Adjusting Compression by Milling Piston.. ?

[IXjamesXI]

Anybody been there done that? I have a set of JE's that measure out about 42 cc of dome volume.. I'd like that number to be more like 40cc if possible. I guess the obvious answer is: order new pistons. But maybe I can avoid that since they would either have to be custom, or I would have to buy pistons for the wrong engine and adjust my rods to suit (i.e. JE makes a piston for the 3.2 rod which has the dome volume I need, but I would have to have work done to my SC rods to use them)

One thought is, just mill it off the top, area highlighted in green. But, the JE's seem rather thin at the top and I would not want to make it any thinner. Have not measured yet, but its not that thick, maybe only a few mms. What about taking material off from the edges around the valve reliefs, circled in red.. it seems like there is a lot of material around the valve reliefs just doing nothing.. seems safer than making the dome thinner. Thoughts?




I have total compression ratio paranoia now.. so I am afraid to run 10.5:1 twin plugs and I am thinking I want to be closer to 10:1 for safety margin. Perhaps that is totally unfounded. Am I a wuss?

[cgarr]

I took the sharp edges off around the valve reliefs regardless of my compression to remove hot spots.

[IXjamesXI]

Hi Craig.. if I recall from countless hours poring over every last post on this forum.. you have a pretty hot 3.4 at 10.5 to 1. Is that correct? What kind of cam and induction do you run, and what octane fuel do you run? Ever had any issues w detonation w/ your setup? Thanks!

[cgarr]

Yea its a 3.4, supercup cams stock intake, twin plugged with an andial splitter and 964 dizzy along with the SW chip. I checked the cylinder volume after assembled, or did one cylinder to get an accurate CR and it came in at about 10.4/1 running 93 octane fuel with no detonation. I think Steve leans towards the conservation side with the chip since he is going off the info I gave him and didn't have a dyno to do any tuning.

[snbush67]

How about adding some thicker cylinder base gaskets?

[IXjamesXI]

Definitely considering that too.. ain't going to be cutting or machining anything until I take some trial deck height measurements... I was blown away when I realized how much a difference a 10th of a mm can make to your CR when it comes to deck height.. about a 10th of a CR point. However, the range is narrow right? 1.25mm would be very sub-optimal. I have heard people say that 1.0-1.1mm (.040-.043) is really the acceptable range. Or less if you are a Swabian Super Hero Motor Builder.

[neilca]

I cut my J&E's from 12.5:1 to 10:1. The crown on my 86mm bore pistons started out at about 0.600 thick. I do not recall right now how much I removed but the crown never came close to my minimum of 0.250. The compression drops quickly as you remove more material so go slow, measure often.

[IXjamesXI]

Just measured.. Appx 7.3 mm thick at the thinnest part of the crown, or about 0.287 in. So not as much material to play with.

And after some crude measuring and math, maybe as little as .10 or .15 mm milled off the top might get me where i need to be. I will call JE perhaps and see if that will get me perilously thin or if its ok to do that. Can I use the specific gravity of aluminum to tell me how much I have taken off, or do I have to measure their volume several times as I machine them? Hmm.

[3literpwr]

Are there not countless people running 10.5:1 on pump gas? I'm currently building a 3.0 with 10.8:1 JE FSR's, DC60 cams and a stand alone...

[IXjamesXI]

I would hope so..! Wouldnt the world be a better place if everyone had the good taste to rip around in hot rod porsches. I just have read posts that scare me... In your case of a DC60 I think the "dangers" are less than with a lower overlap cam such as I plan to run.. Hence my worry. However my understanding of these principles is rather poor.

[Walt Fricke]

10.5 on pump gas with twin plug, and long duration cams, seems to work. Did for me, both at 5,000+ feet at home, and close to sea level when I could travel.

Not clear how far you can go above that. If you had EFI and knock sensing you trusted, you have more room for experimenting, playing with ignition advance, mixing 112 leaded with pump stuff, etc.

Otherwise, how would you know you were getting knock, or detonation, short of something bad happening? Detonation posts feature how quickly rings break or other troubles occur.

[snbush67]

Originally Posted by IXjamesXI

Definitely considering that too.. ain't going to be cutting or machining anything until I take some trial deck height measurements... I was blown away when I realized how much a difference a 10th of a mm can make to your CR when it comes to deck height.. about a 10th of a CR point. However, the range is narrow right? 1.25mm would be very sub-optimal. I have heard people say that 1.0-1.1mm (.040-.043) is really the acceptable range. Or less if you are a Swabian Super Hero Motor Builder.

I think that 1.25mm is on the edge of optimal but I would not classify it as a sub optimal rating for deck height.

Make sure you aren't confusing deck height with squish height.

You are going to get better performance with the tightest tolerance, by getting the piston as close as you can to the heads without touching the valves.

[JFairman]

If you're going to lower compression another option is have offset wrist pin bushings installed in your rods that lower the compression by lowering the piston a little at TDC.
I think most people use those bushings the other way to lengthen the rod CTC a little and raise compression around .3 of a point but you can go either way.
Ollie's in Arizona does it for around $250 last I checked and I think others do it too.

Also round off any sharp pointed edge areas of the valve pockets so they run a little lower temp and don't cause detonation. You can do that by hand with fine wet or dry sandpaper like 400 grit and then finishing with ultra fine wet or dry sand paper around 1200 grit or finer. Weigh the pistons on a scale while doing that so weight stays equal between them if you want.

If you make the crown or top of the piston thinner then it could become too thin to transfer heat to the outer edges and rings and the top of the piston could melt and burn through.

[neilca]

I am going to add this here because no one told me before I blew the heads off my race motor...
The reason you can run pump gas with high compression with twin plugs is because you don't need to run as much timing as a single plug! I was running 12.5:1 2.4 liter motor, twin plug, 30 degrees total advance, using Sunoco 110 octane fuel and detonated my engine blowing 5 of the 6 cylinders. I now run 10:1 and 18 total advance, yeah I am scared.

[IXjamesXI]

Shane - I am confused.. I thought that the larger my deck height measurement, the poorer my squish zone around the edge of the piston? Hence the reason for keeping the deck height at a minimum without creating interference or clearance problems with the head, valves, etc..

J Fairman - Reducing the length of the rod wouldn't really change much would it? It would be similar to adding base gaskets to reduce compression in my mind. If I reduced my rod length but made my deck "correct," then I would still have the same compression. If I reduced my rod length, and left the ensuing larger deck gap there, then yes, I think I would reduce my compression.

I am going to call JE today and see what they can tell me about my idea of skimming .15mm off the top. One would hope they build a little more meat into these things than needed so that engine builders can customize to some degree. At least I hope to find that to be the case.

[PFM]

James,

Are you trying to get to a place more like this?


If you have a good piston vice for your mill you could get closer. This was a piston done by CP to my spec.

Good luck.

[Walt Fricke]

Checking with J&E is certainly the sensible thing to do. But I, too, think there must be room there for this. It is the obvious way for the pistons to be made with the potential for a wide range of compression ratios. They build enough meat into these pistons so that the valve pockets can be made in various widths and depths.

I'm fascinated by the math one would need to calculate how much to cut to remove what volume of material. Where the cut widens at the valve pockets, that would be simple I think. But those rounded sides! Maybe computer modeling can deal with all that so you don't have to cut and try?

[304065]

Of course you have to measure it, that's not tough if you can mill.

I wouldn't cut the valve pockets- you want to cut the ridge at the top, making the piston as flat as possible, to make the flame propogation easier.

If it were my engine I would stick with 10,5 to 1. When you twin plugged, did you CC the heads to take account of the increase in combustion chamber volume as result of the second plug? That will lower CR.

[IXjamesXI]

Originally Posted by 304065

When you twin plugged, did you CC the heads to take account of the increase in combustion chamber volume as result of the second plug? That will lower CR.

I did! I thought I was sooo smart! I measured them ahead of time.. 90cc, just like they should be for a stock SC head. I was really banking on picking up another cc or two.. but of course I did not realize that what cutting another plug hole giveth, flycutting taketh away. My heads were flycut I think 0.010 in.. which was enough to make the net volume of the head 89.5cc.. less than before the twin plug!

I guess I won't sweat it too much.. I think I can get 93 octane almost every place I go, just don't want to have no regrets.

[dbanazek]

You can start with a .25mm thicker base gasket to get what was taken away by the flycut. That will get the chain lengths back to stock.