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Low speed (1500-1800 RPM) driveability issue

General engine tech -- Drag Racing to Circle Track

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

Post by Tuner » Sat Sep 14, 2019 12:16 pm

Dan Timberlake wrote:
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
I think you can consider that as an "open source" description of a highly technical situation, but you may need to use a footnote for the uninitiated to understand the term.

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

Post by MadBill » Sat Sep 14, 2019 1:06 pm

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!!!! :)
Something I like to do anytime there's doubt about how a Holley's (or any 4bbl.) jetting came to be the way it is, is to disconnect the secondaries and tune for cruise and WOT on the primaries alone*, then reconnect and re-tune, changing just the secondary jets for overall optimum WOT results. I often find one half is rich and the other lean and optimizing both has netted me as much as 15 HP while showing about the same overall AFR. (*Keeping the revs down some, since you'll never normally be pulling peak RPM CFM through only the primaries, for example perhaps going to 5,500 RPM instead of a normal max of 7,000.)
Felix, qui potuit rerum cognscere causas.

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

Post by GRTfast » Sat Sep 14, 2019 3:37 pm

Final update...

Car is running like a champ. Had to play with accel pump cams and squirters to get the low rpm throttle response dialed. Here are all the settings

QFT 1050 cfm 4150:
2nd and 4th emulsion holes blocked
IFR moved to bottom, 0.032 in all 4 corners
IAB 0.063 front 0.073 rear
MAB 0.026 all 4 corners
Primary MJ 76's
Sec MJ 86's
Primary PV 8.5
Sec PV plugged
Primary PVRC jets .055
primary squirter 0.032
secondary squirter 0.040
primary pump cam: blue, 2nd hole
secondary pump cam: red first hole
RPM air gap with plenum divider notch blocked
front idle mixture screws 1.5 turns
Rear mixture screws closed
secondary throttle plates all the way closed
primary set for 850 rpm idle, this exposes about 0.020 of the transfer slots


afr's
idle 13.5:1
cruise (anywhere between 1500-3000 RPM) 14:1
light accel: 13.5:1
WOT (2500-5500 RPM) 12.5:1

THANK YOU EVERYONE!

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

Post by Kevin Johnson » Sat Sep 14, 2019 3:48 pm

Still running those hotter plugs? :wink:

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

Post by GRTfast » Sat Sep 14, 2019 3:52 pm

Kevin Johnson wrote:
Sat Sep 14, 2019 3:48 pm
Still running those hotter plugs? :wink:
Yes. Bkr5e-11

Looks like all the cylinders are pretty even mixture wise. Sealing up the two plenums from each other was a game changer. The torque in the 2500-4500 range is nasty.

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

Post by Kevin Johnson » Sat Sep 14, 2019 3:58 pm

http://www.leg.state.fl.us/statutes/ind ... 6.191.html

Now, none of that "exhibition of speed" tomfoolery... :lol:

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

Post by GRTfast » Sat Sep 14, 2019 4:10 pm

Kevin Johnson wrote:
Sat Sep 14, 2019 3:58 pm
http://www.leg.state.fl.us/statutes/ind ... 6.191.html

Now, none of that "exhibition of speed" tomfoolery... :lol:
All my spirited test drives were on a closed course, professional driver, do not attempt at home, warranty void when engine starts, not valid in California. :lol:

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

Post by RevTheory » Sat Sep 14, 2019 4:48 pm

Great thread, gentlemen.

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

Post by F-BIRD'88 » Sat Sep 14, 2019 10:24 pm

Nice job.

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

Post by BobbyB » Sun Sep 15, 2019 7:44 am

I wonder how much time one could spend tuning the area of the opening the OP just plugged. What does it do that is beneficial? What happens if it was only halfway filled? Anybody tried that?

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

Post by F-BIRD'88 » Sun Sep 15, 2019 8:32 am

Best done using various spacers , to get plenum sharing, with the divider fully intact.
The spacers can be tested with various divider types and heights for effect.

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

Post by Caprimaniac » Sun Sep 15, 2019 9:03 am

Always nice to see tuning helps. Great result there.

I’m no Holley- man, bur this thread discuss the same problem my friend has with his Holley 390.
He’s about to thrash it, bo good with a car that runs good on wot only....

2.8 , 221 cui, Ford V6.
Offenhauser dual plane inlet manifold (for US 2.6V6).
Holley 390.
210 Hp on engine dyno.
2550 lbs car.

Spacer? And .... Jets? Any suggestions?
How to turn GURU in an instant.....

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

Post by Brian M » Sun Sep 15, 2019 10:32 am

Just want to share something I tried on Friday and it completely amazed me how well this worked. (Thanks Tuner, this is your idea.)

Cut a piece of .010" stainless shim stock into the shape shown in the pic, sandwiched in between a new 4 hole gasket and 4 hole spacer, with a little bit of contact cement,just below the carb.

First thing I noticed is the engine wasn't so blubbery and missing when I first started it. I use a choke also. After a good 45 minute drive, engine was good and hot, idle speed was up a good 50 to 75 rpm. Idle AFR went from 12.7, 12.8 to 12.1, 12.2. Turned in idle screws almost 1/8 turn to get it back to 12.8 where it idles best. Cruise AFR is slightly richer now to. Will be working on that.

This really smoothed out the idle, accelerating through the gears, slow speed and cruise. Fuel must to be getting improved atomization blasting over those tabs sticking in the bores. Carb is an old 650DP, cam is 225 @ .050 with some pretty lazy lobes 279 @ .006, more overlap than I wanted for my style of driving. Need a different cam. Plus my possessed air gap with the divider installed, heads that have intake runners and throats that are to big, on top of a 406 sbc.

It's working for me, others may have varying results. Yes, this has been very good informative thread. Thanks everyone.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by Tuner » Sun Sep 15, 2019 8:16 pm

Brian M wrote:
Sun Sep 15, 2019 10:32 am
Just want to share something I tried on Friday and it completely amazed me how well this worked. (Thanks Tuner, this is your idea.)

Cut a piece of .010" stainless shim stock into the shape shown in the pic, sandwiched in between a new 4 hole gasket and 4 hole spacer, with a little bit of contact cement,just below the carb.

First thing I noticed is the engine wasn't so blubbery and missing when I first started it. I use a choke also. After a good 45 minute drive, engine was good and hot, idle speed was up a good 50 to 75 rpm. Idle AFR went from 12.7, 12.8 to 12.1, 12.2. Turned in idle screws almost 1/8 turn to get it back to 12.8 where it idles best. Cruise AFR is slightly richer now to. Will be working on that.

This really smoothed out the idle, accelerating through the gears, slow speed and cruise. Fuel must to be getting improved atomization blasting over those tabs sticking in the bores. Carb is an old 650DP, cam is 225 @ .050 with some pretty lazy lobes 279 @ .006, more overlap than I wanted for my style of driving. Need a different cam. Plus my possessed air gap with the divider installed, heads that have intake runners and throats that are to big, on top of a 406 sbc.

It's working for me, others may have varying results. Yes, this has been very good informative thread. Thanks everyone.
Thank you for replying with the detailed results. It feels good to know someone benefits from the simple things. :D Who doesn't wonder what may be the result from a lip like this all the way 360 deg. around the bore?

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. It is probable, even likely, that not every engine will respond similarly, but in the case of the 289 the effect was profound, more than a second quicker in the 1/8th mi. and from 15 to 25 MPG. If you don't try things like this, how do you know what you are missing if you've never had it? If we hadn't removed the lip and then replaced it, just changed that one simple thing, that priceless experience would have never been learned.

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

Post by BobbyB » Sun Sep 15, 2019 8:21 pm

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

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