Compression ratios

[sceneybr]

Hi just wondering if someone can tell me the stock compression ratios for a gj 1984 sigma 2.6 and the earliest magna running the flat tops in the 2.6 just wondering cause putting stock flat tops in my sigma? need some answers soon please.

[webby]

Around 8.3:1, and I'm pretty sure flat tops started in the GN siggy

[84GSR]

Its 8.2:1 for the Astron 1 and 8.8:1 for the Astron 2. TR Magna Astron engine was 9.2:1. The Astron2 detbued in the GK around May 1985. Had a couple of GK wagons with astron 2s. Don't try putting the flat tops from the later engines into the early ones as the pistons will hit the cylinder head.

[sceneybr]

JUST WONDERING ABOUT THAT LAST POST I HAVE AN ASTRON 1 ENGINE 2.6 I HEARD THAT STOCK MAGNA FLAT TOPS WOULD FIT AND GIVE BETTER COMPRESSION CAN ANY ONE GIVE ME DEFINITE ANSWER OR HAS SOMEONE DONE IT.

[GB_BB4C]

JUST WONDERING ABOUT THAT LAST POST I HAVE AN ASTRON 1 ENGINE 2.6 I HEARD THAT STOCK MAGNA FLAT TOPS WOULD FIT AND GIVE BETTER COMPRESSION CAN ANY ONE GIVE ME DEFINITE ANSWER OR HAS SOMEONE DONE IT.

Flat tops will fit in the Aston 1 block but unless you sink the valves into the head or have the pistons skimmed a bit they will hit one another.
Your compression will be up around 11:1

[C_Fernance]

Don't try putting the flat tops from the later engines into the early ones as the pistons will hit the cylinder head.

They don't always hit. I once done a dodgy rebuild in the front yard in a day after destroying an engine (dropped a valve which put a hole through a piston and destroyed the head) and needing the car for work the following day.

Engine started out as an Astron II in a GK Scorp. Block and crank were OK so were reused. Pistons and rods were flat tops out of a TM or TN Magna - with the original Scorp bearings reused and new rings. Head was an Astron I off a GH Scorpion. Put it all back together and got it running that afternoon. Only new parts were a couple of gaskets and the rings.

Apart from the high compression meaning I had to run 98octane and even then had trouble with pinging the engine ran fine and was used like that for another year or 2.

[84GSR]

having said what I did,I have heard of people getting away with it. The engine in my car runs magna pistons and did have an astron on head. Valves were heavily recessed. However,the pitons in mine don't reach the top of the bore so I suspect some skullduggery with the crank. Most magna pistons usually come up level with the deck if not proud of it. Best rule of thumb is don't do it. I have seen one engine where the pistons had to be machined to clear the valves and the head. Then theirs the fuel issue because of compression ratios.
Basically,it looks like experience with this varies considerably. The safe way is not to do it. But you can getaway with it,as has bee mentioned. Sounds like there are more variations with the engines than I new of. Especially when you involve the imported engines. There are a couple of guys on this forum that have a lot of experience with NA high performance mods on these motors. I'tll be interesting to see what they say when they find the thread.

[Superscan811]

There are HIGH and LOW compression flat top pistons for the astron engines.
The high comp have a 1mm higher compression height (36mm) than the low comp ones (35mm).
The TS magna piston (with the kidney shaped recess) has a compression height of 36.5mm

Cheers.

[Rally_2.6LC]

I had Astron II magna block, forged flat top pistons and a Jap spec 2L head in my beast before the bottom end let go.......ran somewhere aroun 12.5-13:1 I was told by someone who did the math. It felt like it too.
I raced it around Wakefield Park (got 12 flying laps on second hand crappy race tyres before it shat itself) but it wasnt running properly, had to shift early as wasnt reving out. This year with my new efi engine (scorp block, scorp pistons, M3 head, TR injection) and propper semi slick tyres i was only getting 3 seconds a lap faster
The current efi setup only runs around 8:1. More compression = more speed.


Cheers Matt