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Compression Ratio - Check my math please 

 

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 Post subject: Compression Ratio - Check my math please
Posted: Tue Sep 11, 2018 8:39 am 
Getting Side Ways
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Ride: EF XR6 Wagon, AU2 XR6 VCT

Power: 148 rwkw

Location: Auckland
New Zealand

I am building an EF Tickford head to go on my AU bottom end. The head has been skimmed once before. After de-shrouding the valves and polishing the chambers, I cc'd the lot. Also, the AU block has previously been skimmed for straightness.

The chambers are 60.6cc.

The Permaseal MLS S2147SSX gasket is .65mm compressed thickness, with a hole of 94.2mm. I calculate this to 4.53cc.

The deck clearance, including the piston dish, is 8.0cc.

The displacement per cylinder is 3984/6=664cc.

So, I make volume at TDC of 60.6+4.5+8.0=73.1cc. And volume at BDC is 73.1+664=737.1cc. From that, I calculate a compression ratio of 10.08:1. Do I have this right?

This is higher than I expected, so I'm doubting my math. Maybe the AU deck clearance is less than the EF. And I understand that the EF Tickford chamber is smaller than the AU (60cc vs 65cc?). Should an EF Tickford head on an AU block with MLS gasket give me this much compression?

Thanks for your help.

 

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95 EF XR6 wagon, 17" FTRs, DBA, QFM, KYB/Koni, AU block, ported 4-angled EF XR6 head, SS Inductions, Territory intake, 10:1 CR, Auckland 1258 cam, Coby headers, no cat, Tickford 2.5", 2850rpm stall, J3 chip

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 Post subject: Re: Compression Ratio - Check my math please
Posted: Tue Sep 11, 2018 1:55 pm 
Technical Contributor
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Age: 54

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Ride: 96 XH/97 EL

Location: South Coast
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Yeah, doubt it - XR's are around 9.35:1 from memory.
There are online calculators as well. I remember using one a while ago. It took account of the inlet valve notch in the piston crowns too.

Have you read all of Tickford_6's and Wagondad's porting info on here ?

 

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96 XH Longreach 'S': LPG, Alarm, 3.23:1 LSD, Cruise, Trip Comp, ABS, Power Windows, Mid Series Dome Lt, Climate Ctrl
97 EL Futura S/W: LPG, Alarm, LED int Lts, Trip Comp, F/Lane Dome Lt, Climate Ctrl, T5 Conversion

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 Post subject: Re: Compression Ratio - Check my math please
Posted: Tue Sep 11, 2018 2:59 pm 
Getting Side Ways
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Posts: 852

Joined: 1st Apr 2005

Gallery: 4 images

Ride: EF XR6 Wagon, AU2 XR6 VCT

Power: 148 rwkw

Location: Auckland
New Zealand

Yeah, I read just about everything on Fordmods about porting before I started.

I just did another search on Compression Ratio and pulled up a thread I posted to eight years ago:
post1424098.html

In it is this little gem I wrote then:
"Yeah, that may be right. In which case, I'm running about 10.0:1 compression :shock:"
I found it hard to believe at the time. But I guess it was correct all along!

I guess my younger self kind of answered my question :D

Thanks for the help.

 

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95 EF XR6 wagon, 17" FTRs, DBA, QFM, KYB/Koni, AU block, ported 4-angled EF XR6 head, SS Inductions, Territory intake, 10:1 CR, Auckland 1258 cam, Coby headers, no cat, Tickford 2.5", 2850rpm stall, J3 chip

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