I'm currently rounding up parts for a streetable/track days ZL1 transplant into my '69. Got everything except the pistons. The stock bore (427) TRW pistons I'm looking at (with the 110 cc heads I have) yield either a 9.07 or a 10.83 compression ratio. I'd like to shoot for ten even, due to octane availablity, and engine durability on track days. I was thinking of getting the higher domed parts, and getting about 10cc (I haven't done the exact calculation yet) milled off the domes to get me to 10:1. How do most shops do the machining, and am I looking at a high cost to do this?

thanks,
Mike

 

Try using thicker head gaskets to refine the compression ratio. Chevrolet even had this in its owner's kit in the C2 cars for travel into areas without high test fuel. You could also use thinner gaskets to raise the lower CR piston upwards. Or go to JE pistons and get exactly what you need. Don't forget to have the assembly balanced.

 

I'd leave the 10.83 ratio alone. You can easily run that if you have a decent cam in it. Mine is 11.06 and races and dynos on 93 octane and no issues even on the street with TX heat.

But if you want to lower compression..you can cut domes easily on a mill. We recently knocked 10cc off a set of 13.0 pistons in a 427 motor. Cost $80 and then we rebalanced it all when done.

I'd shoot for a TIGHT quench. Wouldn't do the double gasket or thick gasket deal. My pistons are .028 from the head including gasket.


JIM

 

If you want to lower CR, cutting down the piston is the best way. The less dome a piston has, the better flame front you have.

Jay

 

Watch out for the thickness of the crown in the areas that you are going to mill. Some pistons are rather thin.

 

How were you able to get .028? From the information I have on the pistons, there is .020 just in the pistons, and another .039 in the composition head gasket I was going to use. What's your secret?

 

Deck the block and use a thin shim gasket.

 

Everything's aluminum, and the head gaskets I've seen listed for this application are between .038 and .051" thick.

 

Though fly cutting pistons for vlv clearance is a common practice I doubt u can find a shop in the local yellow pages. Maybe u just want to trim the pistons down flat a little on the top - i don't know. Depending on your area you may not have a shop that can fly cut the crowns or cut out the vlv reliefs if needed. Most reliable machine shops for performance don't even need to advertize and have all the work they can handle. Next time your at the track u need to ask around and get some references.

Then there is the issue - how do u measure the displacement of the domes after maching/cutting? Trimming a flat-top piston is easier but a dome is hard to measure displaced volume with. I don't know of any rigs or fixtures for measuring dome volumes above the deck.

As far as cost i have never needed piston machine work (so take this for what its worth) but i imagine it has to be less than the cost of custom pistons - what $1500?

My 2cents: don't run a thin gasket with alum heads and shoot for a deck height that brings the piston flat top within 0.035"- 0.055" of the head with a compressed gasket. Find a piston on the shelf for your desired c.r. or do a little chamber work on the heads (contour the chamber, unshroud the vlvs) to get it where u want it.: :

Good luck,
cardo0

 

My pistons are .009 OUT of the hole. I use a gasket that compresses to .037.

Getting them out of the hole cuts the crevice between the top ring and deck as well as adding compression.

Also need to measure a few gaskets.....the Felpro's are pretty accurate...but I've found that some compress a lot more than they advertise. I use Ferrea head gaskets.

We've had Cometic make us some .015 gaskets for some BBC's that worked well.


You need a shop who has the tools to measure crown/dome thickness. The shop I used had one of the old plexiglass *chamber plates* to CC pistons. But we had already measured them by dropping piston 1" below TDC and CC'ing it in the block. We double checked after cutting...dead on 10cc removed! Good guys!


Card00 is right...you can open chambers to cylinder bore and pick up a couple of CC's. you can use an Isky tool to cut valve notches...but again..I'd just leave the 10.83 and stick a good cam in it.

JIM

 

aluminum weights 2.7 grams per CC so removing 27 grams from the piston weight will remove 10 CC

 

I like that idea. It's quick and easy to monitor how much dome volume is taken off. Thanks.

 

that is the way all professional engine builders do it.

 

Won't this cause "Hot spots'???

 

I've never figured out why people dork around thinking of machining this and that. When you can buy the correct quality light weight parts.

Just call up Wiseco and tell them what you want. I had to buy a custom set of 10 for my 434 ci small block for use with 12 - 15 degree heads with big valves.

It is very easy to drive around with over 11/1 CR

I would also wonder about the wisdom of only building a 427 ci motor

 

It's a ZL-1 with a C4 suspension. (I don't subscribe to the "big displacement to try to compensate for big vehicle weight" philosophy.) I don't get outpulled on the straights that often, but with all that cast iron up front and the crappy C3 suspension geometry I was giving up some speed in the corners to some of the more well heeled types in their newer machines. The new engine weight reduction is about 230 pounds (160 pounds in the block, 70 in the heads), along with 115 pounds from the suspension conversion. My present goal is 3000 pounds with a car that looks stock at casual glance.

 

You are correct about weight and cornering speed having a direct relationship. Tires and alignment setting to get the most out of your tires have a big effect.

But road racing is a series of short drag races. Where HP per pound is king.

My boss at one time had the absolute record on every track configuration at our local track in his IMSA Lola Ford. The reason why is sub @3 pounds per hp.

That would be like 1200 hp in a 3000 pound vette

 

As I mentioned earlier, my goal is 3000 pounds, and whatever horsepower a moderately warmed-over 427 will put out.