Thread: S4 Cams and Mid Range - Cam Timing Question

I primarily use my car for autox and recently switched to a bluebird with S4 cams. On my old motor (rr that came in the b15), I ran it with stock 9.8:1 compression, then 8.5:1 pistons. In both cases, I used the same turbo setup I'm using now with a full 3" exhaust. The car had a pretty solid mid range, with plenty of response and torque from 4200 to 5800 rpm.

Now that I'm running the new motor with S4's, the motor does not hit hard until 4800 rpm, but it pulls like crazy all the way to 7700. I'm now running a 3" exhaust that ends with a turn down past the driver seat, so I would expect spool to be the same or faster than before. I can probably tune a little of the lag out with some afr adjustments down low, but I'm looking for options to shift the torque curve down in the rpm range. I'd love to pull it down about 500rpm, but I'll take what I can get.

I know a lot of folks will say get different cams, but I'm wondering how much of an effect cam gears can make on my setup. Does anyone have experience with this? I'm not looking for peak number increases, just area under the curve (especially down low). I've read about retarding intake and advancing the exhaust to reduce overlap giving better numbers on turbo cars. I've also read about advancing the intake cam to get better low end by closing earlier in the compression stroke. But how much of change should I expect and is cam gears worth it for my goal or should I start looking at other options?

Well, I now have a set of CS gears on the way, arriving on Wed (Greg V is the man). I probably will not get to do any dyno testing in the next few weeks, but hopefully the data logs will show enough to dial in the gears for more low end response (as much as can be expected with S4s, anyway).

If anyone has suggestions on where to start, let me know. A lot of folks dial out overlap by retarding the intake 2* and advancing the exhaust 2*. Some go a little farther. There's also the option to advance the intake to increase low end response. This of course increases overlap and may make things worse, so I'm thinking of a nice combo. The -2/+2 approach gives 4* less overlap. If I take that and shift everything by 2*, I can run 0/+4 and have decreased overlap and maintain the intake cam. I could push it even further and go with +2/+6, assuming everything clears in the combustion chamber. Anyone know the maximum I can advance the exhaust cam before contact with the piston? I plan to manually rotate the crank a few times to make sure I don't feel anything.

So cam gears are in and I did a test drive. The settings I went with were 0 deg on the intake and +4 on the exhaust cam. This basically took out 4 deg of overlap without retarding the intake cam. Right off the bat, I noticed a decrease in the lope from the s4s at idle... an expected change with decreased overlap. The test drive seat of the pants feel says the car is more responsive earlier in the rev range, but the top end feels a little less potent (not much). The data log brings a little quantitative data to the mix.

With the stock cam timing, full boost would hit at 4800 to 4900 rpm. The z32 maf would hit 4.15 to 4.18 volts in the 7k+ rpm range. With the exhaust cam advanced 4 deg and the same boost level, full boost hit at 4300 rpm. The maf only hit 4.10 to 4.12 volts in the upper range.

I still have a lot of testing and would like to get this thing on an extending dyno session to explore more settings, but for now I think I've made a move in the right direction for getting usable power lower in the rpm range for autox. I have an event this sunday, so I'll really get to see how the car feels and responds coming out of tight sections compared to last weekend's autox.

well how much boost and are you running the t-25? I ran BC2's with my t-25 on 18psi and it made peak tq at 3800rpms and that was also where it hit full boost.

I'm running a 3071r at 14psi.

I ran an event this last weekend and the car feels MUCH better with the new cam timing. It's going to stay where it is (exhaust advanced 4 deg, intake at 0 deg) until I can get dyno time to fine tune it. I need to focus on a few suspension tweaks now... getting the car ready for a ProSolo this weekend.

your running a 30r for autocross?

Why not? I'm plan to run SC6262.... Nothing wrong with turbo for autocross.

it depends on the size of the track to determine what size turbo would be good for autocross...

Not really... Learn to drive and it doesn't matter. Bigger turbo just means you get back in the throttle earlier, on tighter courses you don't get to really take advantage of it, but if you are smooth it doesn't hurt you.

BTW that was a rhetorical question.

My reasoning is basically because I didn't get what I thought I did. When I bought the turbo about 4.5 years ago, I ordered a 2871r with a T3 flanged housing. The company (innovative turbo, now no longer in business) made the turbo but used a T3 wheel and housing instead of machining a T3 housing to the proper 2871r wheel. I did not know any better when I put it in, because I never drove a 2871r before to notice a difference. It was only when I went to get a new turbine housing that I found the issue.

BTW, the turbine housing is only a 0.48 A/R... so it's not too bad on spool.