xaon

iTrader: (12)

Here's the rundown on my setup
408 Iron Block 4" stroke, 4.030 bore 8cc dish/valve relief piston
AFR 225, opened to 4.030 bore and shaved to 63cc, GM MLS gaskets
Right at ~11.3:1 SCR
LS6 intake, ported throttle body, 1 3/4" longtubes, SLP dual/dual

I'm currently running a TSP Giant Cam, 248/254 .622/.612 114+2 (advertised events at 301/306, ABDC of 82.5, overlap is 75.5)

Currently the car is idling at 950/1000, MAP readings are mid 70's... so I'm only showing about 8.5-9 inHG of vacuum at idle... which is not enough to keep the upper A/C vents open at idle, and I lose my brake pedal if i'm moving around and braking without giving it alot of throttle.

By my calculations, my DCR is ~7.8:1

The car runs great and drives fine, but my biggest issue is the lack of vacuum at idle and the A/C not working.

It appears apparent to me that I need to go with a different cam to get my DCR up and to increase my vacuum at idle. After looking around, I see alot of people running cams with 402/408 setups at around 238/242 that are still making good power.

I ran the numbers on say a TSP MS3 cam, (237/242 .603/.609 113+0), and I come up with a DCR of ~8.3:1. I know the car will definately pick up some low end from increasing the DCR by half a point, and I might lose a little on the big end... But I'm wanting to know if I ultimately will lower my MAP reading and increase my vacuum at idle by switching to a cam of this size?

Advice, Opinions, Questions for me? I'd like to hear from some of you guys that really know your camshafts. I just got this setup back together and the car running again after a year of being down, and the LAST thing I want to do is tear it apart again for a cam swap... but if it will definately fix my vacuum issues I will do it in a heartbeat.

Thanks!

Build #1

Thats just a part of having a bigger cam in a street car. We had a huge cam in an LT1 car and the brakes always were ok. It was 270/272 @ .650/.642 110 lsa There was no A/C though, the whole thing was removed, even the stuff in the dash.

racer7088

I know that the stuff even chuck has done is right on the edge it seems with a cam that big. I just did one too and it was close and it was just like your LME engine which is of course about the same as ours too on compression and stuff. With the faster lobes and less seat timing you have a little less overlap and more vacuum but the lobes you have are not an XER and are fairly large on the seat timing. Basically overlap is what kills vacuum.

SStrokerAce

iTrader: (2)

Quick fix raise the idle.

Second fix is go to a thinner head gasket to bump the compression and DCR.

Third is advance the cam 2 degs more.

Fourth a new cam AND more compression will help. You can not have a large cam relative to the size of the motor and have it drive around IF you don't pay attention to the low RPM (below 2500rpm) part throttle TQ. DCR, SCR, overlap and the rest of the cam specs play heavily into this.

Bret

xaon

iTrader: (12)

I'll be going with a new cam here in the very near future to get my vacuum and DCR back up. Thanks guys.

beardWS6

iTrader: (1)

My set-up is very close to yours. I have NO issues at all with anything other than letting her warm up from starts. Once she idles good, she is ready to roll. You can see my cam specs in sig and numbers. Granted I have ET heads. Mike at G-Force tuned her pretty good for the street!!

98Z28CobraKiller

iTrader: (17)

I have no vaccuum problems with mine and my AC works perfect. My idle is at 1000 w/ the AC on. Make sure that your vaccuum T is not cracked. Mine was cracked and it took me a while to figure out why my AC didn't blow out of the top vents. I put a little rubber cement on the crack and VIOLA! See my current cam in sig