7A-FE Compression Ratio, Looking at changing the CR of my 7afe when I rebuild. Options

[PaukST]

Hey all, I'm trying to decide what to do with my 7AFE (have to take it apart due to a rod knock anyway). I've kicked around a few ideas (or maybe a lot ), and I'm currently researching what an increase in compression ratio might do. Does anyone have any info on this? I'm unsure of the potential difference in performance, and I can't seem to find any hard data on what pistons (4age, smallport 4age, 4agze) result in what CR. I've been trying to make sense of these tools and formulas - http://www.csgnetwork.com/compcalc.html http://s-86.com/s-cr.html and http://s-86.com/s-cr-chart.html - but I'm having a hard time digging out the relevant numbers. Doing the math myself or using the calculator I come up with significantly different numbers than their little handy chart, so I don't know what to think. Is this a futile exercise? Will I even be able to notice a performance difference over the stock 9.5:1 without having to use higher octane?

[Bitter]

9.5:1
Will need cams too
Tune
Higher octane

[PaukST]

My initial thoughts were to replace the 7AFE pistons with 4AGE pistons, something along these lines:
http://www.toyotanation.com/forum/134-6th-...-fe-engine.html
However, I've been struggling to find definitive info on piston dimensions. I already know the bore/stroke and CR of the stock 7AFE's and 4AGE's, and some other info from various places. 7AFE pistons: 12cc dish. Head gaskets - 4AFE: 1.4mm; 7AFE: 0.7mm; 4AGE: 1.2mm. Head CC - 4AGE: ~36cc; 4AFE (presumably close to 7AFE): ~31cc. Also allegedly 7AFE's piston deck clearance is 0.6mm.
What I'm hoping to find out is the dish or dome cc of the various 4AGE and 4AFE pistons. I have the feeling that even blue-top pistons (9.4:1 CR in 4AGE context) may result in too high of a CR in the 7AFE. I'm also figuring that if I end up with a >10.5:1 CR I'd almost definitely need to use higher octane (possibly before that threshold, but I'm having trouble finding info on that in general). Anyone have any thoughts?

[Smaay]

dont bother trying to drop in 4A parts. its not worth the money $$$ just turn your distributor up to about 15 degrees BTC and run higher octane gas. you will notice a huge difference!

[PaukST]

Hm, I guess it may just be the wrong route to go for the result I want. I was hoping to give my engine a little bit more pep without increasing my operating expense... It's my DD and work vehicle. I deliver pizzas in it, so upgrading my octane would cost me quite a bit more in gas. I'm planning to get the head port/polished before I put it back on, and eventually get a header, but I was hoping to do just a little bit of modification to the internals to get a little more 'zip' out of it. Any other avenues I've overlooked?

[stephen_lee]

import performance parts . net sell "HP KITS" that you can pick your CR

id go with 11.5:1 and run supreme gas, get your cams reground while you're getting the motor built and get some sort of tune.

i wouldnt expect any more than 130whp though... but with a very strong midrange and slightly improved MPG if the tune is done right

[Smaay]

the problem is you have a 7A-FE, that was the economy engine and there is really no aftermarket support for it.

[PaukST]

the problem is you have a 7A-FE, that was the economy engine and there is really no aftermarket support for it.

QFT. Honestly, both engines I've had in this car have been pretty tired... One with a rod knock, and its junkyard motor stand-in. I'd probably notice a good improvement with just a good stock rebuild and cleaning. I think I'm going to port and polish the head, and possibly get a header at some point... added to the short ram air intake I already have, it'll probably be an outstanding daily driver.

...And all the time I don't spend on the engine I can put toward freshening up everything else!

[Smaay]

did you see the thread i made in the stickys? its step by step rebuild of a 7A-FE. Erynn's car had a slight rod knock. Luckily the crank was fine, i just replaced all the bearings, new piston rings, new gaskets. and the car ran like a bat outta hell.

[Special_Edy]

My local import machine shop charges $35 to deck a 4cylinder head and $45 to deck a 4 cylinder block. A chemical dip is included in the price for the heads, Im not sure about the block.
This would be the route I would go, you can also get a thinner head gasket for some extra $s. Im not sure how many thousandths of an inch you can remove but with an inline 4 the only major issues you have to worry about are the valves and sparkplug hitting the piston and taking to much material off where the waterjackets are located.
Do some research and find out how many CCs to remove from the combution chamber. I think you could run 10 to 1 CR without needing high octane(unless you try to advance it) and once you get to 11 to 1 CR you will most likely need to go 93 octane. I think 13 to 1 and up requires racing fuel and that gets real expensive.

[PaukST]

As far as I can tell, it would only take 3cc less combustion chamber area to increase the CR from 9.5:1 to 10:1. 4AFE pistons are an 8cc dish (if random internet searches are to be trusted), so the 4cc difference between the 7AFE pistons and those would make the CR ~10.2:1 with the standard 7AFE head gasket.

At this point I'm most heavily favoring a stock rebuild, since it's most likely to go as planned (and takes the least buying of new parts). I really wish I knew more about CR's, detonation, ignition timing and such, though. Oh well, I'll get there eventually!

[Special_Edy]

Just have it decked. Do the stock rebuild.
You are supposed to have the cylinderhead resurfaced anytime you remove/reintall it, which basically involves them grinding a layer of metal off of it. You can specify to the shop how much you want removed if you would like more taken off than the minimum amount needed to bring the head back to specification. I have a '64 chevy straight six in my garage that has an 11-1 CR and all it took was a shop grinding a little off the heads.

As for detonation, there are several factors that can make it happen or help you avoid it.
Ignition advance- more advanced = more power but more likely to detonate. Retarding it will help cure detonation
Fuel octane- Lower octane fuel has more energy per gallon, but lower octane is more likely to knock. Basically octane rating is the fuels resistance to combustion. Increasing octane will reduce detonation.
Air to fuel ratio- should be close to perfect with our EFI systems, but a lean mixture is more likely to knock and a richer mixture will be less likely. Lean mixtures burn hotter and can damage the insides of your engine while rich mixtures will foul your plugs and catalytic converter.
Spark plugs- hotter plugs will burn fuel better, but are more likely to predetonate. Moving to a cooler plug(one that dissipates heat better and stays cooler) will reduce the likelyhood of knocking but will be more susceptible to becoming fouled.
Anyways, theres a few tidbits there I wouldnt worry about overdoing the CR the only thing you should worry about is the clearance between valves and pistons if you do go with a non-standard piston

[richee3]

Fuel octane- Lower octane fuel has more energy per gallon, but lower octane is more likely to knock. Basically octane rating is the fuels resistance to combustion. Increasing octane will reduce detonation.

Quoted for reiteration. Maybe one day the myths will be dispelled....

OP- I would say you're better off with a standard rebuild. The 7A with a higher compression ratio will still not be a high output engine. Even with cams, SAFC, etc. it still wouldn't put a lot of power to the ground. Why be forced to pay for premium fuel for only a little more power than a 5S? If anything, I would personally lower the compression ratio and boost it later.

That's just my opinion, what I would do if I was rebuilding a 7A. Truth is, I'd love to know what a higher compression 7A can do. The sensible side of me says stick to proven methods to make power for the 7A, but that doesn't make me any less curious.

[Bitter]

timing advance is similar to a compression increase in terms of gains.

[mandrek]

My local import machine shop charges $35 to deck a 4cylinder head and $45 to deck a 4 cylinder block. A chemical dip is included in the price for the heads, Im not sure about the block.
This would be the route I would go, you can also get a thinner head gasket for some extra $s. Im not sure how many thousandths of an inch you can remove but with an inline 4 the only major issues you have to worry about are the valves and sparkplug hitting the piston and taking to much material off where the waterjackets are located.
Do some research and find out how many CCs to remove from the combution chamber. I think you could run 10 to 1 CR without needing high octane(unless you try to advance it) and once you get to 11 to 1 CR you will most likely need to go 93 octane. I think 13 to 1 and up requires racing fuel and that gets real expensive.

WOW!!! $45 to deck the block!!! awesome.. my shop here charged $135 to deck the block.. but he also told me that it was because the block is cast iron.. and not aluminum.. is that $45 for iron block aswell?

and to the OP... IMO do a clean rebuild, and then go turbo.. thats what im doing.. and all of my research shows it the best option, if not doing a swap.

[PaukST]

Given what Smaay mentioned about the one he tore down and rebuilt, It's possible that the difference between a tired engine and a clean, fresh rebuilt one may be all I need... My perception of the 7AFE so far has been shaped by 2 tired engines, hardly an objective reference point.

I thought about boosting it, but if it's lively enough to get me around, reliable, and gets good gas mileage, I can always pick up a toy later to go wild on. Or if I pick up a different daily driver, I still have another 7AFE that needs a rebuild - I could always do it differently the 2nd time around.

[Special_Edy]

My local import machine shop charges $35 to deck a 4cylinder head and $45 to deck a 4 cylinder block. A chemical dip is included in the price for the heads, Im not sure about the block.
This would be the route I would go, you can also get a thinner head gasket for some extra $s. Im not sure how many thousandths of an inch you can remove but with an inline 4 the only major issues you have to worry about are the valves and sparkplug hitting the piston and taking to much material off where the waterjackets are located.
Do some research and find out how many CCs to remove from the combution chamber. I think you could run 10 to 1 CR without needing high octane(unless you try to advance it) and once you get to 11 to 1 CR you will most likely need to go 93 octane. I think 13 to 1 and up requires racing fuel and that gets real expensive.

WOW!!! $45 to deck the block!!! awesome.. my shop here charged $135 to deck the block.. but he also told me that it was because the block is cast iron.. and not aluminum.. is that $45 for iron block aswell?

and to the OP... IMO do a clean rebuild, and then go turbo.. thats what im doing.. and all of my research shows it the best option, if not doing a swap.

Yeah the fine print is *prices may vary
the head was definately a good deal.

I would say that for performance mods to your engine, increasing the Compression Ratio is one of those things that doesnt increase your fuel consumption, it just helps you extract more energy from the fuel you are already burning. Increased efficiency plus increased power is a win-win

[PaukST]

It looks like reducing the chamber height by ~ .5mm would put me near 10:1.

If I need to get the block/head decked, I might get it taken down by .3 or .4mm instead of a minimum amount. Taking it down by .3mm would give me a 9.76:1, or .4mm would take it to 9.85:1 (assuming correct math). If I'm already going to get it decked, I doubt I'd run much risk at either of those levels (and it leaves room for error if my math is bad ).

If I don't need to get it decked, though, it's a stock rebuild for me!

[Special_Edy]

It looks like reducing the chamber height by ~ .5mm would put me near 10:1.

If I need to get the block/head decked, I might get it taken down by .3 or .4mm instead of a minimum amount. Taking it down by .3mm would give me a 9.76:1, or .4mm would take it to 9.85:1 (assuming correct math). If I'm already going to get it decked, I doubt I'd run much risk at either of those levels (and it leaves room for error if my math is bad ).

If I don't need to get it decked, though, it's a stock rebuild for me!

Thats the whole point.
You should get it decked just the same as you should buy a new set of torque to yield headbolts. The headgasket leaves indentations in the head, so you should always at least have it checked for straightness by the shop.
Its a performance mod that may not cost you anything since you will probably need the head resurfaced anyways. Just ask the shop performing the labor for their recommendation, they should be able to give the best advice

This post has been edited by Special_Edy: Apr 7, 2012 - 12:53 AM

[celica74]

Let us know what you decide to do, the progress and the results. Im in the same situation. I know i need a rebuild. Wanna have a little fun when i open it up. Was thinking 4afe pistons or getting it bored to 82mm and running 2zz pistons. Wouldnt know my route until i open her up though. Does anyone by change know how to advance/retard the timing with the distributer on the 7a by chance? Might be useful to know.