valve sizes for supercharged engines - 4age

[Dangerous Beans]

Hi
I am building a 4age for use with a twin-screw supercharger. I was planning to leave the valves at stock sizes, but while looking for info on camshafts I found a suggestion that exhaust valves should be larger for the same size intake valves in a supercharged engine. Basically, the theory was that since there is more gas to be removed from the cylinder, and the system lacks the restriction of the turbo, the engine benefits from the larger relative to intake exhaust valves.
The suggestion was, where an NA engine wants around 0.75 intake:exhaust, a supercharged engine would benefit from 0.85-0.90.

Is this theory right? Have others heard this?

In the specific 4age situation; the stock valves are 29.5 intake and 25.5 exhaust, for a ratio of 0.85. if you get 1mm oversized exhaust valves the ration goes up to 0.90.
is anyone aware of any benefit to using 26.5mm exhaust valves with the stock intake size on a 4age?

Thanks for the help

[roadsailing]

The theory is sound but i wouldn't bother unless you are going for a maximum effort engine.

As far as i know 4AGZE heads are identical to 4AGE heads, so toyota thought it was good enough (and cheap enough i suppose) to leave them.

Will you be using aftermarket camshafts?

[oldcorollas]

which is more of a restriction, the valve head size or the port? can you fit them in there and not hit on overlap?

[Dangerous Beans]

which is more of a restriction, the valve head size or the port? can you fit them in there and not hit on overlap?

I have no idea. there should be enough room for 1mm oversize valves though.

The theory is sound but i wouldn't bother unless you are going for a maximum effort engine.

As far as i know 4AGZE heads are identical to 4AGE heads, so toyota thought it was good enough (and cheap enough i suppose) to leave them.

Will you be using aftermarket camshafts?

I am going to be using aftermarket cams.
I agree that it probably isn't worth it, but i'm curious as to how much difference it would make.

[trdee]

most people dont bother with valves and just adjust the overlap with cam gears and/or run staggered cams, from what ive seen anyway

[Duk]

I'd be looking at making a larger diameter, free flowing set of extractors 1st.
With higher induction pressure (from the supercharger) than there is is back pressure from the exhaust, with the right amount of valve overlap a supercharged engine should scavenge the combustion chambers better than any road going naturally aspirated engine and shit loads better than a turbo'd engine.
The key is getting the overlap right . Just enough to get rid of the exhaust gas dregs but not to much that you are wasting air that could have otherwise been mixed with fuel and burnt.

Mmmmm, continuously variable valve timing (and a programmable computer to drive it)

[mullett]

I used a 272 cam on the exhaust, and went back to a stock inlet cam. Made it idle like stock, and pulled almost as hard at high revs, whilst making a LOT more torque at lower revs.

-RM

[TERRA Operative]

4AGZE head is exactly the same as a 4AGE head of the same type, except for the cam covers on the smallport, which have a larger breather pipe fitting and bosses for mounting the DLI coil bracket.

I can't help with the valve sizes, but I did gain about 2psi or so from retarding the exhaust about 3 degrees to close up the overlap a little.

[Dangerous Beans]

4AGZE head is exactly the same as a 4AGE head of the same type, except for the cam covers on the smallport, which have a larger breather pipe fitting and bosses for mounting the DLI coil bracket.

I can't help with the valve sizes, but I did gain about 2psi or so from retarding the exhaust about 3 degrees to close up the overlap a little.

did you gain any power? or was the engine just not sucking in as much?

I am going to have to spend some time playing with cams once i get everything setup, but that is a while away. I have to get cams first (people i was getting them off got hit by an earthquake).

[TERRA Operative]

Dunno, but it felt a bit better for subjective bum dynoing.
I've stopped my development on my 4AGZE (save an SC14), as I am starting a 5SGZE build instead.

[30psi 4agte]

which is more of a restriction, the valve head size or the port? can you fit them in there and not hit on overlap?

You can fit bigger valves with no issues... I run 1mm on intake and 1.5mm on exhaust

( I think.... id have to check its been that long since doing it...... and my members rides thread which had all that info available was mistakenly deleted by a moderator .)

In short yes you can fit them...... but you have to ask. What is easier / cheaper. Machining valves into head or bolt in cams......