7A block/bottom end. Relative vs abolute strength. Options

[Jehuty]

I'm interested in the 7AGE setup, and there is some good info out there, but it's vague in some spots...

Specifically, about strength of the block and bottom end of the 7AFE. Most refferences to the strength of these parts is stated relative to the 4AGE. For example, the fact that the 7A block has less ribs than the 4A, or that the flywheel bolts up with less studs. Yeah, great. It's not as good as the 4A, but what does that mean? I want to know what the actual rev and boost limits are.

Reason is that altho the (7A) block, crank, rods and pistons are all stated as being weaker than the 4A counterparts, the only parts that looks really "weak" (in absolute terms) to me are the pistons. The rest looks like a pretty stout little engine. Am I correct?

So if anyone knows where I can find some info about the rev and boost limits of the 7A (or the 4A for that matter!) please help me out! First hand experience from anyone who's blown a few would be nice too. Heck, I'd even appreciate a little heresay. ;)

[Kwanza26]

I've blown up a 7AFE before. I've blown up 4AGs before also...


The 7AFE is not as strong as the 4AG by a whole lot. If you just compare the size of the rods, you'll see the 4AG rods are significantly beefier in relation to their journals sizes. The 7A rod is slightly larger than the 4A rod, but it doesn't have as much actual "meat" as the 4A rod (with respect to the journals). I personally feel the stock 7A bottom-end is however, strong enough to make a reliable 7AGE and spin at 7500 rpms reliably. It's a well designed bottom-end with a decent rod-stroke ratio. Things such as flywheel bolts and and ribs doesn't make a huge difference. The weakest part of the 7A is the rods. Forget what Toysport says about hybrids (they don't know what they're talking about)... The rods is what will fail first if you try to rev that block too high. They tuners on toymods.org.au will say the 7A block can only do around 7200 rpms stock... but I personally would push as high as 7500 rpms. The Euro spec 7AFEs fuel-cut at 7000 rpms... so It's fair to say, a little above that should be ok. To be safe... 7200-7500 rpms is the stock limits of the 7A bottom-end. With re-worked stock rods, 8000 rpms is do-able.

As for boost... it all depends on tuning. I wouldn't try do more than 15psi on the stock rods and pistons, but with upgrades, it can handle it. It's all about tuning. When it comes to boost, the weakness of the pistons is more obvious. 15psi is really really pushing the envelope on stock bottom-end, but I feel with good tuning and proper fuel upgrades, the 7AFE can do it. I wouldn't do it as a daily driver, but it can hit 15 psi for some weekend fun. Regardless, anytime you look to do a 7AGE, you're gonna want to replace the pistons with 4AG pistons. The later model 4AGEs with 20mm wrists are direct swaps into the 7A bottom-end. With 4AGZE forged low-comp pistons, boost levels can be significantly increased...

Again, anytime you're looking to do a 7AGE, you'll have to use 4AGE pistons. The reason being, the 7AFE pistons are dished so compression will suffer when the 7A bottom-end is mated to a 4AGE head. When 4AG pistons are installed along with the 4AG head however, the increased stroke ratio of the 7A block will increase static compression of the 4AG pistons. Say for example, 4AGE pistons rated at 10:3:1 (smallport 4AG) in a 4A block will make 10:3:1... but when in the 7A block with the longer stroke, it's estimated to be as high as 11:5:1. Big difference. The 7AGE is a decently well documented set-up, so search a bit and learn. Frouto is the only Celica guy to do it (his is turboed) and as I warned him before about the wiring, I'll warn you. Try to get either a JDM AE86/AE92 MAP harness, or a 92 USDM Geo Prizm GSI MAP harness. It'll make tuning the engine a WHOLE lot easier. frouto didn't do this and now he's redoing a bit of his wiring because the stock AFM set-up is not very tuneable.

This post has been edited by Kwanza26: Jan 5, 2005 - 9:59 PM