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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 » Wed Sep 11, 2019 9:58 am

Kevin Johnson wrote:
Wed Sep 11, 2019 9:36 am
Just a thought:
https://www.gsparkplug.com/shop/spark-plug-heat-range wrote:It was common before the modern era of computerized fuel injection to specify at least a couple of different heat ranges for plugs for an automobile engine; a hotter plug for cars which were mostly driven mildly around the city, and a colder plug for sustained high speed highway use. This practice has, however, largely become obsolete now that cars' fuel/air mixtures and cylinder temperatures are maintained within a narrow range, for purposes of limiting emissions. Racing engines, however, still benefit from picking a proper plug heat range. Very old racing engines will sometimes have two sets of plugs, one just for starting and another to be installed once the engine is warmed up, for actually driving the car.
Perhaps use this strategy to stop the plugs on the richer cylinders from fouling in a few days. ???

There is a lot of experience on the board -- is this strategy too risky if the heat range change is minimal?

To be clear: I am suggesting using more than one heat range simultaneously.
Good thought. I went one heat range hotter with the new plugs I put in last night. I didn’t think of doing it only in the richer cylinders though.

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

Post by GRTfast » Wed Sep 11, 2019 12:55 pm

Any thoughts on a slick way to fill in the notch on my air gap manifold, to completely isolate the upper and lower plenum? I was thinking of making a filler piece out of some 1/4" thick G10, then using some epoxy to glue it in.

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

Post by Kevin Johnson » Wed Sep 11, 2019 1:06 pm

Maybe FR4 is a wiser choice?

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

Post by F-BIRD'88 » Wed Sep 11, 2019 1:46 pm

I would make a carb spacer that has this divider attached. The divider would match up to and be also supported by the existing partial divider.
It has to be very sturdy.

Do you have another carb to try? Eg a 750 dp.

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

Post by GRTfast » Wed Sep 11, 2019 1:47 pm

Kevin Johnson wrote:
Wed Sep 11, 2019 1:06 pm
Maybe FR4 is a wiser choice?
Probably, but I have some G10... :D

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

Post by GRTfast » Wed Sep 11, 2019 1:48 pm

F-BIRD'88 wrote:
Wed Sep 11, 2019 1:46 pm
I would make a carb spacer that has this divider attached. The divider would match up to and be also supported by the existing partial divider.
It has to be very sturdy.
That is a good idea.
F-BIRD'88 wrote:
Wed Sep 11, 2019 1:46 pm
Do you have another carb to try? Eg a 750 dp.
I do... doesn't pull at WOT like this one does though. I know, I'm asking for a lot.. :lol:

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

Post by F-BIRD'88 » Wed Sep 11, 2019 2:35 pm

Does the other carb eliminate the drivability issue?

Should have gone multi carbed 650++ hp power capability
Street ability of a single street carb.

The perceived wot difference may be more seat of the pants impression than real measured acceleration difference.

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

Post by GRTfast » Wed Sep 11, 2019 2:41 pm

F-BIRD'88 wrote:
Wed Sep 11, 2019 2:35 pm
Does the other carb eliminate the drivability issue?

Should have gone multi carbed 650++ hp power capability
Street ability of a single street carb.

The perceived wot difference may be more seat of the pants impression than real measured acceleration difference.
I wouldn't say it eliminated it, but it wasn't as prevalent. It wasn't an apples to apples comparison though, because I didn't have the o2 wideband with the 750. I could have just been rich in that zone, which basically fixes the symptom.

I have a fairly calibrated butt dyno, the 1050 is definitely noticeably faster. With the 750, rolling into wide open throttle in second gear pulled hard, but the car hooked up. With this carb, the same throttle application rate from the same speed in second gear will light the tires off. The car is pretty light and rear weight biased, so it still pulls hard when spinning, but there is no doubt that it like the bigger carb.

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

Post by Brian M » Wed Sep 11, 2019 3:44 pm

Here you go. 1/8" thick in the middle. 1/16" thick welded on each side. Fits tight but can be taken out and back in anytime. Used a 1/2" 4 hole spacer with a 650DP. This helped low speed driving but didn't completely cure it...with a 4 speed and a 52 year old sloppy driveline. You know, worn splines and excessive gear backlash.
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Re: Low speed (1500-1800 RPM) driveability issue

Post by F-BIRD'88 » Wed Sep 11, 2019 4:20 pm

The big throttle carb will seem more powerful as more air is allowed for the same throttle angle.

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

Post by GRTfast » Wed Sep 11, 2019 7:07 pm

F-BIRD'88 wrote:
Wed Sep 11, 2019 4:20 pm
The big throttle carb will seem more powerful as more air is allowed for the same throttle angle.
I understand.

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

Post by Firedome8 » Wed Sep 11, 2019 7:22 pm

Tie rap on booster or some other method to correct redirect the charge in the offending cylinder / cylinders ?

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

Post by BILL-C » Wed Sep 11, 2019 8:01 pm

With a light car and 3.25 rear gear, 1500-1800 rpm, and such a large carb, you are still cruising on the idle circuit . You can make 30+ hp before pulling a drop of fuel through boosters if the secondary throttle plate stop screw is opening plates even slightly at idle. It is very common for 1 booster to start pulling fuel before the others, causing light throttle fuel distribution issues. Full throttle fuel distribution has absolutely nothing to do with light throttle fuel distribution. Very common to see the fuel distribution change with rpm and throttle position. Set up a GO-PRO or similar camera over carb and go for a test drive. I'll bet you will be surprised at what you see. Oversized carb and intake ports contribute to problem also.
Carlquist Competition Engines

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

Post by David_viny » Wed Sep 11, 2019 9:09 pm

Light flywheel/clutch? That will definitely contribute to the pulsing feeling at low rpm.

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

Post by GRTfast » Wed Sep 11, 2019 9:10 pm

Brian M wrote:
Wed Sep 11, 2019 3:44 pm
Here you go. 1/8" thick in the middle. 1/16" thick welded on each side. Fits tight but can be taken out and back in anytime. Used a 1/2" 4 hole spacer with a 650DP. This helped low speed driving but didn't completely cure it...with a 4 speed and a 52 year old sloppy driveline. You know, worn splines and excessive gear backlash.
Damn that is slick, looks like the way to go. Thanks for sharing.

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