My curiosity has gotten the best of me. Using O2s, (you still read the plugs anyway, don't you?) has anyone running the Edelbrock 2907 or 2927 noticed that the rear cylinders requiring more jet than the front? We're not just talking a couple of sizes but a differential of up to ten sizes. I've come across three or for racers that have reported the same finding. One told me he was running more jet in the rear and before he told me I said ten right? No, nine. In my case I just give the engine what it wants. It's been the same for the 2907, 2908, and the 2927. Spacer or no spacer.

I've read the entire YB thread on plenums and have yet to come up with an answer. What the heck's going on in there? Is it the manifold design or the plenum?

In the case of the 2908, I got a boxed up new one in a horse trade. It fit my AFR 335 ports perfectly. I'm using it now. The performance difference was zip, for twice the price. Still, the same differential in jetting is required.

Bruce, how are you determining that 10 jet sizes bigger are needed in the rear 2 cylinders? The O2 would average all the cylinders on each sided of the engine so I'm assuming your reading the plugs(?).....I would think you would see a measurable difference in the time slip moving 2 cylinders 10 jet sizes up or down?

If it's really that different cylinder to cylinder, then there is a serious distribution issue going on with that intake....

I agree it's a distribution thing.

I have an O2 in #8 cylinder and #1 cylinder. I use BLP jets exclusively and build all my own carbs. Currently I have 120s in the back and 112s in the front. The emulsion stack is the same front and rear as are all the air correctors. BLP jets are sized by orifice diameter, whereas Holley jets are sized by flow. I'm within a .5 AFR difference between the front half the engine and the rear, so I'm all set. Oh, and I use the 4/7 swap firing order. I use a K&N air filter on all my engines.

My question was if others were seeing the same differences in distribution. Comparing BLP jets to Holley jets is an apples to oranges comparison so I'd like to hear from Holley jet users. Is this a problem with all single plane single carb manifolds or just Edelbrock?

Something is way fuc’d up! My 648 that has powered my dragster to 195mph has a 2.400 blade dominator on it that is jetted 102 square and we are still rich. Maybe it’s that 100LL aviation fuel you run.

That said, I have never found anything on the time slip with staggered jetting or any combination of jetting ran more mph than square on any single plane manifold I’ve run on any of my engines both Firebird and dragster.

I run mechanical injection and 8-O2 sensors. I have made a lot of dyno pulls and a lot of passes with data logger. Got the thing perfect on the dyno then get to the track and the rear cylinders were leaner.

What was happening was the air has more of a ram air effect in rear cylinders so I jetted up those cylinders until everyone was the same. Sounds good but in reality it did not make any noticeable difference in ET which is why we do it right?

It is always good to try to get them all as close to the same as possibly but do not let it get you chasing your tail just to get perfect numbers. Of course you well know a carb is different than mechanical injection but we are still going for same tuning goals.

quote:

Originally posted by 1320racer:
Something is way fuc’d up! My 648 that has powered my dragster to 195mph has a 2.400 blade dominator on it that is jetted 102 square and we are still rich. Maybe it’s that 100LL aviation fuel you run.

That said, I have never found anything on the time slip with staggered jetting or any combination of jetting ran more mph than square on any single plane manifold I’ve run on any of my engines both Firebird and dragster.

Thanks for the 'you're not having to stagger answer'. Whose manifold and is it ported?

I can see why you're rich. a 102H jet has a big hole, .128". Flows 1,780ccs a minute. A 100H jet might help.

This thing was the same way on 93 octane. I reduced the jet for 100LL and the plugs were now easier to read. My jetting is equivalent a 93H 98H, five jet sizes. It's equally fat all around. I'll fix that.

10 or 12 years ago we did dyno testing on three fuels. Renegade 110, Sunoco 110, and 100LL. Same engine, carb and jetting, and flush the system between fuels. Difference in power? Zip. I wrote about the testing in YB's Fuel Section. Why is all 110-octane purple? Well, 100LL is blue and what would make it purple?

I pin my jets and the manifold of the 648 is an Edelbrock’s SV632 #2897. Its gasket matched and the plenum has been blended. Leaning my carb out might produce a quicker/faster time slip but it prints tickets and turns on win lights as jetted. I run Sunoco Supreme aka blue

Here's a 2907 I had Brandywine modify. Ugliest work I've ever seen but it's fast. Cut down about an inch too. All our plugs come out looking the same if that means anything. 1050 Dominator done by Don Garbinski. Had this manifold on 3 engines and it's always faster than what we took off.

I'm looking for another 2907 if anyone has one. I'd like to cut it down but without the blob of shizz to see what it actually does.

Seen a few with the floor raised like that. Cuts the floor puddling too. Plus.. Plus

I used to do a lot of dyno testing for Weiand when I had my shop and having seen what happens in the plenum area at high RPM's I can't believe, or recall ever seeing, puddling in the floor of an intake.
I'm telling you, at 7000-8000 RPM all he11 is breaking loose in that area. introducing smoke to the air-stream was virtually useless as it disappeared below the throttle blades.

Now raising the floor with a blob or one of those turtle things may direct the the airflow into the ports better, but I seriously doubt it would help, hurt, or even influence how the fuel is suspended in the airflow.
But I may be wrong.....

This message has been edited. Last edited by: imakehp,

I have port matched and plenum matched (to the 1250) modified Edelbrock 2927 with a one inch HVC Super Sucker and a APD Max Flow 1250 on a very decent 582.

I slightly leaned the idle circuit with an air bleed change from 46's to 51's but kept the intermediate (63) and high speed (29) bleeds as is from APD. As for the mains I am 16 fatter on the back jets, and the plug readings are near perfect. I should mention that this 1250 has a 4.5 power valve in the primary, hence the wide main jet stagger.
. I attribute this to a plenum issue, something to work on the next time the manifold is off the engine.

During the hotter summer months I tend to slightly lean the mains because the DA where I normally run the car is up around 5500 ft. (sometimes higher). I'm not willing to experiment with the other air bleeds, it runs fine and about all I can do is to skrrrrrew it up.

I've tried Sunoco Supreme, VP C15 and F&L SP3 and SP4 with the results being about equal, not the price but the results. Using a cost per run I would say the Sunoco Supreme produced the best value (Cost per result which equates to consistency).

Bob

I had to get into the lifter valley last year to inspect a possible lifter failure. At that point I changed out the 2927 for a 2908 (an R&M CNC modified 2907) looking for some additional hood clearance. It changed the character of the car. The lesson was that the 2927 was the wrong piece for my combination to begin with. Still the jet stagger persisted. At this point it works.

Couple of thoughts..

First, I question the scoop, they can mess up the air going to the carb causing screwy things to happen to the signal. Put and air filter on it and see what happens. I have seen adding a air filter fix many bad O2 curves!
Next, staggering jets on a single plane intake is similar to pissing into the wind. Maybe you would see something on and intake with a huge plenum, maybe!
The only other thing to look at is the plenum to runner roof transition. I have seen many stock intakes with really bad castings in this area and also the runner common walls. May need a clean up in these areas.