1985 4D55T DNA Head Swap 4D55T

[Sandrailstudent]

Hello and thanks for reading.

I just changed from a protruding valve 4D55t head to a 4D56t head and it is much harder to start and lost a noticeable about of power all over. I am thinking that the difference in squish volume drops the compression. I can get it started with a bunch of cracks and a slow caughing start or laying on the glow plugs for multiple 5 second sessions. This is not awesome, but I can live with it if I have to.

Does anyone know of some adjustments to wake the motor up some? Mine lost its zip with this change and hills that I pulled in 5th gear at 75MPH now require 4th at 60 MPH or less. The fun factor is also diminished.

How much boost should I add above 8PSI to get it back to the stock compression? That is my preferred short term fix for high RPM rejuvenation.

Also, is it worth going to a bigger VE IP head? Amazon and eBay sell some nice options for $150. I also thought about swapping the lift plate, but everyone says to have that calibrated in a shop and monitoring EGT and boost pressure. None of those are bad ideas, but I like to do my own work on the cheap.

I had the swirl chambers crack in the last Chinese head and I would change the swirl chambers out if I could find replacements and know how they were kept in place. I can measure the volume in those chambers and figure out a rough squish volume to get me started at guessing an equivalent boost to get back to stock.

I wanted to ask about all of these ideas first before doing math and measurements. You guys rock.

Any help is appreciated.

Chris

[Sandrailstudent]

Alright, after feeling out a bit and admittedly leaning on GROK for most of the calculation, I get the following. It did really well with just the basic question, but it oversimplified the compression ratio to the 1.3 power by simply multipying by 1.3, which gave an inaccurate approximation. Fixing this one obvious short cut seems like a plausible solution, so I'm going to try it and see how it turns out and I will or baby validate the new compression ratio by volume.

Per Delica forum swapping to flat or recessed valves drops compression from 21:1 to 18:1.

To get back to the same about of pressure as the stock 8psi boost I need to go up to 13.05 PSI of boost (I'm not going that accurate, so call it 13.).

This will not address the starting issue, but will get the top end power back. I also should not have to add any more fuel, but we will see.

The Delica forum seemed to suggest that advancing the timing would help restore power, and some people ended up jumping a pulley tooth or two with a timing light to get it running better.

Thanks for any feedback.

Chris