Engine Air Supply

[Schmidty721]

Somebody put a mechanical boost/vacuum gauge on their ski with the tube connected to the engine bay. Block off the hood vents and lets get some actual data!

haha, there is not factual data around here! It's all theory and "mine works better than yours" principle

Joking aside, I would be curious to see how many setups are pulling a vacuum in the bilge. It's really quite impressive if you think about the amount of air these engines actually require.

for example: A 1000cc engine running at 6000 RPM and a conservative VE of 50% will require 50,000 cc or 3051.18 Cubic Inches or 1.765 cubic feet or air per Second!

 

[DAG]

For what it's worth I have a 1200 in a Footrocket with a Titan pole and no hood breathers and it seems to do fine.

I did not realize the footrockets hulls don't have hood vents. I know a few guys, Frosty, Stanton, Erok, hell even Nick Barton himself all running 1000cc or larger motors in a foot rocket.. I personal rode Frosty's and it was the nicest hitting dasa 1000 I've rode

 

[JT_Freeride]

Nicks rocking a 1200

 

[speedysprocket]

My 950 Foot Rocket put Frosty's 1000 to shame at Daytona...
But yes, only pole breathers for Foot Rockets.

 

[icecoled007]

Doesn't RRP make some hood scoops? Why don't you install those to compensate for loss of air

 

[speedysprocket]

I thought about making a hood liner and cutting vent holes, but why? It works as-is and I don't want to cut holes in my pretty hood that I doubt I could replace quickly if I messed it up.
It breaths enough to flat water backflip. I don't feel like I'm missing anything now that I upgraded to RRP...

 

[DAG]

Perhaps its just a matter of flow rate. If you are running a XMW pole with a 3 cu in pole intake opening your air is traveling into the engine compartment 3 times as fast relative to running a RRP pole with 8.38 cu in opening...???

I'm sure the opening matters as some point, someone stuff a rag down one of the tubes and see if it runs any different.

 

[egbrig]

It's incredible how much air a twin two stroke requires. On my flow bench, I've played with with various intakes, flame arrestors, among my cylinder tests. What I always found funny is that people will put a large cc engine in a hull and figure that two tubes of roughly 2.5 inches will flow enough air to feed two 48mm carbs. NOT even close!!! You lose flow (a large amount) the longer the the tube is. If you want to see the best attemts to feed air to these hungry engines, look at the red Brawler (Brian's) with the 1105 or a KDX/DVX. There may be a couple more but those pop to mind. The next thing is those silly oem flame arrestors. They restrict flow by 31.8%. Yes, they keep out water but they keep out air too, and stand less of chance when feed by a hand pole with cables running down the tubes. So all this talk of power and yet we deny them the very thing that makes power. When things settle down I want to get the flow bench updated and running again. Its so much fun to put things to the test only to find - things are not what they seem. All tests performed @ 28" water.

 

[Schmidty721]

I would not have guessed the OEM arrestors restrict by 31.8%, that is a massive loss! What engine displacement & carbs was that test on?

 

[egbrig]

And NO, an engine is NOT "just an air pump". It's a devise used to get the most energy out of the fuel we mix with air.

 

[egbrig]

I used a 48 Novi as a test base.

 

[Schmidty721]

Is the 31% the difference between no F/A and an OEM F/A? Do you recall what sort of CFM you were seeing through a single 48?

 

[Quinc]

There is an old js550 racing picture where the ski had an air scoop on the pole.

 

[DAG]

Great info Erik, thanks for sharing!!

 

[Big Kahuna]

Tricky is runing an XMW1400 in his Chan with an XMW pole.

 

[DAG]

Who has a picture of trickys hood lol? @tricky1

 

[zimmy]

I used a 48 Novi as a test base.

Erik, can you bench test the RRP and X Metal poles (and maybe any others out there)?

 

[ridetrp]

Definitely saw hood vents too.say what you want but you absolutely need hood.vents to get ultimate power. First hull I ever built had a bone stock sxr engine in it has.the hood had no vents only.vented through rrp.the seal on hood was so tight with hull it would flex took me a few runs to figure out the issue when the bilge fittin started whistlin I knew it needed more air. Night and day difference after I installed 2 hood vents

Same happened to nuklear on his aquabot

Sent from my SCH-I605 using Tapatalk 2

 

[jo5hi3]

Definitely saw hood vents too.say what you want but you absolutely need hood.vents to get ultimate power. First hull I ever built had a bone stock sxr engine in it has.the hood had no vents only.vented through rrp.the seal on hood was so tight with hull it would flex took me a few runs to figure out the issue when the bilge fittin started whistlin I knew it needed more air. Night and day difference after I installed 2 hood vents

Same happened to nuklear on his aquabot

Sent from my SCH-I605 using Tapatalk 2

is that why his versa plug balls were getting stuck???

 

[kevbo]

If you look carefully you will see the flat sides of the hull move inward slightly when the throttle is blipped. It will do this even with large hood vents. The hull inlet does not need to be as large as the carbs because intake pulses get averaged. I you are getting a lot of oil canning and your main jets are unusually small, make the inlet bigger.