What size Carbs for 471 stroker FE

[patrickt]

Originally Posted by ERA Chas

Apologies A98-mea culpa. But....

Hmmmm, I say it's time for a workup. Diagnostic Workup | Alzheimer's Association

[A98Coupe]

Actually, this is my thread if you see who started it.

[ERA Chas]

I guess it's time to take the Rsimoes forum suicide pill...........

[DanEC]

Originally Posted by ERA Chas

I guess it's time to take the Rsimoes forum suicide pill...........

Don't feel too bad - my first thought on reading his update was that the thread was going off the reservation also.

Since he is rebuilding anyway - most of the issues sound solvable. At least the motor hadn't let go and holed the block or something.

[A98Coupe]

ERA Chas,
No hard feelings! My emotions were running high yesterday when I found out the 'bad' news about my block. I agree, the thread was changing tack from the original carb size issue, but I felt I needed to share my grief somehow.

Barry R,
Thanks for your advice. My engine builder (not the original engine builder), also said that the block is useable with a bit of work, but I would be concerned about spending a load more cash making it into a 471 stroker knowing that the block has been damaged in the past. I need to hear what the original engine builder has to say about all the issues before deciding what to do.

[olddog]

To the original question, on carb sizes, I was surprised by the answers.

471 cid at 6000 rpm would pull in 818 cfm at 100% volumetric efficiency.

You didn't say how many rpm you planned to turn nor do we know how good the ported heads can flow.

I would have thought the existing carbs might have been enough. Certainly they are marginal, but may give excellent low end and response for the street.

I'm not arguing against the answers from the experts, but thought I might learn a thing or two here. Why so much carb?

[Barry_R]

Originally Posted by olddog

To the original question, on carb sizes, I was surprised by the answers.

471 cid at 6000 rpm would pull in 818 cfm at 100% volumetric efficiency.

Why so much carb?

Because whatever carb sizing formula you are using is woefully inaccurate, and most likely based on poor assumptions regarding flow measurement. Carbs are rated at a predetermined amount of vacuum drop below the plates in order to define a comparative value. The vacuum drop rating number (1.5" on 4 bbls) was derived from limitations of the flow testing equipment, and not from any particular engine driven value. That number really has only a passing nominal relationship to the air handling demand of the engine.

Ford and others were quite cognizant of this even back in the 60s when tehy installed multiple carbs on otherwise modest engines.

[olddog]

On the block, the groves from the H beam rod strike is most likely below where the rings run with the 3.78 stroke. However when you stroke it to get 471 that may not still be true. This block may be fine for the original stroke, but not for a stroker crank, without major expenses. Maybe you can run a 3.98 stroke, but not a 4.125 stroke.

You could be better off with a new aftermarket block and sell this one to someone needing original parts for a restoration project.

I hope my random thoughts might be helpful. I feel your pain here.

[olddog]

Barry,

I wasn't using a carb sizing formula. I just converted cubic inches to cubic feet. A 4 cycle engine takes 2 revolutions to complete all 4 cycles, so multiply by 1/2 the rpm. This is what the pistons displace, on the intake stroke.

471 CI / 12^3 CI/CF * 6000 R/M / 2 R = 818 CFM

I do understand that it is possible to achieve more than 100% VE, using the momentum of the air to pack the cylinders.

I'm thinking, from what you explained, that the rating of a carb is at a certain manifold vacuum number, and the testing equipment wasn't the best. Therefore the rating of a carb and what will actually flow through it are not necessarily the same, especially in multi-carb applications. Am I understanding this correctly?

I have always wondered why certain engines had carbs that appeared to be way too big. The 340 Dodge Dusters with 3 two barrels, for instance. I suspected the factory put them there so you could install headers, port the heads, up the cam, and you were ready to go, without violating any racing rules. I had always assumed they would have made the same power with smaller carbs, but maybe not.

Anyway thanks for your time and sharing your knowledge.

[WildBill1965]

My MR had a pair of 715's from FOMOCO

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