What's the ideal DCR for LSx Engines
 

What is the ideal DCR for our cars? Is there one?

I know many things affect it (cam ,heads, bore, stroke, head gasket, pistons)

I know there is always a trade off somewhere when you want power, usually at the expense of idle quality.

Would you want a certain DCR for a 346 CI vs. a 408 CI?

Thanks in advance,

Bill

8.5:1 seems to be the limit for 91 Octane and 8.8:1 seems to be the limit for 93. If you're running higher octane than this, you can bring up DCR.

Ideal would be the highest you can get away with without pinging.:)

So basically play around with the calculators till I get close to 8.5 - 8.8.

The values you gave me, are those like an average compression for the whole stroke, just want to understand why it's so important and the full meaning of DCR.

Also, what would be considered range of a bad DCR?

and im also interested in the original question of 346 vs 408 DCR's, i was told you have a little bit higher DCR with the larger motor... is that BS?

At one point you guys will start to run into detonation with increased VE levels. LT1 motors can take a ton of static and dynamic compression due to the good cooling of the GM castings and the revearse cooling. Even with that older chamber design, but if you get the Vol Eff% up high enough you can even have issues with 8.3:1 DCR on pump gas, it's not going to knock at low loads and low RPM but once the motor starts making over 110% VE (210+BMEP) it will start rattling. You cna help this out by reducing engine temps with lower thermostats, colder intake air etc... So a lot of how much your motor can take depends on other factors as well. Different chamber designs can have a impact on the amount of compression you can run and in LS1 land we have a pretty big assortment of chambers, just look at what GM gives us in a LS1, LS6, L92 castings. Not to mention the ET, AFR, TFS and Darts.

Bret

Bret, Have you ever seen an LS1, LS6 make any thing close to 110% VE? How efficient are these motors with their plastic intakes and say AFR 205 heads?

Yep, if you do them right 110VE is there. The intakes make them even more efficient in the midrange.

bret

"Without question, the most common mistake in racing is to buy parts impulsively without having a clear vision of where you want to end up. The result is often a pile of mismatched pieces that will never work together properly." - David Reher


I know its a lot to ask but when buying parts for a strong running LS1 LS2 LS6 346ci street car what would be a wise choice to make? Heads, Intake, and Cam timing

Could you also suggest a good 402 street build also?

Would coating also help? Would one chamber--AFR, TF, ET--handle higher compression better than the others?

David,

The only way to know would be to test it.

Gollum,

PM me about that. A lot of it depends on what you want to do.

Bret

Are you using the seat times or the .050 times? If the seat times, your calcs would show a huge advantage for a solid lifter camshaft with its 15-20° smaller major intensity.

Seat times, definitely. Once you crack the valve open, you're bleeding off compression. Once you close it, the compression starts. Seat times are key and a huge reason why solid rollers perform so well. Generous durations at .050 and .200" vs seat duration.

So why not try your current combo with solid XE Street Rollers instead of LSKs. With the same seat timing you could be a 236/242. Or you could do 228/232 with a much higher DCR.

Cost is the current detriment to going solid roller. Probably at least $2,000. With all the advantages though, I'm sure I'll be there again someday. It's like having your cake and eating it too, huh?

There is an issue with hammering the needle bearings at idle...but yes...the whole hydraulic lifters in performance cars is an emissions thing IMHO.

:judge:

And a valve adjustment thing (every 10K mi?)...

http://www.efilive.co.uk/images/comp-dcr.png

There is your answer (or at least a good view on it). Read the full article as well http://www.popularhotrodding.com/tec...power_squeeze/

As mentioned there are lots of things to consider with max DCR including quench, chamber design, intake temps, efficient exhaust. Engine temps have a large effect as noted in the diagram above.