Compression, calculation and relieving chambers

[Kyle]

Hi all-

Building up a L28 for my '72. It's an N42 block and head, +1mm overbore with flattops. The head has gotten some mild porting, chamber polishing, and portmatching.

I cc'd the head yesterday, and got chambers from 44 to 45.3cc. Obviously, they need to be 'adjusted':stupid:

But that then led to calculating the compression... with 45 cc chambers, 89mm bore, 79mm stroke, +.5mm deck height, and 7.6cc headgasket, I'm getting 10.93:1 CR. Ouch.

My initial plans expected the engine to wind up with 10.5 using as-cast parts. That didn't count on the piston sticking up above the block deck... Now that it appears I need to relieve the chambers, I want to get down to about 10.0:1 That'd require 50cc chambers...

Where can I take the material from, safely? Thus far, I've basically removed the ridge between the valves, and smoothed the transition from the ports to the chamber roof. The large protrusion near the sparkplug hole is untouched. Does anyone know what 5cc really looks like in the chamber? I'll (try to) attach a smaller image of a representative chamber, and the full size (out of focus, sorry) is at http://www.sonic.net/~kyle/Cylinder Head Chamber.jpg

Thanks much for any guidance... this project has taken on a life of its own!

[240znz]

I've never done this but there's a bit on this mod in the "How to modify your Nissan/Datsun OHC engine" by Frank Honsowetz.

It is something that will loose CC's and give you a better flowing head at the same time. I take it that you have already installed the bigger valves and steel seats. If your running 10.5:1 you'll need all the protection you can get from detonation.

I suggest that you go to Amazon and pick it up. It is a good book with some good info.

If you do buy it make sure you goto the Amazon site via a link from this site. Amazon will return a small commission to the club...every little bit helps.

:classic:

[Kyle]

Ahhhh yes... unshroud the valves. Duh, I was just looking at how to modify yesterday, for the Ccing procedure. Tunnel vision, I guess.

My only concern is that when taking a lot of material out of the head casting, I don't take it from someplace ciritical. The area around the valves is quite obvious, and thanks for the :stupid: mallet to the head to remind me.

What about the area near the spark plug hole? I've got a feeling it's necessary to have that semi filled-in area to get the airfuel mix to gather up near the plug, so maybe I don't want to mess with it too much? Maybe just massage the edges a bit?

Thanks again for any tip!

[240znz]

Although the head you have is nearly 30 years old and designed on paper, it will flow pretty feel on its own. Just unshroud those valves, port match, new cam, new rockers, set of flows, EI. Don't go too mad with changing the chamber swirl/qench characteristics or you WILL get a worse result. There are plenty of other ways to empty your wallet...e.g brake upgrade and suspension mods.

I know it's a bit late now, but transplanting a 280Turbo enigine will give you more HP and better fuel economy without spending massive amounts. If you want a stock engine bay then you are on the right track.

If you want all out grunt..2 REAL options for you.

1. V8

2. RB26DETT

:lick:

[Kyle]

Double checked my specs last night - the bore is 87mm, not 89. What a monster difference that makes. The CR with 45cc chambers is now 10.46.

To match the chambers, I did have to do some pretty serious grinding (1cc) off the #1 chamber; I took the material near the sparkplug hole and exhaust valve...

If I was able to get 1cc more out of each chamber, I could get the CR down to 10.25. But the caution you've so wisely given me, plus my urge to get this thing done, suggest I'll just get them all even, smooth the edges, and be ready to assemble.

I hear ya on the L28ET. I wanted to stay with SUs, but even so, and even with a great deal on most of the parts, it's still wound up costing way more than I expected.

On the brighter side, though, rebuilding an L28ET would surely be at least as costly; and if I just got a JY engine and threw that in there, I'd be where I started

Thanks much for your help on this!

[2ManyZs]

Before you take any more material out of the head, have you checked out the compression ratio and such with a different head gasket? There are the 1mm and 2mm headgaskets as well as the stock thickness available. This might make up the difference you are looking for. Just thought I'd ask in case you haven't thought about it yet.

[Kyle]

Good thinking!

I failed to condsider that... but it will bring the CR down to 9.6.

Amazing what 5cc does.

So I guess this makes a new question, what CR is reasonable? That 9.6 would certainly give me breathing room. Guess I'll call MSA and see what it'd run me.

Thanks for the idea!

[2ManyZs]

Anything under 10.5:1 would be reasonable for a street engine, although if you go over 10:1 you may need to use 93 octane to keep it from detonating. I'd say 9.6 would be just about as good as you could ask for. You could possibly even use 91 octane gas and not have to worry about detonation at all.

[hmsports]

Hey, why worry about getting too high compression when 104 octane at $4 a gallon isn't much more expensive the street fuel?! :classic:

[Kyle]

104 octane, sure you bet... If I had a contact at the local airstrip, I might think about it. But this car is to be a daily driver (leastways, it was before it got all taken apart).

The 2mm gasket has been ordered ($150, ouch!) and that will get the CR where it wants to be. The HKS gasket actually has a 91mm bore, so that raises its volume to 13cc, which in turn reduces the CR to 9.5, exactly where I wanted to be. I sure wish I'd had the sense to calculate the CR this precisely before assembly, cutting the pistons a bit would have allowed the use of a cheap standard head gasket. Ahh well.

Someone on the 240Z list raised the issue of quench and swirl. The short version is that the 2mm headgasket might put the piston top too far away from the chamber, screwing up the air fuel mix and causing detonation.

Any thoughts on that? I figure the pistons are closer than stock (.5mm above deck, versus .35mm below deck plus a 2mm dish for a stock N42 engine) so that puts that issue to bed.

What a learning process! Even with lots of surfing and lurking, asking of questions, how-to-modify and how-to-race, I'm still basically making this up as I go along. Nice metaphor for life, there.

[JEFF]

9.6:1 is as high as you can get and still run 87 octane fuel.

( Ford, GM, Dodge - Factory specs )