7A-GTE 20v VS 4A-GTE

[mullett]

Got two options for my next MR2 powerplant. I can rebuild (forgies, ARP bolts etc, balance) my GZE motor, and bolt on the boost, or build a 7A with 20v heads, GZE pistons and (I think) custom rods. I've heard that the 7A will give great torque and spool early, which I'm very keen on, but I've also heard that the cranks break easily. Does anyone have experience, anecdotal evidence...etc? Or even 20V head and quads on GZE

RM.

[4agte]

check this out if you havent allready http://www.turbofast.com.au/mr2.html

[myne]

I remember reading a fairly in-depth analysis of the 7A vs the 4A
One thing I remember clearly was that the 7A crank has it's oil galleries in the cheapest path, vs the 4A that is on the least stressed path. This is supposed to be significant because the cheapest path was not actually ideal for proper oiling, but was good enough for a shitty stocker.

If you're going to go for custom rods regardless, it might be worth using the 7a block with the 4a crank. Because the higher deck means you can have longer rods, which results in less overall conrod angle, lower peak piston acceleration, longer dwell at TDC, and less piston to bore lateral force. According to theoldone.com, the ideal rod is 1.75x the length of the stroke.
Another advantage to the 7a block is that it's slightly taller, so if you really want to spin up fast, an SC12 might fit under the 20v inlet.

[mullett]

But that would kind of kill the main advantage: being that I get more displacement...For the money that the conrods etc are going to cost, maybe I should rebuild my GZE with good bolts, stock pistons/rods, and spend the money on a really nice turbo...Can you get GZE pistons in 20thou?

RM.

[45aken]

i dont know about 20thou, but i know you can get them oversize.
ur looking at about $75 each +rings

[mullett]

So About $300 for a set? Damn, that IS cheap. ANd that's the full set of pistons, minus rings, in the toyota forged/coated form? Sveet.

RM.

[nick.parker]

Myne,
I'm fairly certain the SC12 fits on a 20V 4AGE already, because of the intake runners angling upwards from the head. Out of curiousity I found some after market rod specs for 4AGE so I calculated the rod /stroke ratio is 1.58
Regards, Nick.