Hypothetical Power Increase

[Randalla]

I have a freshly built, SU carbureted, L-28 in my 72 240Z, with about 5,000 miles on the rebuild. The bottom end is a 280ZX block with stock dished (turbo) pistons. The head is an E-88 which has some port matching done, and the valves have been unshrouded. I'm running a stage II cam with total timing set at 34-36 degrees. I run a stock cast iron exhaust manifold connected to 2-1/2" pipes running through a glass pack type muffler. I ran a pair of "SB" profile needles in my SUs for a couple of dyno pulls to assess where I'm at. My goal was to build a very street-able engine, without having to worry about detonation. Of course, now I'm looking at potentially bumping power with some additional mods. The best current dyno run, at the wheels, produced 180.1 horsepower and 224.5 foot pounds of torque. My question is, "hypothetically," how would swapping out dished pistons for flat ones likely impact those numbers. The downside of course is that I may not be able to run as much total timing without detonation and potential risk to my engine. As an aside, I also have a nice triple Weber set up I'm toying with the idea of swapping on. I realize this exercise is pure speculation, but would be interested in any feedback. Thanks!          
 

 

Edited December 27, 2021 by Randalla
update

[grannyknot]

Beautiful engine, 180hp are good numbers but if you popped in flat top pistons and brought your compression up to at least 10 then the engine would really wake up with no danger of pushing it into unreliability 

[zKars]

How many times have we heard “the power is in the head”. Still true.

Cam, lift, porting. Make’r flow like a hurricane!   

[heyitsrama]

already have dished pistons why not slap a little baby turbo in it?

[Zed Head]

16 hours ago, Randalla said:

The bottom end is a 280ZX block with stock dished (turbo) pistons. The head is an E-88 which has some port matching done, and the valves have been unshrouded.

Calculate the compression ratio.  E88's came in several varieties as I understand things.  What size valves are in it?  You could probably come out ahead just by swapping to an N42 or N47 head, designed for dished pistons.  Higher CR and bigger valves.

[Randalla]

Thank you for your thoughts. A bit more info on the head/engine, the stage II cam specs are 480 lift and 276 duration. Good question about which E88 head I've got. Absent removing the head, I don't think there's any way to confirm that.

[Zed Head]

Have you measured cylinder pressures?  That will tell you something about the CR.  Ideally, combustion chamber volume would have been measured when the port-matching and valve unshrouding was done.

Either way, if you did not install bigger valves, an N42 or N47 will be an immediate benefit, due to the higher flow.  2mm bigger intake valve and smaller combustion chambers.  Here's the 1974 260Z specs compared to 1976 280Z.  Even the text is cleaner and crisper.

Edited December 27, 2021 by Zed Head

[dutchzcarguy]

20 hours ago, zKars said:

How many times have we heard “the power is in the head”.

The head of the engine....  or the head from the driver ?  😁

[Randalla]

No, I haven't measured cylinder pressures. I know compression numbers are different, but for what it's worth, compression when checked a month ago was:

#1- 160 psi        #2- 150 psi         #3- 150 psi       #4- 150 psi       #5- 150 psi       #6- 152 psi 

Edited December 28, 2021 by Randalla

[Zed Head]

Pressures can vary depending on the gauge and adapters that are used but those numbers are kind of low.  They're about right for a turbo engine.  I can't find much about the 260Z E88.  I've read about "smog" E88's that have lower CR's.  The 240Z E88's had pretty small combustion chambers.

Anyway, here's an article with some math and comments about what you could expect.

https://www.motortrend.com/how-to/compression-ratio-means-more-power/

Also, the 260Z had a CR of 8.8:1, which is actually on the high end for Z engines.  But you have a larger displacement so even with flat-tops you'll end up lower than that. (edited)

Big picture - it runs well and makes good power, why mess with it for small gains?

Edited December 28, 2021 by Zed Head

[Randalla]

Thanks Zed Head for your thoughts and link to the excellent tutorial. I learned a few things I wasn't aware of. Pretty sure my E-88 came from a 49-state 240Z, so likely fairly small combustion chambers, if I'm following you correctly. Car does run exceptionally well, so small gain with high cost is not really on the table. I had the option of installing flat-top pistons when I installed the re-worked head. My desire to get the car back on the road trumped the piston swap. This post was spurred for the most part by curiosity over what my impatience may have cost me. I may still swap on my Webers, though, I've had my triples on one of my other 240's and, honestly, my experience has been well set-up SUs perform just as well on the street, with a lot less headaches. Thoughts?      

[Mark Maras]

I like the simplicity and driveability of the SUs. Weber's would be fun to fool around with but I'd want a Dyno to check the tuning of them.