Compression - General Engine (Non-Gen Specific) - Firebird Nation

Jump to content is the premier Pontiac Firebird forum on the internet. Supporting Members do not see the above ads.
- - - - -


  • Please log in to reply
5 replies to this topic


    Firebird Mechanic

  • Validating
  • PipPipPipPipPipPip
  • 808 posts

Posted 15 October 2005 - 08:04 AM

So I was looking at buying a supercharger today (man, im going nuts with this car), and was thinkin to myself "hmm, i bet that changes compression a butt ton".

I know that I've heard in some places before you start adding blowers, you wanna upgrade pistons, rods, and crank. Well, I was thinking about how I know changing combustion chamber volume will affect compression, and the LT4 heads have a 54.4 cc volume, compared to the 54 cc of the LT1. That should adjust compression slightly, right?

So basically, A supercharger would probably be too much for the stock pistons, rods, n crank, RIGHT? I mean throwing 6 psi of boost would be ridiculous combined with the other stuff going on in that motor. I guess I'm really just curious if you think the stock pistons could handle it. I dont know when im going to come up with another 3k for a blower, but if I did I know that before that I'd have to up my suspension and stuff, but could the MOTOR handle it?

Just interested. I'm already in too deep on this LT4, but I want to know how a supercharger would change compression. Also, the supercharger is just a belt driven air pump essentially, right? K, im done :dance:

Thanks. :dance:

#2 FBN Firebird Nation

FBN Firebird Nation

    Pontiac Firebird Trans Am and Formula Fan

  • Administrator
  • 36186 posts
  • Gender:Male
  • First Name:Clark
  • City:Sparks
  • State or Province:NV
  • Country:USA
  • Year: 2002
  • Make: Transmission
  • Model: Engine
  • Engine: Engine
  • Transmission: car

Posted 15 October 2005 - 10:10 AM

Here is some info on the topic from powerdyne


    Firebird Newbie

  • Validating
  • Pip
  • 42 posts

Posted 21 October 2005 - 02:19 PM

There are programs out there that can calculate the compression ratio of the .4 cc difference between heads. I'm sure it would have some difference since if you have your heads milled at all you are advised to get slightly thicker head gaskets to compensate.

Most supercharger and turbo kits available off the shelf for any vehicle are low enough psi that a stock engine in good shape will hold up to them. Most of the time, changing the bottom end is only needed if the engine is in poor shape to begin with or if you are going to be throwing 10 psi boost or more into the engine. Once you get into that territory though, you've got some massive power going to the ground, to the point where you need to build up the rest of the car to withstand that kind of power.

And you are correct, a supercharger is basically a belt driven air pump.

I forget what firebird forum I found this through, but it very interesting and is related to the compression topic.


While I have a spare LT1 sitting around the garage, I went ahead and grooved the domes on my aluminum heads. I did not raise my compression as the website suggested, but I think the stock compression ratio and replacement gasket thickness are close enough to what he calls for to see some positive results.

If grooving the domes of your heads do give the suggested results, you would be able to run MORE boost from the supercharger without otherwise modifying the engine. More boost= more air= more power :beer: GRUNT GRUNT GRUNT



    1st and 3rd Gen Master Tech

  • Full Member
  • PipPipPipPipPipPipPip
  • 1460 posts
  • Gender:Male
  • First Name:Ray
  • City:Hacienda Heights
  • State or Province:CA
  • Country:USA
  • Interests:RACING!!!!!
  • Year: 1967
  • Make: Pontiac
  • Model: Firebird
  • Engine: 461 Pontiac
  • Transmission: TH400

Posted 21 October 2005 - 09:56 PM

Try this website.
Your Ferrari won't run with these Injuns!!!
1989 Formula 305 [email protected] mph (March 2005)
soon to be 409 SBC and faster than 15's!!!!!
UPDATE: New 409 SBC 5 speed initial passes w/
2.76 open diff---13.64 @ 105 mph
1967 Firebird [email protected] mph (September 2001)
soon to run with 461 and a tranny that won't grenade in a week!!!


    Firebird Mechanic

  • Validating
  • PipPipPipPipPipPip
  • 808 posts

Posted 22 October 2005 - 12:15 PM

Man, that stuff is trippy with the grooved heads... i wonder how much he spends on blunts.

Its an interesting thought...... I just dont know if they guy I'd have port my heads has heard of this.

I assume since its a "spare" LT1, you didnt test this out?



    Firebird Newbie

  • Validating
  • Pip
  • 42 posts

Posted 22 October 2005 - 09:28 PM

Spare LT1 means I am 80% through an engine swap after the LT1 in my 94 Firebird got some bad bearings and leaked 1 qrt of oil a week through the intake gasket. I bought a 94 LT1 police version from Caprice with 1/2 as many miles as on my TA, swapped all the parts I could. I decided to stick with the aluminum heads from the TA after inspection proved them to be in good shape, and at the same time decided to groove the aluminum heads. If they crack or anything else that means I need new heads, I have the iron heads sitting bolted to the old TA engine sitting on a cart in the garage waiting to be used.

0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users