Choosing compression ratio

[Lilchief]

As far as compression goes , I'd stick with the 10.5 cr. Might have detonation problems with the 11-1 cr. If your using just one injector for your nitrous, I wouldn't go over 75hp shot to minimize distribution problem. I would go with the direct port injection and a progressive controller. You could run a 200hp shot and start with 25% pulse (50hp). I looked at the 1615 cam, looks like a turbo cam. Has more intake lift than exhaust. Nitrous cams like more exhaust lift and exhaust duration and a wider LSA. CompCam makes theirs with 113 LSA. For extreme 4x4 they use a 111 LSA, more low end torque. Just passing on some info, I'm no expert. Hope this helps.

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[Ortie666]

Ohh, I forgot one part, if you do run bigger than a 100 shot its best to use bigger fuel line than stock, then again if ur car already has a 3/8 (I.D.)it should be close enough to what you want to do, on my nitrous car the smallest my fuel line gets to is .450 so its pretty close to the 1/2 inch line.. 180 shot hits strong. next is the presure regulator, I havent seen anyone talk about this yet, so one more thing to take into consideration.. Im not sure what u plan on hooking the n2o fuel up to, but I would strongly sugest a regulator, and gauge b4 the n2o. rule of thumb is 5-5.5psi is on the edge but hits super hard, 6 psi is pretty safe, still makes power, and 6.5-7.0 is too safe and runs kinna weak

[silvergtjrad]

So im trying to figure out what my compression ratio will be using a set of DOHC pistons I scored ($86 for a new set).

These are the values Ive come up with for stock bore using a 3400 head gasket and 3500 heads.

Bore: 3.622"
Stroke Length: 3.307"
Head Gasket Bore: 3.75"
Head Gasket Compressed Thickness: .06"
Combustion Chamber Volume: 32cc
Piston Dish Volume: 4.5cc
Piston Deck Clearance: .02" (proud of the block)

That would give me 13.695:1 Correct?

Im not so sure on the value for the piston deck clearance though. On the CR calculator here, im seeing .0095" as the value for deck clearance on the DOHC. Do I use the value for the 3400 or DOHC?

Clearly 13.6xx is WAY too hight to run on pump gas obviously. If these pistons dont work, im selling them and my forged stock copies, boring the motor and getting some forged 12:1.

[sprucegagt]

You use the DOHC spec. of .0095" for Piston Deck Clearance. But the main thing you need to worry about is cylinder pressure, not compression ratio. Although with that said 13.6:1 is to high for pump gas.

[geldartb]

3500 heads and ironhead 3.4 pistons will net a 11.5.1 CR

thats using the stock spec head gaskets

[sprucegagt]

But I wouldn't use the stock 3400 head gaskets. Leaving .060" quench area will increase KR instead of decreasing it. You really want to keep quench around .040". So when using them your CR raises to 12.3:1.

[silvergtjrad]

Ahh, makes much more sense to me now guys, thanks. The DOHC pistons do give you
12.4:1 then, but that is using the 3400 gaskets. If you use the Ironhead gaskets for better quench, the compression goes up too much to make them useable.

Well, so much for that. Anyone need a set of brand new, stock bore DOHC pistons? $100 shipped in the USA.

[geldartb]

its either making the combustion chamber on the heads bigger to drop the CR a little, with the combo of the .040 head gaskets.

or custom pistons using the .040 headgaskets.

[silvergtjrad]

its either making the combustion chamber on the heads bigger to drop the CR a little, with the combo of the .040 head gaskets.

or custom pistons using the .040 headgaskets.

Im tired of messing around and spending money on things that dont work. Im just going to get my block bored as much as possible, and get some custom 12:1 pistons made. I can still return the DOHC pistons, and easily sell the other forged set for what I payed for them.

[silvergtjrad]

I forgot about this thread, but I ended up going with an 11:1 compression ratio on a set of forged pistons from JE. The shop I used is dicking me around though. Its been about a 4 month wait for them so far. In the last month Ive stopped in once a week to see WTF is taking so damn long. They just keep telling me one more week, two weeks, they should be in by friday, etc..... I wish I could just tell them to fuck off, and go to another shop, but they require a deposit in full for the pistons before they are made. Sigh. The shop has a really good reputation with some pretty high end race cars, but I'll be damned if they gave a shit about me.