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Old 08-03-2008, 08:28 PM   #1
Kosmic
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Default Min. clearances between.....

Help needed: I would like to know the minimum clearances between piston tops and valves. Engine will spin to 8K rpm max. (WRX heads).

I will shave my piston tops to get the max CR I can get. I'm at .013" over the deck atm with 5.180" K1 rods and Wiseco pistons.

Effective quench is important also.

Thx

Kos

Last edited by Kosmic; 08-03-2008 at 08:34 PM.
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Old 08-06-2008, 09:15 AM   #2
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Anyone?

What min. clearance I need between piston top and valves to keep it from hitting. BC cam lift 0.400"

Thx
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Old 08-06-2008, 09:23 AM   #3
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you're running a 1.65:1 rod ratio right?
I'd .032" is what I will target (.028-0.034 are usually pretty acceptable). Leaves room for some stretch. Rotating mass and force (engine speed and rod ratio) are your big factors.

Since you are stroker 2.35 IIRC you are basically a EJ25 with a lighter piston and can stand to be a little tighter. You also shouldn't have an issue with the cam you are using a set that was made for that rod ratio and has the appropriate LCA.

You will still want to clay the engine for clearance when doing changes like this, oversized valves and cams. Better to make sure before you put it in the car

Last edited by Homemade WRX; 08-06-2008 at 09:32 AM.
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Old 08-06-2008, 10:38 PM   #4
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Thanks Micah. I had 0.030" in my head from a read I did a few months back that I can't find anymore. I will clay for sure.

Rod ratio of 1.67 to be exact. (5.180/3.11) and yep a 2.35l stroker.

I will have to take coating into account in my calculation. Gold ceramic on the piston tops and thermal barrier coating on the valve faces (0.002" to 0.003").

My Cometic HG are 0.027" thick. That's the thickness once the heads are torqued right?

Do you have a number for the quench? again I 've read something about this last year....can't find it.

BTW, I will receive my Wiseco (113g) piston rist pins DLC coated in about two weeks. I will post pictures in my DLC coating thread.

Next will be the cams and bucket contact surfaces, DLC coated.
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Old 09-08-2008, 04:21 PM   #5
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Default k there, vave clearance

i there, i m new to this place, and am just having trouble finding the valve clearance measurment for my impreza 2.0 wrx sti 1997....

can u help, many thanks
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Old 08-07-2008, 12:39 AM   #6
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what thickness gasket did you order from cometic? do you have the part number?

you are going to need to mic your deck height too if you wanna solve this on paper.
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Old 08-07-2008, 09:28 AM   #7
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Are you talking about how much the piston is coming out of the cylindre?

From Wiseco (click here) with a 5.181" rod and their K602M975 piston kit the piston will be +0.014" In my case, with a 5.180" rod I guess I'm at +0.013" (over the deck) atm.

I will post Cometic 0.027" HG P/N once I get home.
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Old 08-07-2008, 12:53 PM   #8
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with the 3.110" stroke (1.555" rad)
1.191" compression height
5.180" rod lenght
7.913" deck height

so you have 7.926" of components witha deck height of 7.913 (before decking)...that puts you .013" out of the hole...
.027" gasket will leave you .014" quench height before considering how much you decked the block.

so you'll want a thicker gasket and you need to find out what your actual deck height is.
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Old 08-07-2008, 01:22 PM   #9
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I wouldn't run the piston closer than .035. In some class racing engines I have built in the past you get the piston to "witness" the head even at .035. No two engine parts are exactly the same so if you're getting things that close you need to treat the engine as if each cylinder were a separate engine. As far as piston to valve, you need a margin for error at some point in the engines life. I wouldn't run closer than .060 intake and .080 exhaust... Most builders like to see over .100 because of driver error. Unless it's a auto car and your keep a close eye on it race to race I wouldn't build it that close to crashing itself.

Last edited by AndyDick; 08-07-2008 at 01:42 PM.
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Old 08-07-2008, 01:57 PM   #10
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stupid slow work server...
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Old 08-07-2008, 01:58 PM   #11
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Quote:
Originally Posted by AndyDick View Post
I wouldn't run the piston closer than .035. In some class racing engines I have built in the past you get the piston to "witness" the head even at .035.
what type of engine as I'm quite curious?...also what rpm?
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Old 08-08-2008, 09:28 AM   #12
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Small block fords-4"bore 8.5 max rpm-steel rod-.022 rod bearing clearance-.006 PTW
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Old 08-07-2008, 09:16 PM   #13
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Quote:
Originally Posted by Homemade WRX View Post
with the 3.110" stroke (1.555" rad)
1.191" compression height
5.180" rod lenght
7.913" deck height

so you have 7.926" of components witha deck height of 7.913 (before decking)...that puts you .013" out of the hole...
.027" gasket will leave you .014" quench height before considering how much you decked the block.

so you'll want a thicker gasket and you need to find out what your actual deck height is.
Ok, I get it.

The block never got pulled out of the car before my friend gave it to me, the decks look free of scratches. So I guess the decks re-surfacing will be minimum.

If it's possible, I want to shave the piston tops instead of using a thicker HG.
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Old 08-07-2008, 09:22 PM   #14
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What's the targeted quench height?
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Old 08-08-2008, 12:37 AM   #15
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well that would be your call...I'd aim for 0.032" on your setup if you plan to spin high rpms. of course that would mean a minimum of .018" off the top of piston plus whatever you deck from the block.
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Old 08-08-2008, 01:19 PM   #16
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I think you missed a zero in the rod clearance
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Old 08-08-2008, 01:32 PM   #17
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0.014 is bit low for quench height, but you don't need more than 0.03" to be safe!
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Old 08-08-2008, 07:44 PM   #18
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I'm guessing that is because the lighter valves in a subie don't float as far as the two valve motors because I though I read were .100 was the standard on open valves to piston.

I'm sure every motor will see some driver error. going to 5 and getting 3rd at the top of 4 at 7500 is at least 8800
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Old 08-08-2008, 11:41 PM   #19
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It depends a bit on which engine and which valve you are talking about. You can generally run less clearance on the intake side than on the exhaust side.

When the intake valve makes closest approach to the piston the piston is moving away from the valve and the valve is trying to catch it. The intake valve is under acceleration so there is no slack in the valve train. On the domestic V-8's they typically run 0.050 as a safe side clearance, but can go much lower than that if they trust the build. One rule of thumb is on the intake side run .005 - 0.010 more clearance on the valve to piston that you have between the piston and cylinder head. If you do that the piston will smack the head before it tags the intake valve.

On the exhaust you need more clearance since the Piston is coming up the bore as the exhaust valve is closing, and over taking it at closest approach. If the exhaust valve floats at all it is more extended when the piston gets to its closest approach than under static conditions, and the piston will catch up to it and smack it closed. Also if the exhaust valve bounces off the seat due to an over rev and loss of control by the valve spring it can bounce back as much as 1/3 the max lift.

On the 2 valve heads that means absolute minimum clearance on the exhaust/piston distance of 0.065 - 0.070 if things are stable and 0.090 - 0.100 clearance if you want a good safe build in case you buzz the engine on a down shift etc.

I would be inclined to safe valve clearances since it gets very expensive very fast if you tag them. It simply is not worth the risk and you gain very little increased compression by running tight and you lose very little compression if you deepen a valve pocket to make it safe. The actual point of closest approach is not the whole valve but usually the lower edge of the valve and the edge of the valve relief cutaway on the piston. You can only determine where that point is by using clay and mocking up the engine build and doing some measurements. Once you find the crankshaft angle where closest approach is, then you can set the crank there, and push the valve down until it pushes through the clay and makes contact with the piston. That will give you a witness mark showing the actual point of most likely contact. Sometimes all you need to do to greatly increase the piston valve clearance is to slightly move or enlarge the valve relief cutout in the piston not change the deck clearance of the whole piston.

On our engines we have a large bore (over square) which increases potential for piston rock at TDC and with an OHC configuration you have potential for belt stretch that causes the cam center line to move about a bit under rapidly changing engine rpm conditions. Don't pay much attention to piston to valve clearance at max valve lift as the piston is not near the valves at that time. Closest approach between piston and Intake valve is typically around 10 degrees ATDC. Rule of thumb closest approach between the piston and valves occur at about 8-12 degrees BTDC for exhaust and ATDC for intake. If you plan on using adjustable cam gears better give yourself a bit more piston/valve clearance as your final cam timing may not match the cam locations when you measure your clearances.


Larry

Last edited by hotrod; 08-10-2008 at 01:05 AM.
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Old 08-09-2008, 12:47 PM   #20
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Interesting....thanks for the infos.
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Old 09-08-2008, 05:57 PM   #21
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What type of DLC coating, if I might inquire? Is it a true ta-C DLC? Pure SP3 bond structure?
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Old 09-08-2008, 11:52 PM   #22
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Hi SaabTuner,

I asked the seller at Morgan Advanced Ceramics - Diamonex products and he told me that it was true DLC.

http://www.diamonex.com/

Here's the pictures of my rist pins DLC coated, end of thread.

http://forums.nasioc.com/forums/show...&highlight=dlc

I'll be looking next to DLC coat the cam journals and lobes, buckets. Maybe.

What's expensive is the fixture to hold the part inside the DLC coating unit. I've been quote 1000$ for the crank fixture. I'll ask if I can fab my own fixtures.
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Old 09-10-2008, 03:47 AM   #23
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Excellent! Those look very nice. =)

hah It's funny. I leave NASIOC for a while, and when I start coming back more frequently, so many of the ideas I suggested quite some time ago, but didn't have the money to test, are being tried by others.

I guess I really should be "in the 'biz", since I seem to have some amount of foresight into what things will, at least eventually, be worth testing, or be successful. But, then, my doctors often say that and want me to practice medicine.

Ahh well, I just hope it works out really well for you. Anyone who knows me well will know that I make off-the-wall suggestions, but I'm also careful to state that I'm not claiming knowledge about if they work- merely asserting that they are worth trying, and that there's a high probability that they will work. Very different.

Make sure to keep us all posted on how well it revs!

-Adrian
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Old 09-10-2008, 06:30 PM   #24
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Quote:
Originally Posted by SaabTuner View Post
Excellent! Those look very nice. =)

hah It's funny. I leave NASIOC for a while, and when I start coming back more frequently, so many of the ideas I suggested quite some time ago, but didn't have the money to test, are being tried by others.

I guess I really should be "in the 'biz", since I seem to have some amount of foresight into what things will, at least eventually, be worth testing, or be successful. But, then, my doctors often say that and want me to practice medicine.

Ahh well, I just hope it works out really well for you. Anyone who knows me well will know that I make off-the-wall suggestions, but I'm also careful to state that I'm not claiming knowledge about if they work- merely asserting that they are worth trying, and that there's a high probability that they will work. Very different.

Make sure to keep us all posted on how well it revs!

-Adrian
Will do.

I do read and listen to a few NASIOC members (observations/theories/hints) and you're one of them.
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Old 09-11-2008, 03:41 AM   #25
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Quote:
Originally Posted by Kosmic View Post
Will do.

I do read and listen to a few NASIOC members (observations/theories/hints) and you're one of them.
Thanks. It's nice to know people are at least paying attention.

I try to put as much thought as possible into my posts, as well as information, to help further the advancement of these engines. I do sometimes feel like it's wasted effort, though.

Thanks for the reminder that it's not always just an academic exercise.

-Adrian

p.s. It's a shame we can't ta-C DLC coat the cylinder bores and piston skirts. Because most other coatings are too soft, and wear off, if the DLC coating stays on, it could seriously reduce friction there. That might free up a lot of power.

Also, don't forget the gears in your transmission. Those are prime targets for DLC coatings, as are the bearings, if they can be removed. Gear friction is a major source of transmission losses, and gear/bearing wear is usually the long-term tranny killer, if outright power doesn't kill it first.
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