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Unexpected results using different carb spacer designs
After posting my dyno results on a mild SB 350 a while back, and a couple poster's suggested that I may have an intake restriction I decided to investigate the spacers I was using.
At the time of the dyno I was using an open spacer in combo with a 4 hole spacer on top of a 2101 Performer intake with a q-jet carb, stock air cleaner and K&N air filter.
One run was made with the air cleaner lid off and a 3 HP loss was noted with the lid off vs the lid on. So the current air cleaner set up is not restricting the air intake.
I have modded the stock 77 intake to allow more air into the air box and it appears that it is doing it's job based on the dyno run data.
In this pic you can see my mod. It's just a 2" tube supplying additional air from the wiper tray to the air box in conjunction with the stock single snorkel on the 77.
This 2" hole allowed me to mount an air flow meter in front of it in order to be able to monitor how much air was passing through to supply the carb.
The numbers I obtained are only for a comparison basis and don't reflect the total air being drawn into the carb.
I tested three types of spacers. All 1/4" in thickness.
First the open spacer which I expected to show the greatest flow.
Then the four hole spacer. At least one like this design.
And finally a composite of the two that I made. It has the four hole spacer design for the primaries and then an open spacer design for the secondaries.
The surprising part is the flow data collected from these three different designs was not what I expected it to be.
I don't have a dyno, so all this was done on the car.
All three were tested within a 2 hr window with stable outside temps of 68*.
All three were tested three times with a shift point of 5800 RPM and only the max flows were noted then averaged for the final flow comparison.
The numbers I got were;
Open spacer: 3146 ft/min (I just chose that unit of measure on the meter at random for comparison)
4 hole spacer: 3235 ft/min
Composite spacer: 3251 ft/min
What I expected to see was the open spacer with the highest number the 4 hole the lowest and the composite somewhere in between.
So it appears, at least on this engine, that it flows the most air with the composite spacer.
I have a couple of theories on why this may be but would like to hear from any one else who may have an idea why the open did not flow the highest number.
Engine specs are:
350 CI
9.9:1 CR
219@.050/219@.050 cam
1 7/8" primaries with side pipes STS baffles.
Dart SHP heads. 180cc advertised, 200cc measured.
2.02 intake 1.60 exhaust
My 'guess' is that the separation between the primaries and secondaries minimizes some turbulence caused by the differences in airflow through those venturi. Since the airflow of each secondary venturi is approximately the same, the 'open' design for the secondaries actually allows for an increase of flow.
Nice work! Good testing and good data. Now you have to come up with a patent and go market it to make MILLIONS!
A "confirmation" test would be with a fourth design: Maintain the separation between the primaries and secondaries, but make one larger opening for the primaries, too. This might give you a little increase in flow from the primary set.
My 'guess' is that the separation between the primaries and secondaries minimizes some turbulence caused by the differences in airflow through those venturi. Since the airflow of each secondary venturi is approximately the same, the 'open' design for the secondaries actually allows for an increase of flow.
Nice work! Good testing and good data. Now you have to come up with a patent and go market it to make MILLIONS!
A "confirmation" test would be with a fourth design: Maintain the separation between the primaries and secondaries, but make one larger opening for the primaries, too. This might give you a little increase in flow from the primary set.
I was thinking of going the other direction and restricting the primaries a bit more by making the holes smaller.
The q-jet being a 750 CFM carb and this being a 350 CI engine it's not using all of the capacity of the secondaries so the air flap doesn't open all the way as it is.
So I was wondering if I restricted the primaries a little bit if that would increase the velocity through the primaries without impacting WOT performance in a negative way and further improve part throttle performance. Right now the holes on the primary side on the spacer are larger than the holes in the base of the carb. Maybe make the holes the same size instead of larger.
Go too far and I imagine I would just be forcing the secondaries open when they shouldn't be and impacting mileage negatively.
from everything that ive read, you have to try all the different spacers and see which one responds best to your combination. The best results i've had were from using a 1" open spacer which netted the best results on the strip.
That's what I've read too dawg. I was wondering if the 4 hole was a restriction to flow or not.
From my results on this engine I would say not.
Maybe on a more air hungry engine it would be.
That picture of that POS Mr Gasket heat insulator reminds me of an " episode " in my youth. My buddy really wanted a 69 Z/28 Camaro, I found the ad for one in the local newspaper ( for you kids this is a printed media source that was distributed back in the olden days... ) We went and looked at the Z....it ran like crap but, I noticed a massive discoloration at the Mr Gasket heat insulator below the Holley carb. We barganed hard....$2950 as I recall because it would barley idle and frequently stalled while on the test drive. After the title to the Z was signed off and money exchange, I quickly went into the storage area of my El Camino and whipped out my tool kit....removed all the heat insulator crap....even reused one of the gaskets and started up that solid lifter 302 and quickly adjuted the idle mixture screws and timing by ear....away my buddy went in a sweet running 69 Z and the guy was shell shocked that that was all it took to straighten out his POS! He had spent considerable money at a local performance shop who had a bad reputation amongst knowledgeable cars guys in the area. I was treated to a nice dinner at Sizzlers back when they were a semi-decent steak house....again...Im dating myself.
So are you going back to the dyno or are you gonna make some base line passes at the local 1320'? It's cheaper to make passes all it costs up here is $25.
So are you going back to the dyno or are you gonna make some base line passes at the local 1320'? It's cheaper to make passes all it costs up here is $25.
oops!
Don't have any 1/4 mile tracks here. Nearest one is 180 miles away.
Best I could do is an 1/8 mile track.
Only reason there is even a dyno is the guy just opened shop in June, before that..nothing within 130 miles of me.
No flow benches around here either. Pretty limited in the performance arena.
The cold air intake and wood thermal spacer are a good idea
The local competition carb shop (biz name) with all the flow benches is very pro 4 hole wood spacers. Carb signal and flow. Adding height like a one inch open is for adding plenum volume or in the case of a short manifold increasing the distance to the required nearly 90 degree turn which causes fuel separation on the way to the heads intake ports.
Opening above or cutting down a dual plane divider increases the available flow demand on any of the cylinders. The chevy LT1 375hp was the first production model that had a real tuned and thought about dual plane. ya ya the cross ram.... etc had some power thought into them also
The cold air intake and wood thermal spacer are a good idea
The local competition carb shop (biz name) with all the flow benches is very pro 4 hole wood spacers. Carb signal and flow. Adding height like a one inch open is for adding plenum volume or in the case of a short manifold increasing the distance to the required nearly 90 degree turn which causes fuel separation on the way to the heads intake ports.
Opening above or cutting down a dual plane divider increases the available flow demand on any of the cylinders. The chevy LT1 375hp was the first production model that had a real tuned and thought about dual plane. ya ya the cross ram.... etc had some power thought into them also
Do you run a spacer on either your 383" or your 434"? If so, what gains dud you see? Did you experiment with different spacers.
The cold air intake and wood thermal spacer are a good idea
The local competition carb shop (biz name) with all the flow benches is very pro 4 hole wood spacers. Carb signal and flow. Adding height like a one inch open is for adding plenum volume or in the case of a short manifold increasing the distance to the required nearly 90 degree turn which causes fuel separation on the way to the heads intake ports.
Opening above or cutting down a dual plane divider increases the available flow demand on any of the cylinders. The chevy LT1 375hp was the first production model that had a real tuned and thought about dual plane. ya ya the cross ram.... etc had some power thought into them also
One thing I noticed yesterday while re-adjusting the jetting for the 4 hole now 1/2" spacer (the most I can fit under the hood)
is that the signal to the primaries is stronger. So it wanted to run richer at idle and at part throttle. I adjusted the main jetting with the APT but idle was still a bit rich.
However as I leaned the idle screws it began to pull fuel from the primary nozzles. So it's run rich at idle with the idle screws too far out or from the primary nozzles if I turn the screws in.
I'm guessing I need more bypass air? What do you think.
As you note, your values are in feet/min and not cubic feet/min. If you restrict the size of a venturi, you up the speed of the flow though it, but not the overall volume. While the speed of the flow is important, it is the overall volume of air moved that determines the power, though the speed of the flow typically equates to better throttle response.
Next, it would appear that you are using 1/4" spacers so when you stack them, you now have a 1/2" spacer that is not directly equitable to a 1/4" spacer.
No end to the testing u could do. How 'bout sloting the spacer to match the intake plenum with longitudinal slots rather than a lateral slot? But i guess that would be more like the 4 hole spacer.
Can it repeat the airflow increases with respect to the same spacers on another day? I would be nice to see the raw data before averaging - to see the data spread. As 3 tests is marginal unless all data points are very very close.
But is the airflow a reliable indicator? I am curious as to how it would affect a stock intake manifold. I'm wondering if a good spacer could substitute for a good aftermarket intake - well a dual plane at least. I think if u can develop a good enough spacer to match an aftermarket intake u would have a very good product to market. I have read the 2" spacer has the greatest results - but with our hood clearance its not an option.
BTW did u consider using the stock hood ram air duct? My '74 is a functional unit but not sure what happened in '77. Too hard to retrofit?
I dont expect a modest cam 350 needing more bypass air but some qjets did have a port in the rear for this i think on the larger car like Cadillac - usually its plugged. Ok i find its called a Hot Idle Compensator (HIC) with 3 possible locations - 2 in the rear and 1 inside the float bowl adj on top hat up front. But they kind'a work opposite of your situation in that it open when engine is to hot and vapors flood into the venturis/manifold.
As you note, your values are in feet/min and not cubic feet/min. If you restrict the size of a venturi, you up the speed of the flow though it, but not the overall volume. While the speed of the flow is important, it is the overall volume of air moved that determines the power, though the speed of the flow typically equates to better throttle response.
Next, it would appear that you are using 1/4" spacers so when you stack them, you now have a 1/2" spacer that is not directly equitable to a 1/4" spacer.
Yeah, I realize that I am measuring the speed of the air through that hole not the volume.
I'm figuring that if more air is demanded by the engine through the same size hole then it has no choice but to speed up to meet the demand.
Other than surface friction 1/4" or 1/2" should present the same restriction to flow. Increased venturi effect or pull on the primaries may change, but restriction should remain about the same.
Yeah, I realize that I am measuring the speed of the air through that hole not the volume.
I'm figuring that if more air is demanded by the engine through the same size hole then it has no choice but to speed up to meet the demand.
Other than surface friction 1/4" or 1/2" should present the same restriction to flow. Increased venturi effect or pull on the primaries may change, but restriction should remain about the same.
Other factors at play here too. You change the plenum volume with two vs one spacer.
Also consider that when the carb butterflies open, they extend below the bottom of the carb.
If they extend into the intake manifold, they present a restriction in that air traveling near the inner surface of the manifold can't access the one side of the venturi as easily as the other side because the butterflies are in the way creating a turbulence.
If you raise the carb such that the butterflies no longer extend into the manifold, you remove much of this restriction.
Other factors at play here too. You change the plenum volume with two vs one spacer.
Also consider that when the carb butterflies open, they extend below the bottom of the carb.
If they extend into the intake manifold, they present a restriction in that air traveling near the inner surface of the manifold can't access the one side of the venturi as easily as the other side because the butterflies are in the way creating a turbulence.
If you raise the carb such that the butterflies no longer extend into the manifold, you remove much of this restriction.
I can't really measure, compute or definatively quantify the "butterfly" effect, I was measuring only one variable.
Restriction due to size of the orifice below the carb and how that may affect, or not, the volume of flow into the engine. Used what I had to measure that.
I'm sure there are many other variables, and configurations I could come up with, but time and the lack of controllable conditions doesn't allow for me to test them all.
Take it or leave it, I just though it was of interest.
More plenum volume... yes a little. It's effect is likely minimal in my situation. In any case it could potentially only help to extend the top end if anything.
Can it repeat the airflow increases with respect to the same spacers on another day? I would be nice to see the raw data before averaging - to see the data spread. As 3 tests is marginal unless all data points are very very close.
Don't know the answer to that. I suspect air flow would show similar differences with less speed on a hot day and more on a cold day just due to the density of the air.
I agree the testing was crude at best, being in the car, with shift points not precisely the same each time and conditions not strictly under control.
Would be more valid to see it on an engine dyno under controlled conditions.
BTW did u consider using the stock hood ram air duct? My '74 is a functional unit but not sure what happened in '77. Too hard to retrofit?
I am using the stock ram/cold air intake in addition to the one I installed. There is no hood ram air duct on the 77.
After the rebuild I did add additional bypass air to eliminate nozzle drip caused by the need to open the primary blades more to idle. Adding bypass air allows you to reduce the primary blade angle eliminating nozzle drip.
It seems now that the additional pull on the primaries may be pulling more fuel requiring the leaning of the idle screws. I'm a bit surprised that it would make any noticeable difference at idle. Why fuel begins to pull from the main nozzle the further I lean the idle screws I not 100% clear on but my initial thought is more bypass air is needed.
This would at least should add more air to the idle and lean it appropriately which should allow me to enrichen the idle screws to eliminate the nozzle drip.
That's what I'm thinking anyhow. I'll have to mess around with it tomorrow see what I can see, so I guess I'll find out unless there is a better idea out there.
One thing I noticed yesterday while re-adjusting the jetting for the 4 hole now 1/2" spacer (the most I can fit under the hood)
is that the signal to the primaries is stronger. So it wanted to run richer at idle and at part throttle. I adjusted the main jetting with the APT but idle was still a bit rich.
However as I leaned the idle screws it began to pull fuel from the primary nozzles. So it's run rich at idle with the idle screws too far out or from the primary nozzles if I turn the screws in.
I'm guessing I need more bypass air? What do you think.
How far are your primary throttle blades opened to get your idle?
You could try pulling a small vacuum hose to see if the nozzle drip goes away.
09-06-2014, 11:57 AM
Unexpected results using different carb spacer designs
