Hey all.. I had my car at a shop that did a motor rebuild (knowing it was going to receive a vortec supercharger after the build). well, without getting into a big long story..the shop didn't know their a$$ from their elbow and I took my car to a shop that is competent and treated me like a human being..my concern is that I recently found out the compression ratio is around 9.25:1 being too high to run the supercharger (vortec V2 kit) and still use pump gas. The shop cleaning up the mess from the other shop is concerned about this and let me know. We concluded the only thing to do is move ahead with the build and see what how it runs once everyhting is bolted on. Just lokoing for thoughts, opinions, and any remedies you may offer since I know many of oyu are quite knowledgable. Thanks in advance...

Believe it or not you are a at a perfect compression ration fro a srteet engine to have a power adder. Just remember the stock compression ratio on third gen 350's is 9.3 to 1. You can safely run 8 to 10 lbs of boost as long as you have a nice tune. You can run more aggressively with forged components, aluminum heads, an aftermarket efi to help tuneit in as well. The difference with a much lower compression is that you have to run more boost in order to make up for the loss of compression. I don't see any problem as long as you have aduqate fuel to go along with it.

I'd say you're probably limited to 4-5PSI. This is a link to Holley's Supercharger PDF, its for a root's blower, but I think its a good rule of thumb for boosted engines. Look at page 3 for effective compression.

http://www.holley.com/data/TechServi...ech%20Info.pdf

you will be fine with that comp.ratio.just please use an intercooler.it will really help.....

It sounds like it is already built. I would just run it and then install an intercooler or water/alky spray later on if you want more boost. Play with timing.

In terms of compression at 9.25.............it doesn't really matter without additional info. No one can throw out a decent guess without out knowing quench, chamber shape, chamber finish, cam duration/overlap.

if you don't intercool that car you're going to chunk a piston from knock or be limited to 4-5lbs of boost with 15/16 degrees of PE timing.
5lbs of boost is going to put you somewhere around 11:5.1 if not 12:1 and you'd have to run 13/12 degrees of timing to prevent knock.
IF you intercool you'll be able to run about 8/10lbs with 18/24 degrees of timing depending on how good your tune is.

my gtp is running 13lbs of boost but its on a 8:5.1 motor and thats really at the absolute limit considering I get burst knock every so often.

My shortblock is 9.2:1 compression and I run 6 psi with 30° timing. Cast pistons and crank hasn't blown up yet and I've been running the snot out of it.

Just make sure your tune starts with very little timing and alot of fuel. Work up to it. Doing this will try your patients but it's well worth it.

you realise that if something happens and your motor starts knocking bad that at
30 degrees of timing the stock ECU won't be able to pull enough timing quick enough and your motor is gone right?
theres a very good reason alot of people run no more then 24 degrees of timing.
I'm not saying something WILL happen but IF something does then the potential for damage is much higher.

9:2.1 is that dynamic or static? if you have a descent sized cam thats believable because the dynamic will be much lower.

have you scanned your car for knock after tuning it and have you scanned it multible times considering the older ECU's have a very slow logging rate?

in any case you'd make more power if you cut 6 degrees off your timing and ran another pound of boost.

http://www.theturboforums.com/smf/in...?topic=61219.0

how do you compare a GTP to a gen I small block?

with 8.5:1 CR you should be able to run much more than 13psi aftercooled but then again i am talking about a gen I and you are talking about some "other" engine


gotta love that thread TPI383 read the whole thing and it takes a LONG time

who said I was comparing a gtp to a sbc? and you're right in that you should be able to run alot more then 13psi intercooled but you didn't read my reply properly because the 13psi number was for a non-intercooled roots/twin screw supercharger.
if you want to talk intercooled twinscrew then 26/30psi is "possible" with 9:2.1 compression but you'd need a cam with some serious bleedoff.

lets keep things atleast somewhat in perspective here but let me atleast touch on this for abit.
you say at 8:5.1 I should be able to run alot more then 13lbs? do you have any idea how hot a non-intercooled roots supercharger gets once you start spinning it past its efficiency range? IF my motor was stock I'd be around 20/22lbs but I obviously have enough done that it lowered the dynamic compression thus lowering my boost climax. I'm still limited in the amount of boost I can run because the outlet temps are just to high and I REALLY need and intercooler but I'm cheap

anyways, depending on this guys cam/head choice he MAY be able to run more but I was simply trying to advise him to start with a very low boost number and work from there BUT I didn't see this guy say ANYWHERE that he has anything other then stock heads/cam which WILL limit that amount he can run severely.

if it has an intercooler it should be fine. i run a max of 14psi on my 305 with 9.5:1, but i have a 3 core intercooler and methanol injection, right now i'm running 19 degrees of timing, but i plan to up that.

Keep the tune fat, your IATs low ( intercooler, meth injection) and up that boost. I'm running 8lbs on a cast 9.1:1 motor non-intercooled and have had no problems whatsoever in the last year and a half (driving it like I stole it). Up you timing little by little and listen for knock and when you hear some back it off 2degrees to play it safe. Good luck

I'm at 14psi with a stock 305, stock cam, and 9.3:1 (I guess, if GM built it per spec). No intercooler. With 35 total advance at WOT, and boost retard at a max of 15... I guess I'm running 20 advance WOT. This is using the Vortech kit with FMU and BTM, and 91 octane pump fuel. I started at 8psi and worked my way up from there, through tuning it. The previous owner of the engine also abused it with the same supercharger, and it has more than 100k miles on it now and the only thing I know is wrong with it is valve stem seals (typical GM). Only on very hot days (90+) have I noticed any knock retard going on.

Now with that said, without knowing what specs your engine was built at... hard to say what PSI you can run. 9.25 may be fine, with an appropriate cam and tune... or with an inappropriate cam and tune, and other engine dynamics it might be way too much. The Vortech kits dont come with any boost control though so you'd either need to install something or swap pullies and keep the RPM down or whatever. Best advice is start small and slowly work your way up, if you're stuck with the motor. Post up the cam and other engine specs, if you have them.