What is a safe dynamic c/r to run on 87oct
 

So I'm getting ready to order motor kits and the 355s were 10.14 static/7.56 dynamic.
The 383s will be 10.85/8.13 is this safe to run on 87 no eth?

On the water 93 is not always available

Steel or aluminum heads?

go up a size in head gasket to take the sting out of that ratio... its a tad high for a marine app, aluminum heads or not... IMPO

I ran 10.3 on my 383 aluminum headed engine . 5000lb. boat and no problems but that is riding the edge for 93 . For 87 octane 9.3 max . My blue HP500 was designed to run 87 octane with steel heads and 8.7cr . 10.8 on 87 octane is a recipe for destruction imho .

http://i1243.photobucket.com/albums/...ep/Image11.jpg

exactly!!! thats why I suggested a thicker gasket, to drop the ratio down to an acceptible limit

Sound wisdom my friend !

better to be safe than stuck with a busteda$$ boat!! lmao and the gains from the increased compression are so insignificant, better to spend $$$ on headwork that will yeild much greater returns!!!

They are ported world sportsman heads steel. Ill look into thicker gaskets. The motors run cool with the stainless marine crossovers
What is a good rule of thumb on max dynamic comp?

Ill be running twins in 5800-6000# boat

I'm not sure too many look at dynamic C/R, but I could be mistaken.

My motor is an iron head with 9.45:1 and I can use 87/89, but prefer 91+.

Thanks for the info

Here's my problem static compression is just that. What the volume of the cylinder,piston,head gasket, chamber and valve will compress to on a given amount in a stroke. Dynamic compression takes into consideration the points of the cam values.

I'm not a pro motor builder and haven't built marine motors at all. im trying to learn what will run from people that have done this type of thing so we can have some stout small blocks that will run dependable. I have no clue what a marine small block will take and live.

I have built a few single cylinder race motors. I can tell you that a 13.5 piston running with a stock type cam will build more cylinder pressure than 93oct can handle and will need race gas to survive. the other end of that add a cam with a lot of overlap and compression is bled off so cylinder pressure is not a problem for pump gas and will make more power with a less stable fuel.

In your motors if you can run 87oct your saving money and making more power than with the higher oct fuel. the flame front travels faster gives a more complete burn and should raise the cylinder pressure. We have spent quite a few hours on the dyno playing with many types of fuel from 87pump to 116 and custom oxy or nitro blends when there were no fuel restrictions

This is a simplified run down on what is going on but I'm just looking for a rule of thumb kind of thing to get me in the ballpark.

I usually set them up at 8-8.1 dynamic and expect to run 93 octane. I think you want to get down under 7.8 to run 89 octane. Get on a calculator and see how much tweeking is needed on the cam degree to get you there. If it is 4* or less from straight up I'd probably go that way. I'm never a fan of thicker head gaskets as that works against you in preventing detonation.

I run my 385ci SBC on 87/89. 10.09-1 SCR, 8.3-1DCR, 190psi avg cranking compression. AFR aluminum heads, Mahle -5cc Flat Top pistons. 0 Decked blocked, tight .039" quench. Had to go one range colder plug than AFR recommends to start out with, FRLS2,
Holley HP750 with 80, 86 jetting. 36* WOT timing, WOT is 6000-6300rpms. Ran this setup since 07 with no problems on 87/89 when available.

This is not for the start and go boater. Setup took time to dial in.

I agree with bluethunder that lowering compression by increasing gasket thickness is a bad approach. Your engine wil be more detonation prone with a .55" quench thick gasket piston down in the hole setup. You want as close to 0.39 quench as you can get. 0 decking the block and running the common 0.39" headgaskets makes this very easy on a SBC. Adjust compression by head cc or piston design not by gasket thickness.

Blue thunder and Ryan thanks this is more of what I was looking for.
Thanks to all of the other responses as well. I'm always open to more input;) any other thoughts help. I would like to run this setup for a year or two before something different.

Squish Squish Squish.......

You can run your squish as tight as .036 on that motor unless you run it to 8,000 rpm with aluminum rods..

Squish snuffs detonation, and is also the one commonly ignored factor that makes two otherwise similar motors run differently. Take two motors with the same cranking compression, and the same cams, intakes, carburetion, and similar flowing heads - the one with the tight squish will outperform the other one, will be more responsive, and will be able to run less octane while still not detonating than the motor with squish above .065".

The advice to use a thicker headgasket takes you AWAY from where you want your squish to be.
Your motor may or may not be currently set up with effective squish height, but adding to the gasket thickness will certainly have a bad effect on the quench and detonation snuffing turbulence that you DO WANT.

You really need to be setup with your squish height in the .036" to .050" zone, leaning towards the low end. Sometimes it takes thinner head gaskets, nonstandard piston heights, and/or zero decking the block to achieve modern effective squish heights.

MC

With a mind like a sponge i am always learning . Thank you .

agreed, however I do believe we were talking about an existing engine, and what to do about a slightly high compression ratio... a thicker head gasket would take the edge off it, with likely little effect on the quench, .010 ticker will have a significant effect on compression without dramatically killing the quench, I would assume that the piston is at zero deck, to accomplish the ratio mentioned...

It's cool that your asking about dynamic compression. Most people only look at static.

Ok now were talking!

Thanks for the info! I have a fair understanding on how a engine needs to run to produce maximum usable power and run hard on the most efficient fuel for that motor setup. ( lots of dyno time on every motor that I built with as much info and time spent with the people that were proven winners).

The things I have learned this far.

Each motor has its needs to run great at what you build it for.


Fuel that it will run or that is sanctioned to run for its class

The amount of squish that each motor prefers in regards to bore size vs stroke and fuel

Chamber and piston profiles are huge.

Static vs dynamic compression (13.5:1 stock type cam vs cam with more overlap vs cam timing) pump,race gas or alky.

What cylinder pressure will it like under what load vs time and heat will it dissipate?

Unfortunately I know nothing about marine motors. I have built a few small blocks but never anything over a 450 hp 383 that I thrashed in a car or truck.

On the other hand I have been building small two and four stroke motors most of my life that run very hard after a lot of time and work.
(Bent valves,cracked, burnt,scuffed and seized pistons,broke cylinders, rods and cranks, ect, ect)
Again thanks for all the info!! I'm learning so much and will be ordering my rotating assemblies in the next week.

I've been talking to a few marine and race shops as well. Im trying to make well informed choices. So far the marine and dirt track shops are close in there motor building theory for what I like to think of as "endurance" motors.


Thanks ken