is 9:6:1 to high of a compression to throw 10-12 lbs of boost on a engine?

 

lot of the lt1 guys run that but the reverse flow cooling helps them out. You planning on running pump gas or...? It would be nice to have some type of tuning aid to adjust spark and fuel. So, to answer, no it's not too much but you will probably have to do some other things to make it work reliably.

 

what's the ideal number? what is the typical range that's ok? what's the highest borderline number to run reliably?

sorry for all the questions.

 

Some guys like to run lower compression #'s and run high boost. Problem with that is until you start making some significant boost, your car feels like a slug because of the low static compression. Ever driven a car with 8:1 or 8.5:1 compression? It's kind of like a flashback to the low horsepower turds that were called new cars in the mid/late 70's. It sucks unless you're into that kind of thing.

I've always been from the school that believes in running moderate static compression. I have better throttle response until I start making boost and because of the higher compression to start with, I don't have to make as much boost to run the number. Another benefit is that if I ever decide to go N/A, I still have a respectable setup the way it is.

FWIW, I've been running 9.5:1 in my 406 with a maximum boost of 14 pounds for the last 2 years.

Chris

 

I am running a compression ratio about 9.8:1 - 9.9:1 range. The most boost I currently put on it, non intercooled, is 12lbs. After I get an intercooler I will turn it up to about 14lbs. Off boost throttle response is awsome.

 

so are you telling me the number is ok?

what kind of mileage are you guys seeing through out the year? i plan to put an average of 4,000 a year and i don't want to have to be under the hood every weekend to keep her running. how do your engines run?

 

High compression is not the be-all end-all of making big power. You only give up about 4% for each point of compression. So on a 400 HP engine you're giving away only 16 HP. That's a small price to pay for the added safety margin that gives you if you run lean or over-advance the timing slightly.

I'm running a littl Weiand 142 underhood roots blower on my 383 Malibu at only about 6 PSI. Compression is 9.2:1 with Irone Eagle heads. I accidentally over-advanced the timing on it about 4* while making some runs. THere were little flecks of aluminum on the plugs and the faces of the valves after that. This happened when detonation from the over-advanced timing blew off small chunks of aluminum from the faces of the pistons. Fun huh? That was from ONE LITTLE MISTAKE. Please don't apply my raw numbers directly to your engine- a centrifugal blower is a different animal than a roots. They are not directly comparable.

I decided to rebuild it over the winter for many reasons (mostly I want to go even faster) and I can tell you that I'm switching to aluminum heads and dropping compression below 9:1. Living on the ragged edge with a supercharged engine is NO FUN AT ALL.

Often times a motor with too much compression and too much boost survives by dialing the timing way back, but that's a crutch. A motor with lower compression but able to run full spark advance will actually make more power. Don't be afraid of low compression- it's not the pit of misery some people make it out to be. If you want low maintenance it's better to err on the side of lower compression.

 

<b>Ever driven a car with 8:1 or 8.5:1 compression?</b>

say, dont most 305 engines have dished pistons and like 8.5:1 compression? I think my L03 does... doesnt it?

its no slug around town. heh, in fact, it pulls a blazing .7 G's all the way to 15 MPH... Thanks to 3.73's and a 3.07 1st gear...

But thats not the point. Its no slug, low compression as it is. throttle response and driveability are WAYYYY more affected by intake valve closing point than anything else, compression ratios included.

you wana run lots of boost? you need low compression.
You wana run some boost? You can get away with high compression. but its a "border line thing"... you never know when too much is too much until its too late...

Like Mr. Daemon said, a little mistake with high compression + boost = a BIG mistake. heh.

and for the record, i run 9:1 compression and 10 PSI of boost through my 355. I bleed alot of it off though with my 112 LSA and 303* Duration on the intake valve. thats what kills my dynamic compression ratio early, then the boost makes up for what blows out the exhaust. runs quite well, if i do say so myself. also gets 21 MPG now that i changed the cam to somthing more reasonable... but i had to back the boost down to 8 PSI because of a slight "knock" (Bless my knock sensor!). Its a roots blower.

 

so are you saying i must look to go lower or i can work with this type of setup?

 

Straight and to the point I see.

Im not saying anything except the higher the compression ratio, the more chance of blowing somthing up.

Compression is free fuel economy and free power when your not under boost. more is good.

But UNDER boost things change. its too much when you have too much, so to speak.

If your going to run an intercooler its alot SAFER. if not, its NOT alot safer... lol...

9:1 compression with 30* total timing under boost with aluminum heads can take ROUGHLY 7-9 PSI of non-intercooled, Roots blower type boost. Add about 4-5 PSI to that number for Centrifugal boost, i would think.

Intercool it? Well add about 5 more PSI to that number, typically.

Look at Ati's website. look at weiand's site. they all have typical stuff, but thats just it. its all "typical stuff" you wont find a definite answer, each engine is different. cam overlap plays a huge roll too... so does fuel economy need. the only way to know for sure is to try it and see if it blows up, but there are safe guidlines you can stick to.

9:1, 114 LSA, Aluminum heads, 180* water temp, 30* total timing, 6 PSI of boost is KNOWN to work for people. thats perfectly safe in my book or weiands.... or ati's... or...
but by experimenting you can make your OWN safe guidlines for your engine. knock sensors help ALOT as ive realized. saved me big.

 

just my 02. this is my second s/c car both 350tpi birds the first one i ran 10-1 with afr heads and 6psi centrifical boost boost intercooler.this one i have run for 2 years on a totally stock moter 9.4-1? with no problems and have my new afr's ready to go on again had them milled down to 10-1 so i think if your running iron heads stay around stock cr but 10-1 on aluminum running efi is no problem.
pete

 

well i certainly understand that in this hobby there is no said exact way to do things most of the time and i appreciate all of your comments and suggestions!

thanks for taking the time out to try and explain a few things!

i'll just use my best judgement and see what works.