Choosing compression ratio

[silvergtjrad]

Im starting to get together some parts for a motor build this winter and have a question about what compression ratio would be best.

To start with, this car will be a "daily driver". Ive got a vehicle for work so my car is mainly driven on the weekend, and 90% of the time I wont break 200 miles a week. Second, Ive already purchased a cam, pushrods, and valvesprings from the store here. The cam I have is grind #1615. I want to have something higher compression, at least 10.5:1 , given those two stipulations.

Buuuuutttt... Im going to be using a rather healthy amount of nitrous. At least 100 shot, and in two stages, possibly direct port if $$ allows. The higher compression will be nice because of the added power both on and off of spray. Im worried about being able to tune it to use that much juice, and not put a hole through a piston.

[SappySE107]

Custom forged pistons are the way to go for high compression and nitrous. 10.5 or 11:1 should be more than enough.

[silvergtjrad]

Custom forged pistons are the way to go for high compression and nitrous. 10.5 or 11:1 should be more than enough.

I know custom and forged, but am I going to be pushing my luck with high compression and nitrous? I definitely know boost and high compression dont mix well, but what about nitrous. The main thing Im concerned about is ease of tuning and not having to use race gas so I can spray.

[SappySE107]

I wish I knew the breaking point, but right now I don't know what the peak is if you were to tune it. For ease of tuning, 10.5:1 is fine. I don't see a 100 shot doing the motor in with 11:1 if you tuned it proper. That is just my guess.

[Ortie666]

Yeah, thats deafanatly a big part in running nitrous is having the motor tuned spot on b4 u hit it, and if its tuned really good ur motor should easely take 150-175 shot with the timing bumped back, with high octane fuel.. As for high grade race fuel I would put money on a safe 200, as long as you have a ignition system that can keep up .. And for direct port injection i know alot more nitrous companys are making injectors with the n2o lines going into them, i would look for a set of those, unless u want to drill and tap your manifold witch isnt all that hard.

[Blackrider]

One of the nice things about nitrous is the ability to run higher compression. But one again as has been said a few times already its all about tuning. I would say stick with 10.5.

[El_Diablo]

turbos love high compression engines, its just the fuel that doesn't so you either end up having to run a high grade fuel or retard timing a good bit

have you ever thought of running a nitrous enrichment tank? its a bit of extra work but it will allow you to run a bit more nitrous and timing safely, direct port at those levels is a must but if you go with nitrous injectors that sandwhich between the injectors and manifold save yourself the trouble and DO NOT use rubber/plastic lines as they all seem to eventually blow off causing an sudden and extreme rich condition

[silvergtjrad]

Im not planning on using those injector style nozzles because they are so damn expensive. A seperate fuel cell for the nitrous system isnt worth the money or work for me. Im not planning on spraying it anywhere but at the track and the occasional fun run on an open road. Ive been reading about inecting propane instead of fuel. There is a pretty good thread on lstech.com if anyone is interested. As far as the pistons go, im still a month or so away from ordering them. Im still a little iffy on the whole high compression and nitrous combination. Id really hate to spend all this money and then watch it go boom.

[El_Diablo]

just be sure to retard timing when nitrous comes on, i would look around for a good nitrous controller with retarding abilities (or a stand alone retarding unit) and a bottle heater if i were you

[silvergtjrad]

Ive got a bottle heater already, and already have a way to retard timing. I have the nitrous spray directly at the IAT sensor, and have it tuned to pull timing when it gets below a certain temp.

[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.

[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