most from the engine and then figure out how to make it work.

neilca · 07-04-2009, 05:34 AM

Well, the results from the dyno were quite unexpected. Seems the carbs are running very lean. Air fuel ratio's 16-18:1. We did one run without the cleaners and it got a little better.

New direction. I am running Zenith's with 32mm chokes. Stock jetting. We are going to go back to the 27mm chokes for the next test. I will also have some main jets drilled from a 115 to a 125 (0.047 to 0.052 inch) just in case we are still lean.

So the flatness of the engine appears to be a lean condition.

cstreit · cstreit's Garage

Does anyone have any real data to support raising the distance between the stacks and the top of the watersheild? (or at least any objective suppositions?) I run the tall horns and it's making me wonder...

911st · 07-05-2009, 04:16 PM

Quote:

Originally Posted by neilca

Well, the results from the dyno were quite unexpected. Seems the carbs are running very lean. Air fuel ratio's 16-18:1. We did one run without the cleaners and it got a little better.

If you are more lean with the air filters, wouldn't we want to conclude that the air flow is better with them on?

Maybe the factory "rain sheilds" did more than just keep rain out of the carbs?

J P Stein · 07-05-2009, 04:28 PM

Quote:

Originally Posted by cstreit

(or at least any objective suppositions?)

That's about all you'll get from me. A minimum of the same as the diameter of the stack would seen a good number. If you're running cams with a lot of overlap,
fuel reversion could enter the picture. I wouldn't like that bouncing off the bottom of a cover. Distrubing the reversion doesn't seem to be a good plan. I've read a little of that situation, one in fact....the reversion being blown away by the wind from motion and it was very detrimential.

Plavan · Plavan's Garage

I have been wanting to dump my watershield tops for some time. I look at them thinking the air has to travel different directions and make a couple of turns before getting into the carb. If I went to normal filter tops, would this increase "some" HP in my 2.5L?

cstreit · cstreit's Garage

Quote:

Originally Posted by J P Stein

That's about all you'll get from me. A minimum of the same as the diameter of the stack would seen a good number. If you're running cams with a lot of overlap,
fuel reversion could enter the picture. I wouldn't like that bouncing off the bottom of a cover. Distrubing the reversion doesn't seem to be a good plan. I've read a little of that situation, one in fact....the reversion being blown away by the wind from motion and it was very detrimential.

Sorry, to what end was the interuption of the reversion bad? Losing fuel-charged air and hence a random lean condition? Answer like I'm Forest Gump and I might get it.

J P Stein · 07-06-2009, 07:54 AM

From a story on the build of the "Pooper" to the best of my recollection.

Pete Lovely & crew were running it first time out on the track. They were having problems at speed with the engine running very rough. The carb was semi-exposed to the air flow.
They observed reversion from the carb in the pits while reving the motor.
The fashioned a stack from a paper cup that fit to the top of the carb. End of problem.

Personally, the only time I have observed reversion was on a 2 stroke dirt bike.
I has the air box off, fired the thing off and there is this neat airborne layer of fuel about 1 inch outside the carb.....moving around as I goosed the throttle.
They put that big airbox on there for a reason, I figure.

911st · 07-06-2009, 10:09 AM

There is a video of a fuel injected 3.9 here some where. You could see a cloud of fuel a foot above the stacks at full tilt on the dyno.

That must be what they mean by reversion.

neilca · 07-06-2009, 10:12 AM

We ran a Nissan A14 engine in GT-4 about 15 years ago. We had a very long duration cam and would see the reversion about 2 inches out side of the velocity stacks at 10,000 rpm. It looks like a cloud that just hangs near the inlet. We fashioned our airbox to allow for this.

I am running stock cams so I do not expect this to be an issue now that i have stopped to think about it.

On a side note there is a guy on ebay that is selling main jets for the Zenith. I just ordered 18. I have bought carbs from him in the past. Should be here in a couple of weeks.

neilca · 07-10-2009, 10:46 AM

Just got back from the dyno. Disappointing results. First the air cleaners made no difference. On or off the same numbers. The engine only put out 110 HP and had 110 ft-lbs of torque. Very flat torque curve but the engine signs off at 6000 rpm. It was able to pull 8000 rpm on the dyno, but wasn't making any horspower. Peak horsepower was at 6000 rpm.

The engine does sound good. I guess I will have to work the corners a bit more.