The way I see it is that yes these heads flow very well and are capable of producing high hp figures at high rpm's and to do so they need large carbs. On a street car though how often are you driving around at 6000 rpm's to take advantage of this?
I suspect that since the volume of air being drawn into the cylinder is still the same as with any other 202 that the air speed at lower rpm's when using such a high flowing head and 48mm + webers with who knows what size chokes would be quite low and due to this the low end performance would suffer. It's just my opinion but low rpm's torque is what you'll benefit from more on the average street car. Has anyone driven one of these big hp jzed engines themselves or is the joy just in looking at maximum hp figures on the dyno.
As an example my good old 9 port (with a 12 port conversion) is more than capable of flowing enough to support 45dcoe webers with 40mm + chokes, probably even 48dcoe's with 42mm + chokes and make good power figures at high rpm's but if I set up the carbs to do this then real world drivability suffers, it's great above 3000rpm's but just don't expect much below that because the air speed is simply to low. Then I can go fit smaller chokes and tune it accordingly and it turns into a completely different beast from idle all the way to redline. The redline might not be as high and therefore the maximum hp not as high but it's far more enjoyable to drive.
The JZED does have big ports and when running bg carbs like 50mm carbs yes the bottom end will obviously suffer to a degree- but there are also irving heads that flow nearly as much, but will produce outstanding low to middle torque due to the small but very low turbulence ports.
Its more complicated than just choke area- the choke area is just the end of the drain pipe- but you need to consider the full drain pipe from gutter to spout (or valve to bell mouth).
9 ports and 12 port heads are inefficient in design- hence when ported they are slow ports- especially 9 ports... the air moves slowly, which means we see benefit from speeding up the port speed (ie smaller chokes when running webers or dellortos).
If you have a more efficient port speed, such as with an irving- it will both move air faster and flow greater CFM than a traditional 9 or 12 port head- so 48mm webers with 40 or 42mm chokes are a very good thing on an irving style head.
Granted, JZEDs are not going to move air as fast at lower rpms- but as you can see in the above dyno sheet, with a well designed engine you can work around to at least a little. That JZED engine makes the the same hp as my peak hp for my 9 port (255hp), but only makes it at 3400rpm rather than my 6200rpm. My engine made around 160hp at 3400rpm- BUT thats WOT, part throttle transitions will always prefer smaller carbs/ chokes, but if you use a good compression to cam ratio (dynamic compression) and aggressive cam timing (ie locked dizzy or minimal curve in dizzy like 10 degrees)- you will find you can crutch the part throttle transitions. Or, you can use constant velocity carbs and get some response back that way. There are several ways to skin a cat. Motorbikes run big 45 degree ports and very long duration cams by using CV carbs, just as an example.
The other point to make is that the CFM restricted heads like our 9 ports and 12 ports really benefit from extra duration to give the air more time to fill the cylinders- hence, we already have soggy bottom ends due to the fact that we are running long cams like more than 260@50". These cams are great from 4000rpm and up but are very soggy below. With a better flowing head, like irving, duggan/ JZED- there is less need to run such crazy duration and you could run 250@50 or 254@50 which helps give some drivability back.
Everything is a trade off, and yes, most JZED headed engines will not really be great street engines- but then the people who are going to pay 5k for a head are generally not looking for gentle street engines- they want performance most likely. In all honesty a moderate sized cam (like 238@50 or 240@50) in a YT 9/12 port with SUs will be ideal for the street and regular driver, but it also wont really be all that exhilarating and also wont make much more than an XU1.
Edited by LC-GTR-1969, 21 December 2017 - 11:19 AM.