Carbureted LSX forum - Will a stock LS1 cam pull too much vacuum at idle? Swap cam for carb application?
07-02-2012, 08:56 AM
I'm putting a carb'd LS1 in a my father's Panoz. I had no plans of changing the cam since the car is power/weight limited for its racing class and can only make around 330 rwhp. However, a local acquaintance recently put a carb'd LS6 crate motor in his F-body racecar and is having issues with bogging at tip-in. I.e. the OEM cam pulls too much vacuum and is pulling in too much fuel no matter what they do. He mentioned that they make cam profiles specific to carb'd applications which will correct for this. This is all new to me so any input would be much apprecaited.
My plan is the following:
1. Stock LS1 crate motor with 241 heads and the later 2001+ cam profile.
2. Stock manifolds, but no cats and side exhaust.
3. Edelbrock Performer RPM manifold with MSD 6LS controller setup.
4. Pro-Systems road race carb based on a Holley 750 with mechanical secondaries and all the other road race specific stuff. (Swapping the carb off of the blown up 347 currently in the car)
Any input or direction would be much appreciated. I'd rather not swap the cam if this is a non-issue, specific to his application with the Z06 cam vs. the LS1 cam, or a tuning issue.
Build thread - http://ls1tech.com/forums/conversions-hybrids/1559870-rebuilding-dads-panoz-ls1-swap.html
Pop N Wood
07-02-2012, 01:39 PM
Swapping out the cam might very well cure the problem, but that is definitely the long way around it.
Tell your buddy to reduce the size of his primary air bleeds a step or two and to keep the total idle timing (static + vac advance) down under 20° or so.
Too much timing bumps up the idle speed resulting in the butterflies being closed too far. That caused my AFR's to do squirrly things when slowly opening the throttle off idle. Backing off on the timing allowed me to open up the butterflies so 20 thousands of the transfer slots is visible at idle. Set the primary and secondary butterflies to be open the same amount.
The smaller primary air bleed and less timing cured the off idle bog on my stock cammed LS2 motor. The LS2 has the same cam and heads as an LS6 so it should work for you too. I'm running a standard 750 double pumper. I did need to drill out the air bleed press in restrictors and replace them with brass set screws.
At one point I tried to fix the bog with bigger squirters and a 50 cc accel pump. I don't need either of those anymore. By the way, don't think a pro systems carb will cure the off idle issue. A couple of guys on here have had some issues in that regard.
07-02-2012, 02:13 PM
10-4... I'm new to carbs, but I have a basic understanding of what you mean and have friends that can interpret for me :) I'll just put it together and see what it does. If it's something tuning can't cure I'll worry about a cam swap. Car is a race car so I really don't care about off-idle driving characteristics. Car just needs to run from 3000-6200 rpms at WOT the majority of the time.
07-02-2012, 04:59 PM
I agree with PopNWood, the only thing needed to get that engine to run properly is carb and ignition tuning.
You DONT NEED A NEW CAM, unless you want one...:devil:
07-02-2012, 05:32 PM
Brian is referring to my car. I'm not sure what the lobe separation angle for the LS1 cam is but the Z06 cam is 117.5. My car at idle pulled over 17 inches of vacuum. It was an absolute pain to keep this thing from going diesel rich at idle. I fouled two sets of plugs beyond use, trying to get it right at idle.
Got some dyno tuning time and got everything figured out pretty well but still have the slightest amount of tip in bog. honestly the only time it's an issue is when I'm moving the car around to get it on the trailer.
The tuner suspected that the high vacuum was pulling the fuel out of suspension and slamming it into the bottom of the intake. The idea of a cam swap came up to get something with a little more lobe separation to bring the idle vacuum down.
I ended up blocking off the rear power valve and going with the biggest squirters I could get. The idle air bleeds where brought down two sizes. The Pro-systems carbs have a longer transfer slots in the throttle blade area allowing a lot of air in. So air bleeds that are too big will lean out the idle mixture far too much.
Even then, the idle set screws are crazy touchy. If I even look at them funny the motor will bog.
Brian I'll give you a call to discuss further.
07-02-2012, 05:47 PM
You can just come over and help me adjust it once I get the motor in the car etc. etc. Have a ways to go before I get to that point. I pay in beer :lol:
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