Years ago, when I wanted to slow my car to stay within an ET break, I would simply disconnect the secondaries of my carburetor and maybe short shift too. Never a problem.

However, more than once, I've overheard someone(s) say they would never do it. Something to the effect that it would cause the engine to run dangerously lean. Huh? If the mixture on the primaries is right at WOT, why would the disconnection of the secondaries upset that or even impact it in any way? Have these folks just not thought it completely through, or is it me?

The ONLY bit of logic to it I can think of would involve the power valve. If the airflow was limited strictly to the primaries, could that be enough to generate some manifold vacuum at WOT (and start closing the primary side power valve)? Take care. Tom Worthington

Local racer tried it once and it didn't slow him down at all.

It could cause distribution issues but how much I don't know simply due to the primaries being located closer to the front of the engine. When racing 2bbl dirt classes we would use dedicated 2bbl single planes or adapters centering the carbs on the intake.Some claimed gains by moving the carb slightly forward or rearward.

AS to the power valve closing that was an issue at the end of the straights easily remedied by blocking and jetting up.Or noting vacuum at end of chute and sizing accordingly.

My first bracket race. My Nova was a 6.20 car come to find out.We did not know what it would run but knew it would be way quicker than the 7.50 class I had to run in. We did exactly that ties secondaries down and ran two barrel. Being 1st time in car I had ungodly bad light 1st round. I ran the guy down and wacked throttle 3, 4 or 5 times and still went 7.10,yea broke out just a little bit. Which is what the tower told dad over PA. Still makes for funny story.

Point is it did not hurt a thing, and car ran lot quicker than I would have thought running on 425 cfm carb.

Yep, done that for years. Started out with the old bolt under the gas pedal trick til they came out with the adjustable throttle stop. Use it on my 10 second pro class big block to get to 12 seconds to run sportsman. Finally got tired of raising the hood between rounds so just leave it a the 12 second setting for both classes. Only down side I could see is if the manifold was a closed plenum, otherwise I think the cylinders will get what they want.

Never tried this but with an open plenum intake it should be OK, may need to raise the carb up to help distribution. On a dual plane intake, not so sure how this would work if the carb sat right on the intake. In this case I'd use a throttle linkage positive peddle stop instead.

Curious, what ET is it capable of and what are you trying to slow it too?

Tried it a few times, doing what Qtrrcr said. But it didn't work for me, hardly slowed the car at all, only killed about 0.5 sec.
I think you (Tom) hit on something in your OP, about the Power Valve. I wasn't using one, so did not affect my mixture, but I can see how that could possibly happen. Never would have considered that one.

This message has been edited. Last edited by: FootbrakeJim,

Butch Weinreich has been doing it for 25+ years on his Dart with no problems.

As far as the PV yes, WOT vacuum can be a potential issue. With some running low number PV's(unnecessarily) it is possible to close a PV. With some builders they will actually build a 2 barrel with the intent on leaning it out by having manifold vacuum close the PV, cutting down on the rich trend a 2 barrel normally sees.

Distribution can be a factor but I would be less worried about, the high vacuum you see at WOT does a good job of atomizing and vaporizing fuel helping distribution.

The only one I might worry about would be and external linkage Dominator with stock style 3 circuit metering blocks, fuel flow is limited in some cases thru the blocks and could be a bigger issue in 2 barrel form.

quote:

Originally posted by TOP38:
Never tried this but with an open plenum intake it should be OK, may need to raise the carb up to help distribution. On a dual plane intake, not so sure how this would work if the carb sat right on the intake. In this case I'd use a throttle linkage positive peddle stop instead.

Curious, what ET is it capable of and what are you trying to slow it too?

Nothing specific...yet. I'm just thinking ahead to a point where I might outrun my safety gear. 7.50 in the 1/8 mile is as quick as you can typically go without upgrading to stuff like rollbars, etc. I'm close now, and I'm thinking the fresh engine I put in this year may push me over, at least in good air. The old engine had been in there for 40 years, so there is really no telling. Many years ago when I did it, it was so I could run 8.00 flat and no quicker. It worked out pretty well, back then. I just don't want to risk any damage. It had never occurred to me before now. Take care. Tom Worthington

I do this pretty often for two different reasons. It works great for the bracket finals on my 65 Chevelle wagon. It normally runs around 7.20's give or take .05's. It slows down the car right at 3 tenths. Which puts us right at 7.50 which is perfect for the slower footbrake class. I do this also at one track due to traction issues. The wagon will still lift the front tires just a little on take off but has less chance of spinning on slicker tracks. Plus an added bonus is I don't have to wear a jacket in the hot Texas heat. I only have to have a t-shirt. I sure miss the days when shorts were legal.

7.38 for roll bar.

quote:

Originally posted by FootbrakeJim:
T
And as Top38 mentioned, you probably don't want to try this on a dual-plane intake, (unless you have a tall open spacer), or you won't be feeding half-throttle to all 8 cylinders, you'll actually be feeding 4 of your 8 cylinders at full throttle, and nothing to the other 4! (Talk about strange crankshaft harmonics...)

So when I drive my 65 Impala on the street at part throttle, with a #4779 Holley double pumper and a dual pane intake, I'm only feeding fuel to half the cylinders? Really? Nonsense....

A dual plane intake has the divider running fore and aft, and the primary throttle bores on any four-barrel carb are split left and right. Fuel goes on both sides of the divider from the front (primary) throttle bores. All 8 cylinders get fed equally, as proven by spark plug reading and header tube temps.

By the way, many/most 4-bbl carbs only have idle circuits in the primary side, are those cars only idling on four cylinders?

quote:

I only have to have a t-shirt. I sure miss the days when shorts were legal.

Be careful my friend. The rules and regulations bunch will flog you and demand you wear a HANS device with that re-certified T-shirt. Also, don't dare call street racing drag racing, the sanctioning body bunch has long ago commandeered that term (or so they think).

quote:

Originally posted by Phil Dees:
7.38 for roll bar.

Well alrighty then. I sure thought it was a 7.50 cutoff, but I see that you are right. Take care. Tom Worthington

quote:

Originally posted by FootbrakeJim:
Tried it a few times, doing what Qtrrcr said. But it didn't work for me, hardly slowed the car at all, only killed about 0.5 sec.
I think you (Tom) hit on something in your OP, about the Power Valve. I wasn't using one, so did not affect my mixture, but I can see how that could possibly happen. Never would have considered that one.
And as Top38 mentioned, you probably don't want to try this on a dual-plane intake, (unless you have a tall open spacer), or you won't be feeding half-throttle to all 8 cylinders, you'll actually be feeding 4 of your 8 cylinders at full throttle, and nothing to the other 4! (Talk about strange crankshaft harmonics...)

Whaaaattt?????? Silly.

quote:

Originally posted by Canted Valve:

quote:

I only have to have a t-shirt. I sure miss the days when shorts were legal.

Be careful my friend. The rules and regulations bunch will flog you and demand you wear a HANS device with that re-certified T-shirt. Also, don't dare call street racing drag racing, the sanctioning body bunch has long ago commandeered that term (or so they think).

As I was watching no prep kings the other night I noticed Chuck Seitsinger had no gloves, no Hans or neck collar and no head sock in a low 4 sec twin turbo car while on TV lol
His life and I really dont give a fluck, just thought it was funny is all

Lol. SlyFox and Bad News, my apologies for the goofy post. I had just gotten 3 injections in my spine, and was put under for the procedure. My son had driven me to the doc and back, and he helped me into the house. I had him park me in the most comfortable chair, which is right in front of the computer. And this is no kidding, as I was writing that post, (still in a fog, obviously!), he said to me: "Are you sure you want to be posting anything right now?" An hour later, I was working on the car, and I promise to not do that to you guys again!

Disconnected secondaries AND used a linkage stop to slow my car from high 7.0's to low 7.3's

It's not a good way to get it done, in my opinion you want to kill ET as early as possible, and keep your MPH and finish line throttle response.

Seen a good engine ruined by high vacuum in intake created by disconnecting the secondaries affecting the power valve.BIll C.
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