What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

RV7a Ram air intake or not?

Hi all I'm a newbie on here and a newbie to the RV world.
I'm looking to purchase an rv7 or rv7a, I have noticed that some have the ram air intake and some do not.
In the case of one aircraft an RV7a, it has the io 360 fuel injected M1B with the horizontal induction. I'm told it doesn't need the ram air intake
Is this correct?
Are there any disadvantages with this type of set up?
My flights will be 4 to 7 hours long in north west Queensland area
your advice would be greatly appreciated :)
 
You must have some wicked thermals there to keep a -7 up for seven hours; to say nothing of your bladder. :)
 
Ram air is thought to provide more manifold pressure versus the snorkel but it may depend on having the cold-air sump from Superior.
 
Aesthetics do it for me. I think the smooth cowl looks so much better I don't care about any performance difference - if there is any.
 
Here is a thread that Ram Air Measurement was measured and it was greater.

http://www.vansairforce.com/community/showthread.php?t=136615&highlight=ram+air

I remember a TEST done on an RV-7 with a STOCK 200 HP Angle Valve Lycoming that had both Van's Snorkle and unfiltered Ram air that used a valve that did not allow the Ram air to escape out the snorkle. This was 10 years ago in SoCAL but from what I remember, there was more manifold pressure with the RAM air.

IIRC, the increase over the snorkle may not be worth the effort of the extra work required if one is not going to be racing.
 
Here is a thread that Ram Air Measurement was measured and it was greater.

http://www.vansairforce.com/community/showthread.php?t=136615&highlight=ram+air

Greater than what Gary? There's no comparison with a snorkel in that thread. Don merely reported 0.8 rise over local static pressure. If the available dynamic pressure was 1.16, then 0.8/1.16 = 69%. Plenty of folks equal that in the upper cowl inlet, where we feed a snorkel. They're both "ram air". Methinks the real restriction is the dinky filter.

I remember a TEST done on an RV-7 with a STOCK 200 HP Angle Valve Lycoming that had both Van's Snorkle and unfiltered Ram air that used a valve that did not allow the Ram air to escape out the snorkle. This was 10 years ago in SoCAL but from what I remember, there was more manifold pressure with the RAM air.

It was Dan C. IIRC, MP was about the same. That said, a really well done horizontal intake should be a little better. Look at the one Paulo Iscold made for Anequim, or what Dave Anders is running now on his RV-4.
 
There is an increase in manifold pressure going from a vans filtered snorkel to unfiltered ram air... about an inch. I have yet to see a non-filtered snorkel vs ram air comparison. K and N web site data says vans K and N is to small for a 360 so its possible that the gain is more from removing the air filter and less from the ram effect. I doubt it would make a difference in range. If you want more than 5 hours, you'll need to find one with larger or secondary tanks. They're out there, but not common. Good luck.
 
More than 1" over ambient at 200 mph

I can't compare to other setups but my 7A with Sam James cowl and air box, K&N filter, gives well over 1" greater than ambient at around 8000' DA and 200 mph.

My numbers are from many observations with instruments that were checked during my IFR instrument checks (the MP check was extra).

I have a Catto 3 blade prop and it comes very close to the air intake as the blade goes by. I cannot say if this is a factor or not. I do know that the increase in MP at a given altitude is gradual as the speed picks up even with pretty much equal RPM.

I would expect that the location of the snorkel on the radius of the blade, the 3 blade vs. a 2 blade would be factors.

My engine has vertical intake so the air still has to turn 90 degrees to get into the FI but the intake and air box are horizontal.
 
Range

Well, don't count on a low-drag intake to increase your range much but it could help a little. That's not from increased MP, just lower pumping losses.

Assume a 42 gallon startup with maybe 40 by the time you reach altitude. Assume you need 7.5 hours so as to have .5 hours reserve.

So you would need to cruise at 5.3 gph (US gallons). On my airplane that will get you about 130 knots, maybe more. The problem is that the engine is not at its sweet spot.

For example, the best SFC for my Superior 360 is about 8 gph. The best way to go for maximum distance is probably to climb to where 8 gph is wide open. But you need to take wind into account, of course.

Next best is probably to climb so high that 5.3 gph is wide open. I don't know how high that is but it's pretty high! That way you get minimum pumping losses, can lean to best SFC and take advantage of higher TAS/GS from the thin air. How fast is 130 IAS kts at 12,000'? About 156 kts.
 
Last edited:
Terry,
I probably fly as high as most people on long hops. I have 32 gals capacity in my RV4 and i can get 900+ miles with minimum vfr reserve at 17500'.
The direct ram air inlet is good because it allows you to get higher MAP at a higher altitudes (if it's well done) so you'll be able to use that to your advantage. You'll be able get the increased TAS at higher altitude at the same the same fuel flow that you used at the lower altitude. The dual EI and EFI would allow you to get every mile you can get out of the gph. 10.5 to 1 compression would help a bit too. It will be very high and you will be on oxygen. I can get to 42 or so mpg under the best conditions but it's easy to count on 32 or so any time.
It's actually more to do with mpg then a 7 hr flight.
 
Back
Top