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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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Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Tue Sep 10, 2019 8:54 am

I posted this on another forum, posting here to cast as large of a net as possible.

What I am about to describe is the only remaining behavior that I don't like with my setup.

My engine (494 HP. 532 ft*lb):

0.030 over iron block 454
Eagle Crank
H beam rods
9.5:1 compression
Brodix BB2+ rectangular port heads
Federal Mogul forged pistons
0.558 lift 235 @ 0.050 duration hydraulic roller, 107 degree lobe seperation (1500-5600 RPM operating range)
RPM airgap with 1 inch phenolic 4 hole spacer
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 78's
Sec MJ 98's
Primary PV 8.5
Sec PV plugged
Primary PVRC jets .073
HEI distributor, 18 initial, 39 all in at 3100 RPM, 10 degrees vacuum advance
1 7/8 primary headers into 3.5" collectors into 3 inch glass packs

The car is a 4 speed manual, 3.27 rear end, 2500 lbs without me it it. When I am driving at a constant speed at low rpm (1500-1800) with jetting anything leaner than 13:1 during this condition, the engine seems to have a very light intermitten miss (low level muffled popping sound) and slightly surges. It is minimal, but noticeable and annoying. Cranking compression is 175 psi +/- 3 psi on all cylinders, leakdown less that 2% (engine is healthy).

If I change to 0.035 IFR's the mixture goes into the 12:1 range and the issue is maybe 75% better, but still not completely gone. As you might guess, the spark plugs do not like that rich of a low speed cruising mixture, as it doesn't take long to foul them driving around with that setting.

This engine has always had a bit of a weird fuel distribution issue where number 1 is richest, and number 2 and 8 are leanest when I jet the carb symmetrically from side to side (based on spark plug reading). I've never done a full throttle plug chop, so I can only say with confidence that the distribution issue is present at idle/cruising (normal street driving). I've experimented some with asymmetric low speed jetting a bit, doesn't seem to make much difference.

I suspect that my dual plane intake is the culprit, but I don't want to abandon it if I don't have to, as it makes gobs of torque in the street driving rpm range, and runs great in all other conditions aside from what I have described above.

Questions:
1. I currently have a 1 inch 4 hole phenolic carb spacer, this was needed to solve a fuel boiling issue (Florida summer stop and go traffic is brutally hot). Would an open spacer help to achieve a more even mixture distribution?

2. Is a single plane intake the real solution? If so, what is the best one for a street big block with a cam that signs off at 5600 RPM?

3. Is it normal that the engine needs an overly rich mixture to run smoothly at the very bottom of the cam operating range, or is this rich mixture just a band-aid for the uneven fuel distribution issue?

Any insight/suggestions are welcome and appreciated.

mag2555
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Re: Low speed (1500-1800 RPM) driveability issue

Post by mag2555 » Tue Sep 10, 2019 9:34 am

How much HP would you say the motor is making?

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

Post by GRTfast » Tue Sep 10, 2019 9:36 am

mag2555 wrote:
Tue Sep 10, 2019 9:34 am
How much HP would you say the motor is making?
Made just under 500 on the engine dyno.

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

Post by Circlotron » Tue Sep 10, 2019 9:44 am

What plug gaps?
Wider gaps may help to reduce this problem.
No cost to try.

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

Post by LoganD » 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.

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

Post by BobbyB » Tue Sep 10, 2019 9:49 am

What is your idle speed and vacuum at idle?

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

Post by dustinm » Tue Sep 10, 2019 9:53 am

It is likely going to want more timing the leaner you go.

Get an open spacer

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

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

Circlotron wrote:
Tue Sep 10, 2019 9:44 am
What plug gaps?
Wider gaps may help to reduce this problem.
No cost to try.
stock gaps for the bkr6e11's 0.043

Nice suggestion, I hadn't thought of that. I will open them a bit and see what happens.

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

Post by GRTfast » Tue Sep 10, 2019 10:12 am

LoganD wrote:
Tue Sep 10, 2019 9:46 am

Get rid of the 4-hole spacer.
dustinm wrote:
Tue Sep 10, 2019 9:53 am

Get an open spacer
A theme is forming... :lol:

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

Post by GRTfast » Tue Sep 10, 2019 10:26 am

BobbyB wrote:
Tue Sep 10, 2019 9:49 am
What is your idle speed and vacuum at idle?
idle speed is ~800 RPM, vacuum is around 12 inches

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

Post by BobbyB » Tue Sep 10, 2019 10:46 am

How did you decide on 18 degrees initial timing?
Have you tried higher or lower initial timing while keeping total to 39 degrees?
Do you know the ignition timing when you feel the engine surging?
Can you easily hook up a hand vacuum pump to your vac advance and play with the timing while cruising at the surge speed to see what happens?

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

Post by GRTfast » Tue Sep 10, 2019 10:50 am

BobbyB wrote:
Tue Sep 10, 2019 10:46 am
How did you decide on 18 degrees initial timing?
Have you tried higher or lower initial timing while keeping total to 39 degrees?
Do you know the ignition timing when you feel the engine surging?
Can you easily hook up a hand vacuum pump to your vac advance and play with the timing while cruising at the surge speed to see what happens?
I have played with the timing a lot. I've had as much initial as 30, as little as 15. Idles the smoothest and has the best general mannerisms with the current amount. anything over 28 degrees at idle (18 plus the 10 from the vacuum advance) makes the car "shake" more, and the engine sounds a bit labored at idle and in the lower RPM range. I'm not opposed to playing with it more though. During this recent set of tuning sessions for the issue described in the OP, I tried 16, 18, 20, 22, and 24 initial (all with 39 all in at around 3000 rpm) without it having a noticeable effect.

Advance at 1750 RPM cruising is about 36 degrees based on the current timing curve and the vacuum advance added in.

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

Post by BobbyB » Tue Sep 10, 2019 11:00 am

Sounds like you have the timing well sorted. What about the timing at surge speed, do you know what the timing is when it surges?
Do all the plugs read about the same? If what LoganD says about bungs in the each header tube is happening it looks like you could see the difference in AFR from the lean cylinder to the rich cylinder.

Is your carb 4 corner idle?

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

Post by GRTfast » Tue Sep 10, 2019 11:04 am

BobbyB wrote:
Tue Sep 10, 2019 11:00 am
Sounds like you have the timing well sorted. What about the timing at surge speed, do you know what the timing is when it surges?
Do all the plugs read about the same? If what LoganD says about bungs in the each header tube is happening it looks like you could see the difference in AFR from the lean cylinder to the rich cylinder.

Is your carb 4 corner idle?
Timing at the surge area is roughly 36 degrees (26 mechanical, 10 vacuum) based on the timing curve and how much vacuum advance the distributor has. I do have some uneven mixture issues as stated in the OP:

"This engine has always had a bit of a weird fuel distribution issue where number 1 is richest, and number 2 and 8 are leanest when I jet the carb symmetrically from side to side (based on spark plug reading). I've never done a full throttle plug chop, so I can only say with confidence that the distribution issue is present at idle/cruising (normal street driving). I've experimented some with asymmetric low speed jetting a bit, doesn't seem to make much difference. "

The carb is a 4 corner idle.

I will trying an open spacer next, and some larger plug gaps (both suggestions made in this thread).

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

Post by CamKing » 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
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