Cracked piston?

[slydog 7,873]

Gas likes high comp yes.

[Lord_fahrquhar 2,580]

Yep. me too. And ring expanders are SO expensive...................not.!

I got a tool for that but usually you hear the word ouch thereafter

[Ando81 2,949]

So the machine shop guru suggested I go for 5cc pistons to knock some compression out of it and guessed that it would be 10.6 to 10.8 compression ratio. He seems like a very genuine bloke, not trying to upsell me shit that I don't need or convince me to spend huge amount of money on go fast parts. He said to just do all the internal parts properly to make it reliable and then play with other stuff like exhaust and extractors at a later date. Dropped it off tonight and already getting keen to pick it up like an expectant father

[ando76 4,354]

Where you going to get 5cc pistons. ACL used to do 8.5cc but can't remember ever seeing 5cc. 15cc are available. Got a set in Neil's engine I'm doing atm.

[CHESTNUTXE 7,318]

theres a 5.7 in the precision catlledog

[ando76 4,354]

Cool. Good to know.

[FORD_MAN 1,026]

Yeah there Endurotec 5.7cc but have a comp height of 1.478" instead of 1.547"

[Ando81 2,949]

So that difference in height will lower my compression then won't it?

[ando76 4,354]

Yes. They are .069" under the standard height so unless you deck the block to suit them you will have a reduction in compression, compared to the same piston installed without deck height reduction.

 

What is your current deck height, and bore size?

[Ando81 2,949]

I haven't measured bore size but as FORD-MAN on here told me, my Pistons are + .040 so they are 3.72inch and as far as I know it's all standard deck height.

[LJDB 103]

I'm currently putting one together 22cc prisons standard deck ect with 46cc head = 9.5

[PRO250 1,506]

Im doing one of them as well just enough comp for a 230@50 camtech, I was lucky with this engine came in a corty I bought and ended up having a 10.000km old engine in it decked the block to 0 as the slugs where about 40 thow down the bore

good engine for gramps he is a dog

[LJDB 103]

I have another one with 8cc Pistons an that's over 11. I played with the cc in the head on the comp calculator on summit website. 1cc can make a fair difference each way

[slydog 7,873]

Lets not just get locked on big compression,it just has to suit your combo.The compression in mine went from 12.3 down to 12.1 but the porter picked up flow and dynamic compression and as a result the car is faster.

 

Compression isn't the be all end all,the correct compression for the job is.

[ando76 4,354]

So so true. Said it before. It's all about the package. But I'll say this again. I'd rather use a dished piston and zero deck height than a flat top down the deck. .050 deck height is just puss for performance.

[matt_lamb_160 252]

11.5:1 with that cam and gas will be fine, but slightly less might be better. With the same size cam and over 12:1 compression I could not get full timing (only 26deg total, mind you I didn't want much more. Ideal would still be under 30deg most likely). I was planning on 11.5:1 at the time, but thought I'd see what happened (had another head ready to go on) and then stuck with it

 

If you are building a highway car with 2.77 or 2.92 gears it will be great (assuming no over-drive here).

[Ando81 2,949]

I've left it in the hands of the machine shop to see what they think is the best, I'm hoping to go straight gas and 3:23 gears behind a t5 so hopefully it will be an average improvement on the old stocky

[matt_lamb_160 252]

It will be good

[gerg 10,871]

Let's see it running!

[Ando81 2,949]

I'm keen to see it running too, still waiting to get it back from the machine shop that was busy and now closed over Christmas so I will be getting straight into it when I get it back.

[broken-wheel 659]

i think when it comes to it, compression is the last thing you do, first get the head to flow what you want/need, then that dictates what cam you need and that dictates the compression the cam needs, you then follow with the converter stall and rear gears, get this all lined up and you'll be ok

 

with a 300 cam 12.5 comp didn't ping on premium and 25 degree (27 was ok but kept it at 25 to be safe) 

[ando76 4,354]

Maybe in a full race application but in a street, everyday or even street strip application compression has to be considered in the early stages.

I find it important up here where quality fuel with decent octane is a big problem. I need to factor it into each build at the planning stage because of this. Also a lot of vehicles are used away from 'good' petrol sources so you need to consider that. Yes you could address it with a different tune but not all people carry laptops or have the ability/ confidence to get in and make the changes.

Even in race applications I factor compression in early as it dictates piston choice, head choice, chamber shape, deck height etc. all of which need to be known quantities when the engine hits the machine shop.

If the OP has confidence in his machine shop, I'm sure they will get it close to right.

I sort of get what your saying tho. It's about the package and everything working together to achieve the result