i know it has been talked about before but i can't find it. what is the ideal or max dynamic compression for boosted motors? the set up i'm kinda looking at now will give me between 7.25 and 7.5 dcr. is this low enough? i know too low and it will be a pig around town when not in boost. also i will be running ten pounds on the streets and 15 MAX at the track. i also have water/meth using washer fluid and m15 nozzle.

 

Im a little lost here your compression without factoring in boost would be 7.25-7.5 to 1? no friggin way, 9.5 at least....will work absolutely fine with 15 on pump gas with meth (straight). Hell it'll take 20 on the edge...

 

He's asking about dynamic compression, you're talking about static compression.

4.8T -- here's the thread from internal engine where we hashed a lot of this out: https://www.performancetrucks.net/fo...d.php?t=369817

 

cool, thanks i'll give it a read in a bit.

 

looks like ther was never any real answer on that. people have ran on of boost on stock bottom ends and i know our stock dcr can't be that low. i once read a good mag that said build a good n/a motor then add your boost. not sure how acurate it was.?. i'm guessing useing water/meth can take you higher in boost/dcr/scr due to octane.

 

Meth changes everything. Well, not really, the DCR stays the same, but it adds so much to the fuel that your detonation range is pushed off -- both because of the added fuel, added octane and due to the chemical intercooling effect -- the only attribute of detonation that it doesn't directly effect is timing . (which is to say, it adds more fuel, more octane rating to the fuel, and reduces cylinder temperature, but doesn't make less or more timing from what you set the timing at).

The keys points I got from that thread are:
1) calculate your DCR, then multiply it by your pressure ratio for your "boosted DCR"
2) for unleaded, you still want to be in that magical range -- the problem is, we all run meth, which really changes those limits and no one quite knows them in terms of DCR.

 

well the build i'm looking to do will give me 9.44:1 scr and cam will give 7.5:1 dcr. now that is the high side. if the pistons don't come out of the hole as much then it will be less. i came up with my own cam specs i think will work out good. 206/218 590/563 117.5 its basically a higher lift version of the ls6 cam. i figure since the 317's flow better then might as well use that flow. but with high exhaust pressures i didn't want to open the exhuast valve too much so there is less stress on the exhuast spring.

thoughts??

 

Personally, I'd go with more intake bias and higher lift on the exhaust side (plus really good springs )

 

patriot's??

 

Honestly, it's been so long since I looked at springs, I'm not the definitive answer. I know Richard @ WCCH really likes the Patriot Golds -- definitely worth a phone call to him. I run Comp 921's at present.