dual plane manifold tuning

[dbusch]

found some interesting things on the dynojet today. SBF 310" engine is going pig rich above 7000 rpm. usually the thing will make power to 8000 rpm. A/F is going down to 11.9 at the top of the pulls near 8000. we tried going up several sizes on the air bleeds with very little change in A/F ratio. dropping jets leaned the low end numbers too much. manifold in question is a ported Stealth dual plane as per class rules. Previous manifold was a ported Air Gap with much smaller runner cross sections in the plenum. wondering if the smaller cross sections work better with the dual plane design, since the runner is the restriction, not the dual plane plenum. the Air Gap manifold did not go rich up top....

[let r eat]

sure you don't have a carb pullover problem?

[bill jones]

-What is the relationship of the jets diameters to the power valve restriction diameters?
-What do you have for: choke housing and choke? air filter? carburetor spacers? airfilter spacers?
-How much manifold vacuum are you seeing at those upper rpm levels?
-Is the plenum divider cut away or are you running the left carburetor bank isolated from the right bank?
-How are you measuring the air fuel ratio?

[dbusch]

dont know the specifics of the carb. it has a tall K&N air filter as per class rules. the Air Gap manifold didnt have the rich problem up top so i figured it was a manifold design problem with the Stealth. We used a wide band O2 meter. it has a plenum divider cut open (3" long by 3/4" deep) just like the Air Gap. no choke...this is a 650hp carb. i forgot to tell the customer to get a manifold vacuum reading until after he left the dyno. the carb builder said he could do a set of metering blocks just for the Stealth manifold but i figured it would be like beating a dead horse...

one interesting thing: we pulled jet out and a/f went to 13.4 at peak torque (5300 rpm) and the torque went up. that surprised me and made me think the wideband was shot. even still, it richened up to 11.9 by 8000 rpm.

[bill jones]

-I would suspect either the squirter nozzles are dripping, especially if they have the extension tubes and are .035"s or larger, and I have seen an occasional problem with fuel spewing from the bowl vents from the high air speed creating a vacuum on the vents.
-Did this manifold make any postive power over the other manifold? enough to consider keeping it?

[R.Brown]

Check all of Bill's ideas... but to go from 13.4 to 11.9 is alot for shooter pullover. I usually see 1 A/F number max, and if it is an unmolested HP650, it will have the straight shooters, not the tube type, so I doubt you would even see .4 A/F change.

Sometimes the raw fuel from a plug wetting out (from a fuss point in manifold distribution @ 8000 or wherever you are experiencing the rich point) will get the gauge pointing you in the wrong direction. How does the A/F 'curve' look on the graph. Does it sit steady then go rich only at the top end, or does it gradually go from lean to rich during the pull?

Before going into the carb, try an open 1" spacer and see how that works.

RB

[dbusch]

there was a power loss with the Stealth, approx. 10hp or so. the reason i was concerned was because everything about the Stealth pointed towards more rpm potential and hp. it flowed better than the Air Gap on all the runners, some being 10 cfm more. Cross sections were much better, especially near the plenum. Here is the a/f curve:

5300 12.8
5500 13.5
5750 13.3
6000 13
6300 12.6
6500 12.5
6700 12.3
7000 12.2
7300 12.1
7500 12.0
7700 12
7900 11.9

i dont like it when the math doesnt jive with real world results!!!

[dbusch]

changed back to the Air Gap and the air/fuel goes rich up top again...so it isnt the manifold doing this. One other thing that we changed was the cam since the time when the A/F was right. went with about 20* more overlap at .050" than before. i wonder if the 2.5" exhaust (class rule) is a mismatch with the current amount of overlap, making it go rich at higher rpms when the backpressure starts climbing. Super Stockers run open headers with tight cams and get away with it, but i dont think you can do that with a restrictive exhaust system...

any input here would be appreciated!

[bill jones]

-Several things that I always think about when I see engines too rich at high rpm is soft valve springs, and an ignition system that is laying down and the float levels too high and the fuel pressure too high.
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-Have you ever checked the ignition timing during a dyno run---clear up to 8000rpm?
-What are you running for sparkplug gap?
-What do you have on there for an ignition system?
-Do you have any idea of the what the manifold vacuum is at the higher rpms?
-What about the exhaust back pressure? ever measured it?
-How about explaining what the 2-1/2" exhaust system consists of?
-Is the 2-1/2" that you have on the dyno exactly the same system as what you use on the race car?
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-One thing you might try is smaller needle and seats.

[dbusch]

these dyno runs are on a chassis dyno (dynojet). i tried to get the customer to check manifold vacuum but the dyno owners didnt have a gauge. the exhaust system has 1 3/4" 28" primary Kooks headers, 3" collector that necks down to 2.5" with a foot long section into a Kooks X-pipe, then another 2' section into a pair of 2.5" Dynomax Bullets. i think he runs a digital 6 box for ignition.

never checked timing during a run...maybe it is retarding above 7000 rpm. interestingly, on an engine dyno with open headers, this engine will make peak torque at 6200 rpm, whereas on the chassis dyno with full exhaust, peak torque comes in at only 5300. power stops at 7200 on the chassis dyno, and will go to 7800 on the engine dyno. looks like the exhaust system is the limiting factor here???

[bill jones]

-what is the "class" and what is the "class rule" about the exhaust?
-What did the air fuel ratio do with the open headers?
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-If you are stuck with the small exhaust in the vehicle yet you've seen lots better on an engine dyno with open headers then the entire excersize here is figure out how to get close to the open header numbers with the "rules".

[dbusch]

class is NMRA pure street. the rule says it must have 2.5" exhaust after header collector and extend to within 12" of the axle tubes. must have mufflers.

air/fuel was 12.8 using wideband O2s on Kaase's engine dyno and the open headers.

[bill jones]

-so what are the rules about mufflers?
-Can you handbuild a set of big LONG mufflers that only have a very short 2-1/2" length" pipe and then build the sections of 2-1/2" pipe like venturis?
-what about the wall thickness? exact inside diameter of the the 2.5" pipe.
-does the rule state you have to run ROUND tube?
-I'd make the 2-1/2" pieces out of a thin wall stainless steel or mabe even breyllium, as short and as precise as they can be made and then I'd make'm so that you can remove them real easy so you can keep the inside polished.
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-Do the rules state how long the collector can be? every inch of collector is going to be a good deal.
-Do the rules specifically state that you can only have two collectors? and that you can ONLY HAVE TWO exhaust pipes?

[jdeleon]

Dbusch
I'll second Bill's suggestion on watching your ignition timing. It does'nt take much cam thrust (.012"-.014" walk) to lose 6-10 degrees of timing.
Since you did make a cam change, and it must be a roller judging from the rpm you run.

[Bad95Gts]

Be careful about the wide band and chassis dynos.You need too back it up with EGTs.Also were is the wide band placed.Also with a Hyd Roller you have too be careful on the length of time it spends at those very high rpms.On an engine dyno it needs to be pulled very slow a couple of times starting at 6000 rpm up too max rpm too be sure valvetrain is stable.
Try the chassis dyno in a different gear and see if the richness up top still happens.