FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: c9zx on February 24, 2025, 10:02:34 AM
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Has anyone tested the Edelbrock RPM intake with a notch in the plenum divider like the Blue Thunder intake? Thanks Chuck.
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I cut the divider down on most of my RPM intakes.
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Thanks for responding. Is it a notch similar to the SBF rpm air gap or a notch the full width of the divider? Do you have any information on the difference the cut makes? Chuck
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I cut them down full length and bull-nose the divider edge.
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Thank You! Chuck
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a one inch spacer or a supersucker,would probably help too.
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Does the cut down provide more air flow? Trying to understand why you’d do it. I am using a RPM intake too. Same question on spacer?
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Originally I thought it was for the three barrel
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Does the cut down provide more air flow? Trying to understand why you’d do it. I am using a RPM intake too. Same question on spacer?
You're essentially giving it more of a plenum and you're allowing each plane to pull equally.
The spacer can do all kinds of things, including adding plenum volume, straightening out the air/fuel charge, etc. I've seen a 2" Super Sucker add 35 hp just by itself. Doesn't happen a lot, but they almost always make a positive change of some sort.
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Maybe I am just unlucky, but I have tried all varieties of carb spacers, 1/2", 1", 2", open, 4 hole, tapered, "Super Sucker" style, and I have yet to have ever found any "magic bullet" spacer. Not so much on the dyno, as the slow steady pull on a dyno is very different from how vehicles are typically operated in real life, but rather at the dragstrip, where changes can be measured in thousands of a second, or tenths of a MPH. I have yet to find a spacer that was worth even 3 hundreths of a second over the 1/4 mile, over any other spacer, or no spacer at all. I normally use a plastic or phenolic spacer, mainly to help keep the fuel in the carb cooler, the thickness determined by hood clearance. With engine dyno`s, more than once I have seen more power on the sheet, actually result in slower dragstrip performance.
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You’re just unlucky....hahaha
I've ran them countless times on the dyno and on customer cars at the track. There have been a few instances where I didn't pick anything up, but more times than not, I've seen increases. A good spacer will also straighten out A/F ratios sometimes.
Also more times than not, if you make a good change on the engine on the dyno, it will reflect that at the track.
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One of the things I learned when doing all the testing for my book is that somewhere in most engines there is a horsepower bottleneck. Testing on a 400-425 HP engine, I found that a lot of the intake manifolds were very close to the same in terms of peak power output, suggesting that the heads and/or cam were the limiting factors rather than the intakes. Later, testing the same manifolds on a 600 HP engine showed significant differences between them, indicating that some of the manifolds may have been the limiting factor.
Point is, if the power bottleneck is not in the intake, spacer changes aren't going to help much. But, if the engine wants more plenum volume than the base manifold can provide, adding a spacer can have a huge effect. My spacer testing has ranged from killing 30 HP (on a single 4 tunnel ram when fitted with a Super Sucker), to adding 35 HP (on a strong 460 with an Edelbrock Performer intake). Most of the time I've seen an open spacer add 5-10 HP, but certainly not always.
The only sure way to tell if the engine wants a spacer is to test it.
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Cutting the divider and/or using spacers can sometimes really screw up the distribution in some FE dual planes. Sometimes back to back tests will net a little power, and AFR still looks okay measured in the collector, but individual cylinders have things going on that will scare your pants off. Some of your favorite dual planes, when coupled with the wrong spacer or divider job, can end up making #1 and #4 drastically rich, like in the 10’s, and #6 and #7 dangerously lean, like 17:1! Blend that together in a collector and it looks just fine. Some intakes have better A/F distribution than others, and some are really affected by spacers or divider mods. I would recommend a 1/4” four hole, or open phenolic spacer, for 90% of street driven dual planes. They will be more responsive than some of the changes that appear to show power gains on a dyno. Individual cylinder AFRs can affect long term health of your engine. I have spent a lot of time analyzing thousands of passes down the drag strip, and many years of dyno testing, and I will guarantee that everything that makes more power on a dyno does not necessarily correlate to in-car or on-track success. Intake manifolds and headers are two things that you should test on the track, if you want to be sure something is better.
Also, most of the time, a transition spacer on a dual plane tends to increase booster signal, which usually means the part throttle cruise will get rich on you. Usually some work on the idle/midrange air bleeds is needed to dial in your cruise AFR because it is pulling harder on the boosters.
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My thanks to everyone who took the time to reply and share valuable information. Chuck
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Cutting the divider and/or using spacers can sometimes really screw up the distribution in some FE dual planes. Sometimes back to back tests will net a little power, and AFR still looks okay measured in the collector, but individual cylinders have things going on that will scare your pants off. Some of your favorite dual planes, when coupled with the wrong spacer or divider job, can end up making #1 and #4 drastically rich, like in the 10’s, and #6 and #7 dangerously lean, like 17:1! Blend that together in a collector and it looks just fine. Some intakes have better A/F distribution than others, and some are really affected by spacers or divider mods. I would recommend a 1/4” four hole, or open phenolic spacer, for 90% of street driven dual planes. They will be more responsive than some of the changes that appear to show power gains on a dyno. Individual cylinder AFRs can affect long term health of your engine. I have spent a lot of time analyzing thousands of passes down the drag strip, and many years of dyno testing, and I will guarantee that everything that makes more power on a dyno does not necessarily correlate to in-car or on-track success. Intake manifolds and headers are two things that you should test on the track, if you want to be sure something is better.
Also, most of the time, a transition spacer on a dual plane tends to increase booster signal, which usually means the part throttle cruise will get rich on you. Usually some work on the idle/midrange air bleeds is needed to dial in your cruise AFR because it is pulling harder on the boosters.
Blair, headers were one of the items I am refering to about more power on the dyno equaling slower time slips at the track. Years ago, my buddy had his Super Stock 396/375 HP B Chevy on a Super Flow 901 engine dyno. On the dyno, the engine picked up close to 30 HP with smaller 1 7/8" primary tubes and 3" collectors, instead of the 2 1/8" primaries and 4" collectors that he had been using. But at the track, the "extra" power netted slower ET and MPH numbers on the time slip. Put the big stuff back on, and the ET slips came back. Considering a dyno pull usually starts around 3000-3500 RPM, and , at 600 RPM per second, slowly accelerates to the higher RPM, is opposite what his car was doing at the track. His car was a SS/F 4 speed 66 Chevelle, and he was usually dumping the clutch at 8000 RPM or higher, which dragged the RPMs downward, and then back up, but at a much higher rate than 600 RPM per second. I personally have found the same situation with carb jetting and header collector length. We found that "ideal" A/F ratios on the dyno often made the car slower on the time slip, until we jetted the carb up again. Similar to jet extensions, windage trays, etc, on the dyno the engine is sitting stationary and level, but when you get into a 2 or 3 foot wheelstand, and the G forces that come with a 1.2 60 foot launch, the oil and fuel behaviour is very different. Similar to how the air intake temps, heat from the headers, and space above the air horn of the carb, can be drastically different between the wide open, temperature controlled spaces on a dyno cell, compared to the much tighter confines of an engine compartment, with the hood only an inch from the top of the carb. I guess that`s why we don`t race dynos!
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I am with Jay on this one. I have dyno tested several engines where the spacer made 35 hp difference, and where a spacer made absolutely no difference. It came down to the engine heads, camshaft lift/duration, and compression needing more airflow to make the additional power. The spacer on a higher horsepower engine helped give more plenum volume which the engine was needing. On a lower 400-450hp engine the spacer did not help on a SP intake, but on a 600hp engine with same intake manifold it made 12 more horsepower. Also, on every dual plane intake I have dyno tested with the divider cut down, there was a torque dip that could not be resolved with spacers until the horsepower reached near 600 hp level. At some point, spacers, timing, carb jetting, valve lash adjustment made absolutely no improvement in power or torque. Leaning the carb out or changing valve lash will cause the torque to pick up, but horsepower to go down or vice versa. Just my dyno experiences, but I have to go with what worked or didn't work for me. Joe-JDC
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My thanks to everyone who took the time to reply and share valuable information. Chuck
This is one of those subjects that unless someone has a very specific combination or question, we can only speak in generalities.
It's worth a forum search to see what different members (especially posters in this thread) have found to be successful involving plenum dividers and spacers.
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I guess I agree in generalities that you simply need to try the spacers and see what happens. I used to think that an open one inch was "the ticket" on the Performer RPM packages I was building, until a customer just insisted I try his 4 hole 'cuz it worked so well in his truck. I stuck it on and watched as it made a ton more torque below TQ peak and lost nothing upstairs. Since that lesson it's been "bolt it on and try it", and I usually keep a couple on hand for testing. Then you need to do that same thing on track because in that environment you will be transitioning through the RPM band at varied loads and speeds instead of the steady controlled rate of acceleration on dyno. To Blair's point - I usually pull and looks at a few plugs as well because that averaging in the collector is a "real deal", and sometimes the reason that changes show "no gain" as one hole gets better while another gets worse. Some EMC guys (Scott Clark... )were monitoring eight O2 sensors for individual cylinder tuning.
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Has anyone tested the Edelbrock RPM intake with a notch in the plenum divider like the Blue Thunder intake? Thanks Chuck.
Are you talking about the 1" to 1.250" little notch towards the back of the plenum divider on Blue Thunder Intake ? Which, there were some other aftermarket manifold companies also had cut them.
If so, the companies that did cut those notches, were so that the 3 Barrel Holley could be used on there manifolds.
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In some cases with EFI it's best to notch, as you should have the idle air control able to affect both sides.
With a carb, when Edelbrock first started notching the BBC and other manifolds I called and asked a guy I knew in their dyno shop (not even sure if the shop exists anymore) , and they said they saw no reason not to and most of the new releases would be notchied. However, they told me leaving the ends as a flow director worked pretty well and of course, ensure a blunt edge.
I never did a back to back test, and as others said, other factors in play, but I continue to do it, along with a little rubbing on the plenum roof turn.
I attached a pic of a BT and an Edelbrock RPM for fun
The BT was for EFI, the RPM was on my Mustang and really did well on a 489 inch FE with a 1000 cfm carb, went on to sell the combo to a forum member and had great street feedback as well.
As far as spacers, same as others, some you expect will do great things, some you expect will do nothing, and in the end, the engine decides :)
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is it true that a duel plane intake will take a bigger carb than a single plane?
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At WOT, check what the vacuum reading is on the intake manifold. If it is more than 1.0" of vacuum, then the engine can use more carburetion. Not that it needs more, but if you have 1.5", 2.0" then you definitely need a larger carburetor if you are trying to maximize the set-up. I just got off the dyno Monday with a small engine that made 437hp with a 650 on dual plane, and installed a SP intake with 930 BG carb and made the same horsepower. The dual plane had 1.1" of vacuum, and the BG 930 had .9". When the engine reaches its potential, then go with the smallest carb that does the job and be happy. JMO, but I can live with that. Joe-JDC