4AGE bigport, best way to get more compresion on a budget?

[wiso]

I have just pulled my bigport 4age out of my AW11 and am looking at a rebuild. I don't have alot of cash ATM so this has to be budget, I have about 1k for the whole rebuild, I was hoping it could go later so I could do more upgrades but it has to be done now.

I know the bottom end block is reasonable and can rev abit according to people like Bill Sherwood.

One of the things I really wanted to do to try and squeeze some more power from the motor is raise the compression, Eventually I would like to get some 256 or 264 degree camshafts, but atm they are out of my budget. So to get this motor setup for that I was thinking of trying to get compression up to about 10.5:1 or even 11:1.

Are there any pros or cons on doing this with a standard motor running stock electrics?

I have read here a rough guide is every 1mm shaved is equivalent to 1 raise in compression, so if the stock bigport is 9.4:1 and I want to get it to about 10.5 if I was to take about 0.2 say off the block and 0.8 off the head I should end up with about 10.4? I will most likely be using a standard 1.2mm graphite head gasket.

some have said shaving the block is a bad idea? would 0.1 or 0.2mm come under this? just shaving to make sure its flat also.

the other option I cannot confirm whether its the pistons in the small port motor that make it 10.5:1 if they are the reason, would these be usable in the bigport block? (it is a 7 rib block) I would assume they would be.

If the compression does get raised to one of these levels alot of people say its a good idea to grind the sharp corners in the combustion chamber. from what I have read this will lower the compression ratio. Looking at the standard small port motor they still have these ridges at the 10.5:1 obviously toyota did not see these as an issue at that amount of compression. what about at 11:1?

how much can safely be shaved off a 4age head?


If some can help shed any light on this I am going around in circles not really finding any new info to solve my questions.

BTW this car is my daily so I am not aiming for massive HP, If I could get about 100-110rwhp at the end of the day I would be happy, I still need it drivable hence really looking at the 256 cams

Thanks

[JP]

use a genuine 7A headgasket. From memory they're 0.8mm to start with.

[dnegative]

Whats a stock bigport supposed to have compression wise when fresh?

The one I pulled out of my adub had around 200psi on each cylinder, part of me wants to sell it yet another part wants to pull it apart and see why it pulled so hard. I have a feeling it was rebuilt in japanland once upon a time

[wiso]

that pretty high, the manual says the stock bigport a 9.4:1 should have about 165psi, it would be interesting to work out what the CR on your motor is. but from what I have read to do that you need to know the Combustion chamber size

[Sam_Q]

use a genuine 7A headgasket. From memory they're 0.8mm to start with.

there is some debate on what they actually are, I have measured a replacement one and it was 0.4mm, many others have reported the same. Either way their not compatiable because the cooling holes are in the wrong spot and the engine will run cooler at the front than the back if you install one, for a 20V it's compatiable though.

now wiso you cant use the smallport pistons (officially 10.3:1) because they have a 20mm floating gudgen pin like all the later model gear and unless you have something out of the ordinary you have a press fit 18mm pin.

Shaving the block is trickier because anything you take off brings the pistons closer to the head. Now at worst your pistons hit the head but at best your have a nice tight quench zone which is ideal. A pretty good way to go is to make the piston edges line up exactly with the deck of the block- which they should be close to standard and then un a 0.8mm trd gasket. This will raise your compression to a bit over 10:1. After that a 0.5mm shave off the head and your home free. You likely have to run 98 octane fuel though and readjust your timing to suit.

Are you going to fit new pistons to this? because if so it is possible to order genuine higher compression pistons to your engine, they where released in other countries (Malaysia?).

[Hiro]

now wiso you cant use the smallport pistons (officially 10.3:1) because they have a 20mm floating gudgen pin like all the later model gear and unless you have something out of the ordinary you have a press fit 18mm pin.

7-rib bigports (which aren't "out of the ordinary") run 20mm floating pins (and 42mm big-ends), it is only the 3-rib bigports that run the smaller pins/bigends.

[oldeskewltoy]

Are you going to fit new pistons to this? because if so it is possible to order genuine higher compression pistons to your engine, they where released in other countries (Malaysia?).

Singapore and some areas in Europe offered 10 to 1 pistons in their TVIS engines

this is oversize - .5mm

[trdee]

stock bigport CR = 9.4:1

stock smallport CR = 10.3:1

the car won't run well on the stock ECU if you bump up the compression too high. you say you only have 1k to rebuild the engine? rebuild it to oem spec and stick in a thin headgasket and leave it be.

[Sam_Q]

7-rib bigports (which aren't "out of the ordinary") run 20mm floating pins (and 42mm big-ends), it is only the 3-rib bigports that run the smaller pins/bigends.

no you misread what I meant, I said all the later stuff had 20mm and unless his was out of the ordinary (knowing that he has a 3 rib) then he would have a 18mm

[wiso]

I mentioned I had a 7 rib block in the original post, so with this I should be able to use small port pistons.

still nothing on how much I can shave off the 4age head before its too much?

I was thinking or getting a 0.8mm headgasket but the kit I have has a 1.2mm one, and its cheaper for me to shave it than buy another gasket

[pandaah]

you can shim as much off the head before it comes into contact with the valve seats..

[Sam_Q]

well it looks like it was that risread then, my appologies.

It is a good idea to have the least gap possible between the head and pistons, from what I have seen around the pace a rough guide of about 0.6mm is the least you want to go, 0.8mm is easy as. So if your going to run the original gasket then deck the block to suit.

[knightrous]

I have a set of small port 10.3:1 pistons you can have for $50 shipped

[StuC]

dont forget to get yourself a nice lightweight flywheel and have the crank, fly/clutch and crank pulley all balanced up. This in itself will give you a much smoother, freer revving motor. Lightweight flys make a huge difference

[wiso]

I don't know if crank, pulley and flywheel balancing is within my budget, from my experience having this stuff too light can make issues with the car wanting to stall when clutching in coming down hills, up to traffic lights etc. My old celica had a light flywheel and crank pulley and tended to stall time to time.