Compression Ratios

[ForcedFirebird]

The 885 isn't really a fast burn, it's more of a hemi with angular squish to direct the combustion to the center. The 200 head is more of a fast burn shape (well, what I am used to), and the reason they could have small cams and higher compression (ie higher trapped mass).

[nando]

right, but in relation to previous head designs - it does have pretty good swirl etc.

[digger]

Strictly speaking dynamic compression ratio (DCR) is defined to be dependent solely on Inlet Valve Close (IVC) Timing as far as cam events go, nothing to do with overlap.

In a real engine the actual trapped compression which is what is more important depends on overlap to. DCR is a constant value (yet its called dynamic) if the timing is fixed (no vanos) and does not represent cylinder pressure in a running engine but its useful information to have. Ive heard people say that even if you keep the DCR the same between a 10:1 and 13:1 engine by changing cam events the 13:1 will have a substantially higher risk of knocking as in a running engine the 13:1 will see higher trapped cylinder pressure all else being equal. its only a rule of thumb / guide.

If you are running 93 on a street engine you know you are somewhere close to correct IVC and DCR if you are cranking compression of 190-200psi as rule of thumb which makes the engine more responsive and crisper at part throttle. Advancing the cam will raise the cranking compression because the IVC becomes earlier, it also changes the other 3 events so in terms of how the engine runs would depend how close the other events are to optimal. I know people who have had well over 200psi cranking on 2.8L m20 with OEM pistons and tight squish, does not knock on pump fuel at all, and is stout torquey engine. there isnt much from that era better than the BMW chamber

[Sykohtic]

I'm loving the information, started reading the articles as well.

In the meantime, to add to the discussion:

I've been talking to a good friend who used to work for Korman. He turned me onto the Korman GPU class car. It ram 11:1 compression on pump gas with Webber sidedrafts. He said the cam was like a 300. It was a 3.0L, so had more displacement than I'm running, but the car put down over 300whp.

If I went to custom pistons, and bored the motor out to accept larger pistons to do a 2.9L 11:1, what cam spec am I looking at? Like a 288 or 290 with a slightly lower exhaust lobe?

I understand at the end of the day, I'm talking about comparing a carb'd race motor to a EFI street motor that has to run longer than a single race, so maybe I'm being a bit outrageous.

[digger]

2.8L with ITB 288 seem to work well and totally streetable with proper tuning, something in 290's would start to trade quite some bottom end for midrange and topend as quite they often start to grind the lobes on tighter centres and overlap area increases exponentially. For relatively small engine thats getting to be a big cam. For 3L or more it would be a little more manageable on the street. You can of course still drive it but it might not be as fun when you arent in the sweetspot. id err on the smaller side personally

[ForcedFirebird]

there isnt much from that era better than the BMW chamber

I don't agree with this 100%. GM used it's narrow angle v6's to kind of test designs to be future v8's (they still do, if you look at the 3.6dohc etc). In the late 80's and early 90's they introduced canted/splayed valve heads with high roof ports, extremely efficient combustion chamber. At that time they started actually paying attention to port flow, manifold design and burn signatures. With them nearing the introduction of a gen 2&3 v8, they wanted to get it right, rather than relying on improvement of previous designs.

Sorry to get off topic, that's the engines we first started modifying, flow testing etc. Had a turbo v6 in my Firebird when I signed up on the forums (just kept the old screen name when signing up on e30 forums).