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Low speed (1500-1800 RPM) driveability issue

General engine tech -- Drag Racing to Circle Track

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

Post by GRTfast » Tue Sep 10, 2019 11:13 am

CamKing wrote:
Tue Sep 10, 2019 11:07 am
235@.050" on a 107 LSA
That's your issue
Go down to a 228 on a 110, and your problem will be solved
:lol: I was waiting for that!! We have talked before and I know I made a dumb choice with this cam, I fell for the marketing ploy and got "thumped". A new cam and lifters aren't in the budget at the moment, but are on the horizon, and you will be the supplier.

In the meantime I want to make this one work the best it can.

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

Post by PackardV8 » Tue Sep 10, 2019 11:24 am

235 @ 0.050 . . . 3.27 rear end . . . at low rpm (1500-1800)
CamKing wrote:
Tue Sep 10, 2019 11:07 am
235@.050" on a 107 LSA
That's your issue
Go down to a 228 on a 110, and your problem will be solved
Lost count of the guys who try too much cam and too little gear and wonder why it just won't work together at too low RPM.
Get rid of the 4-hole spacer.
Trying to understand, given it's a dual plane intake and a 4-corners idle carb, why do we think an open spacer will be the answer to his reversion/overlap problem?
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Re: Low speed (1500-1800 RPM) driveability issue

Post by Firedome8 » Tue Sep 10, 2019 12:31 pm

How close are the exhaust to your plug boots? I have seen the spark jump to hei coil screws depending on load and rpm. If larger plug gaps make it worse scope the engine on a loading chassis dyno and don't forget to look at engine in the dark. Just thinking!

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

Post by mag2555 » Tue Sep 10, 2019 12:48 pm

If your only making 500 hp at the flywheel then you have way too much port volume with thosevheads and inturn low port velocity and poor low speed throttle responce.

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

Post by GRTfast » Tue Sep 10, 2019 1:00 pm

mag2555 wrote:
Tue Sep 10, 2019 12:48 pm
If your only making 500 hp at the flywheel then you have way too much port volume with thosevheads and inturn low port velocity and poor low speed throttle responce.
I know, but here's the weird thing... it pull like a freight train from down low, and has super crisp throttle response. Zero hesitation from any RPM. It just doesn't like to behave at a constant speed in that low RPM range. That is really the only "symptom", and it isn't even really that bad, just enough to be noticeable, and that makes it annoying.

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

Post by GRTfast » Tue Sep 10, 2019 1:02 pm

Firedome8 wrote:
Tue Sep 10, 2019 12:31 pm
How close are the exhaust to your plug boots? I have seen the spark jump to hei coil screws depending on load and rpm. If larger plug gaps make it worse scope the engine on a loading chassis dyno and don't forget to look at engine in the dark. Just thinking!
They are 90 degree boots and the wires are routed under the headers (roadster style headers that go straight out from the head). They are as far away as they can be given the proximity of the exhaust ports and the spark plugs. I really doubt that is the issue, but I can set up my gopro and take it for a ride at night to see if there is any "lightening".

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

Post by Belgian1979 » Tue Sep 10, 2019 1:22 pm

LoganD wrote:
Tue Sep 10, 2019 9:46 am
I've seen carb intakes, both dual plane and single plane, with such bad distribution that one cylinder will be 16:1 and another will be 11:1. I'd weld bungs in each header tube and move the wideband around, you can use the jetting to help mitigate it but you'll never get it great.

Get rid of the 4-hole spacer.
I think this is the cullprit.

I have no solution to offer but my experience.
I have 8 WB's on my engine and I can see the difference. Obviously with EFI you can have the ecu correct for this. Corrections made by the ECU vary up to 30% from rich to lean cyls on the extreme side of things at idle and anything which is low speed driving.
What had me stumped is that even when balancing airflows between cyls, it still had the uneven AFR's (mine is an ITB manifold which makes individually balancing cyls possible)

What those imbalances in AFR do is that the lean ones start to misfire making for bad driveability, rough running,...

I thought that it could have something to do with injector angle but after I see this post it seems that a carb does the same thing...

On other issue could indeed be the cam. Although if intake reversion would be the reason I would suppose it would show up in the airflow differences...

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

Post by GRTfast » Tue Sep 10, 2019 1:42 pm

Belgian1979 wrote:
Tue Sep 10, 2019 1:22 pm

What those imbalances in AFR do is that the lean ones start to misfire making for bad driveability, rough running,...
This is what it "feels" like, especially since I can hear the low level intermittent "popping". It's not really a pop like when a plug is fouled, it's like a missing power stroke.. a "hole" in the exhaust note that is only noticeable when you really listen for it. It goes away as soon as you give it the slightest amount of the throttle to accelerate, and it pulls silky smooth.

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

Post by NewbVetteGuy » Tue Sep 10, 2019 2:43 pm

GRTfast wrote:
Tue Sep 10, 2019 11:13 am
CamKing wrote:
Tue Sep 10, 2019 11:07 am
235@.050" on a 107 LSA
That's your issue
Go down to a 228 on a 110, and your problem will be solved
:lol: I was waiting for that!! We have talked before and I know I made a dumb choice with this cam, I fell for the marketing ploy and got "thumped". A new cam and lifters aren't in the budget at the moment, but are on the horizon, and you will be the supplier.

In the meantime I want to make this one work the best it can.
<500hp, very healthy overlap, 1 7/8 primary headers into 3.5" collectors, and low speed driveability issues...

I'm surprised no one mentioned going with a MUCH smaller header (and collector) to get the air velocity up and prop up the bottom-end.

I'm a newb, though so curious what the pros think...

I've heard too much overlap on the cam, heads too big for CU and RPM, I'm going to say headers too big; sounds like a reversion factory at low RPM...
(I'm guessing a single-plane intake would make it WORSE, not better.)

Adam

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

Post by Belgian1979 » Tue Sep 10, 2019 2:53 pm

GRTfast wrote:
Tue Sep 10, 2019 1:42 pm
Belgian1979 wrote:
Tue Sep 10, 2019 1:22 pm

What those imbalances in AFR do is that the lean ones start to misfire making for bad driveability, rough running,...
This is what it "feels" like, especially since I can hear the low level intermittent "popping". It's not really a pop like when a plug is fouled, it's like a missing power stroke.. a "hole" in the exhaust note that is only noticeable when you really listen for it. It goes away as soon as you give it the slightest amount of the throttle to accelerate, and it pulls silky smooth.
The popping you hear is unburned fuel exploding in the exhaust.

The lean and misfiring cyls seem to overal have a bad effect on the motor in the sense that they create unequal rpm (fluctuating) with the lean cylinders pulling the good cylinder down.

Fortunately for me the ecu corrects fueling on a per cylinder basis, so it keeps on running like it should. On a carb you don't have that possibility.
With a wideband you will be able to see it misfire when you can plug it into the header tube, but this is not something that you can do as a way of a test. There is a lot of work to get the bungs in the right place, weld, etc. Aside the fact that you would know which is the offending cyl it won't get you closer to solving the problem.

Maybe another question : how is the manifold to port alignment ?

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

Post by gmrocket » Tue Sep 10, 2019 2:59 pm

It may help to open up the plenum by cutting out the divider and adding as much open spacers as you can.

If you haven't got a crossover in the exhaust, that would also help.

If it was an automatic you probably wouldnt feel a thing, even with that tall gear...but yours , your gonna feel every pulse at that rpm in high gear.

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

Post by GRTfast » Tue Sep 10, 2019 3:19 pm

Belgian1979 wrote:
Tue Sep 10, 2019 2:53 pm
GRTfast wrote:
Tue Sep 10, 2019 1:42 pm
Belgian1979 wrote:
Tue Sep 10, 2019 1:22 pm

What those imbalances in AFR do is that the lean ones start to misfire making for bad driveability, rough running,...
This is what it "feels" like, especially since I can hear the low level intermittent "popping". It's not really a pop like when a plug is fouled, it's like a missing power stroke.. a "hole" in the exhaust note that is only noticeable when you really listen for it. It goes away as soon as you give it the slightest amount of the throttle to accelerate, and it pulls silky smooth.
The popping you hear is unburned fuel exploding in the exhaust.

The lean and misfiring cyls seem to overal have a bad effect on the motor in the sense that they create unequal rpm (fluctuating) with the lean cylinders pulling the good cylinder down.

Fortunately for me the ecu corrects fueling on a per cylinder basis, so it keeps on running like it should. On a carb you don't have that possibility.
With a wideband you will be able to see it misfire when you can plug it into the header tube, but this is not something that you can do as a way of a test. There is a lot of work to get the bungs in the right place, weld, etc. Aside the fact that you would know which is the offending cyl it won't get you closer to solving the problem.

Maybe another question : how is the manifold to port alignment ?
The intake manifold to intake port alignment is dead nuts on the money.

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

Post by BigBlockMopar » Tue Sep 10, 2019 4:09 pm

On pretty much all of my street driven engines I've always improved throttle response by installing a 4-hole spacer.

Have you tried disconnecting the vacuum advance as a test (and maybe put the timing at the 36 you think it should be)?
Perhaps the vacuum advance is fluctuating a bit at that point.

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

Post by gmrocket » Tue Sep 10, 2019 5:54 pm

BigBlockMopar wrote:
Tue Sep 10, 2019 4:09 pm
On pretty much all of my street driven engines I've always improved throttle response by installing a 4-hole spacer.

Have you tried disconnecting the vacuum advance as a test (and maybe put the timing at the 36 you think it should be)?
Perhaps the vacuum advance is fluctuating a bit at that point.
are you talking a 4 hole on a single plane ?

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

Post by BigBlockMopar » Tue Sep 10, 2019 6:19 pm

I was when I typed my previous reply, but I've found some drivability improvements on Performer RPMs too for instance.

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