I thought that some of you might find this info helpful if you're thinking about putting a roots-type blower (144, 177, X-71 series) on your engine. It's very basic, straight forward information if you've never built an engine with a roots blower on it and want to know where "sanity" is at. I'm about to build my third engine screwed togethert specifically for the tiny little Weiand 142 blower. The more experience I get the closer my experience lines up with what they talk about in this tech article. It's good info. If you really know what you're doing and you want to live on the bleeding edge you can probably "push the envelope" a little vs. their recommendations, but it's nice to know how to build a nice safe combo that won't blow up on you but still makes good power.

Pay particular attention to their recommendations on boost vs. compression and on ignition timing. Aside from never EVER running lean, that info is the holy grail of goign fast without breaking stuff. If you're used to high winding N/A combos you'll probably have to readjust your brain to the very different specs a roots blower needs....

http://www.holley.com/HiOctn/TechSer...fo/SCTech.html

 

what do you use for carburetion? I've got a 402 with a 177 now, and I just converted an 830 cfm to a boost referenced power valve version, cause I initally had some popping and backfiring on cracking the throttle. I'm just curious if boost referencing is going to get rid of that, since I'm in the process of putting it in the car, and i haven't fired it up since i modded the carb.

thanks!

 

great info on that site.thanks alot.I wish there would more talk on roots superchargers.There easy to install and dont need an elaborate fuel system setup.

 

Personally, I use a home-tweaked Rochester Qjet on my motor. I have a 6" step-up spring in it (same fuction as a power valve on a Holley) and it is not boost referenced becuase there's no easy way to do that on a QJet. I personally don't think it's necessary on a roots blown engine since the boost happens in almost direct relation to the vacuum above it, under the carb. My combo, through testing, has shown that it goes into boost at about 4-5" of vacuum at the carb. Since I have the step-up spring at 6" it'll never be running lean (cruise A/F ratio) when the engine actually goes into boost- it'll alway be in power enrichment at that point.

Others may disagree but I know this to be true on my engine from actual observation of carb vacuum vs. intake manifold vacuum/boost. I hooked up 2 gagues- one above the blower, one below and drove around for a while making observations.

Overall my carb calibration is VERY similar to when it was sitting on a N/A motor. Just fattened it up a smidge to stay on the safe side with respect to the WOT A/F ratio.

 

One other thing I should mention is that Holley's Compression vs. Boost spreadsheet in that article, along with their recommendation about not going over 12:1 EFFECTIVE compression ratio seem to be roughly right in my experience- it's not BS.

My first motor was 9.2:1 and DEFINITELY had detonation problems at 6 PSI. Effective compression was about 13:1.

My current combo is 8.7:1 and I'm running 5 PSI of boost. Effective compression is right about 12:1 and it's definitely "on the edge". 26* of advance it runs fine. 30* of advance and it's definitely in detonation.

My next combo I'm going to try 8.2:1 and bump the boost back up to 6 PSI. Should put me around 11.5:1 effective compression and hopefully still be able to make as much power as the current combo but without being so fussy about the tuning to stay out of detonation.

 

cool site, ill have to bookmark it. i always envisioned my next rwd car having a 144 with a holley tbi unit. i love injection. i have a grand prix gtp with the eaton m90 supercharger. i think im gonna build another motor like mine and rwd it in a 'bird. if im doing 12.5ish in a 3800lb fwd car on bad tires wonder what it would do in a rwd with drag radials.