Long SBC rods or custom pistons???

[2.8-350TBI]

Hi guys, first of all, I would like to admit I dont know what negative piston deck clearance is. Is it negative when the piston protrudes from the block???

Anyways, this is for my Ironhead 2.8/3400 head hybrid engine.
While I was messing around with the cr clac, I figured I could use ironhead 3.1 pistons with (obviously modified) 6" sbc rods.

With the 6" rod being .3 inches longer, and the 3.1 pin height .32" inches higher, (if I measured correctly) I should end up with .0365 piston deck clearance, an okay 9.85:1 cr, and a measly .077 quench.
Any ideas?

[SappySE107]

.0165" is what the iron head pistons have for deck clearance, which means they sit below the deck .0165". Using a .300" longer rod puts the piston out of the hole .2835". If you relocated the pin .320, .0365" is where the piston would be in the hole. That is low and quench would suck since you cannot get a head gasket that thin.

I cross my fingers for the math to be right

[2.8-350TBI]

Thanx for the clarification Sappy
I was messin some more with the calc, and figured out that 3100 pistons will lie at 0 deck height with 6" rods, a comfy .4 quench, but the piston dish is too big, which nets me an 8.73 cr.

I was planning on a turbo, so this looks like a more or less ideal combo
YAY

Im open to suggestions from you guys.

[bszopi]

What values are you using? Here is what I came up with...

Bore - 3.504"
Stroke - 3.29" (2.8L crank and 6" rods)
HG Bore - 3.622"
HG Thickness - 0.04"
CC Vol - 28.6cc
Piston Vol - 25.5
Piston/Deck Clearance - 0.00"

Results
3118cc
9.54:1 CR
0.04" quench

[2.8-350TBI]

What values are you using? Here is what I came up with...

Bore - 3.504"
Stroke - 3.29" (2.8L crank and 6" rods)
HG Bore - 3.622"
HG Thickness - 0.04"
CC Vol - 28.6cc
Piston Vol - 25.5
Piston/Deck Clearance - 0.00"

Results
3118cc
9.54:1 CR
0.04" quench

These results are great!

Brad, where does 3118cc come from?
Do the 6" rods change the stroke?
I thought rod length and stroke werent inter-realted.

My engine is a 1st gen 2.8 TBI.
I prefer to not have custom pistons made, and that is why I was specualting using 6" rods w/ 3.1/3100 pistons

[2.8-350TBI]

Bore 3.504"
Stroke 2.99"
Head Gasket Bore 3.69"
Head Gasket Compressed Thickness .040"
Combustion Chamber Volume 28.6cc
Piston Dome Volume 25.5cc
Piston Deck Clearance 0"

Displacement 2833 cc
CR 8.73:1
Quench .04"

This is assuming 2.99" stroke.

[bszopi]

I did it wrong...

To compensate for the change in rod length, I added the extra length of the rod to the stroke, which now that I think about it, is wrong. Sorry to get your hope up.

[95Grand-Am]

You could put a 3.1L crank in your 2.8l. Then you could use your rods, but you would have to get 3100 piston. Then the Quench and compression should be right where you want it.

[2.8-350TBI]

I am a 2.8 guy all the way, and that would be the exact same specs as a 3100, which im not trying to build.
I want my little engine to remain at 2.8 liters. Im using this engine more or less as a learning experience for my upcoming project "2800"
With the 8.73 cr, I should be set for some boost, which I have planned. So that settles it. 6" Lightweight small journal SBC I-beams and 3100 pistons coming soon
P&P +flowmatched 3400 heads should be here in 3 days.

Thanks for your suggestions.

[ALLTRBO]

Almost, but you'll need 5.85" small journal rods (fortunately they are made), not 6", with 3100 pistons.

Stock 3100's set the pistons .020" out of the hole with the 3.307" stroke and the 5.7" rods. Basically, with 3100 pistons and Gen III heads, you're just destroking a 3100 back to a 2.8 with the 2.992" stroke, instead of using the 3.307" stroke.

3.307 - 2.992 = .315

.315" is the total stroke difference, meaning half of that is taken away from the top of the stroke, and half taken from the bottom of the stroke.

.315 x .5 = .1575

So, to bring those clearances back up to what a stock 3100 would be (using 3100 pistons), you would needs rods that are .1575" longer (than the 3100's 5.7" rods), which is 5.8575". Obviously that's not practical so if you use small journal 5.85" sbc rods, the pistons will end up .0075" further down from where they were before (5.8575 - 5.85 = .0075). Since the stock 3100 sets the pistons .020" above the deck, with this combo the pistons will now sit only .0125" above the deck (.020 - .0075 = .0125).

So with a 2.8 block (stock bore), 2.8 crank, 5.85" rods, 3100 pistons, 3400 heads, and stock 3100 head gaskets we get these specs...

Bore - 3.504"
Stroke - 2.992"
Head Gasket Bore - 3.622"
Head Gasket Compressed Thickness - .060"
Combustion Chamber Volume - 28.6cc
Piston Dome Volume - 25.5cc
Piston Deck Clearance - .0125"

Displacement - 2835 cc
CR - 8.59:1
Quench - .048"

Rod/Stroke ratio: 1.955:1

Very nice for very respectable boost levels and high RPM's, which this engine will love (you'll surprise lots of people with the power).

If you want to spend a bit more on MLS head gaskets (the way to go for this combo I think), you can change the thickness to .055" and the gasket bore to 3.509" (be very meticulous with the placement on the block), and you'll get:

CR - 8.77:1
Quench - .043"

Even better. With the relatively small bore, slightly better quench (which doesn't help a whole lot under high boost), and that high rod/stroke ratio, you should be able to run 15 psi + with only moderate intercooling even with a stock cam and that compression, on pump gas. All you'll need is stonger valvesprings for the RPM's it will want to run. Lots of power out of that 2.8.
I'd make sure the bores are well within tolerances and probably give it a light hone to help seat the new rings.

That's pretty much exactly what I'm planning for my Fiero's later bottom end, except with a 3400 block .020 over (a sweet "3100"), Diamond pistons, very stout rods, strong hardware everywhere, and lots of boost.

Good luck. Whatever you decide, post up your build!

[2.8-350TBI]

That's pretty much exactly what I'm planning for my Fiero's later bottom end, except with a 3400 block .020 over (a sweet "3100"), Diamond pistons, very stout rods, strong hardware everywhere, and lots of boost.

Good luck. Whatever you decide, post up your build!

YAAAAY, thank you soooo much for this help. Now im decided to do this buildup
Ill be documenting with pictures, and hopefully I can give it to brad to post up on the 3X00 swap site. Im still gathering parts and will be for the next couple months (im jobless as of now), but nonetheless im getting there.

As for your project, I like the sound of that, a destroked 3400, NOW GO AND GETS SOME BOOSTIN GOIN!

[domestikdemon]

Are there any links, to this site or others, that will explain the math you guys are doing. I'm decent at math, basically I'm asking for a formula table.

Thanks in advance,
Chris

[2.8-350TBI]

Chris, welcome to the site (your 1st post), we are using the Compression Ratio Calculator on the left side of the page.
We use known values and plug them into the cr calculator and it does the work for us
We just have to make sure we know the values we plug in are correct

[domestikdemon]

Haha I've been looking at this site for a couple years now. Don't know why my car info isn't there. I have an '00 GAGT w/ 3400 in the process of installing a Getrag 282. Just gotta finish the custom mounts, and it'll be mostly ready to go. Thanks for a nice welcome tho.

And also thanks for pointing me toward that, I hadn't even noticed it.

[2.8-350TBI]

Lol I hadnt noticed it either until I heard (read) of the other guys using it, and then I tried it out. Sometimes it has more useful info than the spec sheets that are posted up.

What specific info are you looking for?

Are you tuning your ecu for the manual tranny? Specifically, dropping the .bin for auto controls? I love to see when people swap manuals for autos. Real men do their own shifting lol.