I've heard both sides of this issues, and don't know what to do. Will big duration cams cause blow through on a boosted motor, and then not produce as much hp? Would running a larger duration intake cam and a regular exhaust cam produce better results? I guess a vtec motor is a good example of this, as the vtec lobs have a pretty big duration. I think John said 290 deg? Don't b18 turbo motors make pretty decent hp....250hp+

Thanks,

 

big cams and turbos go together well when tuned right. but big NA cams will generally have to much overlap. so cam gear tunign is required.

Cam upgrade on my SR netted soem very good gains.

Scott

 

Just like a NA engine using a high duration cam will generally move your power band up higher in the rpms. Whats going to affect cylinder pressure the most from blow thru is probably going to be more related to overlap.

Usually running a wider lsa will result in better cylinder pressure since intake and exhaust valves spend less time open at the same time, overlap. In V8's allot of your big forced induction cams will have lsa's around 114 degree or more for better cylinder pressure while NA engines will run a closer lsa to promote better air flow through the cylinder since it only has atmospheric pressure to feed it.

Big duration cams usually make less power on the bottom end due to a loss of low rpm intake air velocity to help feed the cylinder. Just think about trying to blow a spitball thru a .25" straw vs say a .5" straw. However once the engine is up to an rpm where it can generate adequate intake air velocity to help feed the cylinders effectively things begin to pay off, more air more fuel more power, usually that is, there is such a thing as going too far for a particular application. As an engine revs higher there is less & less time to fill the cylinder with air, this is why you feel most stock cam engines just flatten out on the top end. Even with forced induction there is only so much air you are going to be able to effectively fill the cylinder with in a given period of time and then the effort for the piston to try and squeeze the exhaust out, time for bigger cams I guess.

Any variable valve timing engine is a perfect example of what larger cams do, it's like having a built in comparison of small vs big cams without the loss of bottom end if it even exsited at all. That nice free kick of power is a prime example of what larger cams can do and how effective feeding an engine adequate air and fuel higher in the revs can be.

Anyways I've completely forgotten what the question was and I'm not going back to read it since my eyes feel like they're bleeding.

Check out the results other people have had with different setups, different engines respond somewhat differently to different things, intended use has allot to do with the choice. For a street setup conservative cams will usually prove to be better, loosing your brake boosting at one tap of the brake pedal from lack of manifold vacuum can get annoying at times, not to mention stalling and low rpm cruise engine chugging from the cams loping. For a strip setup with power in mind going bigger would probably be more suiting, rev higher, move more air, waste more fuel, & hopefully make more power, power is always subjective to your particular setup as a whole.

 

Thanks for the info. The set-up is 7.8-1 comp. 1.8l, 8v turbo motor....vw block and head. The turbo flows abour 550cfm.....it's an old t-4 that was rebuilt by Charlie (CRM). I am running 12psi right now. The cam has a duration of 226/225 intake/exhaust at .050" and a lift of .440" The lobe center is 110. The stock cam numbers are 214 .400" 110

I will try the above aftermarket cam and see what happens.

 

Damn, too bad I have been too busy then sick to get into this thread till now.

I have seen a lot of good info here with the exception of one incorrect bit of information in one of force fed's post.

Though it depends on the specifics of the engine, turbine and compressor, in most cases the pressure in the exhaust manifold will be very close to if not less than the pressure in the intake manifold.