Richard Miller

So, on a 4 port engine 12a or 13b running a weber type carb or efi, how does the difference in port timing effect the engine? Considering that with a DCOE or IDA all ports are active at once. It seems weird to have one side of the rotor start intaking before the other. Can the irons even be ported to match or would that open the primaries to much and we would lose velocity of the A/F mixture?

Richard Miller

Seems like I asked a decent question with no answer.

BLUE TII

iTrader: (9)

It is a good question.

Seems to me Mazda made primary ports close earlier and blocked off 2ndary ports for more low rpm power/response.

You are putting them on 4150 TB or carb where all the throttle plates open at once (may be slight delay if running progressive 2ndaries). There will be no advantage to having the primary ports close earlier in this case (perhaps a very small difference in tip in response if running progressive 2ndaries).

I personally close the primary ports a hair later since I believe the smaller ports should have a tad more intake velocity (especially once the big 2ndaries close).

Seems to me Mazda made the primary ports open later on later series engines for emissions (EGR effect).

You aren't worried about emissions, so I would open the intake ports as soon as possible (~24ATDC I believe with pre 2004 rotors, 0ATDC with 2004+ rotors) so the rotor spends less time sucking up exhaust gasses in the early intake stroke movement.

Seems to me Mazda made the primary port volume smaller than the 2ndary port volume (shorter port top to bottom) because the primary runners are smaller, so there won't be the flow to support the larger volume port and maintain the same velocity.

I would never port a primary port down to match the 2ndary ports length unless it was on a tall port 12A center housing.