Magman

iTrader: (6)

Just got my motor running again with new cam/heads and it's making 11" vac at idle. What are some tricks to increase idle vacuum? I have TPS set to .54, IAC sitting at 30 or so and idle set at 850 in park.

New cam is 236/242, 112 LSA w/.579/.584 lift and AFR 1095's.

When I did all my upgrades (Cam/heads/Stall/tubular k-member & A-arms) I also swapped to manual brakes, but after driving it today, I know I need to return to power brakes. Manual brakes are terrible at the slow speeds I drove it at. Couldn't imagine slowing down from 120 mph!

What should I look for for Spark Adv at idle? It's at 26 right now... Any thing else to improve VAC so I can go back to power brakes? Will 11"hg even work?THX

Night rider327

iTrader: (13)

What manual brake system did you use? I swapped over to manual on mine and truthfully it stops as good as the power brake system ever did, just takes more pedal pressure/effort.

I have the small bore strange master cyl, no prop valve, and added a 10 lb residual valve inline with rear brake lines.

Rest of my brake system is pretty basic. Autozone replacement calipers up front with auto zone pads, new rubber flex hoses, power stop slotted rotors, 1978 firebird drum brakes on rear (to fit my ford 9" rear), alum. brake drums.

No real tricks to gain vacuum. The compression and the cam pretty much sets the amount of vacuum your engine will make. You can try adjusting your carb's idle mixture screws to get highest vac levels, try adding alittle more timing, but you will not gain much if any.

If you must have the higher vac levels then you may want to look into a vac pump. They make elec and crank driven styles of vac pumps. Alot of factory cars used an elec vac pump stock so that might be an option

91 camaro racer

iTrader: (2)

what about using a vacuum reservoir? would that help any?

Night rider327

iTrader: (13)

Those vac cans are really just a band-aid. I tried one on my car and yes it did help but the results only last a short time frame.

Like worked great if driving down the road and say someone pulled out in front of me and I had to hit the brakes... No probs..

But having to sit at a red light or even a 4 way stop and my reservoir was only good for a few seconds. With foot on brake you could feel it slowly go down to floor and car start inching forward. Many times I had to just put trans up in park at a red light, then back in drive when it turned green to keep.

Now my engine only has 4 to 5" hg vac at 1,000 rpm idle so that is a big diff in the OP's 11" of vac

sofakingdom

iTrader: (1)

Advance the timing.

If it helps, then adjust your timing curve on your PROM, to give it the same "total" timing (probably about 36°), but ALOT more low-RPM timing. I.e., less of the electronic version of "centrifugal advance". You should end up with around 50° of advance at cruise, when the timing will be the sum of "total" plus the electronic version of "vacuum advance".

You will probably find that it will work best with around 18 - 20° of timing at idle.

Magman

iTrader: (6)

Thanks guys. I'll try messing with the timing a bit. I have the 1.032 Strange Master Cyl with the stock Prop valve and higher tension spring in valve. I just raised my MC rod on brake pedal another 1/4 inch and I'll see if that makes a difference. I did keep most of everything to put it back to stock so I'm still contemplating...

I really need to measure line pressure at calipers to see where I'm at. Also looking into the LS1 brake setup as another option. Manual brakes with my stock setup is no where close to safe on the street.

ex-x-fire

Most cars that have the option of either manual or power brakes will have 2 pin locations for the master cyl. rod hook up. This would give you more mechanical leverage for nonpower brakes.

Night rider327

iTrader: (13)

I have the same master. #B3360T 1.032" bore. I can't remember how far up I moved my rod but at time of install I did the math on it and drilled stock pedal and used a rod end and bolt on end of master's pushrod.

Here's a couple pics of where I drilled new hole.

Magman

iTrader: (6)

That's exactly where I had it. Today I drilled another hole and moved it up another 1/4 inch leaving maybe 1/16 inch clearance before the bolt hits the upper brake pedal mounting bracket. The pedal feels soft now and just about goes to the floor before the car stops. I'm going to put it back down and try bleeding it again. And yes, I bench bled it many times.

skinny z

11" of vacuum operates my power brakes without any difficulty.

25thannivZ28

I only have about 7" vacuum at idle, 18 deg adv. 33 total adv. I hooked my power brakes up to a vacuum pump.

Magman

iTrader: (6)

That's good to hear that 11" hg operates the power brakes. I'm seriously considering going back. Unfortunately, I won't be able to wrench on it for 45 days or so, job has me leaving again.

fireburdluvr85

It may not matter but isuzu NPR trucks have an electric vacuum pump system. It operates when needed or when you hit your brakes. Works well.

ASE doc

I have 14" vacuum with power brakes. My stock brakes weren't great. They worked but being used to brakes on the newer cars I work on everyday, my brakes seemed weak when I needed to stop quickly. I upgraded to Fly N' Bye 13" front rotors with Wilwood 4 piston calipers in front and LS1 conversion in back. The brakes are excellent now. I will swap the rotors out for drilled and slotted at some point but for now they are more than I ever need. Your cam and my ZZX(240/240 @ .050, .560/.560 112 LSA)are very close in duration and LSA(you're obviously getting more power with the HSR and added cubes). I have my base timing dialed in at 22 degrees. That gives me a decent idle with 14" vacuum on the 355.

skinny z

What is the ICL of your cam? I'd like to compare it to my current cam (a custom grind) and in particular the amount of overlap. The 14" of idle vacuum is very impressive. Do you have the advertised duration values as well?
I'm running a custom grind, 274/282 (adv), 224/230 @ .050", 106 ICL, 110 LSA (installed straight up). With my best tune, running a 750 Barry Grant VS carb with 4 corner idle and 16 degrees of initial advance plus 14 degrees of vacuum advance, I can manage, at best 11" of idle vacuum at or around 850 rpm. Seems to me I could get more.

ASE doc

Adv is 290. I only know the LSA is 112 degrees. I don't remember intake centerline, or intake closing. I installed this cam 15 years ago and have long since lost the cam card. It is sold by TPIS as the ZZX. They should be able to provide details on the grind. It is a roller cam, which makes alot of difference.

You say you have initial set at 16 degrees with 14 degrees of vacuum advance. Are you using manifold vacuum for the vac advance? Some OEs did this to improve idle quality and would generally have a restrictor in the line to prevent the vac advance from dropping off on throttle tip in. Usually on performance tunes, I use ported vacuum for advance and limit travel of the unit(generally adjustable by way of a allen screw in the can), allowing for more initial advance, without going too far on total. Try recurving the distributor to achieve 22 initial, with 29 degrees at 2,900 rpm and whatever works best for you up top, generally between 34 and 38 degrees, without vacuum advance. Then bring vac advance in starting at about 7-10 degrees. Vac advance will affect light load, part throttle cruise and serves to improve combustion efficiency and maybe boost fuel mileage just a little. On any build of more than about 1.1 hp per CID, I leave it off to prevent combustion chamber over heating. I have digital EFI on the IROC with ECM controlled timing. Kind of nice to adjust timing from the driver's seat with a laptop, once base is set up that is. I have no additional cruise lead added to the timing map, just for the reason I stated. I have a fairly straightforward curve starting at 22 degrees base, a fairly quick ramp to 29 at 2,900, and 36 degrees by 6,000.
I have tried adding part throttle cruise advance, around 44 degrees total in the lower MAP value(higher intake vacuum) range. In hot weather on extended part throttle cruise(traffic jam), I had it add 40 degrees to the coolant temp. I've had the same experience with customer's cars and therefore no longer add much vac advance on these more serious builds. We make more power by making more heat to begin with. Pushing even more heat trying to save a little fuel is not worth risking the life of the engine. In the end though it's whatever works for you.

skinny z

Advance is full manifold vacuum. I found it improved my part throttle tip-in repsonse and also lowered idle temps by about 20 degrees or so. Even more on a hot day. At cruise, regardless of the ambient temps, I barely get above my t-stat setting which is a cool 160F. I run about 165 or so. Aggressive open track driving will see it come up to 180-185.
Taking even a few degrees of part throttle advance (via reducing the vacuum advance) killed about 2 mpg on the highway. Typical cruise advance at 2500 rpm will be around 40 degrees. At 9.8:1 SCR and 8:1 DCR there should be plenty of tolerance to spark advance. WOT only sees 34 degrees which seems to be what these heads (and the similar Vortecs) want. Of course WOT sees no vacuum advance. Mechanical advance curve is tame. Advance starts at 1500. By 2400 there's 8 degrees and it's all in at 3400 for 18 degrees.
I've found two types of adjustable vacuum cans. I'm using the Crane unit was has the on-set point (not the amount) adjustable via the allen screw. The amount is dictated by a serrated washer that's fixed to the distributor breaker plate.
Without the 28-30 degrees of idle timing (14 -16 degrees of mechanical plus vacuum in varying amounts), idle vacuum is not nearly as good. I haven't run a test lately however from my present 11-12 (using full manifold vacuum) it'll be a couple of inches less with only 16 degrees mechanical. That's with the carb tuned to within an inch of it's life looking for the highest possible vacuum amount via mixture screws with lowest possible idle speed. As it is, I idle at about 850 and I'd like to see it at 650.
For what it's worth I have cranking compression around 185 psi.

Your ZZX cam specs out as a single pattern 290 adv. 240 @ .050", 112 LSA, 108 ICL. That gives an overlap of about 74 degrees seat to seat. 24 degrees at .050".My cam on the other hand is a dual pattern 274/282, 224/230, 110 LSA, 106 ICL. That gives 58 degrees of ovelap seat to seat. 7 degrees at .050".
Seems to me I should be able to see at least another 2" of idle vacuum.

ASE doc

Thank you for finding some specs on the ZZX. I'll write that down for my own reference. I need to call TPIS and see if I can get a replacement cam card. With 185psi cranking compression, you should have better idle vacuum than 11". Is the idle rough? You didn't say if your cam was flat or roller. This does make a difference in ramp rate between .006 and .050, given the same ICL and LSA, and will affect cylinder pressure and inlet signal, but you have 185psi cranking. That tells me you can't be losing much to overlap. Are you sure your vacuum gauge is accurate? This makes me want to recheck my own vacuum. It's been some years since I took that reading.

Going back to your original concern, the OE brakes on these cars are 50 year old technology. No real improvement from GM's first disc brake systems. The same Delco Moraine boat anchor calipers and tiny rotors. Just barely meeting federal safety standards. Even with 19 inches of vacuum, they still suck. They will never be suitable for our high performance vehicles. I lived with mine, gritting my teeth for 13 years after building the rest of the car. Finally, I made it a priority, ordered the parts and made it happen. The brakes I have now cost $1,850. $1,000 for the fronts and $850 for the rears. I know there are less expensive options out there but I wanted to get away from the sliding calipers. I at least did that in front and the 13" rotors with the Wilwood FSLs are great. They were pretty much a straight bolt on, with a little added work on the rears due to my LCA relocating brackets. They are definitely the best of all the upgrades I have made to the car.

skinny z

I don't want to hikack this thread anymore than I have so I'm going to start another and ask around what others have for idle vacuum vs. their particular engine combinations.
Might be interesting.

ASE doc

Lol. I just realized I posted to skinny Z about brake upgrades. Got you confused with the OP. Sorry bro.

skinny z

No problems here. I've set up another thread regarding idle vacuum and am trying to solicit some data.
https://www.thirdgen.org/forums/tech...le-vacuum.html

ASE doc

My IROC is stored for the winter. I don't drive it in wet weather. If I can remember it, I will take my vacuum gauge home and do a quick check of idle vacuum. You've got me curious now.

skinny z

If you could get some timing data that would be great. Curious as to what your FI likes to idle at.