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Big manifold hole & Smaller carb throat.
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thetravman
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PostPosted: Thu Oct 30, 2008 10:06 am    Post subject: Big manifold hole & Smaller carb throat. Reply with quote

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
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PostPosted: Thu Oct 30, 2008 10:10 am    Post subject: Reply with quote

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

http://www2.cip1.com/ProductDetails.asp?ProductCode=C31%2D129%2D711%2D140
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thetravman
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PostPosted: Thu Oct 30, 2008 10:17 am    Post subject: Reply with quote

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
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PostPosted: Thu Oct 30, 2008 2:14 pm    Post subject: Reply with quote

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
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Eaallred
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PostPosted: Thu Oct 30, 2008 4:42 pm    Post subject: Reply with quote

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. Wink
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volksnut
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PostPosted: Thu Oct 30, 2008 5:44 pm    Post subject: Reply with quote

I ran 36 DRLA Dellortos with new CB intakes (which were bigger) with no problems at all, great throttle response and driveablity.
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Das Dragon
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PostPosted: Thu Oct 30, 2008 6:37 pm    Post subject: Reply with quote

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
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PostPosted: Thu Oct 30, 2008 6:53 pm    Post subject: Reply with quote

I would choose matched runners any day over monster holes under the plates.
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thetravman
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PostPosted: Thu Oct 30, 2008 8:00 pm    Post subject: Reply with quote

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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Scott Novak
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PostPosted: Thu Oct 30, 2008 8:21 pm    Post subject: Reply with quote

thetravman wrote:
Matched runners seem like the best course. So now I wonder which linkages work with which manifolds.


The CSP bellcrank linkage will work with Berg, CB Performance or CSP intake manifolds for 40IDF. You'd need to ask Jake Raby if the CSP linkage will work with his DTM fan shroud, but I suspect that it will.

The CB Performance Hexbar linkage will NOT work with Berg intake manifolds unless you remove the heater ducts from the fan shroud, or use a Thing fan shroud without heater ducts, or other aftermarket fan shroud without heater ducts on it. I'm not sure if the CB Hexbar linkage will work CSP intake manifolds for sure, but I suspect that it will.

The Berg fan shroud linkage will work with CB performance intake manifolds, Berg manifolds or CSP manifolds. But Berg linkage will NOT work with Jake Raby's DTM fan shroud.

I prefer the CSP bellcrank linkage design overall.

Scott Novak
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Jake Raby
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PostPosted: Fri Oct 31, 2008 6:00 am    Post subject: Reply with quote

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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Pat D
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PostPosted: Fri Oct 31, 2008 10:19 am    Post subject: Reply with quote

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 Wink
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PostPosted: Fri Oct 31, 2008 11:02 am    Post subject: Reply with quote

My Dellorto 40s measure 40mm on the carb and 49mm on the manifold. They're that way for a reason. Pat D knows. Cool
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mharney
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PostPosted: Fri Oct 31, 2008 12:54 pm    Post subject: Reply with quote

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.
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Bruce
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PostPosted: Sat Nov 01, 2008 1:20 am    Post subject: Reply with quote

Pat D wrote:
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 Wink

OK, I'll bite.
Explain how a big volume under the throttle plate (which causes a massive drop in airspeed) makes more hp?

Then if it is so good to have a big increase in diameter, why don't your manifolds for IDAs have a 60mm hole below them?
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PostPosted: Sat Nov 01, 2008 8:23 am    Post subject: Reply with quote

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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Scott Novak
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PostPosted: Sat Nov 01, 2008 2:11 pm    Post subject: Reply with quote

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
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PostPosted: Sat Nov 01, 2008 3:14 pm    Post subject: Reply with quote

Eaallred wrote:
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.


I would have though the smaller manifolds would have faster gas speeds and more TQ. Confused
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PostPosted: Sun Nov 02, 2008 12:17 am    Post subject: Reply with quote

Eaallred wrote:
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.

I'd like to see that test too.
My guess is that the test with the giant manifold holes will show less bottom end torque due to the drop in airspeed. When that happens, fuel will want to condense out of suspension. To compensate, a bigger jet might be needed.

I'd like to see it done with a 5 gas analyzer. I bet CO and HC will be a LOT higher with the big manifold.
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Pat D
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PostPosted: Mon Nov 03, 2008 4:22 pm    Post subject: Reply with quote

Bruce wrote:
Pat D wrote:
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 Wink

OK, I'll bite.
Explain how a big volume under the throttle plate (which causes a massive drop in airspeed) makes more hp?

Then if it is so good to have a big increase in diameter, why don't your manifolds for IDAs have a 60mm hole below them?
I'ts called a plenum Bruce Wink I guess we could just make all our manifolds 70mm on top and call them Super Plenums Very Happy
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