Author Topic: Obscure SBC Question (Read 6285 times)

Reklaw · « **on:** April 30, 2009, 08:22:27 PM »

Here’s one for the engine guru’s.
I have always been of the belief that a dual plane inlet manifold provides better signal to the carburetor at low revs. It does this by virtually dividing the inlet into two separate circuits. On a SBC, that’s cylinders 2,3,5 & 8 on the RHS and 1,4,6 &7 on the LHS.
On my engine, I am running GM Performance Parts Fast Burn heads, GMPP Vortec inlet manifold (because the heads are of raised runner design) and GM Vortec Inlet Manifold Gaskets. I figured I’d have no problems if I went all GM. Now, these gaskets have a raised section that seals a pair of ports, but doesn’t seal one port from it’s neighbour. Please see the photo below, the blue is the raised section, the black just gives the gasket the right shape and strength. To me, this seems to negate the basic principle of a dual plane manifold i.e. two totally separate inlet circuits.
So why am I interested in such an obscure issue?
I’m at my wits end trying to find why my engine has four “white” plugs and four “black” plugs (reading taken after local driving, not WOT) The four “white” plugs are 2,3,5 & 8, the RHS of the manifold and carburetor.
It’s not a carburetor issue, I have swapped carburetors (and adjusted the idle mixture) and end up with the same result. It’s not the vacuum take offs either. A leakdown test shows results of 10% to 14%, so nothing major wrong down below. The EGR ports in the Air Plenum have now been blocked off, so not that either. When I pressurize one side of the air plenum, the air comes right back out the other side, so they are not separate, presumably because of this issue with the gaskets.
Before I rush out and start pulling the engine to bits yet again, I thought I’d throw the question to the members of the forum.
So my basic questions are …….
*Am I better off finding other suitable intake gaskets that seal each port from its neighbour, therefore getting back to a true two circuit situation?
*Any thoughts on whether or not this is causing the white plug / black plug issue, ‘cause I can’t find anything else that’s causing it?
[IMG=700x525]http://i343.photobucket.com/albums/o477/Reklaw327/InletGasket.jpg[/IMG]

TPI-383 · « **Reply #1 on:** May 01, 2009, 12:46:39 PM »

Reklaw,
What carby are you using. I dont think the gaskets are a prob, I am guessing the blue bits should squash down enough(if there not thick) for the gap to be small enough not to worry about. Only a prob if it does not seal to the outside,you could try new gaskets that are of a different type if it worries you,mine are composite on my TPI and it has alloy corvette 89 mod heads and a holley stealth ram manifold. Most carby's have little machined channels in the base plate below the throttle body that will equalize the manifold a bit, same as a leak across ports. I would have a look for a leak on the exhaust on the side of the white plugs. An exhaust leak will cause a motor to lean out enough to turn plugs white I have had it before on a 6 cyl with twin exhaust,half the plugs white.Mine was on the flange gasket and I could not hear it,only found it when I pulled the headers off to paint them.Maybe worth a look.
Tim

Reklaw · « **Reply #2 on:** May 01, 2009, 01:11:02 PM »

Thanks Tim, normal carby is a Holley Street Avenger (Squarebore) which I replaced with a 650 vac sec (Spreadbore) to see what difference it made. Plug colouring remained about the same. Think I will put some other inlets gaskets on, but won't rush into it. I guess an exhaust leak could let air into the cylinder, depending on valve timing etc. I'll have a look at that, thanks.

87Z51 · « **Reply #3 on:** May 01, 2009, 01:23:28 PM »

I see where you are coming from but when compressed the area available to flow betwween the ports would be minimal.
In any case the airflow on the other size that is "leaking ' across would have the same mount of fuel in it so you should not be getting a lean condition as you would with a external air leak.
As a note I have never seen a SBC intake gasket that does not have 100 % seperation between the ports for the reasons you state
Gasket change is all you can do to prove your theory

Reklaw · « **Reply #4 on:** May 01, 2009, 01:54:01 PM »

Hi 87Z51, Thanks for your input. The leakage from one side of the air plenum to the other is major, like no restriction at all. I have plugged the EGR holes in the plenum, so that can't be the cause. Below is a shot of my most recent, and rather unconventional test setup. I pumped 10lb air into one side, thinking it should just pressurise unless there is a leak of some sort. All rockers were loosened, so all valves closed. The air just came straight out the other side, hence my thought that it's the lack of divider in the inlet gasket. There was plenty of silastic on the plenum divider so it shouldn't have just been leaking under the test plate. Only other thing I can think of is a casting fault in the manifold, but that's unlikely. Think I'll ask some questions about why GM made them like this in the first place. This is my first foray into Vortec, I'm starting to wonder why I did it now. Still have a problem with only 8 small bolts to hold down the inlet manifold.
[IMG=700x525]http://i343.photobucket.com/albums/o477/Reklaw327/InletGasketTest.jpg[/IMG]

87Z51 · « **Reply #5 on:** May 03, 2009, 05:38:13 PM »

Quote

Originally posted by Reklaw
The leakage from one side of the air plenum to the other is major, like no restriction at all. ... The air just came straight out the other side, hence my thought that it's the lack of divider in the inlet gasket.

So it leaks from one side to the other , but both sides should have the same fuel content so that doesn't account for the lean cond .
Edelbrock make a intake for Vortec style cylinder heads (RPM Air-Gap #7516) that has the center divider under the carb cut down 1" to enhance top end power.

The plugs? are they white "normal" or white lean?
The black ones; are they normal or black rich?
What does it run like?
If it is leaning out on 4 cyl ( or running rich on the others ) you would expect to notice a drop in performance
What is idle vac reading, is it constant?

Reklaw · « **Reply #6 on:** May 03, 2009, 08:14:53 PM »

If the Edelbrock has a cut down plenum divider, then this issue of leakage from one side to the other shouldn't be the cause of the "white" plug "black" plug issue, right ?

Which is correct, the "white" or the "black" is a part of the conundrum.

The photo below shows the differences. #2 looks a bit lean because it doesn't have a full ring of colour around the base ring. The "black" plugs are dry, it's not oil.

This motor is new, well it's done 400 miles now and it's on its second set of heads (last set were faulty) same readings before and after head change. Makes it hard to comment on how it runs, but it "feels" that it could be a bit better.

Ignition timing is about 15 or so degree initial, it seems to like that better than the usual 8 to 10 degree.

Vacuum at idle is not totally steady, it oscillates between about 12 and 14 inches. Vacuum take off point is the power brake port, which is across both sides of the carb/manifold.

Went over cam timing with a degree wheel/dial indicator when the heads were off last time. All looks good there.

To check vacuum takeoffs (which are on the "lean" side of the manifold), I took it for a run today with the headlight/heater port plugged. Same result with the plug readings.

Don't normall subscribe to Icons, but :p

[/img]

87Z51 · « **Reply #7 on:** May 03, 2009, 10:44:12 PM »

If I saw those plugs without knowing the story I would say the "black" cyl are runing rich. If running normally I would expect the insulator to be cleaner.
As you say #2 looks a little clean but # 3+5 look normal while #8 doesn't look alot different from the other side of intake.

Bit out there , but what are the plug lead resistances like?
If you were losing spark intensity it could account for the incompletel combustion in the chamber

77CVT · « **Reply #8 on:** May 05, 2009, 09:41:22 AM »

FWIW, porcelain part of the plug should be whitish/grey so to me #2 & #5 look normal to me. The others look too rich IMO.

Agree on the ignition leads. I never fit leads with more resistance than 10k. Good quality ones should be around the 4-5k resistance.

;42

Reklaw · « **Reply #9 on:** May 05, 2009, 11:12:48 AM »

Hi guys,
Thanks for the plug lead resistance info - haven't gone down that path before. Leads are Top Gun, their spec says 5K to 6K per metre. Resistances range from 2.8K on shortest lead to 5.7K on longest lead (1060mm) so I think they are all OK.
One thing remains to be done - get it out on the open road, shut the engine down at about 3K revs, coast to the side of the road (hoping I can turn the steering wheel) and check plug readings again, instead of just looking at local driving readings.