ZCar Forum banner

Status
Not open for further replies.
1 - 8 of 8 Posts

·
Registered
Joined
·
465 Posts
Discussion Starter #1
could you please please please post your thoughts on the cowl induction post below. i was also wondering if you could give me more details on how to do the pressure testing to find optimum placement. you mentioned a cheap do it yourself method but i couldn't understand how to set it up. thanks a lot and feel free to e-mail me the instructions if thats easier.
 

·
Premium Member
Joined
·
40,978 Posts
I did.

Just a while ago.
I use a Magnahelic gauge. They are available from McMaster-Carr or Grainger (they both have online sites...)
The range I use is 0-22 inches of water.
Basically you get some nagalene tubing and make a static pressure probe.

There are two kinds of pressure, static and dynamic. Ram air uses dynamic, cowl induction uses static. Dynamic air pressure is like taking the tube while connected to the gauge and driving along with the tube pointing straight ahead. It would register some ungodly pressure, but that's not a "real" pressure. It's not pressure that's ready to do work for you.
You could notice the smae phenomenon if you improperly ported the turbo boost sensor line.

What you want is a static pressure reading. That is, a reading that is not affected by velocity. What I use is a spraycan plastic top. I cut some holes on the downstream side (windshield side) and some more on the inner cap, and put the tubing in there to read the pressure. What you end up reading is the pressure being built up from the dynamic pressure being turned into static pressure by the windshield (acting as a diffuser).

You can get similar results from placing the tube backwards, but then you create a venturi effect, and end up reading a much less pressure that what's really available (in some cases, if the speed is great enough, you will actually read a negative pressure--vacuum--due to this effect)

What really is advantageous is to hook that gauge to the upstream side of the AFM to measure what positive pressure is available there. Using a compound Magnahelic (reads both vacuum and pressure) you can do an audit of your intake tract up to the throttle body. Anything betweent the intake filter and throttle body that causes a restriction can be altered and the results quantified with this gauge. That's how they design those cold-air packages, they actually test what the restriction is. Many times the drop isn inlet restriction is responsible for as much if not more HP as the cold-air part of the equation!

Hopefully that's a bit better explanation. I have to go right now, but can reply sometime later this weekend (going out of town tomorrow for the rest of hte week) and possibly snap a shot ofr two of my static pressure probe device so you can actually SEE what I'm talking about!
 

·
Registered
Joined
·
307 Posts
SU Question

Someone posted that pressurizing SUs messes up the metering if I remember correctly. Do you have any knowledge of this? If so are there ways to compensate for the range of variables that you can point to.

Thanks in advance,

RDB
 

·
Registered
Joined
·
241 Posts
Re: SU Question

I don't know anything about the su's but if they have vents like the carbs i'm used to, I think it would'nt make a difference if you have the vents hooked to the same pipe that brings the slightly pressurized air in, so nothing will be out of balace??? Just a thought, but in theory.. or mabe the problem with metering comes from the fact that the air is denser... That would be alot of testing to sort through!!

Dylan
 

·
Premium Member
Joined
·
40,978 Posts
Pressurization

The SU has a dome vent, and my understanding of proper aircleaner design has the dome vent in the same pressure zone as the inlet horn. This being the case, there is a chap in the UK who has a blow-through turbo application for SU's that's run for some time, and I thought that was impossible with SU's until I realized that differential pressure is differential pressure, regardless of the pressure that it occurs at...
Besides at most, with a cowl induction system, you are looking at only 20" of water pressure probably as a maximum, and I don't see this as enough to cause any "metering problems", especially if the stock venting of the carbs is retained. The argument of "metering problems" was from a theoretical concern and nobody has been able to proffer any quantifiable evidence to support it, so until then I'd press on and see what comes...
 

·
Registered
Joined
·
957 Posts
Re: Pressurization

okay this might sound too easy to be logical but if you had problems with to much pressure to the SU couldnt you drop like a couple of boltss onto the top of the piston to slow down it's rise? And would'nt it also be logical so that you could slow down the rate of fuewl flow while increasing the air flow for better atomization??
What do you think Tony???sound somewhat right?

Z ya later
Mike K
 

·
Premium Member
Joined
·
40,978 Posts
Pressure Differential

Teh dome riases based on pressure differential. As long as the stock airbox venting is retained, the operating pressure (ambient) should not affect the operation of the dome. This includes pressurizing the float bowl through the ventline also, so the static pressure in the float chamber was correct.
If you didn't do that, the fuel level may be too low, and cause metering provblems.

I mean, at 22" of water boost, that's like 11" of mercury, and that equates to about 5PSI! In such a situation, theoretically you would have to utilize a pressure-biases fuel pressure regulator to keep fuel pressure above the incoming air pressure.

As long as your vents are intact, you should be O.K.
Bolts not required. Besides, you CAN buy different springs to change the opening rate, but that would LEANOUT the mixture under load, instead of enrichenit...
 
1 - 8 of 8 Posts
Status
Not open for further replies.
Top