Compression Ratio vs Blower Boost
 

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 ?

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




Darrell.

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.

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

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.

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.

Check page 4 in the attached doc. Attachment 355409

bl

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

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

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.

With a lower static cr, you are able to pack more air/fuel in to get to your desired cr vs having a smaller chamber so when it lites you have more energy vs a lesser amount of air/fuel?

I have been running 4.7lbs on 9.5:1 static engines. I like the combo because I run the small 177 blowers on 470cid engines and I don't have to spin them too fast to get the desired effective compression. I run no intercoolers. Picked up about 8mph from no blowers.

Bt :cool:

Has anyone found crankcase vacuum to play any part in effecting the balance between boost and compression?

I have never heard of anyone measuring crankcase vacuum in a practical setting? Of course I am not that worldy either. What was your thought CC?

Blue Thunder, I looked at your profile,now cut that out.

However,to answer your jab....For example,we know the new generation of EPA regs has impacted diesel engines.Presently there seems to be definite challenges at the OEM's,who are trying to satisfy several masters. The Duromax seems most successful. Mercedes filed a patent pertinent to the upper end only,which includes aneroid control/boost but excludes the bottom end.

I'm suspicious,a dry sump and or vacuum system could offer some small part in solving issues with the diesels,subject to total design. I'm also suspicious there may be opportunity to apply similar theory to the forced induction marine engine.

Specifically,as crankcase vacuum draws piston blow by the vacuum must absorb some part of the cylinder chamber pressure which in turn may be construed to lower the chamber CR.

Further,I have puzzled over how a few inches of vacuum can increase horsepower as it does...Granted,the short answer is, vacuum scavenges parasitic oil thereby creating less resistance and more horsepower/torque.

My uneducated opinion would be with a vacuum under the piston, less pressure (resistance) on the underside of the piston during the power stroke would result in more power.

This really depends on the blower and intercooler (if available) design. If, power vs. detonation is the ideal goal then you have to maximize compression to the superchargers most efficient pressure/rpm and then add in the intercooler. Roots style superchargers on motors work best between 4-7psi, therefore, depending on the engines flow and supercharger size, this is where you want the boost to be. Some go higher, some go lower, but you certainly get to a point of no benefit with traditional roots superchargers. Therefore, 9:1 compression works pretty good with the roots. Centrifugals work well, so it would be ideal to run 10-20psi, but, you can't typcically run low enough compression to run these high boost levels (can get away with 15 and below on some applications) because there is a major issue with low speed torque. Therefore, 8-8.5:1 compression works pretty well, and then 8-12psi. With a screw, it's similar to the centrifugal, but you can get away with the lower compression. So you can run 10-15psi and setup with 7.5:1 compression.

Of course, there is far more involved in this. If the blower is too small, you may need compression to help bring up power. But the main key is the same thing, you want your motor to be able to flow as much air as possible then maximize the engine with the supercharger.

Thanks

My understanding is the power increase comes from the addtn'l ring seal from the aid of the vaccuum,I think I have heard of running low tension oil rings w/ vacuum pump to aid in sealing also,Smitty

A note of trivia...Auto Verdi is a brand of dry sump oil pumps that several NASAR teams are using.The pump is said to be long lived,produce adequate vacuum and oil pressure and require minimum operating horsepower. The lobes resemble a roots blower vane. The lobe design is thought to produce less pulsation than the gear type pumps. Therefore, the impact on vacuum,oiling and harmonics may be improved.

The question has been asked if a screw type vane would be a further improvement.