compression 101

[PSU 98]

How would you raise the compression ratio in our cars (N/A; or any car, for that matter), and what results would you expect to see?

[Fin]

Decrease the head's cc's (cubic centimeter's), or use thinner gaskets. This will net you some power in itself.

Ex. A head w/a 64cc chamber will have lower
compression than a head w/58cc's.

[ September 06, 2001: Message edited by: Fin ]

[ September 06, 2001: Message edited by: Fin ]

[strange_trp]

you may also deck the heads, but you have to be careful about valve clearance when piston is at TDC. you'd need to use higher octane fuel to prevent detonation. the power increase you'd get would depend on how much you raised the compression ratio.

[Dominic]

Shave the heads/Mill the heads, or raise the pistons...

[PSU 98]

How high could you safely go with the 3800 series II? What kind of results would you expect to see?

[v6maro]

people have done it on the lt1/ls1 all the time, i have not seen that many on the 3.8L's it would be cool to see some dyno results of it tho
-Steve

[black98V6]

to safely do this to the 3800 II I am guessing you would proably have to go with some type of custom pistons to keep the valves from slamming into them.. If you were going to drive it on the street you couldn't go over 10.5 to 1 compression b/c I am pretty sure that is the limit of pump gas before you start to run into detonation. As far as gains no answer here besides it would all be theoritical, to venture a guess.. 30 rwhp tops.. ??

[V6Turbo]

To go really high, you'd need custom pistons. If your running premium, i think you could go up to 11.5.

[strange_trp]

you may have to remove jet module if doing this to prevent detonation. the jet kicks the timing WAY up and relies on the knock sensor to bring it back down when needed. with 10.5:1 and the jet module, you'd knock like hell unless you were on racing gas.

as far as piston clearance goes, i think you have some room to play with. i know this isn't an "interference" engine when stock, where the piston would strike the valve if the valve is open and the piston is at TDC... i'll have to look in the mechanical section of the helm to check on this later.

take the lift of the cam for the largest valve lift and subtract it from the distance to the piston at TDC to find clearance... i'll think about this today when i'm doing mindless brake work. (god i wish i could get paid as well as i am now for doing drivability stuff so i can think at work. but assembly line brake work pays the bills too well.... )

[jmeccia]

how can you measure compression?

im getting a set of 3.8 heads worked on. the guy i took it to is an old guy who's been doing it for years and he said he's going to shave about 10 off and try to increase the size of my intake valve slightly. the valve has to be custom though.
ill keep you guys posted.

oh, and if this is 30rwhp... thats awesome!

[strange_trp]

<BLOCKQUOTE>quote:</font><HR>Originally posted by jmeccia:
how can you measure compression?
<HR></BLOCKQUOTE>

compression gauge. available at sears or autozone...
compression ratio, on the other hand, is the ratio of the volume of cyl + comb chamber at bdc to the vol of cyl+comb chamber at tdc.

[MTMike]

<BLOCKQUOTE>quote:</font><HR>Originally posted by jmeccia:
how can you measure compression?
<HR></BLOCKQUOTE>

<BLOCKQUOTE>quote:</font><HR>Originally posted by strange_trp:
compression gauge. available at sears or autozone...
compression ratio, on the other hand, is the ratio of the volume of cyl + comb chamber at bdc to the vol of cyl+comb chamber at tdc.<HR></BLOCKQUOTE>


You can also do this by taking the reading that the pressure compression meter gives you, and then plugging it into a simple conversion formula.

Example: If it's 29.92 inHg that day, that equates to EXACTLY 1 ATM (14.7 PSI per ATM) of pressure, and if your engine produces 140 PSI in that cyl, that equates to 9.526 ATM, therefore giving you a comp. ratio of ~9.5:1

On most days, the barametric pressure is not significantly higher or lower to affect this reading's accuracy, but if you're really intent on getting it that accurate, you can calculate it as follows:

28.90 inHg
138 PSI Comp

28.90 inHg = .966 ATM
138 PSI = 9.39 ATM

Compression ratio is 9.39:.966. 9.39/.966 gives you 9.78:1 compression.

[ September 08, 2001: Message edited by: MTMike ]

[strange_trp]

good point mike... practical application of theoretical knowledge...

[2kblufbody]

speaking of compression....


(erased)[edited]Will I able to bump my compression more by doing forged internals w/ the Jet Module still bolt on while using premium pump gas?

All throttle no bottle sounds good to me...
:D ;)

[ September 08, 2001: Message edited by: 2kblufbody ]

[ September 08, 2001: Message edited by: 2kblufbody ]

[strange_trp]

the jet cannot increase compression. compression is a physical property of the engine, whereas the jet modifies electrical signals from engine control components.

i would not raise compression too much with the jet. i think you'd be okay with some thinner head gaskets (make sure they can handle the compression you'll be runnning) and the jet, but i wouldn't deck (mill) the heads or anything unless you wanna use racing gas or octane boost with 93 all the time.