Compression, vs blower OD and blower max rpm efficiency

Guys I know this has probably been covered to death but searching I could not find what I wanted to know, I'm playn around with rod length at the moment on what is a 443 hemi 4.125 bore 4.150 stroke. So my compression is not finalised I am however thinking about 11.1, I have a std 6-71 roots that has been stripped, as its not a kick arse blower I'm thinking there would be a max rpm before it either the air gets to hot or cavitation happens and rotors smack together ? Should I run as much static comp as possible keeping piston to valve clearance generous and run the blower slow? Sorry blown alcohol incase people think otherwise. Any help would be greatly appreciated.

 

Just thought I'd add what I was thinking and if you think there is any problem or room for improvement please add. I'm going to limit it to 8000 for component life and was going to run 15% over so the blower would do 9200 not sure about boost maybe if it made 20-25 lbs as its 443 and the cam is probably nit the most suited to make max cylinder pressure and I'm planning to run 11:1 all should be good right ?

 

Opinions are like... my 2cts is 11-1 is max plenty. If this is fairly new program to you DONT go anymore. 10-10.5 is very workable. what are your et expectations? you can always change a head gasket to adjust some compression.

 

Mark, thanks for your response this is a new combo, and i do not have a great deal of experience, the only thought behind having 11:1 was i have 443 cube engine the blower is an old std helix 6-71 and not a huge amount of RPM i have no ambition to put a big blower on as im trying to keep it looking period as its a nostalgia deal. my cam has a 109 lobe center also....
my calculation were based on a 80 thou gasket, as a have a few of them
most guys i know who are running low comp like 10-10.5 run 25-28 some 35 over and i just didnt know if my blower would be up to it! plus it would cause the intake temp to climb up!
does anyone know how quick guys have turned STD GM diesel style blowers? and have them survive without rotors clapping, hitting etc... ?

Thanks

 

I think you need more comp.. The blower isn't going to make much steam so comp. can make up for that. I'd say somewhere around 13.5:1 with a .080 and you can always go down in size/more CR. Just make sure you have enough of piston to head/valve.

Brandon Booher
NHRA TAD #323
Torque MGMT. - Owner

 

I'm with Mark on this one, 10.5 to 11.5 is a good starting point if this is new to you, the tuning window is much, much wider and gives you a good chance to learn what your engine wants with usually only the cost of burnt plugs if you miss the tune. There is always room to upgrade once you get comfortable with your baseline combination. I take it that if your blower is/was stripped, that it is at least an aftermarket setup, probably can take a little more overdrive down the road. I assume with this bore, you're running an early Hemi, correct?

 

Thanks , correct I have an early hemi. It however is a billet block, the blower is a magnesium danakas 6:71 stripped, basically a gm unit, I was thinking now that I'm calculating my rod length I was going to low an extra .040 clearance so I could always go to a .040 gasket later. If I want that more comp, though I'm only shooting for a ballpark 7.50 so if I can do this and only risk burning plugs I'd be happy. Thanks

 

i would set it up to measure your rod length with a .060 gasket at 11 to 1. you can later change gaskets and get from 10 to 1 up to 11.5 to 1 pretty easily. we ran the old Donovan stuff for a while before switching to the late model. you shouldn't have any problems at all running 7.50.