Tuning a modified Big Block 427
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Instructor
Joined: Apr 2004
Posts: 152
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From: South Beloit IL
Tuning a modified Big Block 427
Has anybody ever had this problem ? I built a 427 Big Block (forged crank, crower rods, trw pistons, comp cams solid roller cam 292 duration .666 lift) I have CNC ported Dart 320 aluminum heads with matching Dart single plane intake. A Chuck Nyutten modified 4150 (850 cfm) carb. The fuel system is an Aeromotive A1000 pump with matching bypass regulator. -10 feed line, -8 return line, -6 lines to carb.
The problem is when I read the spark plugs the front two plugs have a nice brown color but as you go back down the cylinders on both sides the plugs go lean. to the point where the rear two cylinders on both sides are almost completely white.
I have changed intake gaskets, checked the fuel system and have adjusted fuel pressure from 6.5 psi to 7.5 psi. I have changed the jetting in the carb even to the point of putting 99 jets in the rear just to see if there was a change. Each time the same results. The front plugs look great but fade to lean as they go back.
I'm running out of ideas and nobody seems to have heard of this problem before. The car runs well and has run 11.3 @ 122 in 1/4 mile. I have a proshot fogger ready to spray away but I don't want to try it til I get the engine running right.
Anyone have any ideas ??????
The problem is when I read the spark plugs the front two plugs have a nice brown color but as you go back down the cylinders on both sides the plugs go lean. to the point where the rear two cylinders on both sides are almost completely white.
I have changed intake gaskets, checked the fuel system and have adjusted fuel pressure from 6.5 psi to 7.5 psi. I have changed the jetting in the carb even to the point of putting 99 jets in the rear just to see if there was a change. Each time the same results. The front plugs look great but fade to lean as they go back.
I'm running out of ideas and nobody seems to have heard of this problem before. The car runs well and has run 11.3 @ 122 in 1/4 mile. I have a proshot fogger ready to spray away but I don't want to try it til I get the engine running right.
Anyone have any ideas ??????
Burning Brakes
Joined: Jan 2003
Posts: 755
Likes: 32
From: Roswell Georgia
Changing the jetting should have had some effect on the problem. If you are sure that fuel is getting to the secondary fuel bowl and the jets aren't blocked so some reason, I would recommend checking out two areas: (1) make sure that the secondaries are fully opening; and (2) make sure that you don't have a vacuum leak somewhere leaning out the mixture to the rear runners.
Vacuum leaks are easy to find. With the engine running, I recommend spraying bursts of WD-40 around all the gasket mating surfaces for the heads and manifold plus any vacuum port(s) in the manifold. If there's a leak, the WD-40 will cause a slight increase in RPM. BTW, if you do this with ether (starter fluid), you run a servere risk of fire if there's a backfire.
Hhope this helps!
- Greg Smith, Perpertual Corvette Mechanic
Vacuum leaks are easy to find. With the engine running, I recommend spraying bursts of WD-40 around all the gasket mating surfaces for the heads and manifold plus any vacuum port(s) in the manifold. If there's a leak, the WD-40 will cause a slight increase in RPM. BTW, if you do this with ether (starter fluid), you run a servere risk of fire if there's a backfire.
Hhope this helps!
- Greg Smith, Perpertual Corvette Mechanic
Race Director
Joined: Jan 2000
Posts: 13,015
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From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
You have a pretty serious high rpm 427 there with that combo....
Usually #6 runs the leanest on BBC's...just the way of the world with firing orders and such. The 4/7 swap cams tend to help overall mixture distribution somewhat.
I find that #2 seems to be the sloppiest fat coolest running cylinder on several that I mess with that use large open plenum intakes. Interestingly the addition of a 1" spacer drastically changes the cylinder temps and plug readings. On the dyno the cylinder temps overall came much closer in line with the simple addition. Hood clearance keeps me from running it on the street, but at the track I use it and it's much better.
I think you're dealing with intake distribution issues. You can spend some time tweeking it by shaping/extending/shortening runners and reshaping entries..but a flowbench would be helpful there. That said, I've seen the #2 issue on a Super Victor, a Dart, a Holley Strip Dominator and a Victor Jr.
My center cylinders run much warmer than the outers under light cruise/no load conditions. But at WOT they all get about the same as well as plug coloring. But I did do some intake work on mine to try and help it out. Again the spacer realy helps the center cylinders too.
JIM
Usually #6 runs the leanest on BBC's...just the way of the world with firing orders and such. The 4/7 swap cams tend to help overall mixture distribution somewhat.
I find that #2 seems to be the sloppiest fat coolest running cylinder on several that I mess with that use large open plenum intakes. Interestingly the addition of a 1" spacer drastically changes the cylinder temps and plug readings. On the dyno the cylinder temps overall came much closer in line with the simple addition. Hood clearance keeps me from running it on the street, but at the track I use it and it's much better.
I think you're dealing with intake distribution issues. You can spend some time tweeking it by shaping/extending/shortening runners and reshaping entries..but a flowbench would be helpful there. That said, I've seen the #2 issue on a Super Victor, a Dart, a Holley Strip Dominator and a Victor Jr.
My center cylinders run much warmer than the outers under light cruise/no load conditions. But at WOT they all get about the same as well as plug coloring. But I did do some intake work on mine to try and help it out. Again the spacer realy helps the center cylinders too.
JIM
Le Mans Master
Joined: Apr 2001
Posts: 5,436
Likes: 1,253
From: Sarver Pa
2021 C1 of the Year Finalist - Modified
Try checking your plugs after shutting down the engine on the track and coasting onto the return road. Your plug readings will be different after returning to the pits. You may just have a distribution problem at low RPM. Also you can experiment with building dams in the intake with epoxy and popsicle sticks. I know it sounds weird , but it works !
Pops
Pops
Melting Slicks
Joined: Feb 2001
Posts: 3,330
Likes: 13
From: Europe , Luxembourg
Hi
Wasn't there a intake insert called turtle or so ?
Would that help ?
Edit due : found it back, here http://www.brodix.com/Turtles.html
Günther
Wasn't there a intake insert called turtle or so ?
Would that help ?
Edit due : found it back, here http://www.brodix.com/Turtles.html
Günther
Last edited by WESCH; Nov 18, 2005 at 11:17 PM.
