could I also skip rebalancing?

 

Another question is do I have to pull the motor to replace the pistons?  I pulled the motor twice when I did the T5 install (which is now a rather shocking 6 years ago), but I still have the hoist and engine stand in storage. So I can pull it again if necessary. I am sure its not reccomended practice, but the truth is I would like to make the piston swap in the car to reduce workload and downtime, if possible.

 

I am still a little leary of running 9.5:1 compression with iron heads, but stock piston choices are either 41cc or 27/28cc.  So, I guess I am going to go with 28 cc pistions unless someone knows of an off-the-shelf brand with a dish somewhere between the two.  After talking to a well respected local machinist, I have come to realize that using off-the-shelf parts keeps costs low whereas machine shop charges for custom work really boosts costs.

 

Thanks,

Dave

 

Can you do pistons in the car? yes you can but it will take you longer to do so. You would have everyting undone but the bell housing because you have to raise the motor, drop the crossmember just to get the oil pan off. You would also have to lean over the fenders to get the heavy heads on & off with the motor in the car too. Also would need to guide the rods and piston in the holes by your self from the top. In my book easier to pull motor and mount to the stand to do all the work and if you balance it has to come out any way!

Dave ----

It will run fine with 9.5:1 with the newer style flat top pistons.  I run that compression ratio and even higher on all my cars running on Phoenix area's 91 octane premium grade panther piss oxygenated "city blend" gas and never have an issue with any of the car's pinging at all.

 

 

I disagree, and have the documents that support my thoughts.

If you have a 9 or 9.5. to 1 with proper quench distance, then drop to say 8.7 but don't have good quench, you can ping worse than the 9.5 to 1.

 

"Retarding the timing a little" won't help with a piston down in the hole .050 - that's HUGE, by the time you add .035" of the head gasket. It's asking for trouble.  Even with compression as low as 8.7 or so - she can ping badly - and "retarding the timing" will do little but make it a really poor performer.

If you take the same engine and simply drop the piston down to reduce compression, it may well have detonation issues. You can actually make things worse by dropping the piston to drop compression.

Been through all this before - looks as if I need to post those documents again.

Basically, you offset the drop in compression with the loss of quench and on these old slower burn chambers will little swirl, you are asking for trouble.

 

If you change piston types, you'll be changing weight, get it balanged. I went with the step dish pistons on mine and the amoung of metal removed from the crank was amazing.

If you change bore, you change piston weight, if you change piston design, material, etc. - you change the weight. Get it balanced. It's CHEAP compared to pulling it down again, and/or finding broken parts, or dealing with weird vibrations.

The gentleman stated he is going to just add a cam and different heads so no need to balance anything. 

 

There is an alternative that may be easier still and that is to use a set of Harland Sharp bridged roller rockers if what you described is all you are going to do to the engine.  They will handle anything you want to throw at them with your combination and are adjustable to accomodate the new cam base circle difference.  The big benefit is you won't have to remove the heads rather you can just bolt them on, and get new pushrods that are the correct length for the new cam and rocker combination and save youselft from having to remove the heads altogether. 

 

The rockers are the ones called pedestal about halfway down the page at   http://www.harlandsharp.com/products.htm" rel="nofollow - http://www.harlandsharp.com/products.htm  and are priced at $488 from their online store

 

 

 

 The benefit of a quench pad on an otherwise dished piston is the cloose proximity of the pad to the head forces air across the chamber. This serves to cool the combustion chamber, and the clooser the piston comes to the head the better. Yes, some have attempted to eliminate detonation "ping" by using thicker or double head gaskets only to make it worse.

 

 Engine ballance: People get rediculous about balancing engines. Would I use a flywheel or damper that was way off, no. Would I rebalance a rotating assembly due to replacement pistion's weight differing from those being replaced, no. If the new pistons are considerably heavier I might look for a way to reduce that some. Otherwise I would just not worry about it. Some people overbalance, some underbalance, besides no additional power is gained no matter what.

 

 SKeown 

 Lighter would be no problem at all, as a matter of fact, that would be a blessing. Were talking recipricating not rotating, besides how does one factor in friction. Like I've said, people get crazy about this subject. I saw a Pontiac engine that was as smooth as silk, but had one wristpin that was 21 grams heavier than the others.  An engine's ability to rev is not attributed to precise balancing one way or the other. Yes, it's good to get the bob weights equal and balance the crank/ flywheel & damper to that, but it's only going to be exact at a specific RPM range anyway. Lighter reciprocating weight is conducive to quicker reving and sustained engine life.

 

 SKeown