CcanDo

One could ask,what is the ideal compression ratio vs blower boost. However,there are several variables that cause that question to be more or less abstract.

Therefore,the question asks for opinion on a window range including invited variables.

For a base line we are wondering if the compression ratio should be as high as possible to safely accept 87 octane. The cam,heads,displacement and etc. will be included in the decision. (92 octane min. may be more logical)

The blower would then be responsible for sufficient boost to create below Sea Level density altitude. The boost number should be low enough to allow significant blower underdrive,subject to blower size.

The slow turning blower would require less horsepower,produce less heat and allow simpler density altitude management.

We realize the general thought is 8.5 compression and higher boost. However,for marine application,perhaps the compression could be 9 + and boost somewhere under 5 lbs.

Is it possible an improved net result would be more safely achieved ?

DMOORE

Info needed: motor size, head material (steel or aluminum) supercharger type, boost#, octane.
All this would halp to dial in a set up.




Darrell.

Scarab28

Guess you could ask what the pros and cons are of building a motor with 7.5 CR and 8 lbs boost versus building the same motor with 9 CR and 4 lbs boost. Which makes more power and which will stay together longer assuming both motors are built with the same alum heads etc.

ezstriper

there is a formula for figuring out static compression ratio, but trying to run a boat(which stays under load) 8-1 and 4-6 psi boost is going to be about it on pump gas with out a intercooler or h2o injection, as you can push the static compression ratio to 11.5 and higher real easy...Rob

CcanDo

The components include,10.2 iron block,4.375 Bryant,6.625 carillo,4.530 bore,aluminum heads,1471 BDS Mag case low helix,5 stage dry sump w/ 10" vacuum and a baseline of 92 octane (87 considered),boost is subject to initial compression,cam,egt and timing.

Thought has included a by pass valve controlled at least in part by an aneroid valve response to crank case vacuum. The purpose includes sensing density altitude and managing excess boost. I don't know if this is achievable but it sounds simple.

bob

I can tell ya I did a pair close to what you have with 10.2 bow tie block, 4.5 Crower, 6.535 Carillo, 5.560 bore, alum heads, 8-71, initially no intercooler, static comp at 9.0:1. Dyno'd one motor with blower at 3.8# and other with dart tunnel ram. HP diff was like 100 at 5700. After dyno added BS/KE intercooler so I am not sure where I am at exactly now. Cam is .678/286 & .651/299. Looking at the numbers in my case, at 5000 torque was about the same between tunnel ram and blower. My thought was that if gas go really crazy I could just run the tunnel rams as I spend my time at 2800-3500.

badluck

Check page 4 in the attached doc. Supercharger Tech Info.pdf

bl

ezstriper

thats where I had gotten my info, running a small weiand and was talking to Tom @ weaind, very sharp on blowers, I was giving him different specs and pulley sizes, and he was giving me static compression...Rob

Scarab28

From page 4:
Additionally, you will make more total power with a low
compression, high boost engine than you will with a high
compression, low boost engine

jeff1000man

I have always heard that it was generally beleived and practiced that starting with 7.5 - 9 with 0 boost and add as much boost as you desire was better than starting with a higher number and using just a little boost.

In my experience though, it has a lot to do with camshafts, flow numbers, carb (air fuel) ratios.

Just depends on how scientific you want to get.

I really would like to hear more on this subject though, as I have one of my experimental engines up against a brick wall right now.