OST-093: My, My, Mystery

[oldeskewltoy]

Essentially there are 3 types of 16V 4AG heads. There are 2 versions of the largeport, and of course the smallport.

The early largeport (1984-1987) was originally designed/built(?) by Yamaha, where the later ones(1987-1989) were changed and made by Toyota(?). You can see some of the changes in the following photos...







If you watch Garage 4AGE's videos you may have seen the earlier head makes a bit more power. This is likely because at the bowls the earlier heads have a better shape as the bowl turns to the valve...



... BUT there is more room in the floor of the later head (arrows).

I've come to the conclusion that the later heads were partially designed to better accommodate Formula Atlantic engine builders....... but that's another story...



The Mystery here is the head I'm working for OST-093. It appears that it is a mixed head, with some newer features, and some older features......




more to come......

[oldeskewltoy]

The Mystery here is the head I'm working for OST-093. It appears that it is a mixed head, with some newer features, and some older features......




more to come......

So... what makes it a mixed head?

Please note that the front of the head is cast as a later type - albeit it isn't drilled/tapped





and yet the intake ports have the earlier shape




more to come.....

[Corey20v]

Fun to see these mixed mfg method parts in the wild! Hopefully the engineers knew and approved

[oldeskewltoy]

While this is a mixed generational head, the intake casting was SUPERB.... note the bowl casting comes all the way out to the edge of the seat.




In all my years working these heads this is among a handful that were well cast..... well at least on the intake side......



The exhaust side... not so much




more to come.........

[Rogue-AE95]

The first 4A-GE I bought was from a 1987 GTS coupe (built in 12/1986 from the VIN). It also has those untapped holes on the head (for the FWD stiffener bracket).

[oldeskewltoy]

2 down... (leftside) 2 to go.....





after discussion about his build, my client decided on a new set of pistons......




more to come.......

[oldeskewltoy]

The pistons I just showed in my previous post.... This is essentially a stronger smallport piston.



Full size image - https://gallery.ostportworks.com/albums/userpics/10005/OST_Traum_high_comp_copy_versus_OEM_high_comp_6_panel_large.jpg


Stock valve sizes because more than 80% of 4AGEs built are built with standard valves. The biggest flaw in OEM type 4AGE pistons (9.4:1, and 10.3:1) is they have a slit for oil return. These forged pistons support the 2nd compression ring far better with a series of holes.

Under a stock chamber, and a stock 1.25mm gasket, they are 10.3:1, under a tighter squish (.032"/ .8mm) they calculate out to 10.9:1.

These are also customize-able, want less/more compression height - done, want more, or less compression(dome volume) - done. These are available in .25mm overbores (81.0, 81.25, 81.5, 81.75, and 82mm)

One of the "olde" reasons for not using forged pistons were they tended to slap in their bores until they warmed up,because piston to wall clearance had to accommodate the pistons expansion. The piston to wall clearance on these pistons are 0.0025" (0.064mm). If you look up the stock piston to wall clearance you'll note it is 0.0039" - 0.0047" - so these should not "rattle" in their bores.

For more info send me an email: oldeskewltoy at yahoo dot com please include "OST/Traum piston" in title

[sirdeuce]

That "rattle" depends on the alloy of the forged piston. "Street" forgeged pistons using a different alloy than "racing" pistons didn't need so much clearance. Wish I could remember the different alloys. The "street" alloy was also less rigid than the "racing" alloys. In the earlier days most manufactureres kept to one alloy, and that was for racing. Most machine shops would only bore the block for the racing application and that led to piston rattle in street engines all the time. For street apps the piston/bore clearance needed to be tighter as the pistons didn't get hot enough to expand properly for the use. These days "intended use" is taken into consideration for alloys and clearances.

I'd like to see hypereutectic castings for street apps. Hyper pistons are lighter than reular cast and forged pistons. They don't have as much eaxpansion as regular cast and forged pistons keeping tighter clearances. Most street performance builds would be fine with hyperU cast alloys.
Hypereutectic alloys seem to wear better as well. Might come from being harder or from tighter grain or both.

Sorry, I do ramble.

[oldeskewltoy]

That "rattle" depends on the alloy of the forged piston.

Typically... 2618, they have been known to make some from 4932, but they don't recommend it for boosted applications.

I'd like to see hypereutectic castings for street apps. Hyper pistons are lighter than reular cast and forged pistons. They don't have as much eaxpansion as regular cast and forged pistons keeping tighter clearances. Most street performance builds would be fine with hyperU cast alloys.
Hypereutectic alloys seem to wear better as well. Might come from being harder or from tighter grain or both.

Sorry, I do ramble.

While I don't disagree..... Castings.... to fit all the possible combinations* would be rather costly.....

* - different bores, different compression heights, different dome volumes.....

[sirdeuce]

The forgings I have for my 22RE stroker, 93 bore, 94 stroke, 2550cc, are 4032. Still gotta idle for a minute before driving to staunch the rattle. Sound normal after the engine gets up to operating temp. Hypereutetic slugs would be better for this engine since it never gets hard use.

Yep, casting cost more. Lot easier to mount up a slug and enter a program (oversimplfied) than to design, and store molds AND to have a foundry.

[Rogue-AE95]

Is it a trick of the camera, or does the forged piston have a little less clearance for the oil squirters?

If you have the forged pistons ceramic coated (skirts & domes), assuming that also changes the piston to wall clearance. I think ( bad memory) that someone told me to add 10 thousandths when having piston skirts ceramic coated. But I don't know if that was the same or different between cast & forged.