Accralite 1341cc compression ratio calculated...

jez1272gt · July 10, 2009, 07:51:07 PM

Just thought i would post this up as i have "cc'd" my 77mm Accralite 1341's. I took these measurements fairly accurately (enough to make me happy) so im not saying they are perfect but this is what i found..

I measured,

Pistons in situe at TDC and they came out at 35cc.

Big valve head with spark plug in place holds 3.25cc.

Metal HG holds 1.9cc.

Total of 1341cc - spot on!

I calculated this to be a total of approx. 40.15cc per cylinder.

After the maths his means that my engine is currently approx. 9.4:1 CR. i.e too high!

Just thought that these figures might be of interest to some people!

Jez

G-spot · July 10, 2009, 08:51:20 PM

wouldn't say too high, depends on your proposed boost/flow rate for them and the fueling etc to match.
the purpose of the forged pistons it to have the strength to carry the higher comp ratio's/boost combinations.

jez1272gt · July 10, 2009, 10:21:01 PM

yeah its not unheard of to run turbos with a CR thay high at all and yeah it could be worked with, but none the less not ideal and not the CR i would like to run. Too high for me should i say!

Nick_S · July 11, 2009, 11:58:43 PM

Doesn't surprise me really, my engine builder worked out mine being 9.5:1 on my 1473cc.

Most modern cars with turbos run higher CR's nowadays, BUT you need very good engine management to cope and keep on top of things.

If you are down south, then just run it on that RON 101 or is it RON 103? if its a second car.

Tommo · July 13, 2009, 05:50:06 PM

9.8:1 is quite high, you can always skim the tops of them slightly.

Im currently running 1.25bar on 9.5:1 with no detonation, and I accidently ran over 1.5bar for a few seconds also with no det.

Metal HG isnt helping I suppose. Is it really neccesary? a lot of people with highly tuned turbo cars keep on the standard HG, as its way better to have a weak HG if you are running on the edge of det. That way if you get too much det it kills the HG and not your pistons.

jez1272gt · July 13, 2009, 06:07:22 PM

Yeah i did think about machining the pistons but its not going to be my first call. Im going to run a decompression plate first to retain an 8.0:1 CR and see how that goes. Been speaking to some ford boys who swear by them and have never had a problem with them whilst running big boost and power so im gonna give it a go and see what results i get! If its no good then i would start again with Wossner pistons probably!

Nick_S · July 13, 2009, 06:15:44 PM

What will the standard G40 HG bring your CR down too? Normal decomp plates are either 1 or 2mm thick aren't they?

breadman · July 13, 2009, 06:43:52 PM

Agree with the comp. ratio being higher than ideal with Accralites if your using lots of boost.
As Nick stated, modern cars do use high c.r. with turbos. However, they use highly efficient combustion chamber designs and very clever management systems to allow this.

I have no experience of a decompression plates, but they seem like a bodge to me. That said of course they may work perfectly! However, I'd rather not double the risk of head gasket failure.

I reckon if you could get the c.r. down to 9:1 by machining the pistons and using a normal head gasket it would be ideal.
I dunno if you're fitting a turbo Jez? If so, I wouldn't drop to 8.0:1 c.r. - a bit too low IMO. You ideally need to run as high a c.r. as you can without detonation for maximum performance.

BTW Jez, do you know what cc the standard gasket holds? Recalculate?

Yoof · July 15, 2009, 11:30:39 AM

If you're going for peak power, and aren't too worried about your off boost driving, then I'd go for 8:1, what you lose through a lower compression ratio you can make back up in ignition advance.

I run 9.3:1 with 22psi of boost and a 25shot of gas- without det. It's all in the set-up/mapping

Aquamist and a decent front mount obviously help too.

Are you measuring a new headgasket, or a squished one?

As Jason says with forged piston your weak point is the valves, moreso the guides.

Engine management being 'good'- yes you can have knock control on Motec etc, but unless you've done a good deal of dyno work you'll not be able to detect knock very effectively, or suffer false detection frequently- having a stethoscope in your ears through to the engine bay is old school, but works.

Digifant knock detection is almost none existent...

jez1272gt · July 16, 2009, 06:59:49 PM

To be honest im not to fussed about off boost delivery or driving at all, im not really building it for that

I want to go with the original 8.0:1 if i can, as you say say Yoofy, then i can get back what i loose in CR with advance rather than chasing retard when its too high. The ideal way really.

The 0.2CC for the metal gasket is uncompressed @ 0.4mm (brand new from VW). I predicted the difference when compressed to be almost negligable. I.e < 0.1cc - in the grand scheme of things not to much for someone like me to worry about!

If the standard gasket is 1.75mm (- uncompressed i assume?) as Victor Reins states and the same bore as the metal version then it will be approx 0.875cc per bore giving an overall CR of 9.56:1. A difference of 0.21:1

I cannot agree more with the fact that a decompression plate is far from ideal, a weak link, a bodge or what ever you want to call it. However it is an option and if its as reliable as im told by those with the experience of running them then its worth a try. If it turns out to be crap and a waste of money then everyone can say those special words - "I told yo....."

Tommo · July 17, 2009, 03:31:04 PM

Ive never really seen anyone use a decompression plate before, Surely it would need a HG on each side making it stupidly thick? Plenty of people just fit two head gaskets, probably dosent last as long as a proper set up but is cheap and they arent exactly hard to replace.

jez1272gt · July 17, 2009, 05:28:40 PM

Nope you only use 1 head gasket as normal. The plate is sealed to the block using a thin layer of sealing paste and then the head gasket sits as normal on the top.

Ordering the plate moday so i'll have it by friday with any luck and illl get the engine built next weekend hopefully.

Only time will tell!

breadman · July 17, 2009, 06:19:09 PM

Quote from: jez1272gt on July 17, 2009, 05:28:40 PM
Nope you only use 1 head gasket as normal. The plate is sealed to the block using a thin layer of sealing paste and then the head gasket sits as normal on the top.

Mmm... that's got me scratching my head Jez. $:-\$ I too thought it meant using 2 gaskets? So exactly how is the plate sealed to the block without one? Paste you say? Surely a paste can't tolerate compression pressures, expansion/contraction/temp changes and water and oil?

jez1272gt · July 17, 2009, 07:25:29 PM

Quote from: breadman on July 17, 2009, 06:19:09 PM
Quote from: jez1272gt on July 17, 2009, 05:28:40 PM
Nope you only use 1 head gasket as normal. The plate is sealed to the block using a thin layer of sealing paste and then the head gasket sits as normal on the top.

Mmm... that's got me scratching my head Jez. $:-\$ I too thought it meant using 2 gaskets? So exactly how is the plate sealed to the block without one? Paste you say? Surely a paste can't tolerate compression pressures, expansion/contraction/temp changes and water and oil?

This was my first impression too mate. But im only going by what im being told by Mike from Ferriday engineering so as i said time will tell. Its all on the Ferriday site if anyones interested to read it....

breadman · July 17, 2009, 10:53:01 PM

Yeah, Ferriday Engineering are well known. However, 'till now I've never looked at their website before.
You're right about the decomp plates, one headgasket between the block and plate then the head sits straight on top of that!

Clever stuff!
Have thay told you how much they charge for a decomp. plate yet Jez?