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suprised 198 in 225 wrapping compersion ratio question
https://www.slantsix.org/forum/viewtopic.php?t=28273		 Page 1 of 2
Author:  oklahoma joe [ Mon Apr 07, 2008 6:46 pm ]
Post subject:  suprised 198 in 225 wrapping compersion ratio question
I am in the assembly stages of rebuilding what I thought was a 225 slant six according to the stampings on the block deck. Today I was caluculating the compression ratio and found the stroke to be 3.64 in. I guess this gives me 198 slant not a 225 as advertized. Suprise :shock:

So how do the 198 slant run compared to a 225? Are ther any redeeming traits of a 198 rather than a 225? I have a super six manifold and carter carb set aside for it which I guess should be plenty of carburation for a 198.


O'yea by the way. The compersion ratio I ended up with was 7.33 to 1 which seems pretty low. Would this be due to some missmached parts or the incorrect head chamber volume. I thought the blocks and heads for a 225 and 198 were the same?

This is my first time cc a head and engine so see if it adds up to you guys.

The numbers I measured on the engine and used to calculate CR were as follows:

deck height = 0.135
stroke = 3.64
Bore = 3.60
head chamber = 59 cc
head gasket thickness =0.057
Author:  polara pat [ Mon Apr 07, 2008 7:06 pm ]
Post subject: 
I know the calculator wants your compressed head gasket thickness. Fel-pro being .045 and many other generic brands coming in around .040. Try that and see where you're at.
There's a reason so many people here talk about decking their blocks, heads or both to bump up the CR to high 8s or low 9s for a good street engine.
Author:  terrylittlejohn [ Mon Apr 07, 2008 7:10 pm ]
Post subject: 
i had the same thing happen and never discovered until the crank was in the machine shop, block clearly marked 225 but had a 198 crank. i never ran it so i have nothing to compare it to but doug(DD) has run them ,the rod to bore ratio is better and the shorter stroke means you can rev it higher. the deck height is about right , i never saw a slant that was even close to the 8.4 compression the all the books say, the head gasket measurement seems on the thick side ,is it factory or after market?
Author:  oklahoma joe [ Mon Apr 07, 2008 7:16 pm ]
Post subject: 
It is a after market head gasket. I think I may go and get a thinner one.
Author:  Bren67Cuda904 [ Mon Apr 07, 2008 8:50 pm ]
Post subject:  Re: suprised 198 in 225 wrapping compersion ratio question
Quote:
Bore = 3.60
Check this number. Maybe 3.4?
Author:  LUCKY13 [ Mon Apr 07, 2008 9:34 pm ]
Post subject: 
You bored this engine .200 over, thats hugh? If that is the right bore I come up with 8.84 cr for your setup. The 198 will rev higher RPM and I think it will make use of the head flow better. It will also make your camshaft act a little bigger if you have put one in above stock.


If that bore is correct that make your engine a 222.

Jess
Author:  runvs_826 [ Mon Apr 07, 2008 10:37 pm ]
Post subject: 
Bren how did you guys get a big bore on Eileen? If I remember right it was pretty large. Also, if you have essentially a 198, you could always find a 225 crank and make a "long rod" engine out of it. I know Doug favors those.
Author:  oklahoma joe [ Tue Apr 08, 2008 4:32 am ]
Post subject: 
oops I forgot the four

bore = 3.460
Author:  oklahoma joe [ Tue Apr 08, 2008 4:39 am ]
Post subject: 
If I use the 198 rods with a 225 crank I quess I will need a shorter piston top. So what pistons can I get that will match the bore I have in the engine now 3.460 in or is this a custom piston job. If so I think that will put me way over budget.
Author:  terrylittlejohn [ Tue Apr 08, 2008 7:27 am ]
Post subject: 
i think a 2.2 dodge 0.020 over works out to a 3.46 bore?
Author:  wvenable [ Tue Apr 08, 2008 8:23 am ]
Post subject:  Stupid question!
Why are so many of our engines coming in with much lower compression ratios then the factory advertised? I have not checked mine yet but according to what I have read it is supposed to be 8.4 to 1. :?
Author:  Doc [ Tue Apr 08, 2008 9:00 am ]
Post subject: 
Quote:
i think a 2.2 dodge 0.020 over works out to a 3.46 bore?
The 2.2 piston, .5mm o/s gives you 3.465 and should hit 9 to 1 compression right out of the box, no head or block milling.

As far as power per cubic inch, the 198 SL6 is the best, I know a few of thes these build-ups that net 1.2 to 1.3 HP per CI.
I posted actual dyno results on one of these engines a while back.
http://www.slantsix.org/forum/viewtopic ... t=198+dyno
DD
Author:  Joshie225 [ Tue Apr 08, 2008 11:51 am ]
Post subject:  Re: Stupid question!
Quote:
Why are so many of our engines coming in with much lower compression ratios then the factory advertised? :?
Poor adherence to specifications for piston deck height and combustion chamber size are the reasons why the compression ratio is so much lower than advertised. NHRA blueprint specs are -.140" piston deck height and 54cc combustion chambers for '72-'75. My pistons were .038" lower in the bore than spec and the combustion chambers were 60cc. Milling the head .040" and the block .038" to achieve the NHRA numbers combined with a .040" thick head gasket my 7.7:1 225 would be 8.5:1. A .020" head gasket bumps it to 8.8:1.
Author:  slantzilla [ Tue Apr 08, 2008 12:40 pm ]
Post subject:  Re: Stupid question!
Quote:
Quote:
Why are so many of our engines coming in with much lower compression ratios then the factory advertised? :?
Poor adherence to specifications for piston deck height and combustion chamber size are the reasons why the compression ratio is so much lower than advertised. NHRA blueprint specs are -.140" piston deck height and 54cc combustion chambers for '72-'75. My pistons were .038" lower in the bore than spec and the combustion chambers were 60cc. Milling the head .040" and the block .038" to achieve the NHRA numbers combined with a .040" thick head gasket my 7.7:1 225 would be 8.5:1. A .020" head gasket bumps it to 8.8:1.
NHRA numbers have nothing to do with factory specs. Those are the minimum allowable numbers for teardown. They do allow some for "modifications". :wink:
Author:  Joshie225 [ Tue Apr 08, 2008 12:46 pm ]
Post subject: 
Yeah, and I even posted that you'd get something other than the factory spec of 8.4:1 if you were to use the NHRA specs.
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