Big manifold hole & Smaller carb throat.

[thetravman]

I have some manifolds that have 48mm intake holes. I want to use some dual 40 IDF's but good grief it seems like a big hole AND a pretty big chamber beneath the big hole. My question is will the air be badly disturbed by the mismatch of 40mm throats on 48mm holes?
This is going on mild 1600-1776 engines but if there is a problem, would a larger engine suffer better or worse under such circumstances?
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[74 Thing]

CSP makes manifold with the right size bore but they are not cheap.

http://www2.cip1.com/ProductDetails.asp?ProductCode=C31%2D129%2D711%2D140

[thetravman]

Your right. I already have some manifolds so new ones seem pricey. Then I'd also have to buy their linkage too $$$. Is it that big of a problem or should I just run the 48mm holes with the 40mm throat carbs?
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[Scott Novak]

Gene Berg also makes straight 40 IDF intake manifolds that have 40 MM inlets to match the 40 IDF carb bores.. The GB GB 580A is shorter than the CSP intake manifold.

It is one of the best quality intake manifolds made. I haven't seen the CSP intake manifolds so I don't know how they compare to Berg.

http://www.geneberg.com/cat.php?cPath=8_291_2873

http://www.geneberg.com/product_info.php?cPath=8_291_2873&products_id=994

Using larger diameter intake manifolds with smaller diameter carburetors does create tuning problems and you should avoid it. It will cause problems with a larger or a smaller engine, but I'm not sure which would be worse.

Scott Novak

[Eaallred]

48mm intakes with smaller carbs is common practice. I've never had nor heard of anyone having difficulty getting thier car to run right and have that end up being the culprit.

It also helps a bit with anti-reversion.
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Eric Allred

You have to remember something: Everybody pities the weak; Jealousy you have to earn.

[volksnut]

I ran 36 DRLA Dellortos with new CB intakes (which were bigger) with no problems at all, great throttle response and driveablity.

[Das Dragon]

Whew, glad to here this as I sit here with my new CB 'big-gass hole' manifolds and 36 Dells.
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[mharney]

I would choose matched runners any day over monster holes under the plates.

[thetravman]

Matched runners seem like the best course. So now I wonder which linkages work with which manifolds. For example, could you run csp manifolds with CB linkage? Or Berg manifolds with Weber linkage etc.?
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Have your car serviced at your local aircooled repair shop today.
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[Scott Novak]

[Jake Raby]

The CSP arrangement will work with my Type 1 DTM, with just a tad bot of clearancing- I used it on my test engines in 2003 and 2004.
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Raby Engine Development
www.rabyenginedevelopment.com
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[Pat D]

Rule of thumb on making H-P. Always run a bigger throat on the manifold than your throttle plate. I'm not going to say how big, it has taken me a long time to figure it out

[DarthWeber]

My Dellorto 40s measure 40mm on the carb and 49mm on the manifold. They're that way for a reason. Pat D knows.

[mharney]

Much of it boils down to intake runner math.. we could argue that the intakes for our engines are optimal for exactly one combination if displacement and RPM for the given guts... we can argue that runner length and diameter both matter more than our intakes will allow us to control. We can also argue that economy would require much different results than power does.

But gross differences in the runner dimensions going down the pipe don't do much to help anything. A negative step in the runner (larger opening under the carb) will not do nearly the damage that positive step on the runner would.

[Bruce]

[Eaallred]

It would be interesting to see a back to back dyno test on a motor with 40mm duals. One test with matched manifolds, and one with the standard 48mm manifolds.

Re-jetted and tuned properly, I would guess the peak HP would be about the same, but the larger manifold would show more torque. Just a guess though.
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Eric Allred

You have to remember something: Everybody pities the weak; Jealousy you have to earn.

[Scott Novak]

It would be also interesting to see how oversize intake manifolds would affect low RPM, part throttle, acceleration conditions, gas mileage, and any attendant hesitations or flat spots.

Scott Novak

[krusher]

[Bruce]

[Pat D]