[Cliff R]

Here's a few pics to illustrate why using an adapter on a factory iron intake with a square flange carburetor is a very poor choice of parts. The air comes pouring down thru the primary side of the carb then slams into the material between the primary and secondary openings. The transition is so poor it actually splits the airflow between the primary and secondary bores and if an open spacer/adapter is used the air doesn't know which side of the intake to follow.

I wanted to stay off Harry K's thread so it doesn't sound like I'm picking on him or being critical of his testing, which I am not.

I'm pointing out WHY one would see less power if they tried to use a square flange carb on a factory intake. Doing this actually requires 1/2" clearance or the throttle plates actually hit the intake, as one of the pics below shows the Edelbrock AFB clone with the throttle plates wide open sitting all by itself on the intake! With a 3/8" spacer it would still need 1/8" worth of gaskets to keep it from hitting at full throttle. Even if it just misses, the incoming air coming off the throttle plates slams DIRECTLY into the flat surface of the factory intake. Do ya think this would/could create some turbulence and interrupt airflow into the plenum areas?

I painted a 1967 factory q-jet gasket white to help illustrate the alignment problems between the parts.

There are certainly a lot of pretty smart folks on this Forum, and some pretty early in the learning curve as well. Since I do this stuff for a living, every single day I see folks spending countless hours and tons of money trying to get the very most from their combination of parts, closely port matching intakes and heads, expensive valve, seat, port and head work, headers to help scavenge the exhaust, modern cam profiles to help the engine breath better. At some point they will have to make an intake and carb choice. I hope the information provided helps them in that regard.......Cliff

[Half-Inch Stud]

Clarity is a good thing.

[67gtospud]

So are you saying if using an adapter you would recommend a 4-hole vs. An open adapter?

[Cliff R]

I doubt in 3/8" to 1/2" if you could effectively re-direct the airflow with either type, and why trying to adapt square flange carbs to spread bore factory intakes is not going to be nearly as good a deal as using a spread bore carb, or getting an intake specifically designed to run either type.

I hope folks notice in particular the fact that the Edelbrock carb HITS the intake at 1/2" when the throttle plates are fully open. Makes me wonder if the recent testing we saw took that fact into account as they were using a 3/8" "spacer"? It may have been hitting before it reached full throttle, in addition to having a piss-poor match to the intake it was sitting on. Do ya think that scenario would cost you some power on a dyno run?..........Cliff

[GOAT WHORE]

Funny thing is a 15 yr old kid could tell you this is not right.

[67gtospud]

This is really interesting and definitely something I never thought about. When I initially assembled my engine with the factory spread bore intake I adapted my edlebrock 600 with a 1/2" open adapter. The engine was never loaded bit did idle quite well and was very responsive. Once I finish my rebuild I'll be replacing the factory cast intake with an aluminum edlebrock performer. No adapter necessary except I've been told that a 1/2" square bore 4-bore adapter with the secondary holes modified so they're together helps the engine idle nicely but yet provides better air/fuel delivery once the secondarys open. Is this true?

[Matt Meaney]

have you ever tested a modified iron intake (like your third pic) with a modified 1" adapter/spacer (similar to the one you've pictured in the past for a Q-jet) with a square flange carb?

would you need to blend/taper the spacer to match the intake opening or would it be ok with a little step?

[steve25]

You can also keep in mind some basic high velocity air flow facts that we deal with in head porting and working out valve seat angles, like the fact that only slow moving air can make it around anything greater than a 15 degree turn without sheering away.

[Cliff R]

I have street tested quite a few spacers on modified stock intakes. Then I backed up the testing at the track to see which one ran the best ET and MPH.

On the street, I found a 4 hole spacer to be the most sensitive at idle and right off idle. The difference was barely noticeable, but the engine did "feel" just a tad smoother at very low rpms for "normal" driving.

The other spacers I tested were fully open, fully divided, and semi-divided (very similar to the one pictured in Jim Hand's book.

At the track when we compared all the numbers, the sem-open spacer was the winner by quite a margin. It ran more MPH and quicker ET than any other spacer tested. The 4 hole and fully open were the worst. The fully divided spacer ran nearly as quick as the semi-open, but still slower in MPH and worse in ET. I would also add here that testing with a fully open spacer also induced some "stumble/hesitation/bog" going quickly to full throttle that I could not tune out. I found the exact same symptoms testing iron and RPM intakes where someone machined the entire divider out of them. One dyno test we got from a customer showed nearly 40 ft lbs LOST with this modification on a 455 making up near 600hp. I keep aluminum flat stock here, and have put quite a few dividers back in Pontiac intakes that have been modified in this way and the owners/tuners were having issues with them...FWIW

I also tested my 4781-2 Holley carb on the iron intake, the RPM, and the new (at that time) Tomahawk intake.

The Tomahawk intake for all intakes tested was the hands down winner in "felt" performance on the street. It delivered strong upper mid-range and top end power much like a shot of nitrous.

At the track, it REQUIRED a 1" spacer to really shine, and ran the best MPH of all intakes and intakes with spacers tested. Problem was that it gave up .09 seconds in 60' times, and got edged out by a couple hundreths in ET compared to my modified factory intake.

For all the testing I've done between my Holley 4781-2 850 DP and my 1977 Q-jet, the Holley runs better in the mid-range, the Q-jet stronger on top end. Both carbs are just about dead for even for ET/MPH every single time I've ran them on any intake. The q-jet typically we be just a shade faster in MPH, and .02-.03 seconds quicker in ET as a result of the stronger top end charge.

I really didn't want to turn any part of this thread into a carb testing/comparison, as that's what I do for a living. I will say that as long as the carb being tested in sufficient in CFM, and correctly tuned at all points, they ALWAYS run about the same, no matter what type of carb it is. Both my 4781-2 850 Holley and 1977 Q-jet are custom dyno tuned for correct A/F ratio, then the numbers backed up at the track with drag strip runs.

I will also say that my Holley was custom tuned with the help of Tom Vaught, who used to work for Holley. Before enlisting his help, it wouldn't run anywhere near as quick as my q-jet, which really shouldn't be surprising, as I've been tuning on the Q-jet for several decades, street, dyno and at the track on several pretty strong running engines.

From what I've seen with spacers and "blending", this pays BIG benefits with single plane intakes if one tries to use a q-jet on one. Without a well made custom blended spacer, we've got our butt handed to us several times on the dyno trying to use them on single plane intakes. It is my opinion that this happens mostly due to the huge difference in throttle plate diameters, which creates turbulence having the rears stick so much further into the plenum areas than the front. The signal from each of the runners are all interconnected, so it's very "touchy" using a spread bore carb on a single plane intake to make best power. The higher you put the carb above the intake, and better blending into the plenum, the more power it will make, every single time I've tested one......Cliff

[Skip Fix]

Let's look at what the square carb would really see on a Q jet intake with 4 holes and not opened up like Cliff's into two side on the normal 3/4" taper adapter. Not quite as ugly as Cliff's pictures show laying it flat. And the open Q jet spacer in Harry's pictures look juist as ugle for air flow to hit the big flat areas.

Now look at an open spacer for a square bore on a 4 hole intake equivalent-almost as ugly.

Can't find my Edelbrock 4 hole in the attic to show it.

But again my car picked up 0.25 seconds over a well tuned Q jet using a square bore AND adapter. Another reeal world test same track day.

Look at the Tomahawk intake-open for a Q jet yet the primaries are so close they can't make the turn into 1 and 2 runners unless you add a spacer-and they aren't hitting anything.

[Cliff R]

"But again my car picked up 0.25 seconds over a well tuned Q jet using a square bore AND adapter. Another reeal world test same track day."

Skip, I woln't say anything negative about your testing, I respect you and your results. I will say that I a very well prepared 4781-2 Holley 850. I've ran that carb on just about everything in terms of CID, power level, and different intakes with and without spacers. It is FLAWLESS.

With that said, every single back to back test I've done at the track with it had it running no faster than, if as fast as the Q-jet. The last documented testing I did with it was on the Tomahawk intake at the HPP Shootout a few years ago at Norwalk. On a 1" spacer it ran .02 seconds and .30 MPH slower than my Q-jet.

I'm not sure how or why you could slow down .025 seconds introducing a SERIOUS miss-match between the carb and intake vs none at all? Maybe your engine liked some turbulence or a little restriction at the intake flange, or both.......Cliff

PS: thanks for the picks, that deal would be at least 200 times better than putting a 3/8" spacer/adapter between the two parts and having the big throttle plates on the Holley hitting the flange on the factory HO intake!.....IMHO

[Firebob]

I also have a specific application to ask about since you have probably tried this setup before. if you could shed any light on what you feel about it?
I will be trying out a holley75ODP on a performer intake. What would be the optimum spacer/adapter arrangement? Sounds like it should be the same as the RPM but it would be nice to see a pic of what was talked about. Just saying a spacer or adapter was used doesn't really specify a peticular design. This is like carbs and intakes 101 for us uninitiated and I, for one, am grateful for everybody sharing their knowledge and experience. It's why I come here everyday.

[Cliff R]

The Performer starts getting pretty restrictive on the 455 engine, but fairs better on the 400 as it isn't sucking nearly as hard on the intake. Since the Performer intake is smaller than a stock intake, it will like a spacer and the additional plenum volume that it provides. If you have room I'd go at least 3/4" on the spacer, and it would be a 4 hole spacer opened up between the primary and secondary openings with the center (divider) left in place (fully divided spacer). Use an open gasket on both sides, and cut a small opening about half the size of a fifty cent piece directly between the two rear barrels right in the spacer.

I still have the Q-jet spacers that we used for testing, if I can find them I'll take some pics and post them here......Cliff

[1969GTO]

unless I missed something on this forum,the only spacer that worked well for me was the Tapered spacer for the Q-jet from Wilson Manifolds.. The intake was modified by SDPERFORMANCE as per Cliff.
In a back to back I gained 2mph and 2tenths in the 1/4 mile with the tapered spacer.

[Matt Meaney]

i wasn't trying to compare carbs as much, as how to get the "best" adaptation/transition of a square flange carb on a factory spread bore intake. while I realize it's not an ideal combination of parts, it would be nice to minimize any driveability issue or loose a substantial amount of power.

I am specifically talking about putting a Holley 3bbl on a modified iron intake. although, I would think it would be applicable to any square flange carb. I like the look of the adapter skip posted a pic of as a starting point and have it ending up looking similar to what jim hand had. open primary to secondary and partially divided. any thoughts or suggestions?

slightly off topic, have you ever put a divider in a single plan intake? it's supposed to improve low end response and driveability in general.

I purchased a car with a 455 and torker intake. it was then that I finally felt the "nitrous" effect of that manifold, as it would break the tires loose as the tach passed 3200 rpm. I don't especially care for "nitrous"

[Skip Fix]

Cliff Q jet was even on a 4 hole 1/2" spacer on the HO intake KRE ported E heads small solid roller 259/264@ 0.050. Same one that got into the 10.90s with a good tranny and convertor. And that was 0.25 seconds slower with the Q jet-might not be as bad now that you have massaged it.

For those running a Q jet right on a Performer double check the secondaries -the plenum divider kind of hits the blades if not ground.

[Cliff R]

Good point Skip, we have to grind on the RPM's, and stock intakes as well to get a good fit between the Q-jet carb and the plenum areas at the rear of the intake.

FWIW, I don't use any spacer at all on my HO or factory iron intakes, due to having a Shaker assembly. I have the ability to use a shorter Shaker lid, and add a 1" spacer. This mandates installing a custom filter in the Shaker opening, as the lid of the air filter gets too close to the carb and hurts performance.....Cliff

[Cliff R]

Originally Posted by GOAT WHORE

Funny thing is a 15 yr old kid could tell you this is not right.

Yes, I didn't want to start a pissing match about it, but the only thing tested on the other thread was not using the right carb on a stock intake vs putting the same carb on an intake that was made for it.

I doubt if any of them even realized that it takes a FULL 1/2" of clearance for the AFB clone or the throttle plates hit the iron intake...and they were using a 3/8" spacer. When I first read the test results I spent a few minutes LMAO, then I saw all the "at-a-boys" and felt I needed to show folks why things may have came out the way they did for power production.

I really hate getting caught up in these sort of things, really not my style, but maybe it will help a few folks out when they are making choices with these sort of parts to complete their engine assembly......Cliff

[Half-Inch Stud]

Originally Posted by Matt Meaney

...slightly off topic, have you ever put a divider in a single plan intake? it's supposed to improve low end response and driveability in general. ..

YES. was really on to something, then i learned that stiff metal and screws are weaker than pulsed 455 air flow, and welded aluminum.

[Skip Fix]

Cliff a 1/2" spacer for my setup and a factory intake was a good fit. The 3/4 adpater kind of bulged the hood up a hair

Some one used to sell a spacer with replaceable dividers of different lengths for sinlge plan intakes.