Confused about total comp. verses octane :/
 

Building my 1st blower (roots) motor this winter and trying to decide on what total compression/boost to run. My confusion came when I was on BDS site and looking at their boost/comp. chart. They said that "Final compression ratios in excess of 12.4:1 are not recommended for use with "pump gas." " Now, I'm more familiar with building N/A drag engine and I've never heard of 12:1 and pump gas used in the same sentence before!!! Now I know you are suppose to retard timing with boost but they don't say that just to run lower octane and sacrifice performance, do they?? I am willing to run a mix of race gas and 92 but I need to understand this before deciding on boost. Help me please....

8.0 to 1 (static) is about where you want to be in the Marine engine world. Then there is how well your cam and heads breath, size blower, etc.
I have 8.0:1 with 6psi and 30degs all in at 2500rpm on 92. As you can see on that chart its around 11.3:1 final. Pretty conservative...I have good size heads and fairly good roller cams so I know all thats getting into the chambers.

This is a very good read...
http://holley.com/data/TechService/T...ech%20Info.pdf

Final compression is when your motor is filling at peak volumetric efficiency
Which is dictated by your cam timing on where in the rpm range your ve peak's that is also where
Your motor makes the most torque

8 to 1 as back4more stated is a desirable mechanical compression ratio for blower motors

I ran 8#'s of boost with 7.5 static in a hopped up 525SC wihcih translates to an effective comp ratio of 11.6. I ran 35* of timing. I never had any issues with 92 octane and the engine had almost 500hrs on it with only a top end refresh and cam at 350hrs.

Thanks alot for that link!
 

Just printed that link, haven't read it all yet but alot of good info! Thanks!!

You really dont wanna use a boost retard system in a boat. In cars its a popular option, because you can add more boost, and normally youd pull timing to compensate. Problem is, in a boat, without enough timing on extended wot runs, you smoke the exhaust valves. Of course there are exceptions in certain applications.

You'll want to be somewhere between 8:1, and 9:1. Alot depends on what blower you plan to use. If your gonna be running a small roots, like a 177, 250, 256, a little more static compression helps. You dont have to drive the blower very hard to make the boost, and the extra compression will make some extra power.

If your going with a large blower, lower compression will allow you to turn the boost up a bit. The larger blower can accomodate. The smaller blower overdriven alot will just make heat and not power when you try to exceed what it was meant to do.

Timing wise, best thing is to dyno it and see what your build tells you it wants. There really isnt a magic number, way too many variables. Too much timing and you will detonate, not enough and the exhaust valves suffer.

BTW, if your building from scratch, go with a designated blower piston. Polish the combustion chambers, use a MLS head gasket providing your head and deck RA finish is suffice, head studs, good rods and bolts, inconel exhaust valves or equivalent, run the engines cold on water temp. Get oil T-stat, to keep oil temp in check, use a good fuel supply, and dont let it get lean. Enjoy!!

I was told from an old engine builder that lower compression & higher boost = more HP. Lots of good info on this thread.

I agree. Low compression and more boost will win the HP war, given the same overall engine setup. And also, dont let anyone tell you ''ahh, it dont matter, on a blower motor''. I can't tell you how many times I've heard this statement. From Cylinder head flow, to exhaust, to cam selections. It does matter.

Keep in mind, boost is a measure of restriction. A well built engine, with good flowing heads, proper cam, good flowing exhaust, good compression, etc, running on 5lbs of boost, will outperform your buddys same sized engine, with $hitty flowing heads, mismatched cam, and crappy exhaust running 8lbs of boost.

So, What are we building!????

Properly spec the motor with good parts, and low boost, will live a long time.

This is some of the best advice you will ever get in regards to a supercharged engine. It is dead on the money. Couldn't possibly have said it better myself.
Eddie

mild thunder sure has a way with words,he could be a politician!