Hi everyone!

I thought of something today, and I want to hear your opinions about it.

So according to Racingbeat, Mazdatrix and tuners a 40 and 45mm downdraught or side draft will be to small to make good HP on a BP or PP ported Rotary,

Ok, so here is what I have up for discussion:

A 40 DHLA Dellorto 38mm Venturi
A 45 DHLA Dellorto 38mm Venturi

It is obvious that a 45mm will flow better than a 40mm, seeing that each butterfly is 5mm bigger.

The circumference math is as follows:

40x3.14159 = 125.663
45x3.14159= 141.371

125x 100 / 141 =88 %

So the 45 has an area that is 12% bigger.

Now this is just the static area of the throats. It has nothing to do with how they actually compare in the CFM side.

What about the flow you may ask? On Carb flow I don’t know much; the smart guys (Sterling and Rx7Carl) will have to chime in.

Please share some insight on the matter; it’s been bugging me forever.

Karis

 

Not circunference, its area that matters

PI*R^2

or

PI*(D^2)/4)

40 >> 1257 sqmm

45 >> 1590 sqmm

26.565 increase

You need to put bigger venturis in the 45 DHLA.

 

Thanks for replying.

It clears it up for me quite a bit!

Karis

 

i am not entirely clear i understand your post. you are comparing a couple of downdrafts but using 38 mm choke tubes/venturies in both? if you use 38s in both you have virtually the same flow. same hp/tq.

i ran a GT3 12a rotary for 6 seasons. during that time the SCCA cut the choke size from 50 mm to 38 mm. it made a significant change in my hp/tq. ( my motors were bridgeported, dry sumped and built by roger mandeville and daryl drummond). in order to regain some hp we raised the operating rpm range to 7-10,000 from 6-8500.

if you have no rule restrictions run 50 mm chokes.

howard coleman

 

Mr Coleman :I`am wondering how much more a 45mm side draft with 38mm venturies will flow ;than my current 40mm with 38mm venturies.

My engine isnt as rough,even though it has alott of work into it.I built it myself. :-)

I`am not in a class restriction,the Rx7 is my Toy.There isnt any 45 or 48 carbs available here.The few we have,no-one can afford.
Karis

 

the choke size determines flow so there will be no diff between a 40 and 45 if they both have 38 mm chokes.

howard coleman

 

Ok,now that i was never aware off! thanks for pointing it out to me!

Yeehaa,so having a 40mm with 38mm venturies isnt bad!

Thanks!

Karis