Biffa

Does boost affect the DCR?
Most engine builders specify a lower static CR for boosted applications, but many also run smaller cams with earlier IVC.
Any advice welcome.

brad8266

iTrader: (23)

NO!!!! Nothing has an effect on compression except for engine/head combo, along with cam design. Nothing, and I mean nothing else has any effect on compression.

With that said, it is better to design a boosted engine to have both lower DCR and SCR, although DCR is what really counts, not SCR. A later IVC will help you get the lower compression.

Biffa

Thanks for response, but let me clarify my question.
I understand that the compression ratio = V.BDC/V.TDC and this is not affected by boost. What I'm looking to understand better is how DCR might need to be reduced to suit boost. There's various guidance telling me that DCR should be <8.5 on a street car with 93 gas, but I've not seen any guidance on how this might need to be adapted for boost.
I understand that FI will increase intake pressure and all other pressures throughout the engine cycle, normally with highers temps from compressive heating. Common advice is to reduce CR to compensate for higher temps at the start of the stroke.
So, is there any guidance for DCR for a boosted engine, with say 1 bar of boost?
(An earlier thread referred to US engine builders running reduced static CR and smaller cams, while several Australian builders run higher CR and larger cams. May be that that the DCR is not that different once respective IVCs are taken into account.)

brad8266

iTrader: (23)

Right, most bigger cams will reduce DCR so in a NA application you would want more SCR in order to compensate for the reduced DCR of the larger cams due to the IVC.

In general you want to lower the compression overall when boosting, so you could run a big cam to get low DCR while also using dished pistons or bigger chamber heads to lower SCR also. It all depends on the cam you are going to use, but usually the bigger the duration the later the IVC happens so the lower the compression.

Christopher Sims

iTrader: (1)

While boost does not affect mechanical or SCR, it does change your effective CR. Think about it, at 15psi boost you have effectively (ok, there will be some inefficiences) doubled the swept volume in the cylinder. Check this out:
http://www.turbofast.com.au/TFcompB.html . Plug in various combos of SCR and boost. While there's nothing like a high boost hit (19psi in a turbo Buick!), I believe (for a DD street car) more compression with low boost yields a more enjoyable driver. Flame away!

Freddyrdzls1

iTrader: (2)

hell of a good *** topic great responses dude

ezrollin

In what aspect? MPG?
If you are saying less boost + more compression = easier to drive then.. who cares about a easily driveable car? (no offense)

brad8266

iTrader: (23)

That does not increase compression, it increases cylinder pressure, there is a difference.