gtred

OEM mazdaspeed decks out at -.005
JE decks out at -.020.
If you add in the gasket thickness, the JE (compression height = 1.205") are about .057-.060" quench. The Supertechs I have are a 1.201" compression height... which puts them down the hole even further. What is the compression height of the Supertechs you sell? One potential point of measurement error on my part could be that I used aftermarket rods. I would think that Carrillo would be spot on for rod length though. Still, a potential for error. The 1st pic is a 2004 msm with OEM internals. 2nd pic is JE 9:1 pistons and Carrillo rods and OEM crank. The deck heights are uncut/OEM.

Savington

iTrader: (31)

Again, can't speak for what JE does. I can only relay my direct experience with Supertechs.

gtred

If you found thsy the supertechs could be improved. would you be opposed to offering this as an option? Do you feel that closing the quench to .035- 040 would be beneficial to your builds?

Savington

iTrader: (31)

Your question makes absolutely no sense, because STs already quench out of the box at .040".

It's like asking if my builds would benefit from adding ARP studs or ACL bearings. I don't know how to respond, because the question itself is non-sensical.

TurboTim

iTrader: (9)

No, he's asking if you think .037 +/- .002 quench would be more beneficial in your builds than .040 +/- .002.

gtred

Tim: With a gasket, I currently have .060 piston to head clearance with the JE's (1.205" compression height). The Supertechs I have show a 1.201 compression height. How is it that I'd expect them to be any better fittment?

I think the question is: does tightening the quench area help a miata motor?

Savington

iTrader: (31)

You would expect that because a motor builder with direct personal experience told you so. You measured your JEs, got .060, that's unfortunate. I measure Supertechs all the time and get .040" give or take a few thou. Your inference based on spec sheets is not equitable to my direct personal experience.

If I sound agitated, it's because I don't like it when people confront my direct experience with conjecture based on specs they read off the internet

DNMakinson

iTrader: (3)

Read post 261. Assuming everyone measures the same way, we have actual measuements of piston to top of block of:
Stock = 0.005" Gteg
JE = 0.020" Gteg
ST = 0.040" Savington

Carry on

Savington

iTrader: (31)

Nope. Total of .040, including the gasket.

DNMakinson

iTrader: (3)

Thanks. Doesn't sound like one can improve on that, for sure.

Leafy

iTrader: (1)

I dont understand. If the JEs are 1.205 from the center of the wrist pin to the top of the piston and the STs are 1.201 from the center of the wrist pin to the top of of the piston how do the ST stick out further? Unless gtred has a motor with a very tall deck and even the ST would be shallow if he actually put them in the block and measured.

TurboTim

iTrader: (9)

Like Andrew implied, spec sheets don't always equal the real world.

Similarly, two items of the same part number, ordered at the same time,
shipped in the same box, may not actually be the same. Setting tool heights is sometimes tedious, or the QC person might be hungover, or china, or whatever.

Savington

iTrader: (31)

It's also worth mentioning that if he got .020" deck face to piston, that maths out to ~.052, not .060 (OEM gasket is .032"). You can get an .027" Cometic gasket, and you can also deck the block a bit if you want to set squish perfectly. The logic behind a piston MFG not setting the compression height dead-on is sound - if you set it dead on, and someone with a block that's had a few thou taken off already buys your slugs, they can't use your pistons. There's also variance in the block heights, enough so that you really want to be on the short side of things lest you end up with a bunch of tolerance stack and a piston that sits flush on the deck surface.

Wossner does this - their published compression height is 31.2mm, or 1.228". No DPE with Wossner slugs, but if I ever do play with them I will be damn sure to be careful about head gasket selection.

gtred

Yes, thank you for your discussion. I did end up buying the .027 Cometic gasket giving me approx .047" piston to head clearance... still not the .035-.040" I was hoping for. Also, buy moving from a .040 Cometic to a .027" Cometic I'd expect to be approx a half point higher than my target compression ratio... and then the problem of what to do with the extra .040" Cometic gasket that was left over from the original rebuild kit.

Seriously, I did not expect this post to devolve into pointed questions about any particular vendors pistons... I measured what I measured and it is just simple math. The nice thing about math is that it is true whether you believe in it or not.

Rather, I was just hoping to have a conversation as to what benefit, if any, a tight quench area would have in a Miata motor; particularly in reducing potential detonation.

Savington: I absolutely love the Trackspeed BBK that I bought from you. A very nice product.

Savington

iTrader: (31)

No harm, no foul. You can't always believe the specs you see for things like compression heights. It's also entirely possible that you ended up with a particularly tall block.

To answer your question, yes, there's probably some benefit to getting down from .050 or .060 to .040. How much benefit is difficult to say, and my guess would be that the gains are diminishing as you get closer to the "ideal". The published compression ratios are going to be inherent estimates, since block heights vary from block to block and chamber volumes vary from head to head. Remember also that the piston MFG has to contend with everything from the guy re-using his stock rods to the guy using Carrillo H-beams, as well as variances in how hard the end user is going to spin their motor, all of which is going to change the ideal clearance target. The squish target is just an approximation, much like ring gap - you are trying to get to a point at max RPM where the rod is stretching, crank is flexing, and the piston and head come very, very close to each other, but never actually touch.

If it's really something you care about, the proper process is to test-fit everything, take a measurement at TDC, and then deck the block to set squish. If you want to build a pointy-ended race motor, then that's the kind of stuff you have to fiddle and set yourself after thinking about the other moving parts as well as your specific application. For the vast majority of engines that are just being used on the street, I wouldn't worry about it.