Slant Six Forum :: CR Question

If you really have 8.8 (I doubt it), then 0.070" mill should get you to high 9s or 10:1. If you have what a stock US 225 looks like (8:1 or so), then you'll want to mill about 0.130".

HOWEVER, I would strongly recommend measuring piston-deck height and head chamber volume as others have suggested. Most stock 225s are below 8:1 comp.

Why such a small cam? Not much above stock, really. If you want to use that cam, then make sure you do a good porting job on the head and bigger valves, and set compression around 9.5:1. Headwork is where you'll see the biggest gains. If you really can run on 95 octane, then 10:1 should be no problem. Really, you might run higher comp, but it depends a lot on how much time you want to spend tuning ign timing and carburetion.

If you will run a stock converter automatic, then this is a good cam choice.

Lou

Author:	68barracuda [ Tue Feb 19, 2008 1:15 am ]
Post subject:	CR Question
Hi Guys OK 68 225 slant six 8.8 CR never been bored skimmed etc. What is the volume off the combustion chamber? Reason being - want to get a second head to have modified - ported etc - want to know what the chamber volume must be for 9.8 CR and 10.0 CR Any help will be appreciated BTW on a 264 seat to seat 215 @0.050 .440 lift intake centerline 106, ex 114 Cam what is the recommended CR? - we do get 95 over here

Author:	Sam Powell [ Tue Feb 19, 2008 4:36 am ]
Post subject:
Send a PM to Dart270. He has build several high compression 225's. Sam

Author:	sandy in BC [ Tue Feb 19, 2008 6:30 am ]
Post subject:
Without actually measuring your piston to deck height and chamber volume you might just as well piss in a gopher hole and guess,,,,,or even just guess,

Author:	DusterIdiot [ Tue Feb 19, 2008 7:01 am ]
Post subject:	The mighty kreskin
Because of variations in the manufacture fo your mill, you'll have to pull the head off and bring a piston up to TDC to measure your deck height then fill a combustion chamber or two to even get close to what you're asking (and you have to settle on your head gasket... shim, composite, O-ring? for the proper calc...) -D.idiot

Author:	Joshie225 [ Tue Feb 19, 2008 9:25 am ]
Post subject:
Yep, just like DI and Sandy said you have to measure. For example my 1973 Vintage 225 had the pistons .177" down in the hole. Using a calculator at www.kb-silvolite.com I found that if I take .100" off the top of the block and use a .040" thick 3.50" bore head gasket that I need 50cc combustion chambers to hit my target compression ratio.

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CR Question https://slantsix.org/forum/viewtopic.php?t=27276	Page 1 of 2

Author:	polara pat [ Tue Feb 19, 2008 11:08 am ]
Post subject:
Quote: Without actually measuring your piston to deck height and chamber volume you might just as well piss in a gopher hole and guess,,,,,or even just guess, Sandy has a good method for checking deck height and CC volume, then go to this site and plug in all the data. http://www.csgnetwork.com/compcalc.html Maybe Sandy will help you with his link since I'm running out the door.

Author:	sandy in BC [ Tue Feb 19, 2008 12:17 pm ]
Post subject:
http://www.slantsix.org/forum/viewtopic ... chamber+cc

Author:	68barracuda [ Tue Feb 19, 2008 9:29 pm ]
Post subject:
WEEELLLLL no gopher holes over here......lots of meerkat holes over here in South Africa and my bladder is way to small I was hoping not to have to pull the head but OK I know it is the best and only way to do things. Thanks guys

Author:	Dart270 [ Wed Feb 20, 2008 6:17 am ]
Post subject:
If you really have 8.8 (I doubt it), then 0.070" mill should get you to high 9s or 10:1. If you have what a stock US 225 looks like (8:1 or so), then you'll want to mill about 0.130". HOWEVER, I would strongly recommend measuring piston-deck height and head chamber volume as others have suggested. Most stock 225s are below 8:1 comp. Why such a small cam? Not much above stock, really. If you want to use that cam, then make sure you do a good porting job on the head and bigger valves, and set compression around 9.5:1. Headwork is where you'll see the biggest gains. If you really can run on 95 octane, then 10:1 should be no problem. Really, you might run higher comp, but it depends a lot on how much time you want to spend tuning ign timing and carburetion. If you will run a stock converter automatic, then this is a good cam choice. Lou

Author:	68barracuda [ Wed Feb 20, 2008 8:33 am ]
Post subject:
Quote: If you really have 8.8 (I doubt it), then 0.070" mill should get you to high 9s or 10:1. If you have what a stock US 225 looks like (8:1 or so), then you'll want to mill about 0.130". HOWEVER, I would strongly recommend measuring piston-deck height and head chamber volume as others have suggested. Most stock 225s are below 8:1 comp. Why such a small cam? Not much above stock, really. If you want to use that cam, then make sure you do a good porting job on the head and bigger valves, and set compression around 9.5:1. Headwork is where you'll see the biggest gains. If you really can run on 95 octane, then 10:1 should be no problem. Really, you might run higher comp, but it depends a lot on how much time you want to spend tuning ign timing and carburetion. If you will run a stock converter automatic, then this is a good cam choice. Lou Hi Lou Thanks for the reply Yes My Converter is stock. With said stock converter - what is the best option after this I notice Erson has a bigger cam 270 seat to seat, 220 at 0.050, .465 lift wich they recommend for std converter app - Good choice - and at that what CR please ? I found a erson dealer over here Yay!! Talking of erson - what is their url please? Thanx again Fanie

Author:	68barracuda [ Wed Feb 20, 2008 8:58 am ]
Post subject:	Double posts sorry
For interest sake What is the compression pressure as recorded when doing a compression test for the different known cr's ? Fanie

Author:	Dart270 [ Wed Feb 20, 2008 9:00 am ]
Post subject:
Fanie, If you have already bought the 264/215 cam, then I'd just go with that, but that Erson 270/220 looks nice too and should work with a stock converter and up to about 10:1 on your pump gas. On either, you'll have to spend some time on ign advance curve and carb tuning, as I said. I would not use a bigger cam than that Erson with a stock converter. Lou

Author:	68barracuda [ Wed Feb 20, 2008 9:31 am ]
Post subject:
Hi Lou, Have not bought yet - bussy getting in quotes - the guy who will be doing the head work loves erson cams - and the erson price is better than comp Already did the recurve, but will have to again once head with higher cr is in place! Am looking at the 9.5 - 9.8 area worried about future fuel quality

Author:	Joshie225 [ Fri Feb 22, 2008 10:07 am ]
Post subject:
Just as a data point I calculated the compression ratio of my stock 1973 vintage 225 and got 7.7:1. The head was milled .080" to get from 62cc to 50cc chambers and the block will be decked .100" to get the pistons up to -.077". With a .040" thick head gasket that gets me just over 10:1.

Author:	BigBlockBanjo [ Sat Feb 23, 2008 10:27 pm ]
Post subject:
Hey Josh...would you mind sharing your engine specs ''when original'?? (Deck height, which head gasket) Just curious....because my stock engine had about the same compression as yours.....Just wanted to compare.

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