Low speed (1500-1800 RPM) driveability issue

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

Post by af2 »

Belgian1979 wrote: Mon Sep 16, 2019 2:30 pm A question comes to mind : how much does overlap on a cam determine the potential of an engine to reach high rpm and power ?
NONE .
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Re: Low speed (1500-1800 RPM) driveability issue

Post by Belgian1979 »

af2 wrote: Mon Sep 16, 2019 3:13 pm
Belgian1979 wrote: Mon Sep 16, 2019 2:30 pm A question comes to mind : how much does overlap on a cam determine the potential of an engine to reach high rpm and power ?
NONE .
Then why do manufacturers produce cams with that much overlap ?
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Re: Low speed (1500-1800 RPM) driveability issue

Post by af2 »

What I meant was there is more to it than overlap. Looking at a cam on a blown Hemi sees less overlap and a very wide LS that shifts at 9500 rpm.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast »

I wonder if anyone has ever modified an air gap manifold with a sliding plenum divider filler section that is actuated by vacuum, or by the throttle linkage? I could see that working well if executed correctly.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by BobbyB »

GTR, As your cruise situation improved with the closed divider, did you lose any top end power?
I wonder how edelbrock decided on the slot dimensions...

I agree that a movable divider seems like a fun thing to experiment with...probably a great way for me to feed some "unintended metal pieces" to my engine though.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast »

BobbyB wrote: Tue Sep 17, 2019 2:35 pm GTR, As your cruise situation improved with the closed divider, did you lose any top end power?
Hard to tell... maybe? The cam signs off on this thing at around 5600, so if it did, I didn't lose much. It definitely feels meatier in the 2000-5000 range. Based on how it feels I'd guess it has more area under the curve in the effective range of the cam.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by F-BIRD'88 »

If you look at the oem bbc highperf intakes from a 454-450hp LS6, 427-L78 425hp 396-375 hp and all the 3x2 bbc 427 intakes they all have full plenum dividers except the 427 L-88 intake. All the recent Hi-Rise Dual Quad RPM air gap 2x4 intakes have fully divided plenums.

A sliding plenum divider would be much better to be employed in a carb spacer vs in the intake manifold.
A good sealing method on the sliding divider $$$$ adds complexity $$$$.

On a high per dual plane intake :
When the carb(S) air flow cfm capacity is BIG the need for a open plenum or cut down divider is diminished.

In this high perf dual plane high output V8 engine application a 6 Venturi 1200-1400 cfm (@1.5") single carb would rock.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by BobbyB »

GTR, how much did your vacuum at idle change from no divider, to filled divider?
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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast »

BobbyB wrote: Wed Sep 18, 2019 6:00 am GTR, how much did your vacuum at idle change from no divider, to filled divider?
Didn't check. I'll check it this weekend and report back.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by BobbyB »

GTR, did you check idle vacuum?

I see your other posts about AFR changing with load.

Are you still convinces that closing the passage in the manifold helped your surging at cruise issue?
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Re: Low speed (1500-1800 RPM) driveability issue

Post by CharlieB53 »

The root cause of the low speed issue is the slow air velocity at minimal RPM. Fuel droplets will fall out causing some cyl To go lean and the wet intake floor can cause other cyl to go rich.

Try installing metal window screen sandwiched with gaskets under the 4 hole spacer. This will force fuel droplets to break into finer smaller droplets, improving atomization and reduce fuel fall out at low air velocities.

You may have to reduce primary jetting.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by turbo camino »

I have 'fixed' similar issues on single plane intakes before with a thin 4-hole shim under a 1" open spacer. I've never tried it on a dual plane with a divided spacer & 4-hole shim, curious if the results would be the same.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by Protech Racing »

On single carb over the RPM manifold, I have seen part throttle lean in certain cylinders due to what I believe are the throttle plates diverting the fuel towards one end or the other. I had a customer SB2 take out the top of a couple of pistons due to par t throttle lean out. A road race car that operates a large part of the time pulling off corners at 50% opening.
If you could change the the plates to simultaneous tip in and use equal jetting , along with 2 power valves I think the part throttle burps may change or disappear.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by Tuner »

turbo camino wrote: Sun Nov 03, 2019 3:00 pm I have 'fixed' similar issues on single plane intakes before with a thin 4-hole shim under a 1" open spacer. I've never tried it on a dual plane with a divided spacer & 4-hole shim, curious if the results would be the same.
:mrgreen:
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Re: Low speed (1500-1800 RPM) driveability issue

Post by BCjohnny »

Tuner wrote: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.
BobbyB wrote: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
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