FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: c9zx on August 05, 2025, 09:31:14 AM
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Does anyone have any information on how good, or bad, the fuel distribution is in the Performer RPM intake? I have not found any porters in my area (central OK) with FE experience. There are one or two that can flow the intake but if there is a problem I don't know if I could trust them to even the flow without ruining the intake. Yes, at times I am risk adverse. Any data at all would be helpful. Thanks, Chuck (S)
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I have seen Joe equalize the intake runners and they do well. I defer to him if he adds detail.
As far as specifically fuel distribution, I am not sure that scratches the itch, but any sharp corner blunted to prevent shear and equal air flow should benefit wet flow distribution.
Barring his experience, I generally clean up an inch-ish into each runner, a little rubbing on the roof of the plenum (not a lot, just a little cartridge roll work where I can reach) and on the big inch ones I notch the divider and leave blunt (which Joe, for the record does not do and sort of pokes at us). I have not flowed every runner afterwards to give you a number, but Joe sure does.
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Even if you just looked at the runners and not the plenum, I know when Joe balances the runners for my stuff, the A/F ratio will pretty much match dead-nuts on for both sides. Even though the intake works well out of the box, there's a lot of room for improvement.
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This is one of the last Edelbrock RPM FE intakes I did for a local builder. This is very similar to the last five or six I have ported for customers.
As cast:
1. 285.22
2. 329.79
3. 334.24
4. 294.13
5. 303.05
6. 300.82
7. 307.50
8. 305.28 for average of 307.5cfm
Ported
1. 374.35
2. 374.35
3. 383.27
4. 374.35
5. 383.27
6. 383.27
7. 383.27
8. 383.27 average 379.93cfm Joe-JDC
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How much does the porting reduce velocity on engines under 428 (like 352 & 390's) and will it allow droplets to form on smaller engines making for less even A/F ratio?
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Incredibly good work Joe, hard to argue the benefit there on a good head
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In short, if you don't really know what you're doing, best leave it alone- you could do more damage than good, and it would be difficult to repair
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My thanks to everyone for your comments. Joe-JDC, thank you for being willing to share hard data, some would not. Mean-Gene, I'm going to take your advice and leave it alone, as well as the heads. It is not a race car and I don't have unlimited funding for this project from the "finance department".
One more question that is kind of related; would a .250-.300 thick open heat isolation carb spacer have the same effect as milling down the plenum divider?
Thanks Again Everyone, Chuck (S)
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The one thing a flow bench is helpful for is taking something as cast, and being able to modify the part in small steps to even out the flow. It does not matter what the actual number is, but the changes made to the part/s. If you have a head that only flows 265cfm, then bolting a stock RPM on the engine will work fine. If you have TFS CNC'd heads that flow 320-330 cfm, then the intake needs some work. I have taken the manifolds and using the highest flowing port brought all the other ports up to that number. Lots of work, but it keeps the manifold velocity where it is useful and not too big. I usually try to get the intake to flow 110% of the heads at camshaft lift for street, and upwards of 120-125% for street/strip. Has worked well for me and many customers for the last 30+ years that I have owned a flow bench. Joe-JDC
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Just note that although it 'is' generally the intention to create equal air flow within the intake, runner to runner, this in itself, for a number of reasons, doesn't guarantee that the fuel (original inquiry) will be distributed accordingly. :o
Scott.
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Just note that although it 'is' generally the intention to create equal air flow within the intake, runner to runner, this in itself, for a number of reasons, doesn't guarantee that the fuel (original inquiry) will be distributed accordingly. :o
Scott.
The whole purpose of my explanation was to show that with 30+ years of flow bench work and equalizing airflow the jetting can be equalized and respond without any one cylinder running rich or lean due to the manifold's design. I have proven this works many times over on dyno testing and actual racing experience. I raced Fords since 1969 on the drag strips around the USA while on active duty, and worked with race shops up until 2004 when I sold my Mach I. I have been to Engine Masters Challenge four times with engines personally, and have helped others for more than 10 years with placements from 6th to 1st, and my own personal 2nd and 4th. I try to be open and honest about what has worked and never gave bogus air flow cfm rates. Truth stands on its own merit. Joe-JDC
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Put an O2 in each primary and you will see that “balancing” the flow by sucking on individual runners with the others blocked off isn’t the whole picture. Scott has a point that I have to agree on. Looking at Mr. Joe’s flow data on a stock and a modified RPM manifold, notice the #6 and #7 runners in both cases flow more than the #1 and #4. If you tune up #1 and #4 to mid 12 AFRs at wide open throttle, you will find out that #6 and #7 are usually lean enough to scare you. Mid 11s on #1 and #4 will generally get #6 and #7 to a tolerable range. The lower side of an RPM has very good distribution, but the high side isn’t as good. Usually they will make more power at WOT if 1 and 4 are a little too fat, which still leaves 6 and 7 a little too lean. Part throttle cruise, lightly loaded, will show nice distribution to all eight cylinders. Fooling around with various spacers can help the high side distribution, or screw it up worse than polio, depending on the path taken.
In terms of distribution, the early 60s iron manifolds, Sidewinder, 428 CJ, and Ed RPM, in that order, have the most consistent AFRs measured in the individual primaries. The Ford PI and Blue Thunder are the WORST for distribution. Both of those will cook #6 and 7. You can’t fix it with jets, because 1,4,6, and 7 use the same piece of the carburetor. I discovered the problem working on Stock Eliminator FEs that are bound by rules to use specific manifolds. After that, I started looking at all of the dual planes. You can fix the 2x4 intakes with staggered jetting. The other neat thing when you start smoothing AFRs is that BSFCs can smooth out, and there is also a by-product of more power and torque. The closer it gets to “right”, it also starts trending towards helping the “terminal” runner design problems. I think pooling of excess fuel in the manifold improves as the overall tune gets better. You can wear one engine with one manifold out trying to correct distribution issues. Making some windows in the other plane into 6 and 7, and using different cam lobes on 1/4 versus 6/7 have shown trends in the right direction.
There is more going on in there than porting and dry flow testing will show. I can say for sure that most manifolds have design characteristics that can’t be fixed without cutting them in two and changing their mind in places you can’t reach too good otherwise.
I will say that it is a plus to have runners that “flow” more and more equally, but the distribution is a different situation altogether. If you look at AFR in collectors, it looks pretty good down there, where two high runners and two low runners mix together to create the number. An old C4 iron low profile manifold isn’t the best for power, but it is the best for distribution.
My two cents
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Blair, how do the HP 390/406 4v and 6v 390/406 manifolds fair?
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Just a read back on this whole thread,
This isn't a port or no port discussion or a flowbench or no flowbench discussion. It turned into two discussions. Are there gains in airflow and are there distribution issues?
The RPM intake has a flow problem, especially with a modern head, which Joe both identified and fixed. I would do that every time if using a modern head. I really doubt the distribution would be worse but that doesn't mean it would be better either, lots going on in a plenum.
As far as the fuel distribution, a dry flow bench doesn't show that and even more not in the dynamics of an engine, but keep in mind, generally getting air to flow minimizes the sharp edges which could pull fuel out of suspension. It could also make a port so fast in areas that it doesn't let it turn a corner. Additionally, you are one spacer away from things being very different from one pull to the next. I just saw the same dimpled poly spacer do nearly nothing on a 630 HP Joe ported Victor then added 18 HP on a Vic Jr SBF, in both cases changing collector A/F ratio significantly.
I have not made pulls with 8 O2 sensors, I have with 8 EGT sensors and realized they have you chase your tail for small numbers too, great data, but none will be exact, you are looking for extremes that could hurt.
I would be hard pressed not to make the weak ports feed the heads, I think it's a great idea and thanks for sharing. Blair, I also will make sure I continue to stay on the fat side seeing what you saw with 6 and 7, and likely worth reading all the plugs for one running way upstairs.
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Quite a while back my friend Scott Clark came to my place to help me dyno the big SOHC that I won with at Drag Week in 2015. I had prepped the headers for an O2 sensor in each primary, and Scott brought his 8 O2 sensor setup with him. Despite having an equal length, sheet metal intake and equal length primaries in the headers, our initial readings showed a wide variation in A/F from cylinder to cylinder, nearly 3 points from highest to lowest. This was surprising, in fact so surprising that we went to the extreme of swapping O2 sensors around to confirm the results. When we finally decided it was real, we began tuning the individual cylinders. This was an EFI engine, so we could change the squirt on each injector, and also change timing on each cylinder.
After spending the better part of the day on this we finally got the A/F on all cylinders within a half point, and all of them in the 13.0 to 13.5 range. The result? All that work netted only a 10 HP difference, from 980 to 990 HP. We adjusted richer and leaner, but 13.0 to 13.5 was where the engine wanted to be.
My take on this, especially with a carburetor and without an 8 O2 A/F system, is to read the plugs and look for anything excessively lean, then richen up the jetting to compensate, so that the engine isn't subject to damage with a lean condition in one or two cylinders. An equal flow intake is good, but it's not the whole story.
By the way, after I thought about the A/F variations in my big SOHC, it was worth noting that the intake was hand built, the heads were hand ported, and the header tubes were different shapes to fit the chassis, rather than all the same. I think these variations must have accounted for the differences we observed...
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I have done wet flow testing, sonic testing, and always go back to individual runner testing before finalizing the averages. IF and I say IF the individual runner is optimized first, then opening up the manifold for all eight runners to flow through the one port, the flow will be nearly identical in flow. I have tested that on single plane intakes, dual plane intakes, 6V intakes, 8V intakes, and TW, TR, HR 4V, HR 8V, TP intakes, etc., and every time if the one port is properly blended in the plenum, shaped with taper, the individual flow will be the same as all eight open ports. BTDT too many times to argue any more about it. I have flow tested Wilson CNC'd $2500.00 intakes that folks were having problems with fuel distribution on, and found as much as 80 cfm difference between end ports and center ports. I always try to get the end ports on single plane intakes to match the flow of the center ports, and on dual plane intakes I try to get those within 5% of each ports flow. The RPM mentioned above has a difference of less than 9 cfm between ports in a dual plane manifold that flows 383.27 cfm. If that is not respectable, than I surrender to anyone who can do better. Signing off. Joe-JDC
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I have done wet flow testing, sonic testing, and always go back to individual runner testing before finalizing the averages. IF and I say IF the individual runner is optimized first, then opening up the manifold for all eight runners to flow through the one port, the flow will be nearly identical in flow. I have tested that on single plane intakes, dual plane intakes, 6V intakes, 8V intakes, and TW, TR, HR 4V, HR 8V, TP intakes, etc., and every time if the one port is properly blended in the plenum, shaped with taper, the individual flow will be the same as all eight open ports. BTDT too many times to argue any more about it. I have flow tested Wilson CNC'd $2500.00 intakes that folks were having problems with fuel distribution on, and found as much as 80 cfm difference between end ports and center ports. I always try to get the end ports on single plane intakes to match the flow of the center ports, and on dual plane intakes I try to get those within 5% of each ports flow. The RPM mentioned above has a difference of less than 9 cfm between ports in a dual plane manifold that flows 383.27 cfm. If that is not respectable, than I surrender to anyone who can do better. Signing off. Joe-JDC
I don't think anyone is arguing with you. It's just you're only addressing one piece of the issue. I clumsily asked a similar question quite a while back and though I got a lot of useful feedback I didn't get to the heart of the matter in the way folks are discussing it right now. Fuel distribution involves bringing a gas (air) and a liquid (gasoline) to the combustion chamber in equal, homogenous proportions.
It helps that gasoline is so volatile but it's density is significantly different than air so complete atomization of the fuel and homogenous mixing with air is difficult to achieve in every runner across the rpm range.
I found Jay's painstaking efforts on the SOHC dang interesting. I'm more concerned about balancing for engine longevity than I am about performance but I think it's all tied together.
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What can you guys tell me about photo of spark plugs below?
Shelby intake copy of RPM, Break in time plus about 6 1/4 mile passes.
428 stock eliminator motor - Autolite plugs, MSD, VP-110, 36 degrees, 89/90 jets, 1” open spacer, 850 vac.
I changed up to NGK plugs 90/91 jets and they look a little better. I’m seeing 2, 6 & 8 slightly lean?
Can’t touch intake but can change up spacer, timing and jets. I’m still experimenting with timing and jetting but would like to bring #8 around. Staggered jets?
Thanks, Mike
(https://i.postimg.cc/nhhWQTPy/20240423-161446.jpg) (https://postimg.cc/svbmkPym)
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Hi Mike, I didn't think that intake was legal in Stock Eliminator. I thought only the 428 CJ iron intake and the C7 PI aluminum intake was all that could be used? They are always changing rules so maybe NHRA now accepts the Shelby lettered (Edelbrock RPM style ) intake? Curious if and when they changed that. I could be wrong, have been before LOL.
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Not sure what year, my manifold has Shelby name plate cast in manifold with no Edlebrock identification. The Shelby name plate manifold comes with a cut down divider. For a ‘67 Shelby in stock eliminator a Holley #3418 850cfm is also legal.
I’m surprised at the jet sizes I’m running and it appears I can go larger although my experience with FE’s is leaner is usually a little quicker.
NHRA Notes:
Cylinder Head Castings
1 - C7AE-A, C6AE-A, R, C8OE-N,(F5WE-A Stock Only)
2 – Carb sizes 1.750 X 1.750/1.562 X 1.562 or 1.686 X 1.686/1.375 X 1.437
3 – Alternate intake manifolds C7AE-C, C7AE-F and Shelby (nameplate cast in manifold)
4 – Alternate intake manifolds C7AE-H and C5AE-E Shelby nameplate cast in
5 – Carb number C5AF-BC 1.686 X 1.686/1.313 X 1.375 or BU & BT 1.686 X 1.686/1.375 X 1.437