Low speed (1500-1800 RPM) driveability issue

[Tom68]

Hardly.. More like smart applied physics.
Vapourization of fuel in the intake absorbs heat.
That heat needs to be replaced to match , or soon vapourization suffers.
Thats why a carbwd engine ,when cold started will fire up initially but then soon stall. The available heat gets absorbed ,vapourization STOPS.
Plenum heat aids quick warm up and low speed driving..
Has nothing to do with a horse nor a buggy.
Poor carbed intake manifold vapourization gives poor low speed and idle quality. Intake manifold Plenum heat is a game changer. Always has been.

Ignorant statements don't change that.
Spark plug and mega volt super lightning ignition box companies love this ignorance. $$$$$

Not to mention Smokey was working on a hot gas engine.

What does that have to do with performance? You want a hot gas engine try LPG. And kill 1/2 your HP so fast your head will spin.

Not my project.

[Tuner]

Let me say it this way. Why the OE’s do what they do isn’t why I do what I do.

I’ll type it slower for you.

If you can’t make a single plane intake outperform a dual plane intake you need to take up putt-putt golf.

Horse and buggy thinking is what it is. Just because you are living in the stoned age doesn’t mean I made an ignorant statement. It means you are behind in your thinking.

ClassAct you are disrupting the class like Little Johnny. Are you forgetting circumstances alter cases? Perhaps a touch of the buggy whip would organize your thoughts?

[F-BIRD'88]

No surprise yiyr holley carb fuel bowls are getting too hot.
This happens on all Big Block Checes with a dual plane intake and holley carb.
The fuel bowls are right over top the intake runners on a BBC dual plane intake.
Back in the day on the 396-427-454 (L72-LS-6 etc) chevs with holley carb the fix was a (service part) carb shield with gasket and a lil air gap between the 2 layers of the heat shield .
Holley sells a version of this heat shield.
You can make your own out of sheet aluminum or sheet metal. Remember the 2 ply with lil space air gap. between the 2 plys.
You can combine with a 3/8" to 1/2" or 5/8" carb spacer to raise the carb up as needed. The BEST material to isolate the carb from excess manifold heat for a carb spacer is WOOD.. ya you read that right. WOOD IS BEST.
.
Back in the day I made my own holley carb heat shield with wood carb spacer combo for my 454 Chevelle.
Paint the 2ply with lil air gap sheet metal shield WHITE..
This will avoid excessive hot soak fuel bowl temps.
Ya looks ugly but it works.
Carb spacer should be divided type on your dual plane.
For a fully divided plenum.
The deep side plenum can use 2 hole (half of a 4 hole spacer). The shallow side plenum can use a D shape hole. You want that full plenum split.
Especially with such a BIG lazy carb venturii size.
A 750 HP 4150 carb would be PLENTY BIG OF A CARB.
on your mild Sub 500 BHP engine.
The venturii are much more responsive for low speed driving (1500-1800 rpm) than a 1050 cfm 4150 carb.
Getting the carb fuel bowl heat under control is critical
Specificaly on the chev big block with a dual plane manifold and holley carb. The intake runners are very close to the holley fuel bowls.
This is the fix.

[F-BIRD'88]

Is you hyd roller cam (107° LSA) a Thumpr Cam?
These cams need a ton of base initial spark timing at idle.
18 base + vacuum advance at idle is NOT ENOUGH.
You need to recurve the diz mechanical adv curve for a SHORT limited 10° advance curve.
This will allow 26-28 ° Base initial timing and 36-38 total at max mech advance..
Then Do not use full manifold vac for the vacuum advance.
Use Pirted vacuum on the carb.
In a high overlap cam the liw manifold vacuum causes the spark timing to jump around and go up and down with changing manifold vacuum.
The ported vacuum works Much better.
The Vacuum advance Should be LIMITED TO about 10-12 degrees max travel.

The diz need a curve with MUCH MORE initial base timing+ a SHORT 10° mech advance curve that is NOT so quick. A more stable curve maxing st 3300 3500 rpm is BETTER than a super quicky adv curve.
Again use ported vacuum for vaccum advance.
The carb setup will be MUCH BETTER.
the throttle responce at your critical 1500-1800 rpm just off idle driving range will be much smoother.

A 18-39 curve (21° advance curve) is wrong for this setup with this high overlap camshaft.
Some MSD diz need a special 10° advance bushing to allow a limited 10° adv curve.
26-36 curve (10°) is much better (ported vac adv is much better on this car.

Every body with these MSD diz runs into the same issue
Lack of diz advance bushing CHOICE..

If where that low speed 1500 1800 rpm low speed driving "drivability" is a critical factor.. That Thumpr Cam was a poor choice for this car and engine and driving purpose. They are intended to have a very nasty idle sound which also effects the very low speed driving range TOO.
Most auto trans cars with a 3000 ++ rpm stall converter can drice right thru it. But a Manual trans car can be even MORE critical in that liw speed driving range.
A 112 LSA cam would have been MUCH BETTER
say 236/236 ° Or say 236/242 on 112. or say a 228/236 on 112 LSA hyd roller cam.
All these cams will EXCEED 500 hp on yiur healthy but mild purpose street BBC
And DRIVE MUCH BETTER.
You don't need the (thumpr cam) extra exhaust duration on this mild purpose car/ engine.
A racey idle cam does not fit the driving purpose well.
You could re- cam this car for much better "drivability" and also MORE POWER TOO. thru a better camshaft choice.
This is where my buddy Cam King can fix you up.
Mike Jones
Jones Racing Cams
Call him.

[Tom68]

Is you hyd roller cam (107° LSA) a Thumpr Cam?
These cams need a ton of base initial spark timing at idle.
18 base + vacuum advance at idle is NOT ENOUGH.
You need to recurve the diz mechanical adv curve for a SHORT limited 10° advance curve.
This will allow 26-28 ° Base initial timing and 36-38 total at max mech advance..
Then Do not use full manifold vac for the vacuum advance.
Use Pirted vacuum on the carb.
In a high overlap cam the liw manifold vacuum causes the spark timing to jump around and go up and down with changing manifold vacuum.
The ported vacuum works Much better.
The Vacuum advance Should be LIMITED TO about 10-12 degrees max travel.

The diz need a curve with MUCH MORE initial base timing+ a SHORT 10° mech advance curve that is NOT so quick. A more stable curve maxing st 3300 3500 rpm is BETTER than a super quicky adv curve.
Again use ported vacuum for vaccum advance.
The carb setup will be MUCH BETTER.
the throttle responce at your critical 1500-1800 rpm just off idle driving range will be much smoother.

A 18-39 curve (21° advance curve) is wrong for this setup with this high overlap camshaft.
Some MSD diz need a special 10° advance bushing to allow a limited 10° adv curve.
26-36 curve (10°) is much better (ported vac adv is much better on this car.

Every body with these MSD diz runs into the same issue
Lack of diz advance bushing CHOICE..

Start retard territory to, or at least a separate ignition switch so you can get the engine cranking.

[F-BIRD'88]

Is you hyd roller cam (107° LSA) a Thumpr Cam?
These cams need a ton of base initial spark timing at idle.
18 base + vacuum advance at idle is NOT ENOUGH.
You need to recurve the diz mechanical adv curve for a SHORT limited 10° advance curve.
This will allow 26-28 ° Base initial timing and 36-38 total at max mech advance..
Then Do not use full manifold vac for the vacuum advance.
Use Pirted vacuum on the carb.
In a high overlap cam the liw manifold vacuum causes the spark timing to jump around and go up and down with changing manifold vacuum.
The ported vacuum works Much better.
The Vacuum advance Should be LIMITED TO about 10-12 degrees max travel.

The diz need a curve with MUCH MORE initial base timing+ a SHORT 10° mech advance curve that is NOT so quick. A more stable curve maxing st 3300 3500 rpm is BETTER than a super quicky adv curve.
Again use ported vacuum for vaccum advance.
The carb setup will be MUCH BETTER.
the throttle responce at your critical 1500-1800 rpm just off idle driving range will be much smoother.

A 18-39 curve (21° advance curve) is wrong for this setup with this high overlap camshaft.
Some MSD diz need a special 10° advance bushing to allow a limited 10° adv curve.
26-36 curve (10°) is much better (ported vac adv is much better on this car.

Every body with these MSD diz runs into the same issue
Lack of diz advance bushing CHOICE..

Start retard territory to, or at least a separate ignition switch so you can get the engine cranking.

Yes especially on a Big Block chev.
The increased idle base timing needs a Start retard or a Ignition power interupt Switch to aid HOT engine Re-Start.
That OEM GM starter motor end BRACE- bracket that everyone tosses is A MUST have On your big blick starter motor. Every Big Block Chev came with this CRITICAL $6 brace bracket on the GM starter motor.
If yoyrs is Missing. GET ONE and install it.

Get a starter motor HEAT SHIELD ALSO.
I'll also add that the MSD diz likely also needs to be checked for critical Rotor to Cap firing alignment phasing. Vacuum advance effects this phasing alignment when running. If the Rotor / Cap phasing is not cirrect within range the ignition will misfire especially if when vacuum advance is too mazed out.
Best to limit vac adv to about 10-12 degress max travel.
Many have way too much travel out of the box.
This creates missfire/ cross fire under the diz cap with excessive vac advance range.
The cap / rotor phasing alignment check is critical.

[Tuner]

I hope this is not too much word salad for comfortable consumption.

The 1" spacer between the carb and manifold on a 225 HP 289 has a 360 deg. lip and the difference between with and without it (the lip) is one of the things I know that gives the most pause for pondering about carb tuning.

Tuner, approximately what is the diameter of the 289 spacer hole and what is the diameter of the lip?

Just bringing this back, missed it first time around ......

Roughly how much smaller ..... what percentage of the bore ...... was the lip on the 289 shear plate ?

Obviously different percentages will have different effects, on fuel shear and flow, be interesting to know the compromise Ford achieved here

Not used a full 360* shear plate as yet, but had good results with a plenum 'divider filler' (stainless sheet) with little fuel shear kick our lips

I have described this two or three times in the last 15 years in this forum, but I can't find the posts I made. Search function in this forum sucks.

I have been trying to find my own posts about the '66 289 Ford 4V spacer with the sharp edges protruding into the flow, the lower half is slightly smaller than the upper half with a sharp edge at the parting line. Removing the lip on an otherwise stock motor caused the car to go from 9.50 1/8 mi. ET to 10.70 and the mileage to drop from 25 MPG to 15. Got another original spacer from a wrecking yard and put it right back to where it was, only change was polishing out the ridge and replacing it with an unmodified original with the lip intact. That one experience is about the worst thing I know about tuning engines and carbs.

I got a PM from 66-408-stang asking the same question to describe the spacer lip, so here is the reply. These two things, the spacer and the advance curve, we (me and my pal who owned the '66 Mustang) learned are nearly the most profound experiences in forming an understanding of tuning engines. The spacer lip is always in the back of my mind when some engine is being stubborn about achieving smooth driveability or has uneven plug color. You just have to wonder. I have put that spacer on a few other engines and it was beneficial on some and had no effect on others.

Tuner,

Thank you for sharing your insight on various subjects, this one including. As a kid I also ditched the spacer with the addition of a larger holley on a 66 mustang 289 and also experienced the loss in torque and mileage, but didn't have the smarts or drive to figure out why. Ford engineers must have been smarter than I thought with that ingenious shear plate design.

Have you played around with different size shear plates? How big of a lip was on that 289-4V spacer?

Bill

The lip in those spacers is usually irregular in width, I guess due to inconsistency in offset of the injection mold dies, and the step or lip varies from 1/16" to 3/32" or so. I think the lower half is about 1/8" smaller than the upper. Also, it looks like the hole is broached to qualify the diameter and the broach is not cleaned so aluminum sticks to it and drags ragged gouges in the hole on the return stroke. All the ones I have seen since look this way. The spacer we replaced the carefully polished one with was the nastiest most ragged looking of the three or four we found in the wrecking yard. It was chosen because the original was ragged looking and we thought, eh, why not?

The guy whose car it was is a lifelong pal and we usually laugh about this every time we see each other.

Another important lesson we learned from that car was modifying the advance curve with light springs for an"all in at 2500" distributor curve hurt the power bad ...... really bad. It absolutely had to have the original advance curve with the heavy secondary spring that only advanced to 26 degrees at 5000. It would not turn over 5000 anyway because at 5050 the valves floated and the lifters pumped up, so it was best shifted at 4900, 3 spd. Koni shocks, Shelby sway bar, clamped the leaf springs, wide tires, it handled great, ran consistent 9.50s x 72 MPH in the 1/8th, in the late 60s we had a lot of fun in that car.

[rgalajda]

Let’s clear up some issues . If your building a BBC up to 500 hp for street use, a dual plane intake with an 850 carb is probably the way to go.
500+ hp and at least 454 cubes for the street, and a single plane intake comes into play , with the right selection on the rest of the build. Especially the camshaft.
For a 107 LSA camshaft you should step down on the duration.

For a true street car that involves building as much torque as possible, and operating seamlessly at lower rpm’s, your probably limited to a 1.4 inch venturi holley carb unless it has annular boosters.
I say this because your not likely using an intake with heated crossover .

Don’ be confused by these 1050 cfm weekend warrior/race carburetors. That’s a race carb. Not intended to operate at low rpms driving. Especially with the downleg boosters. Launching at 4000+ rpm is a different story.
If your running those factory rectangular port heads ( ports are too big problem ) and you can’t get rid of them financially, then you should install an oval port intake.
For the people who say they don’t care about fuel mileage in their street car. They will change.

A 107 LSA camshaft is what I use ( see specs below ) with a modified 3310-1 780 holley on a single plane intake. The quick fuel brawler BR-67258 is almost identical ( venturi size and performance ) which I also have, now sits on the shelf.
Note that I run a 2.73:1 axle ratio, 2000 stall speed converter, and low speed torque is excellent.
I no the 780 Holley is undersized for the motor but it more than makes up for it at low to mid rpm.
Car idles with 12 inches vacuum in gear 750 rpm . 14 inches vacuum in neutral 950-1000 rpm

1975 Monte Carlo LS 454/500 HP

Trans : 400 turbo hydramatic/ 2000 stall speed / Rear axle 2.73:1

Engine: 454 Cu In Big Block Mark 1V 4 bolt main Built by Bob Galajda
casting no:14015445 Stamped: TO419TXC

Balanced and Blueprinted

Block: magnafluxed / sonic tested / bored .030 with plate
decked to .005 inch / line honed mains
casting cleaned to bare metal and flashings removed
Cylinders: brush finished and polished
Lifter bores: grooved

Cylinder heads: Oval port 346236 Valves: 2.190 Intake / 1.880 Exhaust
pocket porting / runners and chambers smoothed
7/16 studs and positive valve seals installed
3 anlgle valve job with blending and cc’d
Comp roller tip rockers
Pistons and Rings: Icon 779-.030 and Total Seal 1/16 ring package / file fit
9.5 to 1 compression with 118cc heads

Camshaft: Lunati – hydraulic custom ground on Voodoo lobes
with lunati spring package and lifters
specs: 273 duration 107 LCA at 4 degrees advance
227@.050 / valve lift .552 inch with 1.72 rockers

Connecting Rods: BBC oem cleaned magnafluxed resized and fitted
with ARP rod bolts

Crankshaft: oem cast crank magnafluxed / ground and polished
.010 rods and mains
Recipocating assembly Balanced

Fuel System: 780cfm Holley on Victor Jr replica by Professional
Products

[Brian M]

For Tuner's information, look up "Intake Distribution Issues". From 2017, page 2.
Sorry, I don't have the patience to try and add a link. I am so called, computer challenged.

[Tuner]

For Tuner's information, look up "Intake Distribution Issues". From 2017, page 2.
Sorry, I don't have the patience to try and add a link. I am so called, computer challenged.

viewtopic.php?f=1&t=50014

[Tuner]

For Tuner's information, look up "Intake Distribution Issues". From 2017, page 2.
Sorry, I don't have the patience to try and add a link. I am so called, computer challenged.

viewtopic.php?f=1&t=50014

I recall measuring an original 289 Ford spacer for this info. It was 6 years ago. How time flies.

The step is smaller than the bore above it, is 360 degrees - all around the bore, and has a very sharp edge, looks finished to diameter with a broach pushed through it, like some carb venturi.

The throat ID at the top side of the plate is 1.675" , the ridge is .200" down from the top and is 1.580" ID with a sharp upper edge, the throat ID at the bottom side of the plate is 1.620". I don't remember the carb throttle bore, whatever the bore is in a 1966 289-225HP Autolite carb .... 1-1/2" or maybe 1-9/16", I think the smaller ??