Is there an ideal piston compression height?

[Squid4life]

Done a search here and on google for info on compression height but have not found much info. I am trying to figure out what an "ideal" CH would be? I am trying to figure out what stroke/rod length I want to run on this stage build.

-I have 153 on center block which is rough bored right now to 3.995
-Deck height is 9.480 (.040 shorter than a typical 109, right?)
-Will be running stage 2 heads
-Would like to make it a stroker (3.59 to 3.625ish)
-Longer rod is typically better.
-Read about wrist pin in the ring land not being the best setup, so thinking it should be a little long on the CH if possible. (Stock is 1.8125 if I read right)

So lets say I find a 3.59" crank, and a set of 6.300" rods.
3.59 x 1/2 = 1.795 + 6.3 = 8.095
9.480 - 8.095 = 1.385" CH

Or,
3.659 x 1/2 = 1.8125 + 6.3 = 8.1125
9.480 - 8.1125 = 1.3675" CH

Is a CH of ~1.38" safe? Anything else to take into consideration that I am missing?

Any guidance is greatly appreciated.

 

[turbofabricator]

I build alot of engines with the wrist pin in the oil ring groove with no issues at all. The shorter the CH the better, as long as you can keep the top groove below the piston dish. If the top goove is at or above the piston dish, the will crack easily at the valve reliefs. The ring pack is critical to keep the heat away from the top ring as much as possible. That being said, the closer to the top the ring land is, the less end gasses that remain. It is an exersize in compromises. Longer rods will (in theroy) produce beter toque and laess thrust loads, but there are alot of serious power engines that have hideous rod/stroke ratio's. Look at some of the mountain motors with 880+cid, they should push the piston through the side of the cylinder wall. There are alot of crank,rod, piston kits avail. Contact DLS, TA, or others for a proven combo. Alot less stress that way.

 

[bison]

Done a search here and on google for info on compression height but have not found much info. I am trying to figure out what an "ideal" CH would be? I am trying to figure out what stroke/rod length I want to run on this stage build.

-I have 153 on center block which is rough bored right now to 3.995
-Deck height is 9.480 (.040 shorter than a typical 109, right?)
-Will be running stage 2 heads
-Would like to make it a stroker (3.59 to 3.625ish)
-Longer rod is typically better.
-Read about wrist pin in the ring land not being the best setup, so thinking it should be a little long on the CH if possible. (Stock is 1.8125 if I read right)

So lets say I find a 3.59" crank, and a set of 6.300" rods.
3.59 x 1/2 = 1.795 + 6.3 = 8.095
9.480 - 8.095 = 1.385" CH

Or,
3.659 x 1/2 = 1.8125 + 6.3 = 8.1125
9.480 - 8.1125 = 1.3675" CH

Is a CH of ~1.38" safe? Anything else to take into consideration that I am missing?

Any guidance is greatly appreciated.

You should easily be able to get a good ring location with those rods. Some guys are running 6.5" rods and 3.625" stroke with no problems though id rather run the 6.3" rod with that stroke.

 

[Squid4life]

Good info guys, much appreciated. Glad to hear you have had luck with the pin in the oil ring as well. Will continue doing research and comparing combos and see what might be best. Will prob put in a call to JE to see what they think as well, since I will most likely order a set from them.

Thanks again!

 

[charlief1]

I build alot of engines with the wrist pin in the oil ring groove with no issues at all. The shorter the CH the better, as long as you can keep the top groove below the piston dish. If the top groove is at or above the piston dish, the will crack easily at the valve reliefs. The ring pack is critical to keep the heat away from the top ring as much as possible. That being said, the closer to the top the ring land is, the less end gasses that remain. It is an exercise in compromises. Longer rods will (in theory) produce better toque and less thrust loads, but there are a lot of serious power engines that have hideous rod/stroke ratio's. Look at some of the mountain motors with 880+cid, they should push the piston through the side of the cylinder wall. There are a lot of crank,rod, piston kits avail. Contact DLS, TA, or others for a proven combo. A lot less stress that way.

Thanks Ken. I was planning on a pin height of 1.12 and was a little nervous about it but it sounds like it should work pretty good.

 

[Squid4life]

So, updating this a little and asking for any further input...
-I have 153 on center block which is rough bored right now to 3.995
-Deck height is 9.480
-Picked up a couple cranks, both 3.625" stroke


Looking at a set of nice Crower Titanium rods at 6.500". Not what I was initially looking for, as I wanted to keep it ~6.3", but they are a good deal where I am hoping to do some horsetrading. May even come out with pistons in the deal, but need to verify they are ok for this.

3.625 x 1/2 = 1.8125 + 6.5 = 8.3125
9.480 = 8.3125 = 1.1675" CH

If I were going to stay with the 6.3" rods:

3.625 x 1/2 = 1.8125 + 6.3 = 8.1125
9.480 - 8.1125 = 1.3675" CH


I know 6.3" may be a better designed package, especially if the car sees street/ highway time, but will the 6.5" be too far off? Will it be a ticking time bomb? I see a lot of builds with pistons in the 1.1" range, but you never hear about street-ability or durability in the long run... If I go with the 6.5" rods, I can pick up the rods and possibly pistons for about the same cost as new Crower billet 6.3's. If I go 6.3's, I still need to shell out $ for pistons. I do most of my car stuff with side jobs or trades, and not too much out of pocket, so $ is important, and I am getting anxious to get this car together.

What do you guys think? Go 6.5's and give her hell, or slow my roll and save more $ for 6.3s and pistons???

Any guidance is greatly appreciated.

 

[charlief1]

If you plan on driving it on the street you'll be better off with a pin height between the 1.2 to 1.5" for longevity.

 

[turbobitt]

So, updating this a little and asking for any further input...
-I have 153 on center block which is rough bored right now to 3.995
-Deck height is 9.480
-Picked up a couple cranks, both 3.625" stroke


Looking at a set of nice Crower Titanium rods at 6.500". Not what I was initially looking for, as I wanted to keep it ~6.3", but they are a good deal where I am hoping to do some horsetrading. May even come out with pistons in the deal, but need to verify they are ok for this.

3.625 x 1/2 = 1.8125 + 6.5 = 8.3125
9.480 = 8.3125 = 1.1675" CH

If I were going to stay with the 6.3" rods:

3.625 x 1/2 = 1.8125 + 6.3 = 8.1125
9.480 - 8.1125 = 1.3675" CH


I know 6.3" may be a better designed package, especially if the car sees street/ highway time, but will the 6.5" be too far off? Will it be a ticking time bomb? I see a lot of builds with pistons in the 1.1" range, but you never hear about street-ability or durability in the long run... If I go with the 6.5" rods, I can pick up the rods and possibly pistons for about the same cost as new Crower billet 6.3's. If I go 6.3's, I still need to shell out $ for pistons. I do most of my car stuff with side jobs or trades, and not too much out of pocket, so $ is important, and I am getting anxious to get this car together.

What do you guys think? Go 6.5's and give her hell, or slow my roll and save more $ for 6.3s and pistons???

Any guidance is greatly appreciated.

I looked back at some of my notes and it looks like I have a 1.210 CH with 6.5 crower Ti rods and 3.59 stroke. I used these rods simply because they were available cheap. If I had to do it from scratch I would go with 6.3 rod.
Edit - re-reading my notes and it looks like this number takes into account for the piston .021 in the hole and 9.534 deck height.

Allan G.

 

[750H.P.V6]

James,

As much as I'd like to make a deal with you on the Ti rods if your block is as short as you say I don't think it will all fit. I think you need a shorter rod.

Allan,

I didn't know you ran Ti rods. I don't know too many people that do. I'm putting them in my new engine if I can ever get my machinist to do any work on my stuff.

Neal

 

[Squid4life]

Back to the top with some new parts and new measurements...

I picked up another block with a 9.512" deck height
I picked up another crank, so I now have a 3.625" and 3.59"
I have 6.5" Ti and 6.5" Carrillo HD's
I have new JE pistons that have a CH of 1.2035"

Now, doing the measurements to see where the piston would sit, the two cranks don't change the equation too much, and quench comes into question:

3.59 crank: 9.512 - 6.5 - 1.795 = 1.217 CH so pistons sit ~.0135 in the hole
3.625 crank: 9.512 - 6.5 - 1.8125 = 1.1995 CH so pistons are .004 out of the hole

So, I think the stroke to rod ratio is safer with the 3.59" crank, and the .0135 in the hole gives a little safety margin (lower compression). But, take into account the head gasket thickness, is the 3.59 still the winner, or is having the pistons closer to the top better? 1026's compress to .040, correct?

I am thinking the 3.59" is still the smart choice, even if the compression might suffer a little, but what do you guys think?

 

[bison]

.013 in the hole is no problem. .010 is typically what I like. Leaves you multiple decking opportunities if needed.

 

[fastblackracing]

My vote would go for the 6.300 rods, in addition to whats allready been mentioned here some of the engines I have done with 3.59 and 3.625 cranks with the 6.500 rods ran into clearance issues between the small end of the rod and the bottom of the piston dish. This will depend on how thick the small end of the rod is and how big the dish is. If your heads end up smaller on the cc and you have to make the dish bigger its more of an issue. I had to have the bottom of the dish cut a bit so the rod could pivot in there with some clearance. Not ideal to cut on the bottom of the dish but it did not cause me any issues either. It gets real tight up there with 6.5 rods, a stroker crank, a dished piston and shorter deck heights. you can use the 6.5 rods but theres not much room to work with. Make sense?

 

[Squid4life]

Makes perfect sense, thank you!

I guess I should have been more clear. I have the 3.59 and 3.625 cranks, have 3 sets of 6.5" rods, and have a new set of JE's. All those measurements are actual, from the parts themselves. The pistons are already mounted on one set of rods, but they are the 6.5" Carrillo HD's, and I am thinking about swapping them over to the Titanium Crowers. I will have to check the pin end thickness; good call!

I prefer(red) the 6.3" rod, but if I go that route I would have to buy rods and pistons. I have everything I need now to run the 6.5"s, but was confirming my decision (or indecision) on stroke.

Thanks again guys, much appreciated.

 

[Squid4life]

Damn.... may be ordering pistons anyhow... Block was thought to be 4.020 and pistons were 4.025 so just a clean up and it'd be ready to go. Got to break out the dial bore gauge as a quick caliper measure of the bore read 4.032...

 

[dr_frankenstein]

must be stage fever!

 

[Squid4life]

Must be! I am ready to get this pig on the road! I think I have pretty much everything I need, hoping I just measured one of the blocks wrong and I can use these pistons on it. Have another 153 block that is 4.007, but really hate to bore it to match these pistons and "waste" cylinder material. We shall see, got to figure something out. Now I have a gang of parts, just trying to put the puzzle together.

Let me know what else you need brother. Can't wait to see your project come along.

 

[750H.P.V6]

Damn.... may be ordering pistons anyhow... Block was thought to be 4.020 and pistons were 4.025 so just a clean up and it'd be ready to go. Got to break out the dial bore gauge as a quick caliper measure of the bore read 4.032...

James,

Double check that measurement. I know I checked it and cam out 4.020"

Neal

 

[Squid4life]

Neal, you are probably right, I was measuring semi-quickly with calipers, didn't get my mics and dial bore gauge out on it like I should have. Was running on empty too from working on the dang bathroom project. Will pull it back out this weekend and check. I hope I was wrong! If you are right, I think I may have everything I need to put it together minus rod and main bearings. Have to doublecheck the cam bearings I have and see if they are good enough, otherwise I will order some coated ones from TA or something.



Like your new sig, but I have a feeling it won't be like that for long.

 

[turbobitt]

Back to the top with some new parts and new measurements...


3.59 crank: 9.512 - 6.5 - 1.795 = 1.217 CH so pistons sit ~.0135 in the hole

I am thinking the 3.59" is still the smart choice, even if the compression might suffer a little, but what do you guys think?

This is my measurements,
3.59 crank:9.525 - 6.5 - 1.795 = 1.239 CH - 1.209 (Piston pin height) = .021 in the hole.
If I didn't have the 6.5 Titanium rods I would have prefered the 6.3 but been running these for years without issues.
No problem.....
Allan G.

 

[Squid4life]

Thanks for sharing your specs Allan, much appreciated.
3.59" BMS crank
6.5" Crower Ti rods
New JE pistons, as long as they measure right

Guess I need to buy rings and bearings and get this thing together.

Any suggestions on rings or bearings? TA for all bearings? I want to say those pistons are cut for metric ring packs. Any suggestions on those?