We are currently running on the new domain and server: www.Speed-Talk.com

IMPORTANT: Update your bookmarks to https://www.speed-talk.com/forum/
(Right-click the URL and select "Bookmark this link")

Low speed (1500-1800 RPM) driveability issue

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Belgian1979
Guru
Guru
Posts: 3865
Joined: Sat Mar 26, 2011 11:34 am
Location: Belgium - Koersel

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

Post by Belgian1979 » 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 ?

User avatar
af2
HotPass
HotPass
Posts: 6546
Joined: Sun Jan 28, 2007 4:42 pm
Location: Grass Valley, CA :Northern Foothills

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

Post by af2 » 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 .
GURU is only a name.
Adam

Belgian1979
Guru
Guru
Posts: 3865
Joined: Sat Mar 26, 2011 11:34 am
Location: Belgium - Koersel

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

Post by Belgian1979 » Mon Sep 16, 2019 3:40 pm

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 ?

exhaustgases
Guru
Guru
Posts: 6463
Joined: Sun Oct 06, 2013 9:03 pm
Location:

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

Post by exhaustgases » Mon Sep 16, 2019 6:01 pm

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 .
Hmm then run out and get a 0 degree overlap cam ground NOW, and then report back here and let us know how well it works.
This is Cam Kings area of expertise maybe he can enlighten.

User avatar
af2
HotPass
HotPass
Posts: 6546
Joined: Sun Jan 28, 2007 4:42 pm
Location: Grass Valley, CA :Northern Foothills

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

Post by af2 » Mon Sep 16, 2019 6:26 pm

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.
GURU is only a name.
Adam

GRTfast
HotPass
HotPass
Posts: 3135
Joined: Wed Jul 05, 2017 8:26 am
Location:

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

Post by GRTfast » Tue Sep 17, 2019 2:22 pm

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.

BobbyB
HotPass
HotPass
Posts: 218
Joined: Mon Apr 15, 2013 1:35 pm
Location:

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

Post by BobbyB » Tue Sep 17, 2019 2:35 pm

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.

GRTfast
HotPass
HotPass
Posts: 3135
Joined: Wed Jul 05, 2017 8:26 am
Location:

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

Post by GRTfast » Tue Sep 17, 2019 3:36 pm

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.

F-BIRD'88
Guru
Guru
Posts: 7265
Joined: Tue Mar 27, 2007 6:56 pm
Location: Ontario, Canada

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

Post by F-BIRD'88 » Tue Sep 17, 2019 7:11 pm

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.

BobbyB
HotPass
HotPass
Posts: 218
Joined: Mon Apr 15, 2013 1:35 pm
Location:

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

Post by BobbyB » Wed Sep 18, 2019 6:00 am

GTR, how much did your vacuum at idle change from no divider, to filled divider?

GRTfast
HotPass
HotPass
Posts: 3135
Joined: Wed Jul 05, 2017 8:26 am
Location:

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

Post by GRTfast » Wed Sep 18, 2019 7:23 am

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.

BobbyB
HotPass
HotPass
Posts: 218
Joined: Mon Apr 15, 2013 1:35 pm
Location:

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

Post by BobbyB » Mon Sep 23, 2019 3:19 pm

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?

CharlieB53
Pro
Pro
Posts: 218
Joined: Fri Nov 22, 2013 2:44 pm
Location: Wright City, Missouri

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

Post by CharlieB53 » Sun Nov 03, 2019 10:50 am

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.

turbo camino
Member
Member
Posts: 136
Joined: Sun Aug 10, 2008 11:51 am
Location:

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

Post by turbo camino » 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.
DON'T PANIC

Protech Racing
New Member
New Member
Posts: 16
Joined: Tue Nov 28, 2017 2:51 pm
Location:

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

Post by Protech Racing » Sun Nov 03, 2019 8:36 pm

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.

Post Reply