[VM0824]

I'm probably gonna end up with alittle over 8.5 compression ratio on the rebuilt motor. is this where I wanna be for a boost motor?

probably sticking with 10lbs boost. maybe 12 in the future.

stock head chamber = 62cc?
deck and heads shaved, not sure how much yet but maybe .005 total.
-12cc pistons
4.030 bore
head gasket 4.1 bore with .04 compressed thickness.

i picked teh dished pistons purely for compression reasons. i had originally picked out -2cc pistons but i think the CR came out to like 9.3+. and im under the impression that i shouldnt increase CR being i have the turbo.



with lower compression, will i see a dramatic power drop while not in boost? mileage? suggestions?

[ryanw]

Im no expert but it seems between 8.5 and stock 9 to 1 would be great, but i wouldnt be afraid to run 9.3

For the most part id think that 9.5 would be highest compression ratio id be comfortable running with boost

[WickedSnake00]

Originally Posted by ryanw Im no expert but it seems between 8.5 and stock 9 to 1 would be great, but i wouldnt be afraid to run 9.3 For the most part id think that 9.5 would be highest compression ratio id be comfortable running with boost

+1


Think of it this way; to prevent detonation, CR and boost have to be inversely proportional. High boost means low CR, low boost means higher CR.

Keep in mind that more boost means more fuel, which can mean worse fuel economy. Low boost and higher CR is generally more street friendly. Just watch out for any pinging.

However a lower CR gives you more room to grow, and you can always compensate for the low compression by just turning up the boost. You also have more safety margin here.

[03_StangGuy]

8.5 is pretty low even with boost. With a turbo you should be able to safely run 20+psi on pump gas.

[VM0824]

That seems really high no?

[ryanw]

I think im running some where around 8.9 to 1 if im remembering right,

[e3sean]

if there were a magic number for what compression to run then thats what all turbo setups would be.

depends on how much CFM and PSI your unit will put out and what the motor can handle. ive herd of cars that are 11:1 compression running a small supercharger.

if youve already bought the pistons you can make them work but its gonna be a dog when not under boost. try to get a good size unit on it to supplement.

[JTs03v6]

I would not build it under 9to1.

[VM0824]

Originally Posted by JTs03v6 I would not build it under 9to1.

alright i'll try to find pistons to keep it near stock. i still have to find out exactly how much material the machine shop removed to be able to calculate it.

[BassMaster5000]

Kennedys DynoTune- Dynamic Compression Ratio Explained

Best explanation that I have seen in a long time.

[VM0824]

Originally Posted by JTs03v6 I would not build it under 9to1.

ive been trying to calculate it myself, but i called the shop and told them to aim for 9:1 and to calculate piston volume from that.

Originally Posted by BassMaster5000 Kennedys DynoTune- Dynamic Compression Ratio Explained Best explanation that I have seen in a long time.

very cool thoery. i kind of figured that CR was some arbitrary calculation that correlated to some real math. with that in mind, the stock cam on my 89 is fairly "small" no? so the DCR would be fairly close ?

[03_StangGuy]

Originally Posted by VM0824 That seems really high no?

Im running 15psi a supercharger with 9:1 and theres still room for some more.

Turbos typically make boost higher in the rpm so the dynamic pressure is less and you can run more.

Originally Posted by e3sean if there were a magic number for what compression to run then thats what all turbo setups would be. depends on how much CFM and PSI your unit will put out and what the motor can handle. ive herd of cars that are 11:1 compression running a small supercharger. if youve already bought the pistons you can make them work but its gonna be a dog when not under boost. try to get a good size unit on it to supplement.

Exactly there are so many factors that play into how much boost you want to run its crazy. Cylinder heads design, cam design and timing, boost per rpm all play into it.

[VM0824]

hmm maybe ill add another spring to my wastegate when this is all done

[ryanw]

How many springs do they give you with the on3 kit ?

[VM0824]

4. they say the spring rates arent 100% dead on and to test em, but i believe they're supposed to be about 9, 5, 3, and 2. i think. i think they're meant to be compounded. as in, i can put the 5 inside the 9 and make 14lbs. they have different diameters to fit inside each other.

[gripthenboost]

With boost come responsibility. Just make sure your ass has enough fuel to ignite all the air. Is your motor down now? I remember u just did the on 3 kit.

[VM0824]

yea i blew my head gasket. when i tore it down i decided to have a machine shop do some work. the work i mentioned in my first post is what im doing now. the other head gasket didnt get along with the boost. although, looking back, im pretty sure it was leaking before the turbo and was just exaggerated by the boost.

[gripthenboost]

ooo.... kool you should be good to go soon then

[VM0824]

yea im waiting for the shop to get alot of the parts and then i get to try and put it all together lol

[ReverendDexter]

I'm surprised no one's mentioned fuel. There's a difference in what compression ratio you're gonna be able to run if you can only get/afford pump gas vs if you can run e85 or 100+ octane race gas.