Musings on the "perfect" 4A bottom end

[myne]

(disclaimer - I'm kina bored at work right now, and despite the 4A being one tough mofo, I've been reading TOO again)

According to popular opinion 1.75 is the ideal rod:stroke ratio (centre to centre) for an all-round balanced engine.

The 4age has a 77mm stroke, with a 122mm rod giving a 1.58 R:S ratio.
Ok so that's "bad", what about the 7a?
85mm with an alledged ratio of 1.5496 meaning it has to be ~131.7mm rod

What about a 4A crank in a 7A block with 7a rods?

131.7 / 77 = 1.7105. Close.

What about deck heights?
Well actually I dont know, BUT with some deduction...
77 + 122 = 199
85 + 131.7 = 216.7
I know that's not the deck height, but since we know GZE pistons can be used in "7AFTE" hybrids, we can assume that it doesnt actually matter what the deck height really is, just the difference in deck height.

216.7-199 = 17.7mm taller block on the 7A

77+131.7= 208.7 so you'd have to deck the block by a massive 8mm - is this safe?

Ok, so what if you got custom 135mm rods (1.753 R:S ratio)?
77+135 = 212 now you're decking the block a mere 4.7mm. Not sure if it's safe but it's theoretically made the "perfect" 4age bottom end.

I guess a good compromise would be finding some aftermarket conrods with the same width, same journal, gudgeon, and a length between ~130.3(1.7) and 138(1.8) and decking the block to suit. I have no idea how much a block can be decked safely though.

Apparently the big reasoning behind the rod:stroke ratio is reduced lateral piston>bore forces, "better" dwell at top dead centre and reduced maximum piston speeds/accelleration.

Basically it's supposed to be more ideal for power and lifespan.

If you want to build an 11,000rpm screamer without going Formula atlantic or a mad 1.6 turbo with stupid boost, I guess that's the only time you might need this.

[Starfire]

Interesting, but I still fail to see a point.

I'm pretty sure that the toyota engineers didnt just pick a couple of random numbers that added up to about 1.6L and just slapped it toghether. A lot of development went in to the engine to make it reliable and practical for just about any application.

[myne]

http://e30m3performance.com/tech_art...ech/rod-ratio/

Have a read. It's not as in-depth as what I remember reading on TOO, but unlike TOO, it's not 404ing.

[Starfire]

I'm not saying that it wont work, just that I doubt you need it.

TOO tends to have plenty of theoretical crap on there. Interesting though.

[Dimitri]

bill sherwood is doing that exact thing right now. (FA 4ag crank, custom rods and pistons).

FA engines actually use a higher piston pin height to increase the road ratio (from what i can gather from internet sources).

i am in the same boat as you. im not an engine builer, but decking a block as far as you sugested i dont think would be a good idea. a series engines are a closed deck design, but i dont know how far down the bores are attached.. you could only be weakening it, maybe not by a measureable amount, but even 5mm seems like alot.

it is extreemely hard to cross referance rods from differance manufaturers, there is one aftermarket supplier that has full imformation ( big and little end diam' and width and length)
but some of it seems inconsistant. i have tried before to use it to work out a good rod for a tradtional 7ag (full 7 stroke). for that purpose ive been suggested to look at mitsubishi and honda rods, by the catalogue i used theres nothing perfect or direct, so i can only assume there is machining of width and boring of big/little ends.

another part of the equation you didnt mention is pin height. if youve found a nice rod thats only a few MM off in lenght to be compatible with the block you might find you can use another piston aswell. thats starting to sound pretty complicated. but i guess its easy enough to atleast consider the piston that goes with the rod youre using (e.g B16/18 rod and piston combo).

cool thread. im willing to try to help work out a nice combo what the hell is TOO?

[Nim]

The B16A has a conrod length of 134mm, giving it a ratio of 1.74, almost exactly what you call perfect ratio. The B18C has a 137.9mm rod (and a ratio of 1.58), but if you put that rod in a 4A-GE that should bring your ratio closer to 1.75. Assuming it'd fit that is. You might be able to get yourself some DC2 Type R rods too. Nice and light.

[mullett]

Something that hasn't been mentioned yet...you can't deck the block that far, because then your piston will fall out the bottom of the bore. Not sure what the tolerance is on the 4a block, you might get away with it. But you ARE adding weight, by using longer rods. So...maybe toyota made the rods as short as they could without adverse affect, in an effort to reduce inertia.
Just an idea.

RM

[Nim]

Something that hasn't been mentioned yet...you can't deck the block that far, because then your piston will fall out the bottom of the bore. Not sure what the tolerance is on the 4a block, you might get away with it. But you ARE adding weight, by using longer rods. So...maybe toyota made the rods as short as they could without adverse affect, in an effort to reduce inertia.
Just an idea.

RM

We're talking about taking a 7A (which as I understand it is a stroked 4A) and putting a 4A crank in it. It wont fall out the bottom, it'll just make the stroke shorter. It'll actualy be further from the bottom of the bore. We're still using the the 7A rod, or even a longer rod, but a smaller crank (shorter stroke).

[myne]

bill sherwood is doing that exact thing right now. (FA 4ag crank, custom rods and pistons).

what the hell is TOO?

Trust Bill to have already figured it all out.
He's the only real 4A guru that's happy to share.

TOO is theoldone.com

[mullett]

Ah, sorry, I mis-read. My mistake

RM.

P.S. What do people know about the 7A-G(Z)E conversion? It seems something worth doing: this is insane http://www.turbofast.com.au/mr2.html