which head to use?

[Stephen Chambers]

Happy spring, birds! 

I'm planning to build an L20B and have 2 heads I could use, a U67 and an A87 peanut-head. Both need to be rebuilt, and they seem to be in about the same (good) condition. Neither has been cut / shaved. 

Whichever head I use, I'm going to run 38mm SUs (port-matching the head to suit the intake manifold), a stock L20B cam, matchbox dizzy, and a cast-iron exhaust manifold (the "good one," full separation of the 2 tubes). I don't know what the price differential would be, but I'm hoping to have the internals balanced. As you can tell, though, I'm not interested in going wild on this build. My goal is to keep it drivable and hopefully reliable for a long time. I want to use regular pump gas, not premium. 

So, the head. From research I've done (mostly on The510Realm) I gather that the U67's open combustion chambers would result in a CR of about 8.5 with stock L20B pistons. If I shave it 0.20 that would rise to about 8.7. I understand that the main drawback to this setup, compared to the A87, is the greater risk of detonation due to the open chambers.  

If I use the A87 the CR would be about 8.9 with those same L20B pistons. But I've read that this would be a poor idea, since the advantage to the closed CC is its quench area, which disappears if I use those stock (dished) pistons. I'd have to overbore the block and use L28 flat-tops in order to take advantage of that quenching, which both produces a little extra power and reduces detonation. But then the CR would rise-- out of the range of normal pump gas? 

What I'm asking is two things. 

(1) Have I got this info straight? I've pieced it together from quite a few sources! 

(2) which way do you think I should go, U67 or A87? 

Thanks! 

Steve Chambers 

Edmonton, Alberta  

[Nathan Marshall]

Steve,

 

I am sure that you will hear several opinions on this topic.  I would recommend that you use the A87 head with the pistons you have assuming that you don’t need to bore the block and get new pistons anyway.

 

After the head has been milled (if and only if it needs to be) use a Dremel or die grinder and a little sand paper to ease all sharp edges in the combustion chamber.  This will prevent hot spots and eliminate any un-wanted detonation.

 

The combination you are talking about will provide you with a nice reliable motor that handles regular unleaded without issues.  Just include a 2” exhaust system in your plans.  A stock tail pipe will feel like a cork with those SU’s on the intake side.

 

Nathan

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[Kelvin Dietz]

On 4/2/2014 7:08 AM, Stephen Chambers wrote:

If I use the A87 the CR would be about 8.9 with those same L20B pistons. But I've read that this would be a poor idea, since the advantage to the closed CC is its quench area, which disappears if I use those stock (dished) pistons. I'd have to overbore the block and use L28 flat-tops in order to take advantage of that quenching, which both produces a little extra power and reduces detonation. But then the CR would rise-- out of the range of normal pump gas?

The stock L18 pistons have a slight dish.  8cc vs the L20B 10cc, if I remember right. 
They worked just fine with the peanut-chamber head.  Someone was throwing out some bogus info for you.

I ran an L20B with L18 pistons and a A87 peanut head. Built a couple more.  They always ran fine on Regular gas.  BETTER than with a stock L20B head.  If I started in 2nd gear with the dead stock L20B I'd get a little ping.  Never had that happen with the higher compression L20B/L10 pistons/A87 head combo.

What I'm asking is two things. 

(1) Have I got this info straight? I've pieced it together from quite a few sources!

No.

(2) which way do you think I should go, U67 or A87?

A87. 

If more people would be happy with and L20B, SUs and a 5spd there would be a lot less Project Cars rotting away in garages around the world.  :^)

Kelvin

[ppete...@comcast.net]

Word!

-------Original Message-------
From: Kelvin Dietz [kel...@datsuns.com]
Date: Wed, Apr 2, 2014
To: Bluebirds [blueb...@googlegroups.com]
Reply To: [kel...@datsuns.com]
Subject: Re: [Bluebird510] which head to use?

[snip]

If more people would be happy with and L20B, SUs and a 5spd there would be a lot less Project Cars rotting away in garages around the world.  :^)

Kelvin

Sent via BlackBerry by AT&T

[Chip Collingwood]

This is the response I got to the same basic question from the Realm guys

U-67 head... Open chamber, best combination of port and valve sizes and will fit the L16/18 exhaust manifold.

Stock bore L20B with stock pistons, U-67 head......................... 8.40 compression
Stock bore L20B with stock pistons, closed chamber (A-87?)....... 8.90
Stock bore L20B with L18 pistons, U-67 head........................... 9.27
Stock bore L20B with L18 pistons, closed chamber (A-87?)......... 9.91
Stock bore L20B with forged flattops, U-67 head...................... 9.90 
Stock bore L20B with forged flattops, closed chamber (A-87?)... 10.68 
86mm bore L20B with L28 flattops, U-67 head......................... 10.30
86mm bore L20B with L28 flattops, closed chamber (A-87?)....... 11.06

Stock bore L20B with stock pistons, closed chamber (A-87?)....... 8.90
Stock bore L20B with L18 pistons, U-67 head........................... 9.27 I wouldn't go any higher than this.

If rebuilding the A87 then stock L20B will be fine. There are some advantages to the closed chamber head but lost when dished pistons are used as the quench area disappears and it is simply is a way to increase compression. If this is the case, the U67 has larger ports than the A87. If going to a larger lift duration cam the A87 will hold it back.

WHAT ARE THE THOUGHTS HERE?

Chip

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Chip Collingwood

theukmotorsports.com

(804) 564-1839

[Stephen Chambers]

Chip's post is helpful. I had read this same info on The510Realm and in fact this was where I drew some of my information from! 

It raises the issue though about compression ratios, which as you know I'm a little concerned about. Kelvin has had good experience with an A87 head and L18 pistons, running standard pump gas without a problem. According to the info Chip shared, that combo would have a CR of 9.91. That seems high to me-- partly because whoever wrote the info that Chip has shared says that he wouldn't go higher than 9.27.

I hadn't mentioned it but yes I also plan to run a 2" exhaust (thanks, Nathan). There will be a roadster clutch, lightened flywheel, and 280ZX 5-speed behind the engine too. Glad to hear that most of you think this is a smart setup... pending this decision on which head to use.

Cheers,

Steve 

[Kelvin Dietz]

The dead-stock L18 that your peanut head A87 head came off of had dished pistons, not flat.  So Nissan had no intention of taking advantage of that smaller combustion chamber?  I'm guessing they probably did...

You'll be adding a little more compression with an L20B bore. I think I calculated mine at 9.3:1 compressions - but it's been MANY years.  ('94?)  I know I never ran anything but regular in my car. 

Do as Nathan suggested and make sure there are no sharp edges in the chamber, just to be hedging your bets. 

[David Levinson]

All these posts are helpful! Kelvin nailed the quintesential answear! One question I have about the info on the 510Realm is: What year was it written? Pump fuel is always changing and is never as consistant as you'd think for Octane rating. I personally run closed chamber A87 or 219 casting heads on my builds, and have found increased drivability and performance on all the engines running pump fuel, even when I used an L20B with Datsun Comp Flat tops, 219 head and Racer Brown SS52 cam with twin SU's . I wouldn't run the same combo today without using at least 100 in the tank. You could still get 94 Ultra then and the combo was borderline for detonation. Running the A87 with the Dished L20b pistons you'll feel a performance increase on 93 with no problems.. ;)

[ppete...@comcast.net]

Last I knew, 92 or higher = Premium

Oh, for the days of Sunoco 260!

Pete

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Date: Wed, 02 Apr 2014 12:25:29 -0400

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[noneofyourbusiness]

Unless you live in California.  The best we get is 91 octane.  In Northern California anyway.....

[da...@datsuns.com]

What's your point, Deets? :-)

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[Matt Dock]

I run 91 here in Cali with my frankenmotor, it's basically just a really big L. U67 head with KA pistons and really lumpy cam (as in needs to idle at about 1100rpm to keep it from cutting out lumpy). It has 38 SUs with fat needles and nozzles. When Troy Ermish built it it he said it would have pretty high compression (my dad can correct me on this if Im wrong) between 9-10:1. 

Runs well with the 91 octane, no real pre-detonation. 

[mattd...@gmail.com]

To clarify, and relate to the actual topic. To my knowledge, when you are running a larger motor, like a bored out L20 or a Frankenmotor, the U67 is preferred due to increased displacement. The A87 is preferred with L18s and such. For a regular L20 it's debatable. 

(Most of this info is coming from my talks with Troy Ermish while we were planning out my 2.1)

[Dan Heil]

Static compression vs dynamic compression. The longer the cam opens the valves, the lower the dynamic CR.

For those of you who have my engine calculator, you can plug in the intake valve closing angle and it will figure out the dynamic CR.

For the guys doing this on paper:

If the intake is fully closed at say 212 degrees, that means its closing 32 degrees after bottom dead center (180) or 32 degrees into the lower quarter of the cycle( 90 degrees). The reduced effective stroke length, when the intake is closed and the cylinder is actually compressing the Air/Fuel, gives you a lower effective or the real dynamic CR.

So to figure out the new reduced stroke length, you add the reduced lower half stroke with a full half upper stroke.
1/2 stroke x Cosin(32) +1/2 stroke = effective stroke length.

Dan Heil
1987 Yamaha 250SRX
And  510 wagons!

[Matt Dock]

We talked to Bruce and Steve at ZTherapy and Steve suggested just running the 38 and making them a little beefier with nozzles and needles. It seems to me that they are very good in the bottom end and are pretty responsive. It gets choked up at the top of the RPM range, but that could just require something massive like some 44s. That's far down the line though, I'm content with working the SUs to get them running as well as they can. 

On Wed, Apr 2, 2014 at 1:15 PM, Kelvin Dietz <kel...@datsuns.com> wrote:

I'd be surprised if you don't have one of the L-series motors that would actually benefit from 46mm Z SUs. 
Big motor, big cam.  I would bet the 38s are actually the choke point in your motor. 

And normally I'd never suggest bigger than 38s on an L-series.

Kelvin