This engine is in a car I am considering buying. Currently the compression is at 12.4:1. I want to drop it down significantly to handle a supercharger, probably down to 10.5:1 atleast. The problem is that this engine was just rebuilt several thousand miles ago (with quality components) and I really want to avoid another complete rebuild. It has the LT-4 Hot kit installed which includes the LT-4 heads. I can get gaskets that will drop the compression another point, so I am told. Does anyone have head suggestions that will drop this down a considerable amount? The chamber is 53cc which I would think I could do much better on, but I`m not sure if I can get the compression down far enough. Anyone have any head reccomendations? I have the stock LT-1 heads sitting here, but I don`t think much can be done with them. The pistons are flat top J&E. Thanks for any help.

 

For a blower, you are going to want your static compression to be less than ~9.5:1. That is not possible with a head gasket or even opening up the chambers.

In that situation it looks like you will have to go with large chamber AFR heads (~76cc), or you will have to swap out the pistons to a dished one.

That is a HUGE compression drop.....alot involved there.

HOpe this helps
Jason

 

If the heads are good you should sell them and get a set of large chamber AFR's, as Jason said. That would be the easiest, most cost effective way,

Rich Krause

 

What about the stock LT-1 heads that I have? I am trying to find the chamber size without luck so far, but I am pretty sure they are much higher than the LT-4 set.

 

Nevermind. I found they are just about the same. Looks like I`ll be spending alot more than I hoped.
I would like opinions though on wether it would be better to go with dished pistons and keep the smaller chamber size, or flat top pistons amd larger chamber size on an engine with forced induction. Also, any opinions on the cam selection would be welcome. I think the LT-4 cam should be ok, since the seperation and overlap isn`t that radical, correct?

 

Agreed, the simplest way to drop compression is with larger chamber AFRs. However, even with 74cc chambers and flat top pistons (say -4cc valve relief), static CR will still be upwards of 9.6:1. To go any lower, having a porter lay back the chambers a few cc's will drop compression to the sub 9.5:1 range.

What length rods in the motor? JE offers both -28cc and -31cc dished shelf pistons for 6" and 5.7" rods respectively, but that requires tearing down the motor and rebalancing the rotating assembly.

 

If you have this much compression now you basically have a flat top piston, stock chamber & the 1074 FelPro gaskets with approx. .010 deck height.

Going with the thicker gasket .051" & enlarging the chambers to 65cc (the most that you can practically enlarge a LT1/LT4 stock head to) will still result in 11:1 compression. Not suitable for much boost on pump fuel.

72cc heads are 10.2:1 & 76cc are 9.8:1

All are basically rather high for a max performance street blower motor. You really need to go lower (by changing pistons) or run low boost (approx 8#) to stay out of detonation. 10 to 10.25 is the reasonable limit for boosted applications unless you like race fuel.

 

I guess I should clarify that I will be running 9# at the very most (for now ) probably 8#. I will also be running 93 and not race fuel, since it will be driven on the street.