Datsun Roadster Forum-311s.org

I need to some data/info (whatever).

My best bud who is working with me on my restore says the 44mm/45mm are overkill. I have to be tuning the carbs down for them to work...

He says the 40mm are the suitable choice and are good for up to 2.2 liter. He even doubts the motor actually sees a significant difference performance from the 44mm/45mm.

I told him it might be due the required air that the motor needs at the top end.

I know the Nissan engineers had to know what they where doing, but I can give a good rebuttle.

Technically based on other vehicle setups, I leaning to agree.

Somebody set me straight, pleaze

Your buddy's wrong in this case. The U20 head flows extremely well and has the bigest valves nissan used in a production engine. 44's with 37mm chokes are standard issue. You could use 40's with 36mm chokes and be okay for most street use but if you're racing at all you will run out of steam at the top end. A stock blueprinted U20 will rev past 8000 RPM so when you get that much air moving you need the 44's. In some pure racing applications the 44's have been used without venturies!

I think BRE was using 50 PHHs. I have been told that the 44s flow 211 cfm per barrel, times four equals 844CFM! Thats more than many SBCs have flow in a well tuned form. By my seat of the pants estimate, I will drop my friends 68 Malibu with a healthy 350 SBC. He is running a large 4 barrel carb on it. The U20 NEEDS the big carbs, plus the induction sounds so cool.

Will

Somewhat off subject, but...

"Back in the days" I built <ahem> a scooter, a 125cc Italian scooter with a 40mm carb. Granted it was a two stroke and readily revved to 11K RPM, but the engine really came alive with that setup. The power at lower revs was significantly less and I had to increase the idle speed to keep it from bogging at idle...The stock carb was 19mm.

Even though they are very different engines, the principals are the same. Larger carbs can support higher flow rates and therefore power at higher RPM. The drawbacks? Poor low RPM mixture control and decreased mileage...

Datsun engineers put the most HP in that they can and keep emissions and driveability reasonable. A carefully tuned car with a small increase in carb size will perform better with very little change in driveability. Now if you go crazy with carb size, place a hotter cam and whatnot - you start to lose the grocery getter type of driveability...

TR

I have the B cam, but I am about to switch to a reground one. With B cam and current carb setup I get a little under 25mph at freeway speeds. I think I could get even better economy if I had a taller 5th gear/ final drive ratio. At 75 I sit at about 4200 rpms. Nissan did their homework when they designed the U20. My bottom end is still pretty strong even with the solexs and the 11 lb flywheel.

Will

the datsun competition department offered 50mm solex carbs.

I really understand why they used BIG carbs with a big cam. I installed my reground A cam (284 intake 290 exh, .485 lift) and my engine now pulls hard through 7000 rpms. I tried the 6000 rpm limiter chip in the MSD and wanted to go right through it, put the 7000 in and it still wants to go . I am limiting it to 7000 and absolutly no more. At 4000 it comes alive. I am running 32 or 34 mm chokes in the solexs, I wonder what would happen if I went to 36 or 38s I really want to unleash this thing at Shasta on the AutoX course.

Will