Kraest

My 396 has a SCR of around 11.7:1, but a DCR of around 8.0:1.

How will this affect the amount of nitrous oxide I can use on my engine compared to engines with a lower SCR/higher DCR?

How would you take this into account when calculating effectiveness in a forced-induction motor?

Mike

Derrick Redd

Regardless if you go with dope or a blower you won't be able to get traction with those futuras until 75mph+ anyway. My vote is for the 200 shot.

Mindgame

As I see it, we're talking cylinder pressure.... more dynamic compression = more cylinder pressure and the more cylinder pressure you have, the more timing you pull to keep the engine out of detonation.
The more compression, the more timing.... the less compression, less timing. So I don't think it's as big a deal as we might like to think it is.
Now for a street engine running around n/a 99% of the time.... I'd want more dynamic comp. for off-the-bottle performance. So that right there would be enough for me to build a street engine at the higher end of the DCR scale... even one that would see occassional nitrous jolts.

-Mindgame

SStrokerAce

N20 engines and Blower engines are not similar in SCR and DCR. They both provide high cylinder pressures, but I would rather have a 11:1 N2O engine, than a 8.5:1 SC engine.

As Mindgame said set up the DCR/SCR for off bottle performance and then just retard the timing alot for the dope.

You can change the cam to help the N2O performance and not hurt the NA performance. Mostly with the exhaust side.

Bret

Kraest

Well, the whole point of this question is to figure out how you can have less cylinder pressure due to valve overlap even though the static compression ratio is high:

Example:
Car A has a 396 with a big-assed cam, but static compression ratio of 12:1.... so, actually, the dynamic compression ratio could be 8:1

Car B has a 350 with a small cam, but static compression ratio of 10.5:1.... but the dynamic might be close to 9.5:1.

While Car B appears to be more power-adder friendly, that is not the case, since the dynamic compression is higher, thus making the cylinder pressure higher.

Is that correct, or am I missing the entire point?

Mike

Damon

DCR as I understand it doesn't have to do with valve overlap per se, it has to do with when the intake valve closes in relation to crankshft position. True, bigger cams tend to have more overlap, but they also have a later closing of the intake valve.

DCR from the simple on-line formulas I have examined basically just calculate the static compression ratio from the point the intake valve closes- usually around 30-50* ATDC given typical street cams. At that point the piston is up the bore X inches. Take that as the point that real compression begins and then recalculate using stadard static compression ratio formulas and you get what they claim is DCR. In other words, it's not rocket science.

That being said, I often wonder if talk of DCR doesn't send a lot of shadetree mechanics off in weird directions with poor results. I, personally, would not lay nitrous to an 11:1 motor on pump gas unless it had a DAMNED big cam.

Long story short- high compression is not the be-all end all of making power. I would MUCH rather undershoot on compression than live on the ragged edge for a street driven motor.

Real life story to follow.....

Damon

As a real world case-in-point I have a friend who comes to this board who bought a turnkey 383 with Dart aluminum heads, a BIG cam (258* @ .050- typical N/A high performance mechanical cam) and 11.2:1 static compression. Your typical saturday night special motor, all parts including intake, exhaust and stall speed relatively well matched. It ran well, but after a few hundred miles of street driving it "pushed" the head gasket and left him stranded on a casual drive one day.

We pulled the heads, obviously, and after examinging the carnage I suggested that he switch from the 64cc heads it came with to the same heads with 72cc chmbers to bring compression down to about 10:1. I suggested this becuase I suspected that there was some detonation going on with pump gas and also becuase he had plans of putting some giggle gas through the motor in the future- I wanted to build some "headroom" for the nitrous squeeze.

But he was concerned the motor would become a "slug" off the bottle due to the large compression drop while retaining the big cam. I had earned his trust enough on other projects that he signed off on the idea anyway, and that's just what we did. Identical Dart heads but with the larger chambers.

Result: absolutely no loss of performance off the bottle. Zip. Nada. Many street miles later there's no sign of recurring head gasket problems, either. And the motor should be much more able to take the nitrous "hit" when that gets added later this year.

I have many such experiences. Pump gas motors, high compression and nitrous don't mix, nor do they need to to make good power.

Race motors running on race gas? Totally different animal. but I don't build race motors.

rskrause

I am sleep deprived right now, but I'm going to take a shot at this. To tired for much theory right now, but the practice is as follows.

If you are building a street/strip motor for nitrous, basically you want to build good NA motor with the following considerations: use strong pistons and a cam with additional exhaust duration but no addtional overlap (IOW, increase the LSA in proportion to the increase in exhaust duration). Choose a static CR that will provide a DCR as high as tolerated with your chosen fuel. The only reason to go low CR with nitrous is if you are building a race motr designed for huge amounts of nitrous.

Rich Krause

SStrokerAce

Having the right clamping force on the heads and the right gaskets is the only way to keep high cylinder pressures in check.


Kraest,

"While Car B appears to be more power-adder friendly, that is not the case, since the dynamic compression is higher, thus making the cylinder pressure higher.

Is that correct, or am I missing the entire point? "

You got it. But anything over 9:1 DCR is not really going to happen very well.

Bret