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Main : Misc A bigger step

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A bigger step
A bigger stepPopular
SubmitterDodgemanMore Photos from Dodgeman   Last Update2004/9/17 3:02    Tell a friendTell a friend
Hits3804  Comments11    0.00 (0 votes)0.00 (0 votes)
The used A15 piston is OEM, the new one is an ACL replacement. Thats an A12GX rod on the left, & an A12a rod on the right

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Poster Thread
Dodgeman
Posted: 2004/9/14 22:06  Updated: 2004/9/14 22:06
No life (a.k.a. DattoMaster)
Joined: 2003/6/27
From: Southern Tablelands N.S.W. Australia
Posts: 8287
 Re: A bigger step
Thanks to Ferral for the calculations. That puts the compression right in the zone for me, so it is all going to plan,... so far.

Thanks also to Woody. The larger pistons used a larger wrist pin & boring the A12 rods would have reduced the wall thickness around the pin more than i had wanted.

This is the beauty of Datsuns. If you do a little homework, we so often find that a factory part, usually from another model, will fit perfectly & do the job that we want.

The combination of 1200 block with A14 crank, using A12A rods & the shorter, bigger bowl A15 pistons, gives me a combo that places the piston flush with the top of the block & a compression ratio that is just what i want.
God i love Datsuns.
EDIT Machining of the piston crown IS in fact required. About a half mill I believe.
This will reduce bowl volume & raise compression slightly.

See my other picture 'one small step' for a closer look at the rods.

Poster Thread
D
Posted: 2004/9/15 3:35  Updated: 2004/9/15 3:35
No life (a.k.a. DattoMaster)
Joined: 2002/10/28
From: under the Firmament LOL no twiglight effect BS
Posts: 10924
 Re: A bigger step
What capacity will this give all up? Sorry if I missed it.
Is it possible to use the a14 version of this 77mm one and machine it down to get higher compression or will it be too much to remove?

Poster Thread
Dodgeman
Posted: 2004/9/15 14:50  Updated: 2004/9/15 14:50
No life (a.k.a. DattoMaster)
Joined: 2003/6/27
From: Southern Tablelands N.S.W. Australia
Posts: 8287
 Re: A bigger step
With a bore & stroke of 77 x 77mm my abacus says it comes out at 1434cc. Thats about 23% more than the 1171cc of the original 1200 engine, & it's built with all stock parts.
EDIT. Piston will need machining a small amount.

The A15 stroke is 5mm longer than the A14, so the piston crown is, i presume, 2.5mm lower on the A15 piston. That means that you would need to machine 2.5mm from the A14 crown to make it work.

Just make sure that there is enough meat above the top ring to prevent the crown from being forced down onto it. With a boost in compression you may have trouble finding a suitable fuel for it, & as a consequence, you may need to bring the spark back a fair bit, which sort of defeats the purpose of the exercise.

The larger bowl of the A15 piston gives me a good compression ratio, plus lots of valve clearance, now thats a bonus.

Poster Thread
D
Posted: 2004/9/16 7:20  Updated: 2004/9/16 7:20
No life (a.k.a. DattoMaster)
Joined: 2002/10/28
From: under the Firmament LOL no twiglight effect BS
Posts: 10924
 Re: A bigger step
Sounds great a completely square bore and stroke ratio now I put a link up a few months ago bout the new hemi and another about compression from the same internet publication but now cant find it
But it explained that the 92 octane unleaded can handle up to 10.8 compression with the right ignition setup and the 98 octane fuel up to 11.3 compression with the same deal.
Ill try and find it but I would have thought that 2.5mm still leaves a chamber for the valves to miss right?
Either way this engine setup is a dream Dodge and we envy you specially since it will be in your 1k and not ours
That is unless you are taking orders

Poster Thread
Dodgeman
Posted: 2004/9/16 11:41  Updated: 2004/9/16 11:41
No life (a.k.a. DattoMaster)
Joined: 2003/6/27
From: Southern Tablelands N.S.W. Australia
Posts: 8287
 Re: A bigger step
Flat top, or near flat top pistons may get a little squeezy if you have bigger valves & an exciting cam, so it would be a very good idea to check this with a dummy assembly run.
I dont have an A14 piston here to check, but the top ring may end up a bit close to the top of the piston after is is machined.

I have some stock A14 pistons over at the other house, so if i can find them, i will check it all out with my trusty caliper.
I do not know how deep the bowl is, but i do not recomend machining lower than the surface of the bottom part, all else being OK.

No orders yet, but with the right parts, the whole thing is a standard rebuild, with the sole exception of the machining of the crank, & any fool with a big enough lathe can do it, so you don't need a specialist machine shop.
Mine was machined in a lathe that was made [not just assembled] by my friends father. He even cast all the major parts himself. It is at least 60 years old & is still flat belt driven. Works a treat.

I will see if i can do a photo essay for anyone that wants it when it's done.

Octane.
The compression ratio that a given fuel can handle is to some degree dependant on the design of the combustion chamber & the amount of spark advance.
A high swirl chamber with a very good port design & squish characteristics will handle more advance & compression on a given fuel than a design with poor port design & poor swirl due to minimal squish area.
My Moto Morini 3-1/2 sport motorcycles ran about 10 to 1 comperssion in an air cooled V twin engine. They have a Heron head design & really good porting for extremely good swirl. Thats a lot of compression for the rear cylinder of an air cooled V twin, but it worked a treat.

Poster Thread
D
Posted: 2004/9/18 4:41  Updated: 2004/9/18 4:41
No life (a.k.a. DattoMaster)
Joined: 2002/10/28
From: under the Firmament LOL no twiglight effect BS
Posts: 10924
 Re: A bigger step
Itll be a great setup if we can get this kind of crank clearance the way youre going about it. I too aint happy with the a14 markings on my engine and more so the non factory looking engine mount so I will have to consider this option soon before I get pulled over and checked for it.
I agree with not going past the dish section when machining but also would be interested to see if 2.5mm off the a14 piston keeps part of the dish etc.

Heres the article on Compression and getting what you can out of different pump gas fuels. It a good read up.....

http://www.popularhotrodding.com/tech/0311_phr_power_squeeze/

Poster Thread
A14force
Posted: 2004/9/18 7:06  Updated: 2004/9/18 7:06
No life (a.k.a. DattoMaster)
Joined: 2003/12/3
From: Christchurch NZ
Posts: 3706
 Re: A bigger step
cheers for the link D, very worthwhile reading. I think you might be scratching a bit to try taking 60 thou (give or take) off A14 pistons.

The dish with the offset would potentially offer superior quench characteristics with the small chamber GX head.

Poster Thread
Dodgeman
Posted: 2004/9/18 8:53  Updated: 2004/9/18 8:53
No life (a.k.a. DattoMaster)
Joined: 2003/6/27
From: Southern Tablelands N.S.W. Australia
Posts: 8287
 Re: A bigger step
Thats a really good piece of journalism, & the 15 points at the end make good sense,... in a race engine, but about half of that would be counter productive in a street engine that is expected to be flexible, smooth, & live long.
On the upside is the fact that the other half is really good stuff for any engine.

I have always said that one of the important elements of detonation controll is good squish. Thank you Mr Ricardo, & thank you D

Poster Thread
dattoman_1000
Posted: 2004/9/14 13:08  Updated: 2004/9/14 13:08
No life (a.k.a. DattoMaster)
Joined: 2002/8/11
From: Perth
Posts: 2689
 Re: A bigger step
Thats a fair amount of dish
Whats the expected CR ?

Poster Thread
Dodgeman
Posted: 2004/9/14 13:16  Updated: 2004/9/14 13:16
No life (a.k.a. DattoMaster)
Joined: 2003/6/27
From: Southern Tablelands N.S.W. Australia
Posts: 8287
 Re: A bigger step
I don't know, but the combustion chamber of the 1200GX head will see a stroke increase of 7mm & a bore increase of 4mm, so the bowl needs to be a reasonable size to hold compression down to pump fuel friendly levels.

I'm guessing that compression wil be somewhere between the stock 9 to 1 anywhere up to 10 to 1 which is the range that i will be happy with.

Poster Thread
B120dat
Posted: 2004/9/14 13:36  Updated: 2004/9/14 13:36
No life (a.k.a. DattoMaster)
Joined: 2002/12/2
From: Brisbane
Posts: 2317
 Re: A bigger step
is there any difference in the RODs?... is one stronger/lighter?

Poster Thread
woodydat
Posted: 2004/9/14 13:39  Updated: 2004/9/14 13:39
Home away from home
Joined: 2004/1/29
From:
Posts: 966
 Re: A bigger step
Yeh have a look at the piston end of the rod, see the hole, its bigger so Dman can run the bigger piston with out rod modifactions.. i think he said he wanted to use the 77 mm piston

Poster Thread
feral
Posted: 2004/9/14 15:23  Updated: 2004/9/14 15:23
Home away from home
Joined: 2002/5/1
From: Millgrove Vic OZ
Posts: 985
 Re: A bigger step
with 31cc chambers, 6cc piston dish and 0 deck with 6cc gasket Clearance volume =43cc
Comp ratio = (43 +358.75)/ 43 =9.34:1 I tip.

Poster Thread
Dodgeman
Posted: 2004/9/14 12:27  Updated: 2004/9/14 12:27
No life (a.k.a. DattoMaster)
Joined: 2003/6/27
From: Southern Tablelands N.S.W. Australia
Posts: 8287
 Re: A bigger step
The bowls in the pistons have pretty much the same volume, but the 77mm ACL replacements have the bowl centered. Why? Beats me, but i now have the right crank, rods & pistons for my 1200GX engine.

Once the head comes back, & is paid for, i will move to the cylinder block.