Don't confuse STATIC compression ratio as meaningful for anything other than cranking [how much power the starter needs to develop]. All that counts is EFFECTIVE running compression after cam [air flow in/out] is calculated.

 

I have always been told that dynamic CR is lower than static CR because of the valves opening. When adding big duration cams you want to add a higher static CR because the valves will stay open longer reducing CR.

Thanks Richie...I plan on getting some cams with more lift than the Nismo Spec 2 cams. Looking at the Cosworth flow chart I think lift will be just as important as duration. Right now I am trying to get my hands on some cams with more lift than is currently available. If I get them I will post info on them and how well they work.

 

Well, assuming we keep the same cams, increasing the flow through the heads is going to increase the volumetric efficiency, raising the effective running compression? Obviously tough to know what an engineer was thinking when he was working on these heads, but do you think the additional chamber space was meant to compensate for the increase in effective compression due to better volumetric efficiency?

*EDIT*

OK, I sat and thought about this, and is it really accurate to say that there is "effective" versus "static" compression? Certainly the volumetric efficiency of the engine is going to change based on any number of factors, such as air pressure, temperature, forced induction, cam timing, engine speed, etc. But whatever you put into the cylinder is going to get squeezed based on the mechanical compression ratio of the motor, which should just be a ratio of the total volume at BDC vs TDC.

*/EDIT*

I'm seriously considering using these heads for my build, and want to bore to 100mm, so I'd like to understand as much as I can.

-E

 

AHHH! Too much to think about. Ok, that makes sense, long duration cams will be open during compression, gonna get some leakage reducing your CR, etc etc. Now I understand what he meant by effective compression while the engine is running.

-E

 

Waiting for the heads to arrive but my decals arrived

 

I am not 100%, I am just going by what I have been told be several people.

 

If I read correctly, you're running a SC, so that sucks. If it were a turbo, the 8.3 would probably be better for you. I know everyone is on a high C/R kick right now for boosted motors, but with a TT set-up, off boost time should be minimal. With the JWT, I think the boost threshold is like 1500 rpm, and who cares about power before then? With the lower C/R, more energy will go to drive the turbines and the turbos will spool faster and easier throughout their operating range. Of course, for a SC system, this does nothing..
Will

 

Were is the diffference coming from in the Cosworth heads? Is it a slightly taller deck height? I wonder if the Cosworth heads will allow for a higher lift cam without the use of interference or dished pistons due to a taller combustion chamber? Just a thought.
Will

 

Got my Cosworth heads in and veryone I show them too is absolutely amazed of the quality and craftmenship.

Bearings are still on hold.... will have to go with OEM. PITA choosing the right ones, and of course there are none to be found locally.

Will be calling you on Monday Sharif for a rush order

Danny

 

Where can I get a price for Dave's heads for the VQ?

 

Wow...no way. Its Dave from Headgames. I didnt know you messed with VQ heads. Anyways, thanks for helping me with the Honda head a year ago. This guy Dave is super nice. I heard you also do 3RZ heads. Is that right? Let me know. Currently Van is doing those for us and he is doing a fantastic job. I'm on the way to E-town as we speak, hope to see you there this weekend. Anyways...thanks for the hospitality.

 

Give him a call... phone: (609) 882-3211

 

increasing the displacement of the engine via boring or stroker kit I don't think has an effect on compression ratio. The compression is the same, there's just more volume of it being pushed. If the piston was the diameter of an oil drum, it would still compress the air to the same compression. There's just more air being compressed overall


I'm curious too about if the heads have more volume how that might change the compression ratio.

I've actually been thinking of going with something like 10.5 or 11:1 CR when I do a built engine with my vortech setup considering I run water/meth injection and 96-101 octane gas anyway.

 

1. increasing bore and stroke can affect compression
2. less head chamber cc's = increased compression, more head chamber cc's = lower compression. Cosworth heads slightly drop the stock compression (vs, say NISMO heads, which raise compression)

 

I dunno, I can see how the cc's in the heads completely affect compression because the air being compressed has more room to expand into with bigger heads
but for the short block with a larger bore and stroke, more air is naturally going to fill the (larger) cylinder chamber and more gas is obviously called for to balance the A/F

I guess what I'm wondering, is if you proportionately increase everything, why would the compression would change??

 

http://www.chevyhiperformance.com/te...ression_guide/

easier for me to post that article rather than post myself

Both bore and stroke have an effect on compression....stroke moreso than bore

 

thanks for the link

so when you buy a stroker kit with 11 CR that was intended for stock heads and I went and got some huge high flowing heads, that'd then lower the CR by whatever amount. So I guess people just need to factor that into their build

 

if coming from a reputable manufacturer, just the opposite..the compression should be EXACTLY what they quote, as they've taken the other variables into account.

Obviously, if you toss that stroker onto an aftermarket head with more or less combustion camber cc's, varying deck heights (can't vary it by much on this motor or it won't bolt together), then compression can be expected to vary slightly

 

^^Correct^^

For instance, when we make our 100mm Arias ED pistons, this results in a 3.8L displacement, so the CR of the piston is set to 9.0:1, and the piston is constructed to acheive this CR, given the CC of the heads, headgasket thickness, etc..etc..

Like Z1 mentioned, CR can vary ever so slightly with different combinations of HG's, combustion chamber shaping/porting, but usually not in a huge way.

Manufacturers will do these calculations for you, so no worries.