Which is better for a rotary (and why?), or is it just a matter of preference?

 

sidedraft with lake city intake...all the way!

 

i would say it's a matter of preference. i don't think any one of them does any more good for the engine itself.

however, i believe the downdrafts are better at producing torque (the Dell'Orto is a special case because although it's a sidedraft, I consider it a hybrid at best, because it draws from the driver's side and therefore has to send the intake charge downwards anyway - but that's just me! ) and torque is better for the street.

 

I would imagine that the downdraft set-up has a shorter intake than the side draft set-ups...that has to make some sort of difference.

 

a Lake Cities manifold has a shorter path than any downdraft manifold. Whether that is beneficial or not is up for debate. However, the Lake Cities is the ONLY manifold that doesn't have to make a downturn to the tall, thin center ports. It's very difficult to make a turn along that axis on a tall thin port, which is why any decent manifold (including the OE manifolds, the IDA manifolds, etc) that has to turn like that does NOT have the center ports going full-height.

But, the Lake Cities manifold CAN go full-height, because it doesn't have to turn like that.

 

Anyone know where I could get a Lake Cities manifold ?.

 

any pics of this Lake Cities manifold?

 

Search!

https://www.rx7club.com/forum/showth...hreadid=110878 (even has "Lake Cities" in the subject)

Specifically:

 

does the atkins sidedraft setup use lake cities manifolds.

 

That is a short intake...the others I've seen curl up and over the engine. You bet the length of the intake path has an effect on torque characteristics.

 

I understood that the down draft was a bigger carb and that this setup was for monster street ports or bridge ports. The side draft works well with stock and street port applications.

Jus one guys perspective

 

Weber makes some BIG sidedrafts I think up to a 55DCO/SP. Port as needed.

 

Any intake that curls over will have to deal with the problem I described above.

BTW - I calculated that the best intake runner length for engines that max at 8500rpm will have a tube length of only about 5 INCHES out of the engine (taking into account the length of runner in the engine). Anything longer is better for low-end. Even the Lake Cities is *much* longer than 5 inches from engine to end of carb!

 

Did you use a formula to calculate that or is there another way? And when you say Anything longer is better for low-end. does that mean that an intake longer than 5" will hinder top end performance?

 

Won't HINDER top end performance, after all people make plenty good top-end power with longer intakes.

I used a runner-length calculator formula and divided the results in half since the rotary has twice as many intake events as a piston engine per rotor at the same RPM. (a rotary at 8500 will have as many intake events per rotor as a piston engine at 17000 will)

I *assume* my math is right. If not, nobody bothered to correct me when I asked about it in the rotary performance section...

 

On the atkins side draft system, they mention 45,48, and 50 DCO.

What do those numbers mean, and what does "DCO" stand for? Thanks!!

Geoff

 

That makes sense so far peejay, but don't the formula results change if you go from a 4 cyl to a 6 or 8 cyl engine? A 6cyl will have more intake events than a 4 cyl if they are both running at 3000rpms just because there are extra pistons in a 6cyl right? If what I think is right, then my next question is how many cylinders did the engine have that you compared to a rotary in your equation? If I'm way the hell out in left field, then don't answer that.

 

Per cylinder. The runner length was for individual runners... a single cylinder in a 4-stroke piston engine only has one intake event every 2 rotations, no matter how many cylinders total the engine has.

 

I gotcha!

 

peejay, thanks for the education. now, if i am understanding you correctly, then you've just explained why for my MR2 Turbo (which is a 4 cylinder, with 8 port runners) when you graft the runners so that you have 4, you don't feel a loss in low end torque, but what i'm unclear about is would it breathe better upstairs? theoretically, of course ...

if i understood your explanation, then the answer should be no, right?

 

oh yeah ... if this is going to be lengthy, then just PM me, i don't want to hijack the thread with my MR2 stuff ... hehehe ...

 

Not familiar with that engine, sorry... my guess is that one set of runners has a throttle valve in them, which is a common trick on multivalve engines. Mazda even does this on the rotaries, with primary and secondary intake ports.

 

oh my God!
sorry, just threw me off there for a sec, anyway, yeah it's one of those staged intake jobs, where the secondaries are controlled by a vacuum switch and open under load ...
okay ... no worries. i was just curious 'cause of all this intake and port dimension talk. thanks anyway ...

 

Diameter of the intake path also makes a difference. Long narrow for low end...short and fat for high end. Lot's of modern cars use both employing a variable inake path system.