I guess I'll ask here since there are no answes yet in the NA build section:

 

wat?

 

do higher lift cams and changing valve timing (overlap) alter compression ratio?

I guess I just asked the same thing twice... what part am I not asking correctly?

 

you mean the over all compression as in measured by a gage or compression at a point in the rpm range?

 

I realize compression is typically determined by cylinder bore and piston size… but considering higher lift cam with longer duration and more aggressive cam timing – more air and added fuel would produce higher pressure right? So would the compression ratio be higher than OEM 10.3:1?

The reason I’m asking is to determine the level of octane needed for maximum benefit… whether 105 would prove better than 100 for example.

Even if the compression ratio is slightly higher, it’s probably not enough to make a considerable difference… but I don’t know for sure which is why I’m asking.

 

I answered it there...

We gotta keep all the NA business we can get...

 

thanks man, I replied (actaully asked more) there... I'll keep the topic in that thread - probably better suited there anyway.

 

you are no where near the threshold where race gas will do anything except cost you more than pump gas

 

how about the compression ratio question

 

My answer is of a general nature.

A higher lift & longer duration camshaft will lower the active CR @ idle to lower mid RPM range.

Assuming camshaft is designed correctly & the motor has the required intake & exhaust system, as the new camshaft gets into the higher RPM range there will be more cylinder fill & hence more active CR.

Most modern 4 valve production vehicle performance motors achieve close to 100% cylinder fill @ there peak tq rpm.

A check of the advertised PeakTq output of some production performance motors including Porsche & Ferrari

EG: 360 Ferrari 3.6lt Tq 275 ft lb's @ 4750 rpm
Porsche 911 3.6lt Tq 273 ft lb's @ 4250 rpm
Nissan 350Z 03 3.5lt Tq 274 ft lb's @ 4800 rpm
Nissan 350Z HR 3.5lt Tq 268 ft lb's @ 5200 rpm

Now look at a race motor
Porsche RS spyder LMS2 3.4ltr 273 ft lb's @ 7500rpm

So at there peak Tq all these similar sized motors are achieving max cylinder fill and so similar Tq output yet @ peak HP there is nearly a 200 HP difference from a 03 Z to the RS spyder motor.

What the production Ferrari & Porsche are able to do compared to the Nissan is get a better cylinder fill at higher RPM's.

In the case of the RS race motor it would be getting less cylinder fill in the 4k to 5k range because of camshaft & intake design that has been optimised for high RPM

To achieve higher HP & Tq from a set engine capacity you need nothing more than a bigger bang in the combustion chamber. Get more in & if it can be controlled you will get that bigger bang.

A higher lift & duration cam is a way of achieving a higher active CR

 

Overlap has nothing to do with dynamic compression. That is determined by intake valve closing. The later this event happens, the lower the dynamic compression will be, and vice versa.

 

for those interested, this discussion was active and basically concluded (at least for me) in this thread:

https://my350z.com/forum/na-builds/3...ion-ratio.html

 

for those intersted, here is a very good thread specific to the topic I initially asked about... it hits all the questions and reason for the questions I asked: cams profile/valve timing - DCR - high octane fuel/race gas:

http://speedtalk.com/forum/viewtopic...er=asc&start=0