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Low speed (1500-1800 RPM) driveability issue

General engine tech -- Drag Racing to Circle Track

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BobbyB
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Re: Low speed (1500-1800 RPM) driveability issue

Post by BobbyB » Fri Sep 13, 2019 7:04 pm

Nice work. Thanks for posting the picture.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Fri Sep 13, 2019 7:31 pm

BobbyB wrote:
Fri Sep 13, 2019 7:04 pm
Nice work. Thanks for posting the picture.
New removable splitter filler.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by Tuner » Fri Sep 13, 2019 8:48 pm

About 40 years ago I made an insert like that for an Edelbrock C454 intake by filing the three sides of the notch in the plenum divider, both ends and the bottom, to a sharp edge on a 90 deg. angle (each 45 deg. from the manifold divider axis) and fabbing a 3/8" aluminum plate with sharp 90 deg. slots in the ends and bottom, file fitting until it was a good press fit in the manifold. Worked fine but after about a month it got loose enough to start buzzing then rattling with the engine's rhythm of 180 deg. suction pulses alternating sides in the manifold. The aluminum plate was like a bar in a xylophone or wind chime, LOL it rang and changed pitch a little as you played with the throttle while listening to the WTF iszzat ...!! ?? !!

Therefor, I caution you to only run that for a small while before you pull the carb again and examine to see if it is coming loose, because it looks small enough it could make its way through the manifold to a valve and do some real damage.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Fri Sep 13, 2019 9:45 pm

Tuner wrote:
Fri Sep 13, 2019 8:48 pm
About 40 years ago I made an insert like that for an Edelbrock C454 intake by filing the three sides of the notch in the plenum divider, both ends and the bottom, to a sharp edge on a 90 deg. angle (each 45 deg. from the manifold divider axis) and fabbing a 3/8" aluminum plate with sharp 90 deg. slots in the ends and bottom, file fitting until it was a good press fit in the manifold. Worked fine but after about a month it got loose enough to start buzzing then rattling with the engine's rhythm of 180 deg. suction pulses alternating sides in the manifold. The aluminum plate was like a bar in a xylophone or wind chime, LOL it rang and changed pitch a little as you played with the throttle while listening to the WTF iszzat ...!! ?? !!

Therefor, I caution you to only run that for a small while before you pull the carb again and examine to see if it is coming loose, because it looks small enough it could make its way through the manifold to a valve and do some real damage.
I made it snug where I had to tap in down into place, no play. I’ll heed your warning though, and epoxy it in.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Tuner » Fri Sep 13, 2019 10:30 pm

The one I made was a press fit, it had to be driven in, and it came loose in about 500-750 miles of daily driving. The engine was an LS-6 454 crate long-block. The cycles of heat and cool expand and contract the manifold so it squeezes the insert like a milkmaid's fist and the insert and the manifold worry each other until a little clearance develops and then the manifold pressure pulses slap it back and forth. Once it started making a little buzzing sound it was only a few more miles for it to get really loose. Our pal with the TIG talent fixed it.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Fri Sep 13, 2019 10:33 pm

Status update... the combination of the divider filler and the 4 hole space is the hot ticket. The car is running smooth in the range where I was having the problem, and the low speed cruise afr is stable right at 14:1. Also, I am having the reduce the main jet sizes and the PVRC restrictor jets to get the AFR in the mid 12's at wide open... this is telling me that the boosters are getting a stronger signal. I did a test drive earlier with 76's in the primary, 86's in the secondary, and 0.065 PVRC jets in the primary (secondary power valve is blocked). It was in the 11.2:1 range on a third gear pull through the powerband. I reduced the PVRC's to 0.055, as this is the size that gives me equal jet cross section jetting in the primary and secondary circuits. I can't do a test drive tonight because it just rained and I've had a couple beers. I will report back tomorrow. It seems I am on the right track, thanks everybody!!!! :)

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Fri Sep 13, 2019 10:35 pm

Tuner wrote:
Fri Sep 13, 2019 10:30 pm
The one I made was a press fit, it had to be driven in, and it came loose in about 500-750 miles of daily driving. The engine was an LS-6 454 crate long-block. The cycles of heat and cool expand and contract the manifold so it squeezes the insert like a milkmaid's fist and the insert and the manifold worry each other until a little clearance develops and then the manifold pressure pulses slap it back and forth. Once it started making a little buzzing sound it was only a few more miles for it to get really loose. Our pal with the TIG talent fixed it.
I have a TIG and a small dose of talent... :D

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Re: Low speed (1500-1800 RPM) driveability issue

Post by MadBill » Fri Sep 13, 2019 10:57 pm

Tuner wrote:
Fri Sep 13, 2019 8:48 pm
About 40 years ago I made an insert like that for an Edelbrock C454 intake by filing the three sides of the notch in the plenum divider, both ends and the bottom, to a sharp edge on a 90 deg. angle (each 45 deg. from the manifold divider axis) and fabbing a 3/8" aluminum plate with sharp 90 deg. slots in the ends and bottom, file fitting until it was a good press fit in the manifold. Worked fine but after about a month it got loose enough to start buzzing then rattling with the engine's rhythm of 180 deg. suction pulses alternating sides in the manifold. The aluminum plate was like a bar in a xylophone or wind chime, LOL it rang and changed pitch a little as you played with the throttle while listening to the WTF iszzat ...!! ?? !!

Therefor, I caution you to only run that for a small while before you pull the carb again and examine to see if it is coming loose, because it looks small enough it could make its way through the manifold to a valve and do some real damage.
Many years ago, a bud of mine built a 'notch filler' like that out of thin gauge aluminum. Next time he had the carb off, there it was: gone! No hiccups, no compression loss... :shock:
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Fri Sep 13, 2019 11:08 pm

MadBill wrote:
Fri Sep 13, 2019 10:57 pm
Tuner wrote:
Fri Sep 13, 2019 8:48 pm
About 40 years ago I made an insert like that for an Edelbrock C454 intake by filing the three sides of the notch in the plenum divider, both ends and the bottom, to a sharp edge on a 90 deg. angle (each 45 deg. from the manifold divider axis) and fabbing a 3/8" aluminum plate with sharp 90 deg. slots in the ends and bottom, file fitting until it was a good press fit in the manifold. Worked fine but after about a month it got loose enough to start buzzing then rattling with the engine's rhythm of 180 deg. suction pulses alternating sides in the manifold. The aluminum plate was like a bar in a xylophone or wind chime, LOL it rang and changed pitch a little as you played with the throttle while listening to the WTF iszzat ...!! ?? !!

Therefor, I caution you to only run that for a small while before you pull the carb again and examine to see if it is coming loose, because it looks small enough it could make its way through the manifold to a valve and do some real damage.
Many years ago, a bud of mine built a 'notch filler' like that out of thin gauge aluminum. Next time he had the carb off, there it was: gone! No hiccups, no compression loss... :shock:
:shock: :shock: :shock:

This piece is 1/8" thick, and I made the slots where it fits pretty deep. I will either epoxy or weld it in though, within the next week.

I can see how that would happen though. Engines can chew up aluminum and spit it our without missing a beat. When I was young we raced 500cc two stroke methanol powered karts on dirt tracks. Pulled the motor down to do a top end and the piston skirt on the exhaust side was missing. Kart was running like a raped ape, never missed it. :lol:

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Re: Low speed (1500-1800 RPM) driveability issue

Post by superpursuit » Sat Sep 14, 2019 1:24 am

GRTfast wrote:
Fri Sep 13, 2019 10:33 pm
Status update...the combination of the divider filler and the 4 hole space is the hot ticket.The car is running smooth in the range where I was having the problem, and the low speed cruise afr is stable right at 14:1. Also, I am having the reduce the main jet sizes and the PVRC restrictor jets to get the AFR in the mid 12's at wide open... this is telling me that the boosters are getting a stronger signal. I did a test drive earlier with 76's in the primary, 86's in the secondary, and 0.065 PVRC jets in the primary (secondary power valve is blocked). It was in the 11.2:1 range on a third gear pull through the powerband. I reduced the PVRC's to 0.055, as this is the size that gives me equal jet cross section jetting in the primary and secondary circuits. I can't do a test drive tonight because it just rained and I've had a couple beers. I will report back tomorrow. It seems I am on the right track, thanks everybody!!!! :)
Happy to see that you're sorted now. Enjoy! :D

Allan.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Geoff2 » Sat Sep 14, 2019 5:23 am

I have had the same experience as Tuner using a slot-in plenum divider. It came loose. It just goes to show how much pulsing is going inside the intake manifold.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Belgian1979 » Sat Sep 14, 2019 5:36 am

Geoff2 wrote:
Sat Sep 14, 2019 5:23 am
I have had the same experience as Tuner using a slot-in plenum divider. It came loose. It just goes to show how much pulsing is going inside the intake manifold.
On and ITB at low rpm idle you can see the vacuum fluctuate wildly between a minimum and a maximum. I can only think that with the different times during which the intake pulses take place that one cyl is going to draw air out of the one of the other runners that has its valve closed. The higher the rpm the higher the frequence this takes place. So this would have an effect on intake filling imo.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Dan Timberlake » Sat Sep 14, 2019 9:35 am

" like a milkmaid's fist "

With your permission I'd like to have that in the ready for use in technical discussions at work.

thanks,

Dan T

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Dan Timberlake » Sat Sep 14, 2019 9:45 am

[/attachment]
GRTfast wrote:
Fri Sep 13, 2019 7:31 pm
BobbyB wrote:
Fri Sep 13, 2019 7:04 pm
Nice work. Thanks for posting the picture.
New removable splitter filler.
=================

I wonder how many manifolds with beautifully rounded, faired in carb openings are throwing away some drive-a-bility unnecessarily.

Back in 1972 the folks at AMC said this to their SAE buds.
You do not have the required permissions to view the files attached to this post.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Tuner » Sat Sep 14, 2019 12:11 pm

Dan Timberlake wrote:
Sat Sep 14, 2019 9:45 am
[/attachment]
GRTfast wrote:
Fri Sep 13, 2019 7:31 pm
BobbyB wrote:
Fri Sep 13, 2019 7:04 pm
Nice work. Thanks for posting the picture.
New removable splitter filler.
=================

I wonder how many manifolds with beautifully rounded, faired in carb openings are throwing away some drive-a-bility unnecessarily.

Back in 1972 the folks at AMC said this to their SAE buds.
That AMC info is good stuff. It isn't the only source where sharp edges in the intake are described as beneficial for vaporization and distribution (to be fair - shapes of various radius are important in some cases, as outlined in the Mopar engine book carb and intake chapter) Obert's and Taylor's Internal Combustion Engine textbooks both describe sharp edges as useful in many cases. In spite of the ballyhoo about 'air gap' manifolds for performance, exhaust heated intake manifolds are much more desirable for street driving.
Tuner wrote:
Mon Nov 06, 2017 10:18 pm
prairiehotrodder wrote:
Mon Nov 06, 2017 11:21 am
so i noticed on the used BBC tunnel ram that i purchased that the ports got opened up where they meet the head. The intake port is lower than the port on the head. So now there is a 1/8" step up to the head. I have to take the t-ram back off to fix the end seals where the ultra-black did not seal. Will use "right stuff" this time. Should i epoxy the bottom of the intake ports to get better alignment ? To get rid of the step ? If so what and where do i get this epoxy?
Brian
With the port floor in the head higher than in the intake manifold, I would leave it.

There are more than a few examples of a step facing into the flow that indicate it can be beneficial, particularly a mismatch at the floor.

For starters, it is well known that, in spite of flying in the face of “conventional wisdom - common sense - it just doesn’t look right”, a square-port intake on an oval-port BBC head runs great, gets mileage and makes good power, better power than some supposed to be correct oval-port intakes.

Anybody who was paying attention and is old enough to have played with the engines when they were new has seen it.

I had an experience with a 225 V-6 Buick in a Jeep that had a near 1/8” step from the stock intake manifold floor to the head; the head floor was higher than the manifold. The manifold was off to do a valve grind so porting the floor to match the heads seemed the thing to do.

This engine had a small Isky hyd. cam, headers and a 500 Holley 2bbl that had been tuned and fiddled with so it would 4-WD good at crazy angles and climb steep hills without stalling, it got 25+ MPG.

After the valve grind and porting job, which included a bowl job in the heads and matching the heads and manifold to the intake gasket, the formerly snappy engine with a good torque and tractor-like low speed lugging abilities and the aforementioned good mileage was sluggish junk and did not respond to carb or ignition tuning attempts to return it to its former good performance.

Efforts to tune outside the engine were obviously getting nowhere, and having had a previous experience with a step facing into the flow on a 289 Ford carb spacer that was extremely beneficial (is an understatement), we pulled the intake and ported the floor down to mimic the original mismatch and the original performance was restored.

Go figure.

The ’66 220 HP 289 has a sharp step in the carb spacer bores at the casting parting line, the lower half is smaller than the upper half with an irregular 1/16”-3/32” step. On a carb overhaul/tune-up, removing the step on a ’66 Mustang that got 25 MPG and ran 9:50’s in the eighth caused it to fall to 15 MPG and 10:70’s. After much anguish and fiddling with the carb and timing, etc, replacing the spacer with the ugliest one we could find in the wrecking yard put it right back to its original performance.

Go figure.

Those two incidents are the scariest thing about carb tuning I know.

Another example is a dyno test done by Joe Mondello back in the late '70s with the 460 Ford marine engine. One of magazines, Power Boat I think, had numerous Joe Mondello tech articles about modifications of the 455 Olds and 460 Ford jet boat engines. The high performance 460 that was married to Berkeley pumps in the popular little jet boats had standard small port heads with a large port Cobra Jet intake. The intake mismatch is about 1/4" all around the port, about half as "bad" as the BBC oval port head with a square port intake. One of the tests Mondello did was to port the head to match the CJ intake. Eliminating that step mismatch at the gasket flange killed the power bad, 40-50 HP as I recall,

I think the most important areas to match are the port roof and outside radius wall.

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