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!

Or a used car salesman.......lol

:thankyouthankyou:

Pat yourself on the back though, lots of what I learned came from what you've shared. I just saved you some typing, so you can get back to work on that bad a$$ skater project. :evilb:

Well thank you. It's nice to know that someone beside myself is reading what I type.:lolhit:

glad to see you state this. I have wasted too much time with guys trying to explain that. they all think that because they are making 8lbs of boost they are making 1200hp with their stock crate motor because they read a magazine where someone with a small block did it. of course the magazine motor had bigger heads, a proper cam, exhaust etc. I asked some kid once what he thought his motor made and was told 500hp. I asked how he figured that and his answer was "the intake said it was good for 50hp, the headers said they were worth 40hp, etc, etc, so I added them to the 250hp I started with and came up with 500hp." I just walked away after that.

Thanks again everyone for the advice. Im starting to get a better idea of my build now. But still not completely sure. I've got the zz502 w/ between 9-9.5 comp. (not ideal I know and haven't cc it yet) with the 320 RHS alum. heads. Thinking a 10-71 w/ 2 850 holleys and 6psi boost. I'll run a mix of 110 and 92 octane, since i'm looking at around 13:1 no big deal OR is the 2 psi even gonna be worth it. Haven't looked into cams yet but just ran across a thread talking about "exhaust revirsion" w/ a 502 and EMI manifolds, which is exactly what Im running?! What cam company or builder should I go w/ to pick my cam and talk about this with? And do u think I can pick a cam that will work for this 502 AND the 540 im building that will have all the "right stuff" to replace the 502 later.
AND I previously asked in another thread about using my 355cc pro 1 heads instead of the 320 RHS and was suggested to stay w/ the 320s. U agree? Once again, I won't be puttin alot of hrs nor be running this boat hard until I upgrade from my Bravo1.

an 8-71 with 750's would be better.

Im going with the 850's because I already have them and the 10-71 because when I finish the building the 540, the 10-71 will be better suited. Kinda using this 502 as a test to see if I want to stick with this blower thing for my 540 build. I am New to the power adder/ supercharger thing. Im sure Im gonna love it though?!

850's will work fine. IMO, Thats the carbs i would choose. Even in the Holley/Weiand catalogs, if you were buying a 6-71 or 8-71, twin 750 for small block, twin 850's for big block.

Im running 850's on top of a 468 with a 420 B&M blower. Just slightly smaller than a 8-71. Engines made 804HP non intercooled and 6lbs of boost, and run perfect. Fuel Curve is great, not even boost referenced. Idle all day if I have to. Granted, 750's would work just fine for me, but the 850's sure arent hurting a thing.

Pretty much, the theory of too big a carb causing sluggish low end and soggy performance, you can throw out the window. Roots blower like lots of carb. Or let me rephrase that, they DONT like any restriction. Ever notice the twin 1050 Dominators on a merc 900SC? Merc Racing Engineers weren't dummies. A pair of 4150 style 850's might have helped it idle a little better, over the dominators, but i'd be very shocked if they made any more power over the 1050's on that engine. (540 W/10-71 and chiller). Overcarbing ( I mean like a pair of 1150's on your setup), would most likely cause tuning issues, more than anything. Too much carb causes poor signal thru the carbs, and you end up having to run alcohol jets to get a acceptable AFR.


BDS has a calculator for blower carbs. Based on a 509CI, turning 6000RPM, running 8lbs of boost, required CFM is 1364. They dont suggest more than 30% over carb needed on a blower. so 1364CFM, +30%, 1774CFM.

You see guys all the time with big roots blower stuff. They Gut their flame arrestors, put big old Pro Stock scoops on, trying to get more air into those motors. But then they want to run small carbs.

As for the 320cc RHS heads, vs the big Darts, tough call. I don't know much about the RHS Heads. While the 320CC port size works nicely for what you're doing, you need to think about flow also. I'd Talk to someone like JIMV here on the boards. If you have both sets of heads available to you, you might be able to weld up the floors on the Darts, to get the port velocity up, and smoke the RHS heads in performance. I'd hate to tell you to run the RHS heads based on the intake runner size alone. Those Darts move some air.

Figure out which heads you're gonna use on your build, and then have Bob Madera (RMBUILDER) here grind a cam for what you plan to do. If you take your time and do a nice build, I think you'll be pleasantly surprised, and make a lot of power. You may not even feel the need to spend the extra few grand to install a stroker crank and rods. The extra 30-40ci you gain, can easily be outdone with a properly set up 509. Then you can use that cash to beef up your drive.

Great thread

+1
I'm learning more... That im set up way too conservative.

Thanks again, I typed in Bob Madera (RMBUILDER) and found another Forum which many people were all talking about what a great cam he ground for them. Sounds like he really knows his $h!t.
As far as the heads go, I think I'll just stick w/ the 320's on the 502. I already have the 540 w/ Dart block and the stout rotating assembly I just need to bore it and ditch the 12.5:1 pistons for ssomething more like 8:1 and would like to go w/ a dry sump system on that motor. How well I can get carbs to work will help me deside if I'll go with EFI or not.