650cfm carb vs 750cfm?
Lead Driver
Joined: Sep 2002
Posts: 9,540
Likes: 2,031
From: North Texas
CID x max RPM / 3456 ... 460 x 5000 / 3456 = 666cfm
What is done to the engine to make that HP? No accessories? 410hp is basically gross rating...I would run the 650 and install/borrow a vac gauge to check vacuum restriction at WOT. I would assume the place should have this data on hand if they have the ability to dyno the built engine. If you have restriction, then yep would probably need to step up in size. Also, depends what you consider max RPM for the thing since the HP peak is 4800rpm.
What is done to the engine to make that HP? No accessories? 410hp is basically gross rating...I would run the 650 and install/borrow a vac gauge to check vacuum restriction at WOT. I would assume the place should have this data on hand if they have the ability to dyno the built engine. If you have restriction, then yep would probably need to step up in size. Also, depends what you consider max RPM for the thing since the HP peak is 4800rpm.
Tuned
Joined: Dec 2017
Posts: 387
Likes: 22
CID x max RPM / 3456 ... 460 x 5000 / 3456 = 666cfm
What is done to the engine to make that HP? No accessories? 410hp is basically gross rating...I would run the 650 and install/borrow a vac gauge to check vacuum restriction at WOT. I would assume the place should have this data on hand if they have the ability to dyno the built engine. If you have restriction, then yep would probably need to step up in size. Also, depends what you consider max RPM for the thing since the HP peak is 4800rpm.
What is done to the engine to make that HP? No accessories? 410hp is basically gross rating...I would run the 650 and install/borrow a vac gauge to check vacuum restriction at WOT. I would assume the place should have this data on hand if they have the ability to dyno the built engine. If you have restriction, then yep would probably need to step up in size. Also, depends what you consider max RPM for the thing since the HP peak is 4800rpm.
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Cross-Country
Joined: Dec 2023
Posts: 72
Likes: 6
Yup... near as I can read thats it.....so any fedback on if it'd be good/ bad and how so if I went with recommended 750 cfm carb up from 650cfm?
EDIT... I went back and re read your input. Makes sense even to this non tech guy. i did some research YouTubing carbs... kinda sounded like carbs are all about air flow and engine is the thing pulling air so maybe the builders strongly recommended 750 cfm due to 460 engine? Im guessing going up from 650 to 750 would make the already gas mpg even worse? If thats even possible lol.
EDIT... I went back and re read your input. Makes sense even to this non tech guy. i did some research YouTubing carbs... kinda sounded like carbs are all about air flow and engine is the thing pulling air so maybe the builders strongly recommended 750 cfm due to 460 engine? Im guessing going up from 650 to 750 would make the already gas mpg even worse? If thats even possible lol.
Last edited by HubbaHubbaLife; Dec 22, 2023 at 02:05 PM.
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Tuned
Joined: Dec 2017
Posts: 387
Likes: 22
Yup... near as I can read thats it.....so any fedback on if it'd be good/ bad and how so if I went with recommended 750 cfm carb up from 650cfm?
EDIT... I went back and re read your input. Makes sense even to this non tech guy. i did some research YouTubing carbs... kinda sounded like carbs are all about air flow and engine is the thing pulling air so maybe the builders strongly recommended 750 cfm due to 460 engine? Im guessing going up from 650 to 750 would make the already gas mpg even worse? If thats even possible lol.
EDIT... I went back and re read your input. Makes sense even to this non tech guy. i did some research YouTubing carbs... kinda sounded like carbs are all about air flow and engine is the thing pulling air so maybe the builders strongly recommended 750 cfm due to 460 engine? Im guessing going up from 650 to 750 would make the already gas mpg even worse? If thats even possible lol.
Logistics Pro
Joined: Aug 2010
Posts: 3,716
Likes: 583
From: Chillicothe
What are you using the truck for and is it going in a truck?
A 650 isn't going to totally kill performance, you may lose 15 hp way out on the top, but if you are just driving around or towing you dont need to go buy a 750.
A 650 isn't going to totally kill performance, you may lose 15 hp way out on the top, but if you are just driving around or towing you dont need to go buy a 750.
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Cross-Country
Joined: Dec 2023
Posts: 72
Likes: 6
no your mpg will not decrease by going from 650 to 750. Can you run the 750, yes you can with no ill effects. Will there be any benefits? No. Unless you're spinning up real high there's no reason to go high. To somewhat answer your question, you can have issues going to a carb that's too large as you'll run into lean conditions and stumbling, even when trying to compensate with larger jets. On the flip side, going smaller then what is recommended will cause no issues besides power decrease and could lower max rpm.
So I guess there we have it. stays a 650 CFM and I learned something. Now onto all these leaks
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Cross-Country
Joined: Dec 2023
Posts: 72
Likes: 6
Now there we go..... I heard back from the previous owner who had the engine tweaked.... he basically said the same thing. He's a retired Ford Race Engine guy. Guessing maybe he knows whats up... sold it cause of medial issues ... appreciate your input brutha
Folks love these old trucks down south
Hotshot
Joined: Jul 2020
Posts: 10,469
Likes: 2,631
From: Near Salt Lake City
Increased RPM is the reason for bigger carbs
Think Boss 302 and Z-28 Chev
Both 302 CID and both with a Holley 780 vacuum secondary
Both turned 8000 rpm (well the early Boss Mustangs blew up with their cast pistons)(and had a rev limiter)
Run them both and check the throttle response
I think you'll like the 780 but jet it down
Think Boss 302 and Z-28 Chev
Both 302 CID and both with a Holley 780 vacuum secondary
Both turned 8000 rpm (well the early Boss Mustangs blew up with their cast pistons)(and had a rev limiter)
Run them both and check the throttle response
I think you'll like the 780 but jet it down