You won't feel it's too big til you bolt on a smaller carb, or a vacuum sec carb. Then you'll notice the bottomend come alive.
Without knowing how the whole car is set up (weight, trans, gears, etc.) you can't just say a 650 DP is too big. If you know how to tune a Holley a 650 isn't too big. I know of a guy that has a custom Holley that flows over 900 cfm on his little 289. Very driveable on the street and runs low 12's in the quarter. In my case I could never get the 600 vacuum secondary carb to open the secondaries quick enough, even with the lightest spring. The 650 DP cured that and runs just as well down low as the old 1850. To the OP, I prefer a 4-hole spacer if it's for a street car. Something a lot of people don't think about, did you ever look at the bottom of your carb's base plate and see the passages that connect each of the 4 throttle bores? Holley's intention was/is that those passages are closed at the bottom and the only openings are at the throttle bores. This is to provide even vacuum at the throttle bores but not allow spent gases from a cylinder on one side of the intake plenum in overlap to dilute the mixture of a cylinder on the other side during the intake stroke when running a dual plane intake. A 4-hole spacer will help keep those passages sealed.
I didn't say it was too big. What I said was: "you won't know if it's too big until you bolt on a smaller carb" It's really easy to tune a vac sec Holley to open the secondaries, all it takes is their spring assortment. If you cannot get a vac sec to open soon enough, then the carb is too big to start with, or something is obstructing the secondary vacuum passage to the vacuum pod, or something's wrong with the linkage. I ran a 650 DP on a mild roller 302 (in my 89 Ranger with a Toploader 4 speed) and it ran great, but I could feel a noticeable drop in accellleration when I worked the secondaries slowly, opening them gradually, the accelleration dropped off as the secondaries opened in the lower rpms. You don't really notice it when you nail the pedal, opening them instantly all the time. I replaced it with a modified 1850 600. I removed the choke horn, replaced the bowls with center hung bowls, changed the sec spring to a purple (next to the lightest spring) and rejetted it. With this carb, the secondaries would open a second or two after you nailed the pedal, you could feel them kick in, boosting the accelleration, instead of slowing it. When a carb is too big, it looses it's ability to pull fuel from the bowls in the lower rpms, due to the loss of the vacuum signal when the throttle blades are open too far, the only way to compensate for this is to use a huge accellerator pump shot, which in my opinion only a crutch to fix the problem, not to mention wasteful.
per:Holley instructions for setting the secondaries... those who “feel” a kick when the secondaries engage are actually feeling a flat spot during initial acceleration because the secondaries have already begun to open and have weakened the fuel delivery signal to the primary boosters. The engine is struggling to increase speed and what they actually feel are the secondary nozzles “crashing in” as the engine finally reaches the speed where it provides the proper fuel delivery signal to primary and secondary venturi. opening the secondaries early causes the situation described.
i did say it and i stick to it. sure a 650 will work really well. it wont make it hard to drive. the 650 may even make more power. i use 600 double pumper on my car. i dont like the vac secondaries either. they are ok for daily drivers but not all out performance. the low end responsiveness and efficiency will be better with a smaller carb. the smaller carb will get off of the idle circuit faster. regardless the original poster is talking about carb spacers. the 650 will work just as good on the top end of the rpm band but can use help on the low end. the 4 hole spacer will do this. the open space will hurt the low end and help the top end. i doubt they will have room for the spacer unless they have a scoop or cowl hood
With our circle track engines, we spent a whole day once playing with different spacers on the dyno. We messed with everything from 1/2", 1", 2" in both open and 4 hole. We even had the high dollar ones that taper into the plenum. Several pulls with each, optimized each time. We found very little difference between a 4 hole and an open. We did find some differences with heights. Patterns do develop based on the intake run but the gains were fairly small. From what I have seen in all our research, and considering you have a dual plane intake and very poor flowing heads with a too large carb, I think you should save your money. It would be better spent in correcting the intake to match the compression and ignition (single plane), better heads and a smaller carb. It's only a 302, correct? Work on the heads first. I don't think you will get anything out of a carb spacer other than bragging rights that you run a carb spacer. An open spacer may possibly help a bit by enlarging the plenum and giving the fuel a better turning path, but you still have stock flowing 289 heads and there goes anything you might gain. JMO, SPark
I've been around Holleys for 40 years, What I described was not a flat spot. The engine was not struggling.
This is obviously one of those smaller nuances of fine tuning an engines power curve but IMO, depending on what version cam you're using here.. it may be best to use an open spacer for your apparent midrange/upper end type combo. Adding compression and converter usually means that off-idle torque(which is not the same as responsiveness although they can run hand in hand) isn't something you usually want to trade for midrange and top-end power gain. And if that's the case.. a single plane won't affect the combo as much as it would others with less of both(compression/converter stall) and it seems that it would be better suited here. As you may already be aware.. a properly designed cam can easily help crutch weaker flow on both intake and exhaust. Especially on the exhaust side when using a greater amount of duration and lift on faster ramps. What many don't realize.. is that at higher rpm's even a full length mufflered street engine's intake tract will see noticeably greater pull as the intake valve comes off it's seat as a result of that extra exhaust scavenging. This is because the mass and velocity of gasses becomes the greater driving force at initial intake valve opening rather than piston pumping simply because there is not enough time(crank degrees) for the piston pumping to do that same job effectively. Adding cam spec's would help us to give more specific advice. And also IMO, there's absolutely no need to swap that 650DP with a smaller street oriented vac sec since it's just fine with higher compression and proper calibration on an engine this size. Depending on the gear and available traction.. you obviously just need to ease into the throttle based on what the engine/drivetrain/chassis allows in it's current configuration, is all. Vac sec's are more for the "set it and forget it" type crowd that can't.. or prefers not to.. modulate throttle angles based on what the cars combo allows.. and like to just "mash and go". Driving well.. can be just as important as how well the engine is built too and you certainly need to tailor both to work together. Basic rules are this: 4 holes will give better response and midrange torque while also helping to reduce reversion in the process. Split plenum spacers(2 hole) will help midrange and even give slight upper end gains as a reslt of that extra plenum volume. Too small/short a dual plane for the size motor being used(especially true for higher compression stroker motors) can also gain low end torque and resonse as well. Open spacers will help midrange and top-end while also potentially causing some reversionary disadvantages in the process with some cam designs. Except for overly large motors.. you almost always hurt response and off-idle torque. Merge style spacers will help midrange and top end as a result of increased volume/plenum sharing just as the open spacer would.. but also gives a boost to overall carb/manifold flow because air/fuel schear and turbulance is reduced at the end of the carbs throttle bores and as it ebbs back and forth between both plenums/banks of the motor(taller spacers give greater turning radius off the bottom of the carbs bores and merge styles maximize this for any given spacer height). Unfortunately, they also prove to increase reversion/reduce carb signal at low flow/rpm to some small degree. Despite the merge styles shortcomings.. the overall benefits usually far outweigh the downsides and more power is made if everything is tuned around the airflow changes. I use them whenever the engine is cammed well enough to warrant them(and not cause undue reversion in the process.. but even that can be dealt with by using anti-reversion plates too).. in undersized intake/carb situations.. and especially when I really don't give a damn about shaving a few ft lbs of torque at lower rpm ranges when using a looser stall. In fact.. I use this mod on just about every dual plane config I run on my street cars these days(I'll also be doing an RPM Air Gap mod like this for initial/interem use on my Comet until I go full tilt with an epoxied/flowed Parker intake with bigger solid roller cam).. because it gives the best of both worlds with very little loss at the bottom of the rev range. A best of both worlds hybrid type design that produces VERY close to the rpm/flow potential of a single plane street intake while still producing a fatter power curve down low and well into the midrange as any good dual plane wass designed for in the first place. Aside from modding your dual plane into something like this.. I'd go with as large a 4 hole merge style spacer as you can fit under the hood. And if the current air cleaners size is sufficient enough already.. I'd even trade some of that height for more spacer volume under the carb. Then progressively tickle it off the line.. mash it.. and enjoy a fatter and noticeably extended power band. If this mod didn't work at all?.. none of the big time engine builders would be doing similar plenum mod's to win engine masters challanges.. and I certainly wouldn't be spending 15-20 hours to do it on my own junk. It can be done even quicker than that.. but I prefer to weld mine on since it allows greater taper at the bottom of the spacer as it meets the intakes split plenum and takes longer. Anwho.. for this combo(although without cam spec's and more details.. I'm just stabbing blindly with much assumption) my vote is the 4 hole merge style. Then on to the open style as second fiddle choice. Then the split spacer(2 hole).. then the 4 hole.
This is your opinion.. It's making a generalization that's just plain wrong. DP's are the carbs that most of the "set it and forget it" crowd uses and who rarely if ever modulate the throttle opening to suit the engine's needs.(which is the whole reason for tailoring a vac sec carb to open sooner as opposed to Holley's settings and the net result of what you suggest he should do with the DP in modulating the throttle) The accellerator pump on the secondary side is primarily there just for the simple reason that the users of these carbs are the "mash the gas" types who want the secondaries open instantly. Modulating the DP carb to better suit the engine's demands on a DP is the same thing as tuning a vac sec carb's secondary opening, only the net result of using a vac sec to do so is better fuel economy as the vac sec carb lacks the accellerator pump on the secondary side that's constantly squirting fuel needlessly when the throttle is worked too hard.
Well of course it is. As is most of the other input around here. The point you make could be generalized just the same if we say that each carb can be adjusted and tailored to fit the same fuel curve. BUT.. a VS carb can be set to "out think" an overzealous right foot.. whereas the driver must adjust throttle angle to achieve the same thing with a DP. And regardless of all that fiddling.. you can never adjust driving style to make a vac sec do what a double pumper can on some engine combinations. The basic fact is this. A double pumpers dual accellerator pump design is intended to be used with larger plenums and wide open throttle conditions because it fills the gap as the circuits transition from one to another. It can also more easily be made into a street carb.. than a vac sec can be made into a race carb because of that simple fact. Accellerator pumps fill holes as throttles transition and a VS can never do that in racier situations(larger plenums/heavier vehicles that need immediate enrichment) no matter how well you tune them. I know many, including myself, that have tried doing it with larger hi-flow power valves, 50cc pumps and cams that prolong the pump shot and they never.. ever.. run like a DP will. Which makes a lot of sense considering that Holley says the same thing about the reason which it created the DP design in the first place. Not to say that a well tuned VS carb can't be made to work well enough on most hot street cars.. just that.. IMO.. a DP can be made to run even better, is all. Without that second pump shot to fill the transitional voids.. you'll never be able to completely work out that flat spot. If that wasn't the case.. almost ALL would be using VS carbs on racier builds. Which we all know just isn't true if you see a lot of popped hoods on the faster cars running carbs. Anyways.. the thread was about the type of spacer which would best suit this particular engine.. not which carb would be best. Plus.. he already has the carb.. and apparently he likes it well enough to not want to spend more cash just to change it out because "him, them, or they" might think it's better suited for his combo. His car is light, and if he has enough gear, the DP will be faster and funner to drive from my experiences so far. Of course, it's all in the tune and combo used, so YMMV.
I love these kinds of discussions. You get to read about many years of experience, lots of engine theory, a good dose of opinion tossed in for good measure. And in reality, no matter what spacer this guy uses, he might get 5 hp increase? He could do THAT with a strategically placed JEGS sticker! Oh, and IF he can get the hood to close. By the way, beautiful looking manifold a couple posts up there!!!
another thing with the hood clearance is the air filter size. back in the early days when i was first starting to play with hot rodding cars, i was playing with air filter heights. at the time me and my friends were doing the air filter shuffle with different sizes. so the 4 cars were 2 69 mustangs and 2 chevelles. we had 14" drop base filters. all filters were k&n. so we started out with a 3 on the mustangs and figured out we could fit a 4" under the hood. we felt a noticeable seat of the pants acceleration increase, both in the drivers seat and passenger seat. so put the 4 inch in the chevelles and both had the same type of gain. next we got a 5". it would not fit the mustangs but the chevelles could easily fit it, and again noticeable gain over the 4". then we did a 6" and again you could feel the difference. so my intended point here is instead of spending the money on a spacer and loosing air filter room, get the tallest filter you can. also find the flattest lid. that usually gives you 1/2" more filter room. i would be interested to see if scooper can try a comparison on his new filter set up with a traditional 14" round air filter. i understand he doesnt have room under his hood but if he leaves the hood off he can do it.
