I was curious if anyone have any suggestions for the best carburator to use with a 274 high energy comp cam? I'm running the cam in a sec 350 with 10:1 compression. And stainless steel valves with z28 springs on the heads. The intake manifold is the spread bore q-JET manifold that was on the 73 vetts. I was thinking about a holley 650 spread bore double pumper. Would that be sufficient or should I look into a different carburator?
The spreadbore Holley's work good I think the other size besides the 650 in an 800, (unless there's different sizes in modern designs), so the 650 should work really well. I ran one on a 355 with a 494 lift cam, 10-1 compression, 4.11gears in my t-bucket and it would make you lose your stomach when you nailed it.
years ago when I played with a couple spread bore Holleys, they had problems with being old, and leaking from places that can't be easily fixed. And that was 25 years ago. If you can find a good Q jet, and can get it working properly, it would be great. Otherwise you're better off with a square flange intake and a Holley 1850 or 3310 or an Edelbrock 600 or 750. Which, is up to you, they'll all work, they'll all take some messing with to get working right.
My first choice would be a Q-jet. Second choice would be a Holley, but if I had the Holley I'd probably wish I had gone with a Q-jet
The Summit Racing is a good carb. Dual inlet, no gaskets below the fuel level so no leaks. Comes with vacuum or mechanical secondary's, 650 to I think 850 cfm. I'm running the small one on a Ford 460 and one on a Ford 302 and am happy with both.
You size your carb to the intake. Unless you are looking to impress someone with your size. In '73 the vette would have run a carb in the 600-700 CFM range so the Holley spread bore in 650 CFM should be close to what the intake was designed to use. You really only need a double pumper carb when your engine does not pull enough vacuum to pull substantial fuel through the circuit. If you are nailing it from a dead stop that could cause you a need for the second pump. This is just idle information here. No need to pay any attention to it.
My 1st choice is always the Q-Jet, that's what's on my Coupe, been using them for 45+ years. Now, if your not comfortable in finding a good, not messed with core, and investing the time and money in learning and building your own carb, then you can go to the folks at Chevrolet Quadrajet (quadrajetpower.com) and buy one for your engine setup. I would strongly suggest picking up the phone and giving them a call and have a conversation with them. They will make the carburetor for your engine so it will idle correctly and produce the drivability your looking for. I know Summit is selling there branded Qjets, but these are all being done to 1 recipe, so what can you expect from it? Same goes for all the Edelbrocks, FSP, Quick Fuels, Holleys, etc. You have to have the knowledge and parts to change and tune them to work properly. Or ..... what we end up doing on many of our customers cars at our Hot Rod Shoppe is buying the "Brawler" Street series carbs (when you can find them these days), 600 or 650 cfm double pumper units depending on the engine. Availability of carburetors, intake manifolds, camshafts, steering columns, wheels and a huge list of other items are hardly available for the last year.
The stock Q-Jet was 750cfm. Why would you want to go smaller with a 650??? My first choice for you would be the original Q-Jet, if you still have it. My first choice for me would be Carter 9800s 800 CFM TQ. It is pretty much a bolt on swap; and is factory calibrated for a medium performance SBC. This would be my second choice for you. And remember the rule when choosing carburetors: my dog is bigger than your dog! Jon
He was asking about using a 650 cfm double pumper, of a size appropriate for his engine. The 750 cfm Qjet has a small primary, and large secondary side with an air valve, so it pretty much "sizes itself" to the air demand of the engine. The carbs I suggested in my first post also have a demand based secondary side. I'm sure you know all this already....but some folks might not, and it's a good thing to understand. Not all carbs with the same CFM rating will behave the same on different engines. Displacement, as well as engine modifications, and drivetrain characteristics, as well as intended use, all affect the size carb that an engine will work best with. If you can find a good Thermoquad, yeah, go for it! They also have a small primary, and a demand based secondary.
Jim - the 750 Q-Jet is fixed (primary) 150, variable (secondary) 600. The 800 TQ is fixed (primary) 200, variable (secondary) 600. I do not know the relationship on the spread-bore Holleys. By 1976, GM was using the 800 Q-jet (200/600) on virtually everything. Chrysler was using 800 TQ's and 850 TQ's, and the engineers found the transition from primary to secondary was smoother with the 200 primary instead of the 150 primary. Personally, I am running the 800 TQ's slightly larger "brother" (850 TQ 250/600) on my 350 Pontiac, but it is probably a tad more radical than the OP's SBC. If I ever get caught up with orders, will try a 1000 TQ on that 350 on advice from an ex-factory Pontiac racer. Jon.
274 high energy comp cam is rated to 6K rpm. No need to go larger than 650 cfm. If you're running a short gear and a stick, go DP. Joe
Max 650cfm. 600cfm would be perfect. Easier to adjust if not too big carb and You really don´t need bigger than 600cfm or max.max 650cfm. Elelbrock 1406 + needle & spring set. You need to work evening or more, but You can have it perfect. Easy choise is Holley 600 - 650 db. Just change right size jets and testdrive right powervalves. ah
This is the reason I prefer square-bore double-pumper carbs over spread-bores w/ vacuum actuation. Lean spikes can occur w/ spread-bores during WOT that give you that 'seat-of-the-pants' feel that is actually quite deceiving. A square-bore w/ secondary pumper will completely cure that problem. Joe
Joe - not arguing, but have not seen this on dialed in spread-bores (Carter and Rochester). The biggest mistake even relatively experienced Q-Jet rebuilders make is failure to replace the secondary cam AT EVERY REBUILD. Feel so strongly about this, I include them in all our Q-Jet kits. Have seen many with 0.025 plus wear. Due to fulcrum placement, 0.025 wear is 0.050 lift NOT done to the secondary rods. The lean spikes we have seen are failure to lift the rods at the correct time due to the wear. Jon.
Running that same cam in my 289. .5 higher compression, TF aluminum heads, Edelbrock RPM Manifold….and a Holley 625 cfm…here’s my carb info…
No argument here, King. You definitely have more experience w/ the spread-bores than myself. The Holley is a preference I have for my current combination after extensive tuning problems w/ the Edelbrock AVS recently. They are great carbs for economy cruisers but lean out when they are kicked in the pants. Any thoughts on the AVS? I'm not the first to experience this problem. O2 sensors pick up lean spikes on these things. I'm assuming the same experiences w/ the Q-Jet are for reasons you covered. Good stuff. Joe
Joe - I have no experience with the new carbs from Edelbrock. The original Carter AVS carbs were (opinion) the worst 4 barrel Carter ever made. Even Carter thought so. Lots of folks like the Holleys, as today, the variety of tuning parts for Holley for most enthusiasts is beyond compare. I have somewhat of an advantage when it comes to genuine Carter or Rochester parts. Jon.
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