Question regarding modifying compression ratio after the fact...
Thread Starter
Bench Racer
Joined: Feb 2003
Posts: 4,201
Likes: 15
From: Palm Beach Gardens FL
St. Jude '03 thru 14,'25
Question regarding modifying compression ratio after the fact...
Perhaps I should be asking this in the Forced Induction section but is there any way to bring the compession ratio down to the 9:x level from the 10.x level by modifying the heads alone? I guess what I'm asking is if one were to build a new engine to initially run naturally aspirated (but with a bottom end that could withstand supercharging) with the intent of supercharging it at a later date, do you have to replace the pistons to get the CR down to a safe enough level to run the supercharger or can you somehow modify just the heads to get the CR down to an acceptable level? Thanks....
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Re: Question regarding modifying compression ratio after the fact... (tobijohn)
A lot depends on how you build it initally. You can always use thicker head gaskets to lower the compression ratio, but that will screw up your carefully (hopefully) executed quench area, actually increasing the chances of detonation. The bast way to increase the volume of a combustion chamber is to unshroud the valves, out to the edges of the bore. For more, you may be able to remove material adjacent to the spark plug. Depending on how large an adjustment, you intend to make in the future, careful planning now, will make it easier, later. Good luck, and...
RACE ON!!!
RACE ON!!!
Racer
Joined: Jun 2000
Posts: 294
Likes: 0
From: WA
Re: Question regarding modifying compression ratio after the fact... (tobijohn)
You will need two sets of heads to do this. AFR heads for LTx motors can be purchased with 74 cc combustion chambers. You could easily switch between the AFR heads and your stock GM heads to make a change of approximately 2 points in compression ratio. Uing flat top pistons with 4 valve reliefs and 0 decked block will give approximately 11:1 with the GM heads and 9:1 with AFR 74 cc chamber heads. This makes it easy to switch between NA and supercharged. You really should use two different cams as well. The pistons and ring package should be set up for supercharging/nitrous - the ring lands should be located further from the piston top and ring gaps set wider. This won't be optimum for NA but is needed for a strong supercharged application.
Edit: This assumes 350 cu.in bored .030. If you don't care about switching back to NA you could build it with your stock heads and then have the chamber enlarged but you can only change compression ratio by about 1 point this way. Maybe start with 10.25:1 and then go down to 9.25:1. You might want slightly higher CR if going low boost (but why bother) or slightly lower if going very high boost, but the spread will still be about 1 point.
[Modified by torquejunky, 1:46 PM 2/3/2004]
Edit: This assumes 350 cu.in bored .030. If you don't care about switching back to NA you could build it with your stock heads and then have the chamber enlarged but you can only change compression ratio by about 1 point this way. Maybe start with 10.25:1 and then go down to 9.25:1. You might want slightly higher CR if going low boost (but why bother) or slightly lower if going very high boost, but the spread will still be about 1 point.
[Modified by torquejunky, 1:46 PM 2/3/2004]
Thread Starter
Bench Racer
Joined: Feb 2003
Posts: 4,201
Likes: 15
From: Palm Beach Gardens FL
St. Jude '03 thru 14,'25
Re: Question regarding modifying compression ratio after the fact... (torquejunky)
...The pistons and ring package should be set up for supercharging/nitrous - the ring lands should be located further from the piston top and ring gaps set wider. This won't be optimum for NA but is needed for a strong supercharged application.
Drifting
Joined: Feb 2003
Posts: 1,978
Likes: 2
From: San Antonio Texas
Re: Question regarding modifying compression ratio after the fact... (tobijohn)
I agree with EFI-CFI...unshrouding the valves in the chamber would be the best. He forgot to mention that this might also increase power power at the same time, due to increased air flow around the valves. I trust you already have forged pistons; if not, that would be the way to go. The type of pistons would depend on expected boost levels.
:chevy
:chevy
Thread Starter
Bench Racer
Joined: Feb 2003
Posts: 4,201
Likes: 15
From: Palm Beach Gardens FL
St. Jude '03 thru 14,'25
Re: Question regarding modifying compression ratio after the fact... (Curveit)
I agree with EFI-CFI...unshrouding the valves in the chamber would be the best. He forgot to mention that this might also increase power power at the same time, due to increased air flow around the valves. I trust you already have forged pistons; if not, that would be the way to go. The type of pistons would depend on expected boost levels.
Racer
Joined: Jun 2000
Posts: 294
Likes: 0
From: WA
Re: Question regarding modifying compression ratio after the fact... (tobijohn)
...
Thanks! When you say that this setup wouldn't be optimal for NA, where exactly would that show itself?
Thanks! When you say that this setup wouldn't be optimal for NA, where exactly would that show itself?
For the supercharger the rings need to be located further down the piston to reduce heat in the rings. The piston crown needs to be thicker to handle higher temperature and pressure. Ring gaps should be larger to compensate for increased expansion from higher temperatures.
The supercharger piston/ring will work fine normally aspirated as these are all fairly minor differences in regard to power. It would have slightly higher ring leakage when run normally aspirated and slightly higher friction.
