Low speed (1500-1800 RPM) driveability issue
Posted: 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.
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.