The Truth on Frankenmotors?

[Ironfoot]

It's awesome to see the love and respect that Datsun has enjoyed since its very early years. Of course racing success and style go along way in the minds of motor mad, and Datsun has always had a lot of both. Still it's nice to flip through coverage of this year's SEMA show and see countless iterations of Datsun bliss, including this very interesting OS Giken build of a LD28 block with a version of their tc24 head done by Z Car Garage. This NA setup is purportedly making over 400hp and begs the question are all the blocks we've been ripping cranks out of for years going to be the next hot ticket. Probably not for the average Joe but it seems that with the taller block you could run an even longer stroke forged crank than you could otherwise. Of course it would require some ingenuity in the head department as well.

This brings me to a question that has been rolling around in my head for a while, and I think it's been done, but I'm having trouble tracking down any real world info. I'll preface, this is a budget build question. OK, so here it is: If I grab a late flat top L28e from the junkyard and stick the crank and rods out of my L24 in it will it work? I would have figured this would have been one of the most common back yard builds since you end up with a motor that has only slightly less displacement that the L28, but a compression ratio of between 10:1 and 11:1 depending on head and gasket, along with the more favorable B/S and R/S ratios people are so keen to focus on. Any input or real world experience would be appreciated. I just love the idea of an all Nissan build that can be done with junk yard parts and result in this kind of compression. Any thoughts, or am I totally out in left field on this one?

 

[Derek]

I used the OZDAT calculator and come up with a 2.5L with a 9.3 CR.

 

I think your calculations might be off. Other than that I'll leave it to others to comment on advantages/disadvantages. 

 

Derek

[Ironfoot]

Cool, ya what a great tool that is. To get the real nice numbers I put in +1mm L28 flat tops, peanut head (E31), and NISMO .6mm headgasket, producing a 2.63 liter with 10.6 to one. Ya realistically a back yard crank/rod swap would be right where you quoted. Still a nice options for the money, thanks for the reassurance and help on the figures. If anybody has done this please chime in.

[Chickenman]

Just an FYI.  OzDat calculator seems to have the wrong deck height in their database for the L28 FT pistons. According to real world measurements, which give stock L28 FT a negative deck height of approximately .020" ( plus or minus .005" for stack  dimensions ). IE: FT pistons sit above deck approximately .020"

 

That reduces the OZDAT compression ratio calculation substantially.

 

Other data bases and other calculators show an L28 with FT and stock Nissan head  gasket of having a CR of around 10.8 to 1. Thin Nismo .6mm head gasket would put you well over 11 to 1 CR I do believe.

 

http://www.viczcar.com/forum/topic/4286-overview-of-l6-engine-heads/

 

Here is a site that I use for engine calculations. Has a massive amount of calculators:

 

http://www.wallaceracing.com/Calculators.htm

 

Here is the CR calculator:

 

http://www.wallaceracing.com/cr_test2.php

 

Edit: I used the Wallace calculator inputting a std bore L28 with Flat-top pistons ( -.020" deck height ) and E31head with stock head gasket thickness ( .047" or 1.2mm compressed ). CR came out as 10.9 to 1... which seems to jive with what other sites say.

 

Here are my figures:

Edited January 28, 2017 by Chickenman

[Chickenman]

Delete. Wrong screenshot.

Edited January 28, 2017 by Chickenman

[Chickenman]

I plugged your combo in and got 11.22 to 1 CR: Used Nismo head gasket with compressed thickness of .6mm ( .023" )

 

Edited January 28, 2017 by Chickenman

[JMortensen]

Type this into google: "destroker site:forums.hybridz.org"

 

Lots of threads on it. Simple answer is that the stroke of the L28 is not the limiting factor. You can build it to rev as high as you can your destroked motor, but with the destroker you're losing displacement. 

[Ironfoot]

Thanks guys, that would be great if I could pull those numbers. And yes I agree that the stroke of the L28 should in no way be the limiting factor of the engine as far a high red line is concerned or more importantly usable power at revs. Even the LD cranked strokers are still oversquare by definition. The main goal of my inquiry was to see if it was possible to get a nice compression ratio for little money by swapping some junkyard parts, and it seems very feasible.

[madkaw]

Dizeased over on CCZC does these de strokers all the time - I think he has done 3 .

Could find 81-84 maxima and use that head on your L28 if you want junkyard CR!!

[NewZed]

Diseazd, I think you mean.  Isn't there an old saying - "There's no replacement for displacement"?  "Destroking" gives a buzzy motor but you end up with less power.  Nissan lengthened the stroke to add displacement, giving more power.

[Chickenman]

Thanks guys, that would be great if I could pull those numbers. And yes I agree that the stroke of the L28 should in no way be the limiting factor of the engine as far a high red line is concerned or more importantly usable power at revs. Even the LD cranked strokers are still oversquare by definition. The main goal of my inquiry was to see if it was possible to get a nice compression ratio for little money by swapping some junkyard parts, and it seems very feasible.

There are much easier ways to get a reasonable CR without running swapping cranks and rods. I take it you're worried about detonation on pump gas?  If your running Flat Tops on a L28, do a trial assembly to get an ACTUAL measurement on piston deck height, and then machine the piston to get a zero deck height ( or what ever you need ) .

 

It's no big deal to machine .020" to .040" off a piston top. Have your machinist measure the piston crown thickness, but there is usually more than enough meat there. Some hybrid motors machine the piston tops .100" with no issues. If you have to machine valve reliefs for a big camshaft, that usually involves quite a deep cut.

 

Measure twice... cut once.

[JMortensen]

I would expect an L28 at 9:1 to make more power than an L24 at 10.5:1, especially on pump gas. If you have to pull back the timing to get it to stop pinging, you're fighting a losing battle. There is some hp in those last few degrees of advance.

 

EDIT- My L28/E31 had about 11:1 compression and needed 95 octane to not ping with mid 30s advance. That was with a bunch of headwork done including grinding off the spark plug threads that stuck out past the plug in the chamber and smoothing out every other sharp edge that might lead to detonation.

Edited January 30, 2017 by JMortensen

[Ironfoot]

So with the right head and headgasket I can get over 10:1 with my l24? What is the main reason for detonation in the l-gata, and what internally can be done to reduce it, and how do sharp edges come into play? I would suspect the head design (non-crossflow), plays a part but let me know. I am aware that it is a problem with these engines and have been considering converting to E85 to overcome it if I go all out in the future. Even so I would like to get the compression as high as possible with pump gas. A more aggressive cam would help offset it as well, but what little things like grinding the plugs as mentioned above, can I do to alleviate this tendency?

[NewZed]

You should read all of these threads - http://forums.hybridz.org/forum/90-l-series/

 

You seem to be hearing what people are saying then deciding to do what you wanted to do anyway.  The engine's don't like high compression ratios.  And people don't just "convert to E85" on a whim, it's a whole other world.  If your thinking that picking the right head gasket is the way to set a compression ratio, E85 is going to take you forever.

 

Seriously, read the threads in that link, they're full of answers.

 

 

p.s.  I see that I basically copied and reworded JMortensen's comment, #10 and #7.  Overlapping posting, I think, I missed his.

[JMortensen]

Any sharp angle in the combustion chamber will get hot and that leads to pinging and detonation. You can smooth the corners of the valve margin, remove the plug threads, take off the sharp edge where the combustion chamber meets the flat surface of the head, things like that. Make all the chambers the same size so that one isn't higher compression than another one, you can unshroud the valves, notch the block. There's a million little things to do, only limit is how anal retentive you are. When you get them all done, that L24 at 10:1 still won't make the same power as an L28 at 8.3:1 with similar cams, induction, exhaust, etc.

 

You might try the How to Modify Your Nissan Datsun L Series book. If you pay attention, there's a lot of info in the pictures. Again, I did all that and my 11:1 L28 required 95 octane.

[madkaw]

There's no magic bullet and no absolutes . There are builders running 11:1 successfully , but it's the small details that get it. No secret combos , just tuning and much research .

[Ironfoot]

Thanks guys I have a lot to think about. The sharp (thin) edges getting hot makes a lot of sense. I haven't decided where to go with the motor right now. I'd like to not destroy the factory engine so eventually I'll be purchasing a 28, but I figured if a head swap was worthwhile then no harm done. I guess we'll see how she runs after a good tune and go from there.

[NewZed]

Don't miss the fact that pre-ignition, caused by hot spots, can cause detonation, but it's not a necessary precursor.  Detonation can happen without it.  Detonation is caused by pressure and heat of the fuel-air mixture.  It's an explosion of all of the remaining fuel-air mix at one time.  Combustion chamber shape comes in to play because a bad design can create pockets of high pressure.