bschonman

Hi,
I am finishing up a built 99 motor that will have a Rotrex c30-94 feeding it. I also did the "exhintake" cam mod. I decked the head a massive amount for compression (.060") and some research indicated that my cam timing would be changed. Long story short, I now have every tool I need to properly time everything.

My question is about what the best desired cam timing setup would be for a supercharger particularly.

According to Patsmx5's thread, he recommends clocking the intake cam to these specs:
intake valve opens at 10.5* BTDC
closes at 51.5* ATDC

This increases overlap by 2.5*, which reading says is bad for TURBO setups, citing higher exhaust back pressure, but I am struggling to find any data about supercharged setups. I am assuming that higher overlap is still counter productive, but I am hoping some of you may have some insight into this situation.

If less overlap is desired, what is the best approach? Advance intake cam and retard the exhaust cam by the same amount?

Any and all input is appreciated
Thanks!

18psi

iTrader: (76)

It's gonna be really difficult to dial in any of this properly without a dyno to properly measure the effects.

bschonman

Yeah, I understand. I was hoping to come up with an educated starting point, and use my adjustable cam gears to fine tune when the car eventually goes on the dyno.

Bmiata40

I'm working on the same questions... Check this Supercharger Tuning Through Cam Selection and Cam Timing
Good luck

Bmiata40

Take note of 'exception to the rule' chapter with the high compression of your motor. Don't know if it applies, just sayin.

k24madness

iTrader: (6)

Madjak

It depends on so many factors... the only way you'll know is on a dyno with adjustable cam wheels. If you have decked the head make sure you mark on the wheels if interference occurs, between valve and pistons and valve to valve if you run oversized valves. Mine is scribed into both cam wheels so I can see clearly the window I can move the timing within. FYI I only get a few degrees each way with my cams and decked head before things start to touch.

A Rotrex supercharger is a different beast compared to a turbo or N/A. The amount of overlap you can run will depend on the dynamic compression, exhaust backpressure and amount of flow through the head and cams. Moving the cam timing should also move the power around on the RPM curve. What is better will depend on the type of driving you want.

bschonman

Madjak, how much did you deck your head? I'm a very beginner engine builder (first time), and of course didn't measure any piston-valve clearance, I just did research on what others have done. You are starting to make me think I may have bent a set of valves. Engine builds 165 psi on all cylinders, but has consistent missfires at idle and part throttle. Doing another compression test as well as a leakdown today to confirm.

Full_Tilt_Boogie

iTrader: (1)

The overlap is not a problem for your supercharger. The idea that you dont want much overlap with a supercharger is only applicable to a positive displacement supercharger which builds boost only by pushing against restriction. A dynamic compressor like a centrifugal or twin screw will, for the most part, build the same amount of boost regardless of overlap.
A turbo is also using a dynamic compressor, but the difference there is you have very poor scavanging due to to the high exhaust backpressure, which can sometimes be higher than the boost pressure. You wont have that issue with a supercharger. Your best cam specs and cam timing is going to be closer to what an NA car would run than what a turbo or roots SC car would run.

Bmiata40

Sounds like low compression considering .060 decking. But it's also even across, so probably not bent valves. May be the intake cam profile. Patsmx5 suggested intake cam timing is close to what I've ended up with in the past on several NA (naturally aspirated) 4 cylinder dohc engines with after market stage 3 race cams. The last motor had .040 skimmed off a 9.5 to 1 compression ratio engine; my pre-dyno timing procedure was to advance the intake cam 2 degrees at a time checking static compression till I found the highest compression without pushing it past 10 degrees advanced. Every motor is different, on this particular one I saw the highest change in compression between 6 and 8 btdc. Once on the dyno, we advanced it 2 further to 10 btdc. Some will also start with exhaust cam, which does not normally have the authority of the intake, where the magic happens. Any movement on the exhaust side for power/compression will be slight compared to the intake side. Hopefully others will critique this procedure, just wanted you to have some more info from my past experience. Madjak's suggestion on knowing where valve interference occurs is a good one. I run a fully charged jumper box as a helper to get the most consistent compression checks. I also pick a number of engine turns whether it be five or six, or more, even until it doesn't go higher, as long as it's the same each time.

Madjak

Yes with compression testing the key to accurate results is a fast cranking speed. I turn off fuel, spark, pull all plugs and hook up a dual 4cell RC lithium battery. Cranking is fast and it builds to full compression in 4-5 cranks. My engine has lots of static compression but also lots of overlap... but it still reads around 245 - 250 PSI.

My head is decked 2.75mm so 110 thou. At that level the timing belt is loose so you need to move the idler to get full tension.

With stock cams I'm not sure at what point stuff starts to become interference. It will depend on the piston dome I think.

Rotrexes can certainly handle some overlap better than other FI but you will still loose low and mid range power just like a N/A engine with big overlap. Sometime soon I'm going to drop a big Rotrex on my engine and then we'll see what overlap does. I'm guessing lots of flames out the exhaust at low revs as the boost blows out the exhaust. I'm running lots more overlap than stock cams though... like 55 degrees or something stupid.