Idle vacuum with big cam
Idle vacuum with big cam I'm getting what I think is lower than normal vacuum at idle, but I'm not sure.
This is a 5.3 that was rebuilt 6500 miles ago. And approx 1000 of that after the cam swap.
It was an LMG. It has 799 heads, tbss type intake, 92mm tb, s ummit 8708 cam (10 degrees overlap) LS7 lifters with .090 preload. Summi t springs 140 seat 390 open.
Ebay 1 5/8 headers to sum mit 2 1/2 dual exhaust. Running a 2002 silverado base tune in a 411 pcm.
Warm idle is 65 to 70 kPa absolute which is 8 to 10 in hg on a gauge. It's steady. When it gets hot like a long drive or city traffic it'll get down to 75 kPa. It has ac and a fairly loose converter and idles at 800. IAC counts are 60 to 80 in gear. 0 to 40 in park.
I've tuned it fairly well to about 15:1 idle afr.
Changing the idle timing doesn't help. It has idle spark correction so the advance is all over the place at idle. If I shut this off with hp tuners vcm controls it will surge and stall soon after.
I'd pretty much consider the tuning done but I don't know what to expect for idle vacuum. Shouldn't it be more? Like around 12+ in hg.
This is a 5.3 that was rebuilt 6500 miles ago. And approx 1000 of that after the cam swap.
It was an LMG. It has 799 heads, tbss type intake, 92mm tb, s ummit 8708 cam (10 degrees overlap) LS7 lifters with .090 preload. Summi t springs 140 seat 390 open.
Ebay 1 5/8 headers to sum mit 2 1/2 dual exhaust. Running a 2002 silverado base tune in a 411 pcm.
Warm idle is 65 to 70 kPa absolute which is 8 to 10 in hg on a gauge. It's steady. When it gets hot like a long drive or city traffic it'll get down to 75 kPa. It has ac and a fairly loose converter and idles at 800. IAC counts are 60 to 80 in gear. 0 to 40 in park.
I've tuned it fairly well to about 15:1 idle afr.
Changing the idle timing doesn't help. It has idle spark correction so the advance is all over the place at idle. If I shut this off with hp tuners vcm controls it will surge and stall soon after.
I'd pretty much consider the tuning done but I don't know what to expect for idle vacuum. Shouldn't it be more? Like around 12+ in hg.
You'll likely need more compression to help with idle vacuum. 10 degrees for a 325cid motor is going to idle worse than it would in a 346. Probably like a 13-14 degree overlap cam would in a 346. So 65-70 kPa is about right.
You'd need something in the 2-4 degree range to idle in the 55-60 kPa world without changing compression. The 8706 from Summit would drive better and not give up much power given the mods you have. Would make more torque and more avg power actually. It's a 226/230 113 cam. Yeah, it's for turbos, but turbo cams are spec'd like an NA cam if your turbo setup works 1:1 or 1:2 on backpressure.
You'd need something in the 2-4 degree range to idle in the 55-60 kPa world without changing compression. The 8706 from Summit would drive better and not give up much power given the mods you have. Would make more torque and more avg power actually. It's a 226/230 113 cam. Yeah, it's for turbos, but turbo cams are spec'd like an NA cam if your turbo setup works 1:1 or 1:2 on backpressure.
Last edited by JakeFusion; Mar 19, 2020 at 03:15 PM.
IMO that's way too much cam for a 5.3 and you could get more avg power like jake said and all around better everything if you put something smaller in it.
Probably be faster down the track too unless you're revving it way out over 7k and keeping it there.. aka have a 6 or 8 speed in it with minimal rpm drop/keeping it way up in rpm between shifts. Or a big stall keeping the rpm way up between shifts
If the converter is loose enough you can try bumping the idle to help it out if you don't wanna change the cam compression etc.
Probably be faster down the track too unless you're revving it way out over 7k and keeping it there.. aka have a 6 or 8 speed in it with minimal rpm drop/keeping it way up in rpm between shifts. Or a big stall keeping the rpm way up between shifts
If the converter is loose enough you can try bumping the idle to help it out if you don't wanna change the cam compression etc.
Last edited by 00pooterSS; Mar 19, 2020 at 04:07 PM.
What kind of dynamic compression do these engines like?
When I put it together I calculated 10.3:1 static. It has true flat tops .010 above the deck, and a GM .051 gasket. With this cam IIRC dynamic is 7.4:1.
I've run older engines in the 8.5:1 DCR range on 93 octane.
This was put together to stay mostly stock, but then I went with this cam based on this http://www.cpgnation.com/the-100-horsepower-upgrade/
That cam is nearly identical in specs, not sure of lobe profile. And since the rest of the engine is almost exact I thought why not.
Maybe I'll go for more rpm.
When I put it together I calculated 10.3:1 static. It has true flat tops .010 above the deck, and a GM .051 gasket. With this cam IIRC dynamic is 7.4:1.
I've run older engines in the 8.5:1 DCR range on 93 octane.
This was put together to stay mostly stock, but then I went with this cam based on this http://www.cpgnation.com/the-100-horsepower-upgrade/
That cam is nearly identical in specs, not sure of lobe profile. And since the rest of the engine is almost exact I thought why not.
Maybe I'll go for more rpm.
Dynamic compression doesn't have anything to do with any type of engine. All engines like the highest you can run. The fuel you run is what you compare to what dynamic you run.
I'd swap the cam to something that works better with your setup man. Or put a 6.0 in with that cam, and bump compression on it a little.
I'd swap the cam to something that works better with your setup man. Or put a 6.0 in with that cam, and bump compression on it a little.
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I'd like to get a six liter. I have 4 LKQ u pull it yards and a u pull and pay within 1 and 2 hours from me. They average 8 six liters a week between all 5 yards. But by the time I get there they're gone, mainly because I work during the day.
Thats $175 for the short block, I have everything else. Thats what I payed for a 5.7 hemi short block. All I have to do is try harder.
This cam is kind of disappointing given the description they provide. I've had better results putting the cam that worked for that guy in a gen I, one of the last ones I built I did a cam swap to match a magazine article and that alone went 1.5 seconds faster at the track.
When I was building big blocks I would do it right and start with what rpm I wanted to run, then pick a cam to get it there, then get the pistons to get in the mid 8's DCR, the fuel system to feed it, the rotating assembly to handle it, etc.
I haven't yet had a worth while experience with an LS engine, but I'm not blowing money like I used to either.
I'm probably going to put the old cam back in this, it wasn't that slow with it, and it got way better mileage.
This cam here is a great choppy idle only for this engine. I've never had so many random people come up to me and ask about it. Got some thinking to do.
Thats $175 for the short block, I have everything else. Thats what I payed for a 5.7 hemi short block. All I have to do is try harder.
This cam is kind of disappointing given the description they provide. I've had better results putting the cam that worked for that guy in a gen I, one of the last ones I built I did a cam swap to match a magazine article and that alone went 1.5 seconds faster at the track.
When I was building big blocks I would do it right and start with what rpm I wanted to run, then pick a cam to get it there, then get the pistons to get in the mid 8's DCR, the fuel system to feed it, the rotating assembly to handle it, etc.
I haven't yet had a worth while experience with an LS engine, but I'm not blowing money like I used to either.
I'm probably going to put the old cam back in this, it wasn't that slow with it, and it got way better mileage.
This cam here is a great choppy idle only for this engine. I've never had so many random people come up to me and ask about it. Got some thinking to do.
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The low vacuum reading seems reasonable to me. Sum 8708, 231/242 113+3, .600/.600 Lift in a 325 cube engine.
The 282HR example from the link below is a 230/236 in a SBC 350 cube and had 9" Hg @ 800 RPM & 11.5" Hg @ 1000 RPM w/ no load. While not LS based I think this gives a fair idea of vacuum vs cam duration vs motor size.
Comp Cams SBC 350 vacuum & dyno with various cams
Raise idle to 1,200 or so? New cam? Vacuum pump & tank? Live with it as is?
The 282HR example from the link below is a 230/236 in a SBC 350 cube and had 9" Hg @ 800 RPM & 11.5" Hg @ 1000 RPM w/ no load. While not LS based I think this gives a fair idea of vacuum vs cam duration vs motor size.
Comp Cams SBC 350 vacuum & dyno with various cams
Raise idle to 1,200 or so? New cam? Vacuum pump & tank? Live with it as is?
Last edited by 99 Black Bird T/A; Mar 19, 2020 at 07:31 PM.
I'd like to get a six liter. I have 4 LKQ u pull it yards and a u pull and pay within 1 and 2 hours from me. They average 8 six liters a week between all 5 yards. But by the time I get there they're gone, mainly because I work during the day.
Thats $175 for the short block, I have everything else. Thats what I payed for a 5.7 hemi short block. All I have to do is try harder.
This cam is kind of disappointing given the description they provide. I've had better results putting the cam that worked for that guy in a gen I, one of the last ones I built I did a cam swap to match a magazine article and that alone went 1.5 seconds faster at the track.
When I was building big blocks I would do it right and start with what rpm I wanted to run, then pick a cam to get it there, then get the pistons to get in the mid 8's DCR, the fuel system to feed it, the rotating assembly to handle it, etc.
I haven't yet had a worth while experience with an LS engine, but I'm not blowing money like I used to either.
I'm probably going to put the old cam back in this, it wasn't that slow with it, and it got way better mileage.
This cam here is a great choppy idle only for this engine. I've never had so many random people come up to me and ask about it. Got some thinking to do.
Thats $175 for the short block, I have everything else. Thats what I payed for a 5.7 hemi short block. All I have to do is try harder.
This cam is kind of disappointing given the description they provide. I've had better results putting the cam that worked for that guy in a gen I, one of the last ones I built I did a cam swap to match a magazine article and that alone went 1.5 seconds faster at the track.
When I was building big blocks I would do it right and start with what rpm I wanted to run, then pick a cam to get it there, then get the pistons to get in the mid 8's DCR, the fuel system to feed it, the rotating assembly to handle it, etc.
I haven't yet had a worth while experience with an LS engine, but I'm not blowing money like I used to either.
I'm probably going to put the old cam back in this, it wasn't that slow with it, and it got way better mileage.
This cam here is a great choppy idle only for this engine. I've never had so many random people come up to me and ask about it. Got some thinking to do.
The only thing wrong with that cam is it's a really poor match up to your combo. To be blunt, it was just a horrible choice. Not the fault of the motor or cam.
If you built an LS properly, like you way you described the big blocks, they would run right. Also, you need to do a bigger LS and do it right, going from big blocks to 5.3's is going to be depressing. But a well built 6.0 or 6.2 is pretty damn impressive and can easily stack up to some big blocks. A well done 6.0 or 6.2 can put down 500 ft lbs and 600 hp (flywheel) without getting to expensive or wild, needs to be properly planned out but probably doesn't cost as much as you might think. Good compression, LS3 heads and intake and the right cam will put you damn close and all that dont cost much at all. (flywheel). 5.3's are just too little. I can talk **** about them, I have one. I call it my baby dick motor.
Thanks for all the replies everyone.
I think it's just never mattered before, vacuum was what it was. But now that I've tuned it with a factory PCM, I'm realizing how much it effects how it runs.
I think I have it tuned as best as it's going to get, but yet there are some very minor quirks to fine tune if I want to. Or call it good. But it's either not fast or I got used to it.
Is a 4.5 second 0 to 60 mph fast for this set up in an S10 with pretty good traction but not from a stop? The problem is I also have a video of the old cam (a '02 LS1 cam) doing a 4.7 0 to 60 mph. AND I have an '09 Charger with a 0-60 timer that will get 4.9, 148,000 mile ex patrol car with 13,000 hours on it and 4000+ pounds. Nothing seems fast I think because my crap is all slow.
I think it's just never mattered before, vacuum was what it was. But now that I've tuned it with a factory PCM, I'm realizing how much it effects how it runs.
I think I have it tuned as best as it's going to get, but yet there are some very minor quirks to fine tune if I want to. Or call it good. But it's either not fast or I got used to it.
Is a 4.5 second 0 to 60 mph fast for this set up in an S10 with pretty good traction but not from a stop? The problem is I also have a video of the old cam (a '02 LS1 cam) doing a 4.7 0 to 60 mph. AND I have an '09 Charger with a 0-60 timer that will get 4.9, 148,000 mile ex patrol car with 13,000 hours on it and 4000+ pounds. Nothing seems fast I think because my crap is all slow.
That's the truth right there. Even my first Gen I was a 400, I'm going backwards here. 
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It is possible in extreme cases to have so much more duration than an setup can handle, that you make less power than the heads have potential to deliver. I think you’re there.
Either put the cam in a 6L with the 799 heads (between the displacement and compression it should at least work better) or go with the cam JakeFusion suggested. Might even be happier with the 8720.
Either put the cam in a 6L with the 799 heads (between the displacement and compression it should at least work better) or go with the cam JakeFusion suggested. Might even be happier with the 8720.
Last edited by 68Formula; Mar 19, 2020 at 10:29 PM.
The vacuum definitely always mattered. With a carb I would say it's even more important for proper fueling.
If you want to keep this cam put a 5k stall in it and a short runner intake and turn the idle speed up quite a bit and rev it out to 7500
In my opinion 4.5 isn't very good at all for a stalled cammed v8 s10. I'd want sub 3 second 0-60
I bet you would be faster with a stock cam from 0-60 if it's all setup well.
If you want to keep this cam put a 5k stall in it and a short runner intake and turn the idle speed up quite a bit and rev it out to 7500
In my opinion 4.5 isn't very good at all for a stalled cammed v8 s10. I'd want sub 3 second 0-60
I bet you would be faster with a stock cam from 0-60 if it's all setup well.