Just searching around the boards and see that a few are runnning blowers on pump gas with compression ratios of 9:1.1 or better. I was told by a Weiand rep that my compression was way too high to run a blower, and with the cam I have, I'd be blowing most of the boost out of the exhaust......is that a true statement?? I believe my compression is around 9.3:1 or so as it stands...

Also, I have'nt really seen anyone with a supercharged 400 sbc. Why is that? Are 400's not good candidates for boost, and if so - why?

I hope these aren't silly questions, but I'm trying to learn more about blowers..........

EDIT: BTW, I've been looking at roots type blowers just for the ease of installation and tuning. I can't seem to locate a centrifugal blower to use with a carb, most I've found are for TPI's.

 

votec sells a carb centrifugal kit

400s aren't good with boost because their cylinder walls are so thin they're prone to cracking.

the guy who tries to sell people superchargers, would deffinately not steer you in the direction of not purchasing one if you had the right setup for it.

9:1 is way high for a roots blower. sub 8:1 is average. I think 7.5:1 or something.

9:1 is fine for an intercooled centrifugal though.

 

I agree with NightRider87, go with a centrifigual type, they produce less boost at low RPM's and it keeps things from detonating on higher compression engines, plus doesn't use as much fuel just putting around town. Also, the Vortech system that I got has a Crane Hi Fire ign included that has a under dash mounted adjustment for timing, with a light in it that shows when the computer is pulling timing due to the knock sensor. It only runs 6 lbs of boost on an LT-1 which is higher compression than yours, and I have no problems.

Troy
So Cal

 

You beat me to it lol.
There are differences on how you can build the engine between centrifugal or roots superchargers. I have a roots so I have more knowledge on what is required for it. This site has a lot of good info http://www.holley.com/HiOctn/TechSer...fo/SCTech.html
As far as centrifical maybe some one will chime in that has experience but here are some sites that I have found with comparisons

http://www.chevyhiperformance.com/techarticles/49378/
http://www.sporttruck.com/techarticles/1436/
http://www.automotiveforums.com/vbulletin/t425.html

 

I appreciate the info and the links guys!! Good info.

Just for informational purposes:

I plan to tear my engine down during the winter due to smoking problems and blow-by. I suspect that the shop that built the thing didn't clean it up properly after they wiped two cam lobes, and I now have scored cylinders - I suspect. I'll know much more when I pull the heads.

If it turns out the cylinders are scored deeply, I may end up getting another block, because this block already has a .040 overbore, and going to .060 over the machine shop doesn't recommend. They did say that if the scores aren't too deep, they may be able to hone them out however.

But if it turns out I'll have to bore, then I'm considering throwing lower compression pistons in a Motown block to support boost down the road.

Sounds like by what you're telling me about stock blocks though, I'd be better off staying n/a if the block can be saved.

Thanks again!

 

Most people run too much compression with boost… there is a mistaken belief that the only way to get good response from an engine is to keep compression up and then a lot of people also look at build ups like some done on aluminum headed LT1’s (which will tolerate ½-1 point more compression) or even serious race cars and figure “that’s what I’ll do.” If you want to run big boost with compression in the 9’s or higher figure that you’ll have to run that boost with race gas or only well above the power band of the engine (extend the power band with boost), like some of the Honda brats are doing.



Mostly because in these circles most people don’t have 400 blocks in their cars so there are proportionately fewer people that will boost them.

Beyond that I’m not a big fan of running big boost and big bores…. The primary reason that a big bore (vs longer stroke) gets you more power is that it unshrouds the valves allowing for better breathing. With a supercharger this becomes somewhat of a non issue, since you can use the blower to make up for it in most situations. The advantage with smaller bores is that it’s easier to control detonation in a smaller bore, ever notice how detonation tolerant 305’s are compared to 350’s or 5.0’s? Or even more so, as compared to most big blocks?



assuming that you don’t have any excessive core shift, that’s not really the case, especially since on average 400 blocks should have thicker walls then non siameezed bore blocks. If you’re cracking cylinder walls you most likely have a detonation problem or are making A LOT of power (like say well north of 750hp)… OTOH, the .040” over and larger stock 400 blocks are pushing things a little bit (it’s pushing things for most OEM blocks)

Again, run as little as possible, and make up for it with boost. There are tons of oem turbo engines out there with compressions in the 7.5:1 range that had acceptable response (for example, the little 1.8 and 2.0 turbo used in some sunbirds). I’ve always wanted to build something with compression in the low 7’s or even high 6’s, at that point even larger displacements won’t cover up some of the potential saggyness down low, but could you imagine the boost that you could push through it? Ohhhh Yeaaaa!