How do adapters and spacers affect airflow, and vacuum?

[Harte3]

While tinkering recently I removed one spacer from under my carb and got a 1.25 drop in manifold vacuum. I still have the adapter required to match my spread bore carb to the square bore manifold...all of which got me to thinking (dangerous) and making a few assumptions and ending up with more questions.

What affect does more or less spacer have, if any on air flow and velocity?
Some say spacers are power adders and others say power robbers...I would guess that...it depends.

This also led me to the question of what affect, if any there is when the throttle bores on a carb like a 390 cfm Holley (1-7/16) are setting over the bores of a square bore manifold that accomodates a carb like a 600 cfm Holley (1-11/16) What affect does the difference in the throttle bore diameter and the diameter of the bores on the manifold have on air flow and velocity?

In my own case my spread bore 450 cfm Holley has 1-3/8 primary and 1-7/16 secondary bores using an adapter the accomodates a Q-Jet with 2/1/4 secondary bores so the a/f mix from the carb is entering in to areas somewhat larger than the throttle bores of my carb. What affect, if any is this having on airflow and velocity? Is the affect similar to the difference between using a DP manifold and an open plenum manifold?

Which brings me around to how did I lose 1.25 of vacuum by removing the extra square bore spacer?

 

[addo]

I think that what you've experienced is an increase in local turbulence. It's then impeding the overall airflow.

 

[XPC66]

If the flow was impeded, the vacuum should increase. The manifold runner vacuum, afterall, is the pressure drop of everything upstream = runner, spacers, carby, etc.

 

[StrangeRanger]

Did the spacer you removed have 4 individual holes, one for each venturi, or one big square hole?

 

[Geezer 300]

The new Car Craft that just came out has an article on spacers, haven't read it yet but should have some info you can use. Alex.

 

[ludwig]

Hot Rod did a side by side dyno comparison a couple years ago of several intake mods and the single greatest gain was achieved by a spacer and nothing else. IIRC it was at least 2" or so.

 

[Harte3]

It's 4 hole adapter, spacer, and manifold.

I could not detect any difference in the running...extremely smooth from idle even as low as 500 rpm right on up.

The configuration of the spread/square bore adapter would most likely create turbulence but it's necessary for the carb I am running at this time...which I like quite well.

So then, if the addition of the one inch spacer increases the vacuum and I assume the velocity, possibly a two inch spacer would even increase the vacuum and thus the velocity even more. Vacuum is not bad...18.75 at 600 rpm with the spacer and 17.5 without it which holds steady even down to 500 rpm.

I am interested in the physics of what is going on here but I am not at all versed in the subject...All part of the fun of tinkering.

And I thank you gentlemen for your comments.

 

[66 Fastback]

I know that a spacer tends to hurt low end torque. The spacer adds additional air volume and mass and it takes a bit longer to get it moving into the cylinders. It can sometimes create a low end bog because there is lower air velocity. But a spacer will not increase velocity at idle. Remember that conditions of high engine manifold vacuum typically occur when the air velocity is low, because the plates are closed. Now venturi vacuum is the opposite. No flow, no vacuum. High flow high vacuum. But we rarely measure venturi vacuum. Drag cars with their high rise dual quad ram manifolds overcome the low end torque bog by leaving the line at high engine rpms and air velocity.

As I remember, spacers tend to increase horsepower on upper rpm range. I am not positive why, but I imagine that again it has to do with the increased volume and mass of the intake. At higher rpms's the throttle plates are open, and the air mass is flowing fast. It has gained momentum now and it does not want to stop. Hence, it will pack a cylinder and gain volumetric efficiency. At WOT, manifold vacuum is lower because there is a relatively unobstructed connection with the outside air.

Years ago I had a Rochester carb that needed rebuilding and I had an extra AFB on hand. So I replaced the Rochester w/ the AFB on a 283. The carb was a bit big for the engine and it required a spacer. There was a bog at low end, but it gave the engine more top end.

With regards to the 1.25" of vacuum loss, maybe you have a vacuum leak now that you did not have before or the throttle plates are not sealing as well at idle. Basically, spacers tend to degrade street drivability but enhance strip performance.
Doug

 

[Anonymous]

Yeah, I was wondering if it was a certainty that the vacuum drop was due to removing the spacer. That's the kind of thing that can make tuning frustrating!

But Doug, you have a different idea than I about the effect of making a plenum larger or smaller via spacers. I always thought that a larger plenum HELPED each cylinder get the initial gulp of air/fuel, sort of like drawing a quick breath from a paper bag rather than through a restrictive tube.

I also had an idea that a plenum, like a balance tube, tended to even out the mixture delivery between cylinders to some extent and dampen pulsations in the runners.

Adding a spacer is thought by some to give the mixture coming out of the carburetor throat a better shot at getting around that sharp, square edge in the manifold.

But these may be just notions, mine and/or others, and may be all wrong.

 

[66 Fastback]

Seattle Smitty,
I don't think we are far off on our views. It is just at off idle or lower rpm's that the larger plenum will not help the motor. There is that lag in air velocity as the valve opens. At idle, the air in the plemum is not moving much and is hesitant to move faster. Newtons laws at work with a body at rest wanting to remain at rest.

At higher rpm, the air mass is rushing through the carb and into the intake runners. Now it is more like a freight train that does not want to stop. Again Newtons laws regarding a body in motion wanting to stay in motion. So if there are any sharp bends etc, the more mass of air you have behind it pushing as it feels this friction, the better. With more air mass behind it, t can "force" more air around those restrictions in the runners etc. .
I agree, that it should work like a balance tube and help dampen the pulsations in the runners. There is more mass and volume to distribute the pulses so the effect is reduced and the momentum of the mass flow is not "dampened" as much. Therefore you get better cylinder fillage.

Pulsation dampeners on piston pumps are very similar. We put a volume pot on the suction and discharge of a pump. It has an air or nitrogen cushion in it such that the fluid is allowed to expand up into the additional volume. It helps keep the piping from vibrating and breaking welds.
Doug

 

[XPC66]

Four reasons I consider for spacers:

a convenient way to get extra tappings;
increase the distance between the thottle plates and the manifold turn for straighter flow, especially under part throttle;
thermal barrier to stop heat soak into the carby (keep the manifold hot for better vapourisation and reduced fuel condensation);
increase manifold header volume to cushion peturbations/runner_reversion and reduce parasitic draw between active and inactive runners.

 

[66 Fastback]

thermal barrier to stop heat soak into the carby

I stand corrected. Spacers can also help improve streetability if the they provide a thermal barrier and it prvents vapor lock of the carb.
Doug

 

[Harte3]

Great discussion!

Who would care to address the affect of small throttle bores mounted on larger spacer and/or manifold bores?

Here is my theory using water flow: Assume say x psi pressure at the tap flowing through a one inch garden hose at y gallons per minute. The flow of gpm does not change at the end of the hose but the velocity does. Add a nozzle to the end of the hose and the flow may not change much but the velocity can be increased due to the restriction of the nozzle. On the other hand, attach a two inch garden hose to the end of the 1 inch garden hose and the velocity is greatly reduced but the flow remains the same.

So, is this somewhat the same affect one gets when mounting the small throttle bore carb on a larger bore spacer and/or manifold? If so and if it is detrimental to velocity, would reducing the size of the spacer and/or manifold bores to closer approximate the size of the small carb bores be of any benefit such as maintaining velocity even though actual flow may be the same either way?

 

[66 Fastback]

It seems to me what your are setting up is a restrictor plate which will choke the flow.

In your analogy, the water hose is fed at a certain pressure dumping to atmospheric pressure. Restricting the end of the hose increases the friction and now some of the pressure used to push water through the hose is consumed and converted to heat. The flow will be reduced. But as you noted, when it exits the nozzle the pressure has been converted to increased velocity. But velocity and mass flow are not the same.

In a carburetor the maximum amount of pressure available to push air into the engine is atmospheric pressure. (Unless you use forced induction) The driving force is the pressure difference between the atmospheric pressure and the lower pressure in the cylinder and manifold. Placing a restriction in the manifold will increase the friction and less air will be able to flow through the carb due to the smaller difference in pressure.

There probably are benefits to having the openings in the plate match those of the throttle bores. Too small creates a restriction. Too large, and you create an area of additional turbulence and loose some of your ram tuning effect.

The difference with the garden hose and the carb systems is that the garden hose has quite a bit more energy available to move the fluid. When you start talking of fluid flow at 0 psig or vacuum conditions friction starts eating your lunch. At 100 psig, a 2" pipe can deliver quite a bit of volume. In order to move the same amount of gas at 1 psig would require much larger pipe. The gas is expanded and the velocities are much higher and as a result the friction is higher.
Doug

 

[XPC66]

Harte3, a few basics that might better help you understand flow characteristics:

Tp = Vp + Sp

Tp = total pressure. Vp = velocity pressure, Sp = static pressure.

Two main losses : Friction loss (given off as heat) and dynamic loss (flow separation)

Significant reynolds number limits: laminar, transitional and turbulant. When laminar there is negligable friction and dynamic losses, because the vast majority of the flow is axial (concentrated in the centre of the pipe). Very rare for any part of the intake side to have laminar flow under full load

Carby works on static pressure drop in favour of a rise in velocity pressure. Velocity pressure is related to the flow rate. To take advantage of air tending to the centre of the venturi and being less turbulant, an amplifier is commonly manufactured into the throat.

.

 

[Anonymous]

Carb spacers are easy to make. Maybe make some and test them, tell us about it.

Have you got a dyno-hill nearby? That's a long, fairly straight, fairly constant grade with minimal traffic at some point during the day or week. Use some marker at the bottom of the hill as your starting point. One usual way to test and tune is to roll past the starting point in second gear at some given low speed at the bottom of your usable rpm range. As you pass the marker, nail the throttle and start your stopwatch. Click off the watch as you reach your target rpm, which takes into account your red-line and the practicalities of your hill, the law, neighbors, etc.. Make your change and try again. It's better than nothing, and cheaper than time on a real dyno or dragstrip, so it's not bad for second-stage tuning, after you've dialed things in as well as you can in your driveway. It's quite good for seeing if your engine is happier with a different initial ignition advance setting than what the book told you.

 

[Harte3]

All good info gentlemen and I thank you :!:

The building of an adapter/spacer is exactly what I had in mind as the Trans Dapt and Edelbrock are rather...configured not well inside for the transition from spread bore to square. I did do some smoothing of the ridges/bulges inside before I mounted it.

The other thing I thought about them is they are made (and spacers too) as a one-size-fits-all type part. For example; the spread bore side accomodates 2-1/4 inch secondary throttle bores which has my 1-7/16 throttle bores dumping in to a large area which is then narrowed down to the standard square bore opening. The square bore adapter and manifold bores will accomodate probably up to 1-3/4+ throttle bores which means (I think) a reduced plenum effect prior to the A/F reaching the runner area when using the larger carbs.

A somewhat similar situation happens with those using the 390 cfm on the Offy DP 4 hole manifold with 1-7/16 bores dumping in to larger manifold bores possibly creating a premature plenum affect at the base of the carb which may explain some of the tuning problems encountered with those installations. I am theorizing that by reducing the spacer/manifold bore size to something closer to the size of the carb bores that the point of turbulence will be moved away from the base of the carb to the base of the manifold bores. Whether this will prove to be beneficial or not I'll have to wait until I get off the road this winter and do some experimenting as SS suggested.

Thanks again for a great discussion!

 

[AzCoupe]

I am theorizing that by reducing the spacer/manifold bore size to something closer to the size of the carb bores that the point of turbulence will be moved away from the base of the carb to the base of the manifold bores. Whether this will prove to be beneficial or not I'll have to wait until I get off the road this winter and do some experimenting as SS suggested.

You are correct in your theorizing. This increases the upper low end and mid range power, and is great for a daily driver which operates within these rpm ranges most often. It is also theorized that adding tubes which extend into the manifold plenum, has an even greater effect. An open spacer increases top end power, and is better suited to a drag car which operates in the higher rpm ranges.

 

[Harte3]

Thanks AZ.

When I take every comment posted here and put them together I'm fairly certain I'm on the right track in creating something like a port match from the carb into the manifold to maintain or create higher air flow velocities which according to Emanuel my present carb is well suited for with it's small venturi and extremey long booster venturi.

You gentlemen make fordsixperformance very interesting and a lot of fun!