Ordering Pistons TODAY - CR suggestions

[winstontj]

I want to order a set of pistons from JE today. Will be S14 84mm crank, stock valves, mild porting, stock M10 rods. Will use e30 318i EFI intake, sthal header and megasquirt (spark & fuel).

There's really not that much difference in cost between something 13:1-92mm and 9.5:1-89.5mm so not knowing exactly what will make this motor/combo happiest would anyone have a suggestion? I'm thinking 9.5 or 10:1 CR and staying at ~90mm bore (one or two oversize) depending on what JE has to offer, and price. I have a feeling I'll have to pay more for the oddball smaller size but less for machine work, less for the 92mm piston but more in machine work.

Would run poll but with bore and CR I didn't know how. Thanks!

[Guest]

Two things you should consider before you place your order....

Where are you going to get gas to feed your motor? Pump or blend? 10:1 C/R won't like most pump gas.

Has your machinist told you how far he has to overbore to get a nice cylinder surface? Don't order pistons before you KNOW.

mac.

[winstontj]

Two things you should consider before you place your order....

Where are you going to get gas to feed your motor? Pump or blend? 10:1 C/R won't like most pump gas.

Has your machinist told you how far he has to overbore to get a nice cylinder surface? Don't order pistons before you KNOW.

mac.

mac, that's EXACTLY what I was looking for. Machinist says I can just re-hone if needed. I could go with a standard one-over or as big as I want. It's all up to me. This motor had a blown head gasket and looks like the PO used orange silicone on both sides of the head gasket??? So the hash-marks are good. I'll only take bore into consideration with cost factor, like I said - more $ to machine out to 92mm but less $ for pistons as 92mm is more common and a bit cheaper, less $ for machine work = more money for the standard size pistons...

How's the fuel thing work? I have access to leaded race fuel @ $5/ U.S. Gal. but I'm not sure I want that. This is a DD car and even more important than horsepower and high compression is the ability to run it on regular or mid-grade pump gas. I don't even want to run premium pump gas... (maybe i'm dreaming though??)

I don't really know anything in this department. I know about motors and rebuilding but not about things like what CR requires what octane rating, etc. Any elaborations would be much appreciated.

[hpsenicka]

This is a DD car and even more important than horsepower and high compression is the ability to run it on regular or mid-grade pump gas. I don't even want to run premium pump gas... (maybe i'm dreaming though??)

I'm no expert, but I would expect that you are looking at premium (high octane) pump gas even with a 9.5:1 CR....

[boxy02]

I don't know much on these motors yet, but from my flat-6 experience, if you are at ~10:1 CR you need to run the highest octane pump gas that you find in your area (premium). If you're ~12:1 or higher you'll be running race gas only (100 octane, usually). The last thing you want to do is run regular (lowest octane gas) in a 10:1 motor. YMMV...(Your Mileage May Vary).

Good luck on your motor! I'm soaking up knowledge from your posts. Thanks for asking the questions and posting your concerns during your build, it helps us newbees to go forward with motor rebuild plans of our own!

Cheers,

[Mike Self]

premium, but I suspect I have too much advance on my timing. Gotta back it off a tad to see what happens...I certainly wouldn't go any higher for a daily driver; if I had the choice I would have gone with a 9.1, but I got a good deal on these.

BTW, I have a brand new set of standard size 9.5:1 piano tops for an E12 head that I don't need ('cause I overbored) that I'll sell at a good price...

cheers

mike

[peteinjp]

with my tii here in japan with 9.5:1 it pings with the advance set to spec. I think the gas here is probably about 90-91 oct in the states. Out there Sunoco used to have a good 94 octane. At any rate who knows what is going to be available. I guess you could bump the compression down with a thicker headgasket. Never done it myself though so take it for what its worth. Then again if you run the megasquirt for edis you might be able to compensate as needed in terms of the ignition advance.If with the stock dizzy...? How about asking about spark control and octane over on the megasquirt forums?

[hpsenicka]

... in a pinch, you could always add an octane booster additive to regular pump gas, but that would get tiresome (and expensive) fairly quickly for a DD.

[winstontj]

premium, but I suspect I have too much advance on my timing. Gotta back it off a tad to see what happens...I certainly wouldn't go any higher for a daily driver; if I had the choice I would have gone with a 9.1, but I got a good deal on these.

BTW, I have a brand new set of standard size 9.5:1 piano tops for an E12 head that I don't need ('cause I overbored) that I'll sell at a good price...

cheers

mike

Mike, depends on what pistons you have - I'll be running an s14 crank (84mm) which is +2 up/-2 down. I need a custom piston with relocated wrist pin due to the different stroke. If this is what you have then please email me!

thomasjwinston at yahoo dot com

[winstontj]

with my tii here in japan with 9.5:1 it pings with the advance set to spec. I think the gas here is probably about 90-91 oct in the states. Out there Sunoco used to have a good 94 octane. At any rate who knows what is going to be available. I guess you could bump the compression down with a thicker headgasket. Never done it myself though so take it for what its worth. Then again if you run the megasquirt for edis you might be able to compensate as needed in terms of the ignition advance.If with the stock dizzy...? How about asking about spark control and octane over on the megasquirt forums?

Will do ASAP as i will be going MS - also I'd imagine that JE will have some thought on the matter. I'll call them in a few and chat. Also your email bounced or I didn't hear back with an address to ship those inertia switches???

[Mike Self]

CR for the E12 head...

mike

[Pinepig]

When you buy them check these folks out

http://www.jrcmotorsports.com/

They had the best price I could find on JE pistons when I ordered the custom ones for my Corrado.

[winstontj]

Just spoke to TEP.... again I'd almost rather say nothing than say something bad - BUT... They lacked the customer service I was hoping for. Seemed like they couldn't be bothered to talk - just wanted a deposit for the $650 set of pistons, and litterally "they are custom. you tell us what you want" was the response when I asked about CR, bore, etc. I was a little taken back but this is 2/2 with this vendor so there goes that order.

Turned around and spoke to Jeff Ireland - he was busy you could tell but he knows his stuff. You could tell the wheels were turning... Jeff's suggestion was to go no higher then 10:1 (which was what I thought) and to stick with a 90mm bore because of gasket availability and price. Jeff's prices were also better but honestly I thought that the pistons would be more stock-ish and not as pricey. I didn't expect there to be one flat-rate cost regardless of bore - which leads me to the next thing - how much more would I get by boring from 90mm to 92mm? Will this mean I need a VAC MLS head gasket?

I've narrowed it down to 9.5 or 9.8:1 CR depending on what the MegaSquirt guys say.

Thoughts on bore anyone? Also anyone know of a good source for a decent set of pistons closer to the $500 mark? I'd really like to get the deposit paid for today/ASAP due to the 3-4 week wait and my pressing needs.

TIA,

TJW

[Jaycen_PNW2002]

In end your engine's compression ratio is going to depend on this. The shape of the dome on your pistons may also depend on this. Is the head you currently have rebuildable? What if it isn't and you go with another casting?

[Harv]

I don't think I'd go over 9.5 for a DD. And at that point, your going to be using at least 91oct. CR depends on many things and one of the biggest is combustion chamber design. Now the M10 has a decent combustion chamber design but it's not as good as the 911.

I'm going for 11.5:1 but I'm going to use a mix of 94 and avgas.

John