The Stovebolt Site Forums The Shop Area Engine Shop The Cadillac 261 question with an 848 head.

I read in the article that was written that there was no need to find "848" head (which requires steam hole drilling), for the modification of the 261 engine. I believe the 848 head has a compression ratio of 8 to and the other heads were slightly lower. Since I have an extra 848 head in my garage would it be better to use the 848 head and drill out the steam holes or the standard 261 head?

Would that change the C/R from 8 to to another C/R when using the 848 head?

Secondly, I read again in a different book that on the 235 you can mill the deck .020 and shave off the head if I remember correctly .040 and you could still run pump gas and get a little more performance. Forgive me I not sure of the head shaving number. Now would you also want to do this with the "Cadillac 261" or will that open another can of worms?

What's the advantage of using moly pistons vs. The standard ones ?

The 1954-55 261 block do they have shims under the main caps like the 1955 235 block?

Just stroking a 261 engine and using the same head on the "stroker" as the stock 261 engine will gain almost a half a point of compression by itself. So you will need to do some number crunching first and find out some absolutes as far as final combustion chamber size, bore size, block deck height, head gasket thickness, etc...so you can zero in on a target number for compression. Stroking an engine changes the compression, all else being the same, as does boring the block. So determine all these values you will have and this will help let you know what path to take.

I think you mean moly rings vs. standard. Either are fine, but if you want moly, just make sure they are available in the bore size you end up boring the block too, sometimes they may skip that size and only the standard cast ring is offered.

Cast iron rings: Cheap, tough and break in/seal fast. No harder than the cylinder wall so wear is more on the ring and less on the wall. Moly advantages? Fast break in on new perfectly machined cylinder. harder face than cast iron so wear is lower on the ring, but the trade off, is more wear on the cylinder. Biggest advantage i have heard reported is less oil required on cylinder wall. So better oil control. Any other thoughts out there? Oh by the way, cost is very different for moly over cast.
Small block chev cast ring set 20.00 dollars. Moly ring set cev small block 30.00 and Cast pistons 80.00 for a set, forged 95.00 a set.

Moly rings also require a finer grit stone for the finish honing. So if you hone the cylinders for a cast ring and install moly rings, you will be disappointed.

Yes 12 Port, Tim from a different thread that I was on provided me a link to the Cadillac 368 Pistons and when I clicked on the part number it brought up another window. I did not realize it was a link for rings until I verified it just now duh.

The caddy pistons are 47 dollars each moly rings 64.99 and the cast 36.99 trw pistons

Ok so determining if the 848 head is an option would really depend on the final compression. So I would have to play with the options and numbers to make a decision on what I would decide to do such as shaving the head and decking the block which from the article in theory most of the number are provided. I get the stroking thing now since it change with the offset on the crank.

I assume there is a compression number that I would want to stay under while running pump gas. Does anyone know that number and once again I will try my math skills and then run it by you guys to see if I am on the right track. I am not trying to reinvent the wheel I just want to try and be creative.

There is a compression number in mind that reflects the absolute maximum that is allowable for pump gas, but it is based on the engines DCR(dynamic compression ratio), and not the SCR(static compression ratio) which is only the theoretical compression you have, and what everyone says their engine has when they say "My engine has 9:1 compression", for instance.

The DCR is the actual compression value the engine sees taking into account the camshaft timing events, which greatly affects the engines cylinder pressures and its ability to run on pump gas without issues. It is also usually a lesser value than the engines SCR, but is probably the more important of the two of them when it comes to building an engine that needs to meet certain criteria, such as using pump gas. The highest DCR that can be used for pump gas in most cases without problems is determined to be between 8.1:1 and 8.2:1(based on some piston manufacturers research). So once you start mixing and matching heads and pistons around and stroking the crank, this is a number that should become your target, and not the SCR number of 9:1 or 9.5:1.

You can have a SCR of 10:1 or higher and still only have a DCR of 8:1 or less based simply on the cam you select, so cam selection is very critical for tailoring an engines needs to a specific fuel use. And since cam choices for the Stovebolt may be limited compared to a Small Block Chevy for instance, you may find yourself building the engine around the cam, instead of using the cam as the tool it was intended for. But you can still get where you want to go with your build, so don't sweat it.

You can contact the builder of the original Cad 261, Tom Langdon, at www.langdonsstovebolt.com and I am sure he will help you with your questions.

Yes Drew that's good advice. I have spoke to Tom a few times. He convienced me not to change my gear in my old 3.90 rear in and go with the T-5 plus he sold me my dual carbs a few other things. He really makes sense at least to me.

Yes 12 port the compression thing is not as simple as I was lead to believe in my little mind. I really need to make a plan and run lots of numbers and do some more reading. I guess instead of just taking the what was before I was trying to squeeze a few more ponies cheaply out of all the things I read. However I will continue doing research on for my next motor.

I was reading my new issue of Car Craft last night. Jeff Smith has a great article talking about static and dynamic compression ratios. Its a good read if anyone is interested in more of what 12 port is talking about in his post.

The only reason to have a static compression ratio much above 9:1 is to compensate for the closing point of the intake valve of a long-duration camshaft. If the valve doesn't close until the piston is 1/3 of the way up the compression stroke or further, the remaining cylinder volume must be squeezed tighter just to get the compression pressure back to where it used to be before Harry Hotrodder installed that radical cam to get the lumpy idle note he wants.

Unless the engine is going to see a LOT of full throttle high RPM operation, a high static compression ratio and a relatively mild cam grind can cause a lot of problems with detonation at low to moderate engine speed and a little bit of "lug" where the throttle is opened up and the engine can't accelerate freely. Other than nuclear, gasoline engine detonation is the most destructive explosive force known to mankind. Ounce for ounce, gasoline vapor has the potential for more explosive power than dynamite, TNT, or C-4.
Jerry

Great explanation, where I got my brain storm was from a book called "The classic Chevy truck handbook" and then Tom's article on the Cadillac 261. I figured that if I combined the two thoughts I just might be on to something. Page 12 "The classic Chevy truck handbook" the writer states that "you can take off 20 thousandths to give a little more compression and to make sure the deck is flat without hurting a thing on most engines." He continue to write , "with a single-head straight six , shaving .040" off will help raise the compression and not hurt the head (more can be taken off, but combined with the decked block, your compression maybe become too high for pump gas if you do). Just make sure you have the valves recessed a like amount to avoid having the tips of the valves come in contact with the tops of the pistons in operation." I am sure he was talking about the 235 not the 261 however I was thinking the same principles would apply.

Now when I always here the guys talking in the shop about their engines I always hear the phrase I am running a --------.-- compression. I assumed and you know what happens when you assume that's where I got into trouble because I was assuming they were talking about SCR(static compression ratio) and not the DCR(dynamic compression ratio). I have been skooled along with some Drain Bramage. I was hoping it was really that simple. However I now understand that it involves a little more complicated math.

I just was trying to have a specific plan for the machine shop so when I picked up my parts for me to assemble (the fun part). I really admire you guys who have been doing this a long time reading your guys responses make me envious however I am glad to learn from you guys.

One of my friends back in the USAF learned the compression lesson the hard way. He bought a set of custom-made 11:1 compression pistons for his Morgan Plus 4 sports car which ran the same engine as a Triumph TR-4. When I suggested he should also step up to a longer-duration cam, he told me he didn't like the lumpy idle. We ended up setting the static ignition timing a few degrees AFTER top dead center to avoid severe detonation on acceleration! Now, the Triumph engine was a "wet sleeve" design- - - - -the coolant is right against the back side of the replaceable cylinder sleeves. Within a month, he had cracked three sleeves top to bottom from detonation. The third time we tore the engine down, he decided to go back to running stock-compression pistons. High compression MUST be used with a radical cam grind, but it's destructive to an engine with a standard cam.
Jerry

Ok I think when I do my next motor the 261 I think I will just follow the proven plan.

Thanks for all the input it got me thinking real hard now.

Good luck with it! A lot of people seem to never realize the first time around that all the components in the engine have to work together and be matched for the same purpose. As Hot Rod Lincoln mentioned, the other side of that coin has many people putting a big huge monster cam in an otherwise stock compression engine and wondering why they have to rev it to 3 or 4000 RPM just to take off from a red light and why it doesn't have any power.