Q. Do you need trumpets when you run K&N filters on twin 40's?

[Wile7]

My car has run without trumpets since I've owned it. 12 years. My first seven (a 1600 gt) was also trumpet-less from memory...

I've just cleaned/oiled the filters and was browsing posts on K&N fitment and it seems that the advice is that you should have trumpets, as long as possible, within the K&N filter to leave 20mm behind the face plate or so, to aid the flow of smooth air into the Venturi thus improving torque.

Is this true? My 'common sense' hat on says that if you have enclosed K&N filters you already have a good air space/flow so why 'restrict' with trumpets? 

I've never experienced any issues but maybe I'm missing something....do I need to invest in some trumpets to improve performance on my 1700 xflow? If so, what length?

Answers here or on a post card please....

Dave

[Nigel Riches]

Hi Dave.

This is all down to airflow into the carb, if the air has to negotiate a hard 90 deg corner at the carb intake the airflow will become turbulent, think of it as trying to take a 90 deg bend at max chat, but if the air flow can be drawn in across a larger opening, ie the end of the bell mouth, which then directs the air in an orderly manner into the carb, the more laminar flow will aid getting more air to the cylinders, with more fuel, like taking a large radius bend at higher speed, if you follow, so the potential for an increase in performance is there, the trumpet length is a bit of a lottery, Jenvey do a variable length set up, which can be tuned to your engine and driving, however like most thing to do with intakes, trumpets are a bit of a compromise, longer for greater torque, due to the weight of air in the "tube" and shorter for higher engine speeds.

This is inter related to your cam profile also. 

Not sure what length trumpets on my 1660cc x flow, but around 60-70 mm, which with a 234 cam 40dcoe, 30mm chokes gave around 107ft/lb @4500 rpm, at 85mph, 103hp at 5700rpm, allegedly at 106 mph, on a chassis dyno, not a rolling road. does any of this sound like your engine?

Do a google search for  Carb intake trumpet lengths, lots of stuff out there, and as someone posted, it's a bit of a black art.

Have fun.

Nigel.  

[Wile7]

Nigel - you are a star! Thank you for that info. and very well explained.

My car is a 'standard' 1700 Supersprint on 40's. Not sure of the chokes...need to check, otherwise similar engine spec I think.

Will look up the 'black art' on Google but sounds like I need 50mm +/- for my needs given what you outline regarding torque and Revs.

I've just got the car running really well so will do research, purchase trumpets and then wait until they drop out of sync a bit (Webers), fit the trumpets and then get them 'tuned' up to suit. At least I think thats the right route.....

Black Art Googling starting now.

Thanks again Nigel....

Dave

 

[Smithy77]

http://www.emeraldm3d.com/articles/emr-adj-length-intake/

Good reading to explain the importance of intake trumpet length and effects on engine performance.

In my experience (K-series) it has a dramatic effect on the shape/position of the torque curve, making an engine peaky at the top end or have more bottom end grunt, smoothing out flat spots etc etc. Then with air filters, you just want to give as much space as possible around the trumpet mouths to make the filters as "invisible" as possible.

[abbot]

Not sure I have the correct calibration of the seat of my pants to pick up that difference.

 

[Jonathan Kay]

http://www.emeraldm3d.com/articles/emr-adj-length-intake/

Good reading to explain the importance of intake trumpet length and effects on engine performance.

Not convinced by some of the physics but the experiments and results are fascinating. Thanks for posting.

Jonathan