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Calculating slant six compression
https://www.slantsix.org/forum/viewtopic.php?t=55703		 Page 1 of 1
Author:  1930 [ Sun Jun 29, 2014 12:18 pm ]
Post subject:  Calculating slant six compression
I have gathered enough info now I believe to calculate how much I need to remove from the cylinder head in order to achieve my goal of 9.1 compression ratio.

Duster made this comment ...........The .0066 per cc rule is spot on as long as the head being surfaced is above the 44-46cc chamber measurement...once it dips in that range the cut starts into the shelf and the measurement is no longer close...
With the head casting variances, it's typically safe on builds under 10:1 SCR to be able to "round" your cut up to the nearest whole as it will not change things much (i.e if you are wanting a 10cc reduction on an untouched head you'd take .066...most machine shops will charge you by the depth, so .070 will not cost any more than the .066 and still get you what you want...and cover a lack of target compression if the head gasket doesn't truly compress to .039...).
....................

I am hoping someone can look at this document Image that Ive put together, check my work and make comments.

Looking at Dusters comments above I guess I am going to be safe using the .006 thou rule.

I am guessing that at this point I would really not need to be too concerned with pushrod length but I will still check this.

Using Dusters comments I am going to ask that .090 be removed from the head. ( round up )

Thanks
Author:  1930 [ Tue Jul 01, 2014 2:27 am ]
Post subject: 
73 views and no comments? Id like to know I am on the right track before going any further.
Author:  Dart270 [ Tue Jul 01, 2014 3:10 am ]
Post subject: 
Those numbers all look fine to me. I did not recalculate everything, but it all looks very close to what I expect. In short, 0.175" deck ht with a 0.090" head cut and 0.039" gasket will get you close to 9:1 static compression.

Happy building,
Lou
Author:  1930 [ Tue Jul 01, 2014 3:21 am ]
Post subject: 
Thanks Lou, next questions are

1-How much the engine builder oversized valves are going to affect the CC rating/compression rating. Not even sure which direction things will go I.E less or more compression because of the change?

2-What is next literally, I plan to re-work the head which will include shaving the surface the 90 thou, porting and installing the oversized valves.

3-I plan to do the porting myself but not sure if I should have the valves installed first and see how much that changes the compression ratio ( by CCing the heads again ) before I go ahead and have the 90 taken off.

I guess to get to my target compression ratio I may need to either have less or more removed cause of the valve change.

4-Can someone tell me if installing the engine builder valves is going to require having all new valve seats installed or is it just a matter of cutting what is there.

5- Will there be any valve shrouding using these valves. I have read that typically when installing larger valves the chambers will need to be swept/valves de-shrouded after their installation, any truth in that when it comes to the slant engine?

Im going to guess that installing the larger valves and then sweeping/unshrouding the valves ( if that were needed to be done ) would cancel out one another if bigger valves were to reduce chamber size at all but that is just an un-educated guess.

So many questions :)
Author:  Dart270 [ Tue Jul 01, 2014 7:08 am ]
Post subject: 
Do the porting BEFORE you shave the head. Have the valve seats "rough cut" to just larger than the Engnbldr valve dimensions, then port away, then get the final valve job done. You do not need hardened exh (or int) seats installed for those valves, but some folks like to do that to minimize seat wear.

No idea on more or less cc's with those valves vs. stock, but likely pretty close to the same. Those valves are not so big, so shrouding will not be a major issue. If you have a stock bore, then maybe...

Lou
Author:  1930 [ Tue Jul 01, 2014 5:03 pm ]
Post subject: 
Thanks again, makes sense now that you mention it on getting the seats at least rough cut.

Someone on the A-body forum said...........On the exhaust side you'll probably need insert seats. The original induction hardening on 72-later heads was only a few thousandths deep and could only ground away with a light cut.............

Makes sense that I should look at replacing at least the exhaust seats I guess, what do you think?

The heads I am working with are all hydraulic engines ( 81 up ) so maybe by that time a better seat would have been installed?
Author:  Dart270 [ Tue Jul 01, 2014 7:28 pm ]
Post subject: 
For a true street driver that will see high temperatures, and assuming not a "maxed out" port job, I would put the hardened exh valve seats in. However, on a few of my engines, I did not run hardened seats and the heads have done just fine and not eroded the seats. Mike Jeffrey never puts hardened seats in race heads. Sorry I cannot be more definite, but here is info to help your decision.

Lou
Author:  SlantSteve [ Tue Jul 01, 2014 9:09 pm ]
Post subject: 
If you get hardened seats installed get them done prior to porting,these heads are very thin in that area and it's pretty easy to hit water.....ask me how I know :oops: There is a post here somewhere that lists the size of the seats,but the guy that did mine used the thinnest seats he could get and still broke through on the last one he cut. I've just finished porting a head with the engnbldr intake valves and hardened exhaust seats,as Dart270 said,the volume hardly changes. I would recommend getting the head pressure tested after you port it,just incase thee is a thin spot or some porosity. I think for a street port job it's usually fine,but I just want to be sure. It's a pretty common deal to mill a lot off the heads from what I've read here,and you won't have issues....but if you have time and money in the head perhaps leave a little extra up your sleeve for future milling so the head can be used on other builds...if the engine is in pieces,perhaps hit the deck and block equal amounts? Just a thought. Happy porting....the sound of a die grinder is still ringing In my head...and I had ear muffs,I'm sure the neighbours loved it too!
Author:  CNC-Dude [ Tue Jul 01, 2014 9:41 pm ]
Post subject: 
I'm with Lou, I am not seeing shops push the hardened seast like they used too probably because the quality of gas is completely different from when it was necessary. The alcohol blended gas must not be as harsh on the seats as it was previously without it.
Author:  DusterIdiot [ Fri Jul 04, 2014 10:07 am ]
Post subject:  2 cents...
Quote:
I would put the hardened exh valve seats in. However, on a few of my engines, I did not run hardened seats and the heads have done just fine and not eroded the seats. Mike Jeffrey never puts hardened seats in race heads.
Just to add my own experiences with this since I've done quite a few engines now:

I put hardened exhaust seats in all my street/strip heads, I have lost quite a few stock seats due to leans outs from super tuning the carb for more mileage, I'm pretty sure if you run EFI on the fairly lean side it will take it's toll on a long trip, carbs being not so tight on control tend to err on the rich side for safety. Once they regrind the seats in a late head you will have no hardening left or just some spots (My machinist after doing the first couple regrinds just said "Good Luck with that!!!", and I started asking for a set of hardened or stainless exhaust seats.)

Race heads take a different tack since the carb will be calibrated to be rich all the time for max power and to take heat out of the cylinder with the excess fuel, some racers run leaded gas which coats the seats and everything else where it burns, and there is a marked difference between street and strip in this application...not too many cars on this coast with Jefferey equipped heads would be taking a 1 hour trip to the next city, most get trailered and endure a short fire up stage, shut off, fire up, burn off, stage, run, idle back to the pit (or get towed), shut off routine (rich idle, rich run, warmup, slightly rich lumpy idle, high rpm run in the PV and main jets, return to rich idle).

I now solely use the 1975-1987 heads because they have meat in the seat area (and ports too) and I have never hit water with any of the late heads, I have not seen a problem in the 1974 and 1972 drool tube head I used, but have had problems with a 1963 head and a 1967 head so shun the 60's heads for my performance builds that will be street driven.

-D.Idiot
Author:  Sam Powell [ Fri Jul 04, 2014 10:39 am ]
Post subject: 
Interesting. Thanks DI.

Sam
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