what compression??

[grumpyvette]

I am in the planning stages of my engine build at the moment, and plan to go with a 496. It will be a stock block, all forged internals, 8-71, and aluminum heads. Street toy with no track time. I want to be able to run pump gas and unfortunately 91 octane is as good as it gets around here. Where should my compression be ideally? I have heard everything from 7:1 all the way up to 9:1. Where is the sweet spot?

Thanks

OK, first fact! the piston can,t compress anything until both valves fully seat, static compression is based on the volume compressed between the piston starting at bottom dead center and compressing everything into the combustion chamber , head gasket quench,volume, that remains when the pistons at TDC
dynamic compression is the ONLY compression the engine ever sees or deals with, it measure compression from the time both valves seal the chamber,and that is always lower simply because the valves always seat after the piston is already moving upwards on the compression stroke.
an 871 roots supercharger is potentially capable of moving a good volume of air, at fairly low rpms and looses efficiency when spinning much over about 5500rpm so your drive train gearing and belt drive pulley ratio, used also plays a significant roll in matching the boost curve to the engine rpm.
theres always a compromise, that needs to be made balancing related factors based mostly on operating temperature,DYNAMIC compression, quench,and fuel octane used.

http://www.xcceleration.com/cr-boost%20101.htm
http://www.superchargersonline.com/inde ... page&id=46

RELATED INFO
viewtopic.php?f=86&t=9594&p=35513&hilit=871+roots#p35513

viewtopic.php?f=86&t=1250&p=2680&hilit=roots+supercharger#p2680

viewtopic.php?f=55&t=142&p=176&hilit=supercharger#p176

viewtopic.php?f=86&t=9078

http://www.summitracing.com/parts/wnd-7186

 

[Indycars]

It's not clear what type of compression is represented by the BLUE
compression lines on the graph.

Are they dynamic or static compression ratios???

 

[grumpyvette]

lets take the last blue line on the right marked, and starting at the base of the chart, 10.00:1, if we follow it up to where it crosses 8lbs of boost then drop vertically , it indicates an effective compression of about 12.5:1 on the graph! does that help? or am I simply making it more confusing?

 

[Indycars]

It's not about how to read the chart, but what compression ratio(SCR or DCR) does
the blue line represent.

If I have an engine with a DCR of 8:1 and an SCR of 10:1, What is my effective CR
with 8 psi of boost ..... is it 9.9:1 or 12.3:1 ???

 

[grumpyvette]

I could EASILY be wrong, but AS I read the chart, I would think an 8:1 dynamic compression would result in about a 10.5:1 effective compression with 10 lbs of boost, which seems rather LOW to me!

 

[grumpyvette]

getting back to the original question,and thinking about the intended use, (CRUISER NOT RACING) Id be looking to use an 8:1 static compression, an over drive trans like a 4l80E with a 2600rpm-2700rpm stall converter and a 3.43:1-3.55:1 rear gear ratio tires as large (tall and wide) as I could fit in that 1965 chevelle, If i remember correctly thats about 27" tall, and a hydraulic roller cam on a 112-114 LSA with about 225-230 duration at .50 lift with at least .550 lift, and Id sure try to back cut the valves, have a 3 angle valve job, a low restriction 3" exhaust with an (X) pipe behind long tube headers of at least 2" primary tube size if it was my chevelle.a combo like that will provide impressive power levels in the rpm range youll use while driving on the street and allow impressive acceleration in the 30-80mph ranges youll see as useful.

viewtopic.php?f=71&t=555&p=703&hilit=tire+size#p703

viewtopic.php?f=87&t=1946&p=43927&hilit=tires+printed#p43927

http://garage.grumpysperformance.com/index.php?threads/cooler-denser-air.8961/#post-54528

http://www.wallaceracing.com/gearcalc.php

http://airflowresearch.com/articles/art ... /A-P1.html

http://www.prosystemsracing.com/calculate.html

viewtopic.php?f=52&t=6414&p=20213#p20213

http://www.hotrod.com/cars/project-vehi ... -build-up/

http://airflowresearch.com/articles/art ... /A-P3.html

 

[65MLBU]

getting back to the original question,and thinking about the intended use, (CRUISER NOT RACING) Id be looking to use an 8:1 static compression, an over drive trans like a 4l80E with a 2600rpm-2700rpm stall converter and a 3.43:1-3.55:1 rear gear ratio tires as large (tall and wide) as I could fit in that 1965 chevelle, If i remember correctly thats about 27" tall, and a hydraulic roller cam on a 112-114 LSA with about 225-230 duration at .50 lift with at least .550 lift, and Id sure try to back cut the valves, have a 3 angle valve job, a low restriction 3" exhaust with an (X) pipe behind long tube headers of at least 2" primary tube size if it was my chevelle.a combo like that will provide impressive power levels in the rpm range youll use while driving on the street and allow impressive acceleration in the 30-80mph ranges youll see as useful.

viewtopic.php?f=71&t=555&p=703&hilit=tire+size#p703

viewtopic.php?f=87&t=1946&p=43927&hilit=tires+printed#p43927

http://www.wallaceracing.com/gearcalc.php

http://airflowresearch.com/articles/art ... /A-P1.html

http://www.prosystemsracing.com/calculate.html

viewtopic.php?f=52&t=6414&p=20213#p20213

http://www.hotrod.com/cars/project-vehi ... -build-up/

http://airflowresearch.com/articles/art ... /A-P3.html

Grumpy, many thanks for your help on this. A bit of additional info I failed to ad to my original post, Car is tubbed and running 31x18.5 street radials, 3.50 gears, and a 700r4 transmission. This transmission has been beefed up, and is surviving fine behind my mild 468, but I know with this engine I am proposing it's time would be quite limited. So I am considering a 4L80E as you mentioned. However, I am wondering if a OD trans is even needed? I have a built TH400 on hand that I could swap in if I decided 3 gears was enough. My 700r runs this car about 2100 rpm in OD, and only about 2650 rpm in 3rd. at 70mph. What rpm do these blower motors like to cruise at? I am thinking with the tall gears and tall tires I would probably be fine with the TH400? Fuel mileage is not one of my concerns with this car since it will only get driven for fun. I currently have 2 1/8 primary long tube headers, which seem too large for my current setup, but sounds like they will fit the bill nicely for this engine we are discussing.

Thank you!