Confused on G4 port timing

[francisjohn]

I'm confused regarding the port timing on the G4 cylinders (specifically the open transfers)

I have done cylinders in the past that have closed transfers. Using a degree wheel is super easy on these, right as the transfer port is a hair open, right down the opening degree and calculate your duration.

With the open transfers what is confusing is the two transfers (primary and secondary) are at different heights. I'm confused on how do you calculate port timing when you have two different transfer durations

It ends up seeping into problems calculating your blowdown (Transfer opening ATDC - Exhaust opening ATDC = Blowdown)

Is there some sort of formula I don't know about?

 

[Chainlube]

The start is when the transfers just begin to open.

 

[francisjohn]

The start is when the transfers just begin to open.

I understand that, the issue is on the G4 cylinders you have two transfer "ports"

For example, if it were a closed transfer that opens at 116° ATDC, and the exhaust opens at 92° ATDC

116° -
92°
---
24° Blowdown

The issue is you have two transfers that open at different times. I mean, you can't have two separate blowdowns. I wasn't sure if you only pay attention to the primary transfer (transfer that opens first) and not the secondary?

 

[Chainlube]

I understand that, the issue is on the G4 cylinders you have two transfer "ports"

For example, if it were a closed transfer that opens at 116° ATDC, and the exhaust opens at 92° ATDC

116° -
92°
---
24° Blowdown

The issue is you have two transfers that open at different times. I mean, you can't have two separate blowdowns. I wasn't sure if you only pay attention to the primary transfer (transfer that opens first) and not the secondary?

The degree starts when the transfer just starts to open, so the highest side. The low side can be corrected by filing it a bit, it wouldn't take much.

The biggest issue with most of the transfers of these engines is the casting, the side angle of the transfer isn't very good.

 

[francisjohn]

The degree starts when the transfer just starts to open, so the highest side. The low side can be corrected by filing it a bit, it wouldn't take much.

The biggest issue with most of the transfers of these engines is the casting, the side angle of the transfer isn't very good.

When you say by "filling it" I assume you mean removing material? I took a junk G4 cylinder I have and attempted to raise the secondary transfer to the same height as the primary transfer and it's such a tight space that the bit kicked back on me.

The port map I have for the stock G4 cylinder I have is this:
Exhaust : 103° ATDC (154° Duration)
Transfers : 120° ATDC (Duration 120°) - Yes, that looks odd but 120 ATDC is a 120 duration
Intake : 60° ATDC (120° Duration)

Just going off the exhaust, if I remove 2mm of material from it, it will open at 97° ATDC (96.5 technically but if more than .5, I round it up)

The exhaust opening at 97° ATDC, with a transfer opening of 120° ATDC will give me 23° of blowdown which will be very nice. It appears on these G4 cylinders they remain constant in the sense of port map. That low of a blowdown is ideal on a stock setup etc. Going up to 23° of blowdown works best with weaker hitting pipes. Good mid rang torque & power up to 9K-10.5K RPM range.

 

[Jayraye39]

I'm confused regarding the port timing on the G4 cylinders (specifically the open transfers)

I have done cylinders in the past that have closed transfers. Using a degree wheel is super easy on these, right as the transfer port is a hair open, right down the opening degree and calculate your duration.

With the open transfers what is confusing is the two transfers (primary and secondary) are at different heights. I'm confused on how do you calculate port timing when you have two different transfer durations

It ends up seeping into problems calculating your blowdown (Transfer opening ATDC - Exhaust opening ATDC = Blowdown)

Is there some sort of formula I don't know about?

open is open and closed is closed nomatter how many tranfers their are. as soon as the highest transfer starts to open thats where you start when the high side closes thats where you stop

 

[Jayraye39]

I run heavily ported g4 cylinders on all of the motors I have built. raized exhaust 1mm and widened it by 2.5mm. transfers i did not touch. intake port also widened by 2mm. ramped the crown on my pistons for all 3 ports. My rpm can hit 10,500. the G4 still retains good low end also