vacuum reading vs Paxton

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vacuum reading vs Paxton

Postby gjz30075 » Thu Jun 15, 2017 11:02 am

Hi all, I was planning on taking a vacuum reading from the runner port on my GT350 but I'm told I need to disconnect the Paxton in order to be accurate. So, do I
leave the engine as is, ie, with the Paxton running or, remove the belt and possibly the plenum (for more air) to take my reading?
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Re: vacuum reading vs Paxton

Postby zray » Thu Jun 15, 2017 6:55 pm

Don't disconnect anything. I'd post some photos of vac. Gauge reading taken on my '66 GT350 but this forums restrictions on posting photos, the mandatory resizing is just just too laborious. I can email them to you if you wish. I will try to resize them later ,

With a Paxton friendly cam, not that much differt than the HiPo cam, I had 18-19" Of vac. at idle, with or without the Paxton.

I can tell you from experience, the vacuum readings are not going to change whether the Paxton is connected or not connected. The Pistons don't know or care what the atmospheric pressure is or whether it's been boosted. The pistons are going to be moving down the cylinder bores, creating a vacuum regardless of the incoming charge pressure.

Who is telling you to disconnect the Paxton ? What hands on experience do they have ? Or do they just have internet opinion experience ? ? Sorry, but misinformation drives me batty.

But let's just say for the sake of the discussion, say that you do disconnect the Paxton and take readings of the vacuum gauge, what good would that information be ???.

Normally, you are driving down the road with the Paxton installed and connected, what possible use would any vacuum readings be with the Paxton disconnected ? Since that's not how you are normally using the engine, any vacuum data provided with the Paxton disconnected would be useless.

AT IDLE

Image

AT WFO in 3rd gear. Couldn't take photo WFO in 4th gear, going too fast ! ! note gauges are different. The Paxton bird gauges gave up the ghost.

Image


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Re: vacuum reading vs Paxton

Postby gjz30075 » Fri Jun 16, 2017 4:30 am

Thanks Z, I knew you'd chime in. My source was a casual mechanic who sounded somewhat convincing with his, now obvious to me, fake facts about supercharging :D

My readings, with the supercharger working, wavers between 14 - 15 with a fairly lumpy idle. I'll P.M. my email and attach the cam sheet with what I think the cam is
currently installed, but not sure.

Thanks much! Makes sense to me now.
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Re: vacuum reading vs Paxton

Postby zray » Fri Jun 16, 2017 2:37 pm

Just as a general indication, cams with more duration (more overlap) vs. the stock cam are going to have the more lumpy idle and less engine vacuum. So 14 to 15" vacuum at idle is pretty good. What vacuum and low end performance is lacking with a lumpy cam is compensated (one hopes) by better performance on the top end .iii

BTW, The guys that are running power brakes that require moderate engine vacuum to openate normally would kill for the vacuum you have. They go for extreme cam producing 8-9" vacuuum then are all in a tizzy because their power brakes don't work !! Too bad, I tell them, should've done your homework before awarding yourself an honorary engineering degree.


Z

PS. are you trying to troubleshoot something via the vacuum gauge ? ?
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Re: vacuum reading vs Paxton

Postby C6ZZKGT » Fri Jun 16, 2017 4:33 pm

zray wrote:
BTW, The guys that are running power brakes that require moderate engine vacuum to openate normally would kill for the vacuum you have. They go for extreme cam producing 8-9" vacuuum then are all in a tizzy because their power brakes don't work !!


I had a customer with a 65 Mustang standard non Hipo engine who had a lumpy cam with very little vacuum for the power brakes. I installed a Cadillac belt driven vacuum pump which resembled a smog pump as I recall. This cured the power brake vacuum issue. I did this over 35 years ago so my memory is not totally up to snuff on all of the details.

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66 K GT Fastback Signal Flare Red / Black Pony Interior
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Re: vacuum reading vs Paxton

Postby gjz30075 » Fri Jun 16, 2017 5:13 pm

zray wrote:
PS. are you trying to troubleshoot something via the vacuum gauge ? ?


Nah, just having a 'need to know' moment. I really don't know what's inside my engine and I'm really just curious because you get asked this kind of stuff at shows and I'd
like to know answers. The cam sheet that I got with the car shows an Isky RPM-300 solid, which has a 108 deg lobe center, 270 deg duration , 228 deg at .050, valve lift of
.467.

So, how does this relate to a stock Hipo cam or LeMans cam?
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Re: vacuum reading vs Paxton

Postby zray » Fri Jun 16, 2017 7:16 pm

Despite the fact that Vacuum gauges can reveal a myriad of engine difficulties , they are seldom installed. Having one permanently installed has given me a baseline to follow whenever I am having a problem, or better yet, the gauge often can be an early warning device, that can "foresee" an issue even before the seat of the pants is telling you something's wrong.

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