Going 521 w/Blue Thunder 2x4

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Post  the tree man on December 1st 2019, 12:13 am

To maximize the potential of the ported SCJ heads I just got I've decided to upgrade to a 521 from 466 and will also be adding a custom,larger SFT cam from Cam Research.I'm really want to retain the current intake for appearance on the T-Bolt but curious if it will service the above combo.It's the small port version but I'll hog it out.On the 466 I'm running 2x600VS Holley which I think may be a bit small on a 521 but possibly not with this intake.Wondering if anybody has run this intake on the 500+ cube engines and what carbs?

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Post  Mark O'Neal on December 1st 2019, 8:46 pm

Is Art still out there?

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Post  the tree man on December 2nd 2019, 10:43 am

Art?

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Post  rmcomprandy on December 2nd 2019, 11:14 am

the tree man wrote:To maximize the potential of the ported SCJ heads I just got I've decided to upgrade to a 521 from 466 and will also be adding a custom,larger SFT cam from Cam Research.I'm really want to retain the current intake for appearance on the T-Bolt but curious if it will service the above combo.It's the small port version but I'll hog it out.On the 466 I'm running 2x600VS Holley which I think may be a bit small on a 521 but possibly not with this intake.Wondering if anybody has run this intake on the 500+ cube engines and what carbs?

A.T. (Art) Francis at Blue Thunder offered both Standard port size and Cobra Jet port size dual-quad manifolds for the 385 series Ford big block engine.
I believe that only the ports were different for the last 2 inches and the actual flange heights were the same. This may have changed in later released manifolds but, I don't think so.

Just measure the height of the flange ... if it is tall enough then it will cover the taller SCJ ports.

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Post  the tree man on December 2nd 2019, 12:21 pm

That's correct.Lots of meat on this one at the port to match the CJ hole.When I bought this intake I could not find any dealers with a new one in stock and nobody,including Blue Thunder,seemed have a date when they would be available again so I spotted mine on Ebay and scored it.Another thing I noticed with this intake was a significantly wider swing in exhaust temps than the Ford Racing dual plane,single 4bbl that was on the motor initially.I wanted to try a couple open spacers to see if that would help with distribution but even with a 4" teardrop scoop the air cleaner is tight to the hood so they won't fit.If that would help with distribution I'd cut the divider in the intake a bit but I need to be sure before doing that.Anyone with thoughts on that?

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Post  rmcomprandy on December 2nd 2019, 4:00 pm

the tree man wrote:That's correct.Lots of meat on this one at the port to match the CJ hole.When I bought this intake I could not find any dealers with a new one in stock and nobody,including Blue Thunder,seemed have a date when they would be available again so I spotted mine on Ebay and scored it.Another thing I noticed with this intake was a significantly wider swing in exhaust temps than the Ford Racing dual plane,single 4bbl that was on the motor initially.I wanted to try a couple open spacers to see if that would help with distribution but even with a 4" teardrop scoop the air cleaner is tight to the hood so they won't fit.If that would help with distribution I'd cut the divider in the intake a bit but I need to be sure before doing that.Anyone with thoughts on that?

That manifold, (as any 2 plane, dual quad, lower profile manifold), will never have good port to port distribution if ANY fuel is not fully atomized; especially at part throttles.

The carb flanges and under-bore radius' can be ported to get it a lot closer; (To detailed to even start directions here).
Other things which will make distribution closer is to mount the carbs backwards, similar to an FE Ford engine using a bellcrank for throttle actuation and making them actuate together, 1 to 1, not staged.

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Post  the tree man on December 2nd 2019, 5:56 pm

Thanks for that info.I read a couple posts where you had some experience with this intake and commented on it.I mounted my carbs conventionally for ease of throttle linkage and 1 to 1 but wondered if because the intake was based on the FE design that backwards may make a difference.A bunch of extra stuff was included with the intake when I purchased it including the reversed linkage so I'm going to spin them around and see what happens.Tuning the carbs also seemed fussier than usual even though it has a solid 11" of vacuum at 1000 rpm,I modified the Duraspark/6AL with a decent curve 22 at idle/36 all in at about 2000 and I'm running an AFR gauge so that cuts down on the guess work.I've had a number of dual quad set ups on street cars,love them,but on tunnel rams so this was new territory for me.I've reduced the IFR's about 25 percent and put jets in the metering plates but I also had to change powervalves,secondary springs,pump shot and cams,etc.Nothing went untouched yet I've always found the 2x4 600VS versatile and very forgiving.Any thoughts on sizing for the 521?Appreciate the input and any other suggestions.

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Post  rmcomprandy on December 2nd 2019, 6:45 pm

the tree man wrote:Thanks for that info.I read a couple posts where you had some experience with this intake and commented on it.I mounted my carbs conventionally for ease of throttle linkage and 1 to 1 but wondered if because the intake was based on the FE design that backwards may make a difference.A bunch of extra stuff was included with the intake when I purchased it including the reversed linkage so I'm going to spin them around and see what happens.Tuning the carbs also seemed fussier than usual even though it has a solid 11" of vacuum at 1000 rpm,I modified the Duraspark/6AL with a decent curve 22 at idle/36 all in at about 2000 and I'm running an AFR gauge so that cuts down on the guess work.I've had a number of dual quad set ups on street cars,love them,but on tunnel rams so this was new territory for me.I've reduced the IFR's about 25 percent and put jets in the metering plates but I also had to change powervalves,secondary springs,pump shot and cams,etc.Nothing went untouched yet I've always found the 2x4 600VS versatile and very forgiving.Any thoughts on sizing for the 521?Appreciate the input and any other suggestions.

I have never used those manifolds on anything other than "street rod" type engines or Cobra kit cars maybe raced only once in a while; not anything for competition. Using 750 vacuum secondary or 660 center shooters or Carter/Edelbrock carbs have been my experience although 600cfm Vacuum Holleys should be fine for most anything.
Driveability and throttle response was really the only criteria. Carter/Edelbrock carbs seem to work fine forward but, a Holley or derivative seems to like being being reversed.

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Post  the tree man on December 2nd 2019, 7:11 pm

It's lots of fun,not fussy but still slightly fat at idle and has a very mild bog if you mash it without enough rpm (converter is a bit tight too);however,overall starts/idles/cruises fine and pulls pretty good,just not good enough.I'd like to see at least 620HP or more if possible.In your opinion,do you think that goal is attainable with the 521,ported SCJ's,SFT cam and this intake.Thanks again.

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Post  rmcomprandy on December 2nd 2019, 8:52 pm

the tree man wrote:It's lots of fun,not fussy but still slightly fat at idle and has a very mild bog if you mash it without enough rpm (converter is a bit tight too);however,overall starts/idles/cruises fine and pulls pretty good,just not good enough.I'd like to see at least 620HP or more if possible.In your opinion,do you think that goal is attainable with the 521,ported SCJ's,SFT cam and this intake.Thanks again.

Easily ...

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Post  gt350hr on December 3rd 2019, 2:21 pm

I  use the BC/BD carbs (710 cfm 427 MR) on my 521 with early CJ aluminum heads.They were designed to run together on a dual plane intake so the calibration is good. I do run them backwards as Randy noted as that is how Ford ran them. Only the small blocks ran forward facing carbs.
Mine run very nicely no flat spots or stumbles. The linkage is still progressive but I will try the one to one next year at the track to see if the ET changes. I am comfortable that it makes over 600 hp on 91 octane.

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Post  the tree man on December 3rd 2019, 5:58 pm

Thanks,boys.Good info to start.I sprung for these carbs new last winter,sizing them for the 466 assuming it would have enough HP.Pinhead,I know enough to know it's never enough.I'll fiddle with them for the time being.They are a smidge rich across the board as I have them set now so they might suit the extra demand of the new engine OK but I still feel they may be a bit small.Since another set will be about $1000 regardless what they are they'll have to wait.Looking forward to the extra HP/fun regardless.

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Post  gt350hr on December 4th 2019, 4:57 pm

Rich across the board makes sense as the "signal" ( vacuum pull) is greater on a larger CI engine and still pulling through the same size hole. That's the other reason I went with the 710's. Careful "shopping" can find them at reasonable prices like I did.

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Post  Carbguy on December 6th 2019, 10:54 am

Ran the intake recently on a 572" P-51 headed engine. Certainly down on power vs. a good single 4bbl, but can make fair numbers. Fuel distribution is not perfect, but with a little work can certainly be made better.

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Post  the tree man on December 6th 2019, 12:48 pm

This is a street car so I'm not looking to set the world on fire.It will see at least a few thousand miles a summer and it's a Thunderbolt so the dual quads are mandatory.Car already has a built trans and diff but I don't want to go looking for the weakest link either so the 600+ range should be a good compromise between performance and reliability.A little extra bite to go with the bark.

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Post  the tree man on December 6th 2019, 12:49 pm

What carbs did you run on that 572?

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