Air horns how much performance

[mayitin]

Last time I asked a question about the intake manifolds 240 vs 260 and I did not get a clear answer. My question now is air horns. The 240 air filter assembly comes with 2 air horns, well a lot of us took that out and put on K&N or WEIAND air filters to get better performance... did we really? has anybody tested the difference? performance economy etc. I have K&N filter now but MSA has air horns and claims performance gains, so I am wondering if I would get better top end or mid range performance one or two horses but I would use the old filter assembly.:stupid:

[Arne]

I don't know how much performance you've left on the table, but the need for air horns on this type of variable-venturi carb is a known characteristic, not just on Datsun SUs, but on similar carbs on English cars as well. These cars meter the fuel dependent on the air flow through the carb. If the air flow isn't smooth, the carbs can't do their best work.

[Bruce Palmer]

The air horn question you raise is going to be somewhat intuitive in it's answer, just like the manifold question...... Look in one and then the other and tell your self that one manifold might not flow a little better.

Now the same thing holds with the airhorn question. When you look at a naked SU the first thing that jumps out at you is a 90 degree edge at the opening. The air is going to arrive from the side and have to break over that edge creating, as Arne said, tumbling air moving by where the fuel metering takes place.

Now place something on the face of that carb to roll that airflow across a curving surface and ask yourself which may be the better way to go. If you want something minimal to do that rolling effect, check www.aptfast.com for their 1/2" stub stacks which I run on my 46s and like them a lot. They also have several other stack offerings as well.

[Bcalvosa]

When air flows into a square edged tube, the edge creates a large disturbance or boundary layer that reduces the effective diameter of the tube by a significant percentage (depends on tube diameter, edge profile, fluid viscosity, and velocity). For a Z carb, assume about 20% at full throttle. A radiused opening (aka venturi, air horn, snub stack) smooths the air entry, reduces the boundary layer, and allows a greater percentage of the actual carb diameter to be used. The difference is significant enough, that one should always provide a smooth radiused entry in the direction of flow (be it intake, exhaust gasses, coolant, etc.). An optional secondary benefit is the ability to use different length venturi tubes to extend the overall manifold length to tune the intake to a lower RPM.

[Rising Sun]

It has been proven on the dyno that you will Lose hp when you remove the stock aircleaner .Hp goes back up using only the backing plate .The velocity stacks in side are the reason .This is why several people that Race SCCA IT cars will use the stock aircleaner .Even use it as an airbox and duct air from the core support directly to it .I have a few customers that ran that setup for years .When compared to individual K & N's ,Weiands and Ram Flows with No Stacks that are also using under hood heated air for induction ,yet another power loser for your engine . Stacks inside the K&N 's will work but under hood air temp for induction will be to the detriment .Consider non stock off the shelf items "All show and No go !" A properly designed and built airbox with velocity stacks and ducted air from in front of the radiator using a large K&N is the best and used by the racers in the know .Coupled with good exhaust manifold heat shielding and proper intake air management ,your performance will noticeably increase .Good luck,

Steve

www.risingsunperformance.com

"The Z car Experts " and the Oldest Datsun /Nissan Specialty and Z car shop in the state of Texas

[mayitin]

Thank you guys its very interesting your answers. Steve thank you for your information, I always wonder if anybody did a dyno test. well I do not race but I will sure go back to the original set up. Thanks again

[dspillman]

www.aptfast.com ... I have not used this product, but close.....

I agree with post #4....

This type radiused horn or something similar is worth whatever you have to pay......stock horns are better then no horns.... will make a difference.

David

[sleepyzzz]

i have posted this before, but can't remember if it was this site, or hybrid z.

i found an old filter box, incomplete, and took the back part, with the air horns, and cut them out to fit inside my K & N filters.

very easy to do, using a jig saw or saws all, and then smoothing them out with a bench grinder. i am sure several of you dont care to take a hard to find part and cut it up, but as i said, it wasn't complete, and i prefered to go with this set up instead of keeping the car original.