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Author Topic:   3.562 vs. 3.48 stroke , etc..
goinrcn44h
Member
posted September 06, 2004 07:47 PM
I have been thinkin on somethin that would work I suppose, but figure someone here will know for sure. currently a 3.48 stroke crank, if I were to switch to a 3.562 stroke that would be an increase of overall length of .082" then divide that by 2 would be .041"....
So if at present the deck is 9.005 and has the piston in the hole .005" it would end up at .036 ( best case scenario ) out of the cylinder. I doubt the ring would be at risk of comin out so realistically I could just go to a thicker gasket and accomplish my compression needs that way as long as my valve / piston clearances were good and the piston was the .035" from the head surface. so I'd need a gasket that is .065" to .070" and the world would keep spinnin and all would be happy if I figure all that correct...??? someone tell me if that a no-no, can -do , etc...
thanx 44H


Ego Racing
Member
posted September 06, 2004 08:56 PM
The overall stroke difference would be 0.164 not .082. The difference in the crank is .082. .082 on the top of the stroke and .082 at the bottom of the stroke. I if there was any streach or looseness in the package you would get catrostrophic results. By popping the piston out of the bore you would also be subjecting it to forces it was not desighned to take. The piston needs the support from the cylinder. That is just my two cents. We have had some engines where the piston came out .002-.010 and they never lasted.


outlawstock17
Member
posted September 07, 2004 04:45 AM
ego, you lost me...

44h, unless you're running a high dollar lightweight piston, you should be able to clip .036" off the top of the piston as long as it's a flat top. i've clipped .055" off hypereutectics lots of times. this will eliminate any intake misalignment. i do this when using long rod/stock piston combos. i like to get them to "zero deck" clearance or up to .005" out of the hole. i have run them as much as .010" out of the hole with the standard .042" thick gasket without problems. this is based on my experiences building ford engines but should apply to any other make as well. you should check piston to valve clearance very carefully.

forgot to add....you should mock up your assembly without clipping the pistons and measure EXACTLY how much of it is sticking out of the bore to determine how much to clip off the piston. every assembly is a little different. trust me, i've already slipped on that banana peel...

[This message has been edited by outlawstock17 (edited September 07, 2004).]

Ells949
Member
posted September 08, 2004 07:07 PM
JE/SRP 11 cc Dome forged . so I guess that is out of the question, besides at the compression these run and the fact I want to bump it up slightly I would like all I can get on top for stability... im at 13.8 or 13.9 at present, would like to see 14.5 or 14.6 just for a lil extra. but thats the least of my concerns at this point...
As for the math, I know the stroke diff. is .082 , you would then divide the diff by 2 half up, half down.... that leaves the .041 number. A stock deck is 9.025 which would be closer but still out slightly, so I am guessin I'd need the 1.22 comp. height pistons to make it work on my present block ( if it in fact isn't dead of course) if it is, well, thats just more hurdles...
44H


awkwardjeff
Member
posted September 09, 2004 09:02 AM
Is this the smae motor we are taling about on the other thread? If so, the point is mute it sounds like.....

One thing nobody has mentioned is the quench area, when the air/fuel mixture is fired it's like a bomb going off, idealy the piston should be down the bore ABOUT .008 that is open to debate.......but all good motor builders agree the piston needs to be down the bore. the deck of the block protects the head gasket when the piston is down the bore. The other thing is piston rock, with a dome piston when it fires the flame travle tries to rotate the piston in the bore.......this again is helped when you leave the piston down the bore a few thousands.

I currently run a motor that is 3.562 stroke, most cranks today are 3.550 stroke, you can use the same rod/ piston combination. and only change the deck .006 There is a little to be gained on stroke, but given a limited budget of not wanting to build a whole new motor I would look into a differnt cam. I don't know what you run for pumps but most pumps can be turn around to run a reverse rotation cam....that would keep you from buying new pumps... then look into some boat drag racing cams..... they seem to have a nice profile that works good in a 360 motor small injection motor.



outlawstock17
Member
posted September 09, 2004 10:59 AM
quote:
Originally posted by awkwardjeff:
One thing nobody has mentioned is the quench area, when the air/fuel mixture is fired it's like a bomb going off, idealy the piston should be down the bore ABOUT .008 that is open to debate.......but all good motor builders agree the piston needs to be down the bore. the deck of the block protects the head gasket when the piston is down the bore. The other thing is piston rock, with a dome piston when it fires the flame travle tries to rotate the piston in the bore.......this again is helped when you leave the piston down the bore a few thousands.


this is the role of timing. it has nothing to do with where the piston winds up at TDC. they all fire while the piston is down in the hole because advanced timing mandates this. a thicker gasket, in effect, raises the deck height.

i wouldn't go with the thicker gasket unless you can get a solid copper one that is thick enough. a thick composition gasket is going to cause problems with that kind of compression and you're probably going to have intake alignment issues either way.

[This message has been edited by outlawstock17 (edited September 09, 2004).]

awkwardjeff
Member
posted September 09, 2004 07:29 PM
[QUOTE]Originally posted by outlawstock17:
[B] this is the role of timing. it has nothing to do with where the piston winds up at TDC. they all fire while the piston is down in the hole because advanced timing mandates this. a thicker gasket, in effect, raises the deck height.

Not to argue with you or anyone else, This is the role of timing..... why do some motors use 32 degrees BTDC and some use 38?? you and I both know there are MANY reasons......but basically it called flame travel, the more effecient the flame travel the less timing needed...... the BEST example of this is with a locked advance.......if you lock the advance and turn on the ing. before spinning the motor it tries to push the pistons back down before the piston gets to the top, thus bucking against the starter. But when you have the advance plate working so the timing is retarded it starts easy and total advance is the same.......so whats the differance? we all know the flame travel is the same.what is different is timing and WHY, the time it takes from spark to pressure pushing on the top of the piston doesn't change what changes is the time it takes the piston to travel a certain distance. The starter turns the motor as say 800rpm so the time it takes to reach TDC from 36 degrees is longer then it take for the flame to travel across the piston and start pushing down..... this is the bucking against the starter... WITH ADVANCE PLATE WORKING say 12 degrees timing starts fine because piston reaches TDC before flame travel starts pushing down on piston.

With that said the most powerful part of the fire stroke accures with the piston at TDC.....not before. the spark fires before TDC.....but the flame travle doesn't try pushing the piston down until TDC....if it does you break parts, cranks, rods, bearings, main caps/block webbing.....

I totally agee with you about gasket use

[This message has been edited by awkwardjeff (edited September 09, 2004).]

[This message has been edited by awkwardjeff (edited September 09, 2004).]

goinrcn44h
Member
posted September 09, 2004 07:42 PM
It is the same engine I guess in a way, but the thing is I can have a 3.562 stroke crank, (43 lbs) for nothin that needs a grind to .010 and thats it, its already been magnafluxed and is fine that way and is also still straight. but thats why I am asking. I know what you can and can't do with 2-strokes, but not these as of yet entirely. I'd rather learn through someone elses knowledge than blow stuff up , cause I won't last long that way. I mean its no big deal to just get the proper piston which if the block is ok would be a 1.22 comp. height that is if they make a 1.22...? but I was just askin for reference if it ends up being somethin that just decided to fail and didn't destroy other things what I could do to get it together and run a few more races maybe.
it still all hinges on what I find when I peel it open anyway...
but thanx
44H


mavrik82
Member
posted September 11, 2004 07:17 AM
I've build alot of motor with pistons running up out of the bore, mostly drag motors, but also some circle track. Yes ideally your better off staying inside, but in the real world I've seen no problems. As far as piston rock goes, the slight amount coming out of the bore should make little difference and if you are rocking that much your clearance is way off. What you need to maintain is minimum of .050" clearance to the head and proper valve clearance. That high of compression you should use copper gaskets and o-ring the block or heads. SCE sell copper gaskets in many different thickness'.


Kromulous
Member
posted September 11, 2004 09:39 AM
If you can get a 43lb 3.5625 stroke crank for free i suggest building it. We run one currently built by an old school sprint car guy and that combo is awesome. 6.0" rod (small journal) and a domed piston 13.2 to 1 is the compression were running. 23* heads (Dart Pro 1's) and a mech roller. I wouldnt trade it for anything.


goinrcn44h
Member
posted September 12, 2004 05:36 PM
I have a guy about 3 miles from me that I ran around with his wife's brother in school. He does nothing but crank and head work at his dads shop which has been there 30+ years with his brother. He figured if it wasn't too bad , to recheck straightness to .001" straighten if nec. , grind to .010/.010, magnaflux, polish, the full rework on the crank would be under 200 bucks.
he even said its possible to offset grind and go from a large journal to a small journal. which I wouldnt do cause my rods are large but he can do it.
Finding a copper gasket shouldn't be too tough, there is even a place near here that makes them to whatever you want ( there is a pretty big drag strip in the valley but no dirt ovals...?? go figure) but finding a gasket that is proper shouldn't be too tough. If its a real good quality it should almost act as part of the block, as they can make them any size bore you want which I assume would help even if only slightly.I will have to have the heads slightly surfaced from the ridges forming from repeated torques and it will re-erify if they are good and flat as well.
I am gonna call Tim at schwanke tomorrow and give him the scenario and see what he says as well, he built this shortblock for me last year, besides I have to tell him the crank went T.U. anyway..... I also found the pistons I'd likely use if tim didn't have any, if it comes to that as well.
I don't claim or strive to know what happens where and how many degrees before or after anything, thats what those guys that make cams and such are for, I'd go bonks tryin to sort that stuff out anyway. I am used to way way less moving parts and am still amazed that they stay together as well as they do specially when ya turn them 8000 to 8300 rpm.. boggles the mind what stuff is doin in there. I likely don't want to know really...lol
anyway, I still have to go get the rig, anyone wanna go to washington and drive it to idaho for me???
44H


mikeymike0007
Member
posted September 13, 2004 09:28 PM
If you try to run the 3.562 stroke you may run into more problems then you know. With around 13.5-14:1 compression to start with the top ring is going to be exposed to some extreme heat, but what is worse is that, if you notice at the very top of the bore the cooling jackets do not come up this far. What you will encounter is that the rings will stop @ what is now new top dead center and will be out of the cooler area of the block. Also this is going to place the 2nd ring in the general region of were the top ring use to stop at. If you were dead set on running this combination I would strongly advice you to increase the ring gap of the top ring. In this hotter area of the bore it will be placed in, it's bound to getter hotter, and may evently **** together and freeze in the bore.


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