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-   -   Vacuum reading with new cam sbc 350 (http://www.hotrodders.com/forum/vacuum-reading-new-cam-sbc-350-a-183773.html)

eric32 09-02-2010 07:26 PM

Vacuum reading with new cam sbc 350
 
Hello guys well today I finally got my cam swap done and hooked up my vacuum gauge and was surprised at my reading. First off new engine specs, 350 sbc, world sportsman 2's, edelbrock eps intake, holley 600 vacuum secondary, 260/270 212,218 @ 50 (custom hydraulic roller cam made after the lunati 60120 voodoo cam) 510/510 lift with a 106 center line with 112 LSA and is 6 degrees advanced. Timing is set at 16 mechanical and with full manifold timing with vacuum advance is around 28-30 degrees. I hope that is enough for engine specs.

Ok my old crane roller cam specs at 284/292 222/230 @ 50 509/528 lift and it gave me about 14 to 15 inches at idle as with its overlap it kind of went back and forth between the two.
The overlap on that cam was about 66 degrees. Ok now the part that surprised me when I put my vacuum gauge on and I got all of my timing and distributor curved at a 900 rpm idle I had a very steady reading of only 15 inches and I was guessing I would have around 18 or so?

My new cam only has about 41 degrees of overlap seat to seat timing and with a 112 lsa and short duration I figured I should have more. All the valves are set correctly and there are no vacuum leaks and timing tab I verified myself when i installed my timing chain and tdc locater.

When I checked it with just 16 degrees of mechanical timing I had only about 10 to 11 inches of steady vacuum and this cam is far from being a radical cam. Its a lot smaller then my previous one and looking at lunati's website with a similar flat tappet voodoo cam the 60101 they claim 19 inches at idle.

I know that cam is not exactly mine but the duration at .050 is about the same as mine and very close specs. Hope someone cam maybe help me out and let me know is this a normal reading for this small size of a cam.

Thanks
Eric

Custom10 09-03-2010 09:01 AM

So this was just a cam swap? or did you have the old cam in the motor before the rebuild using the new one in a fresh build. I assume just a swap.

Someone else may elaborate better on the scientific facts, but relationships between cam lobe design and the effect that they have on vacuum levels is not solely limited to lift and duration numbers. Take for instance the Rhoads lifter, it effectively lowers cam duration at lower RPM to increase the vacuum produced. My point is that there are other things go'n on that will effect what the gage reads. Id say that 15 is not that low for this cam, I had a comp 12-235-2 specs 210/218 @ 50 .447/.447 on a 111 LSA in a 350, with the same EPS, vortec heads, edlbrock 1405 it only pulled 16 at the very most.

I would not be too concerned, you can't really "guess" but interesting question.

eric32 09-03-2010 04:42 PM

Yes this was just a cam swap. This motor is only a year old with about 2000 miles or less on it. I went with a smaller cam for better street manners going through town and for a little less overlap for a better idle and other factors. I needed something better for cruising:) I have driven my truck for two days now and man I love the difference. I went from 160 psi to 180 psi and this cam really works well with my compression and other setup stuff and really has a nice torque curve from bottom to midrange and is no longer soggy on the bottom end and pulls very strong clear up to 5 grand:) I was just surprised though about only 15 inches of vacuum but the needle was really nice and steady at 15.
Eric

eric32 09-03-2010 04:45 PM

Also custom10 thanks for your info about your cam experience so maybe I was just expecting more. I read on a post about the voodoo cam I got my custom grind made after and they said it maid about 18 inches but maybe they was wrong. I guess there is more to what a cam will make for a vacuum level then LSA and duration. Hopefully someone can elaborate more on this.
Thanks
Eric

cobalt327 09-03-2010 06:08 PM

Add 10 of timing from a vacuum advance connected to manifold vacuum and you'll get the idle vacuum you want.

Custom10 09-03-2010 06:28 PM

I think he had his vaccum connected at one point and then disconnected another, not sure what his vaccum is with it, 15? which is what I assumed.

Quote:

Ok now the part that surprised me when I put my vacuum gauge on and I got all of my timing and distributor curved at a 900 rpm idle I had a very steady reading of only 15 inches and I was guessing I would have around 18 or so?
Quote:

I have driven my truck for two days now and man I love the difference
Yeah in general terms it seems that somewhere around 218-220 @50 deg duration is were cams start to loose some street manners and power/torque curves start to get pushed up into the higher RPM ranges. My new 383 tends to tame the lunati cam I have now which is also very close to your original, it is at 221/230 @ 50 with only .454 lift though cause its flat tappet, don't think I would like it much in a 350 driver. Not sure how a roller compares to a hydrualic flat tappet in terms of this discussion though? Glad to here your happy with the new setup.

cheers :thumbup:
TJ

eric32 09-03-2010 06:44 PM

Well to answer the one question I do have my vacuum advance hooked up to full manifold vacuum. I have 16 degrees initial without vacuum advance and I get about 11 inches of vacuum and when hooked up to vacuum advance which I have set around 12-14 degrees I get a total timing at idle of 28 to 30 degrees with vacuum advance and I have a reading of 15 inches of vacuum and the gauge is very steady. I have my idle around 850 rpm. It does not seem to want anything lower then 800. I have a way bigger more radical lunati voodoo roller cam in my other 350 and it does not make much less then this cam. I figured from reading other pages I should have around 17 to 18 inches of vacuum. Pasted below is where UD Harold himself said about the cam I got mine exactly made after. My cam was made 99 % like the lunati 60120 cam.


taken from chevelles.com

this is straight from the Designer of the Voodoo line, also designed the Comp XE line, and also owned Ultradyn cams..


Quote:
The one I remember dynoing was the VooDoo 60120, the 262/270 cam vs the Xtreme Energy 264HR. The VooDoo was 211/219 at .050, the 264HR was 212/218. The VooDoo lift was a little higher, and LSA was 112 vs 110.
From 5000 to 5600 the VooDoo averaged 10 BHP higher that the 264HR.
From 3200 to 4200 the VooDoo averaged 8 ftlbs torque higher that the 264HR.
But the best news was at 2500, where the VooDoo had 30 ftlbs torque MORE than the 264HR. It was 20 ftlbs more at 2800, and 3" more vacuum at idle.
With 9:1 CR, the 270 VooDoo is as big as I would run, and it works with 3.23 RAR.
Computer simulation programs assume that all cams flow air the same way, but they do not. If they did, one company's 300*, .500" lift cam would make power like another's. They don't.
A computer simulation program that gave an accurate Horsepower/torque curve would be similar to those used by the major car manufacturers, and would require the exact cam/valve lift curves, not just a couple of numbers.

UDHarold

That acording to him is why I expected more vacuum then 15 inches. At comp cams site there cam mentioned above made 15.5 inches of vacuum on there dyno readout.

When I got my cam I wanted the exact same thing as the 60120 voodoo cam but I had it made on a billet steel core and the guy said it will be very close and only difference is 1 degree @ 50 duration and 2 degrees seat to seat timing on the intake.

The actual cam timing numbers are a difference of only .5 degrees. Hope that explains why I figured I should of had more.

Eric

cobalt327 09-03-2010 06:54 PM

Different gages can give different results, but that wouldn't explain having basically the same reading (except for the 2in/Hg fluctuation) between the first cam and this cam.

Was the cam degreed in? Was the intake c/l installed at 106 (the advance of 6 ground in) or did you install it advanced an additional 6?

Custom10 09-03-2010 07:03 PM

me
Quote:

I think he had his vaccum connected at one point and then disconnected another, not sure what his vaccum is with it, 15? which is what I assumed.
Thanks for that clarification

Quote:

That acording to him is why I expected more vacuum then 15 inches. At comp cams site there cam mentioned above made 15.5 inches of vacuum on there dyno readout.
Good info, but I can't see how the engine vac can be calculated or expected to be a constant from one engine to the next with the same cam. There are allot of other factors.

eric32 09-04-2010 05:08 PM

When I installed my cam I installed it straight up cause it already had 6 degrees advance already. And no I did not degree it in. I usually just put them in and have no problems. I know you should but I don't have a degree wheel but in the future I will get one.
Eric

Custom10 09-04-2010 05:28 PM

1 Attachment(s)
Yeah all mine to this point all have gone in straight up, as long as you verifing TDC, I use just a basic piston stop to find it in relation to the TDC timng mark and pointer, first time I did it I remeber having to scrtach my head cause I was trying to line the marks up with crank gear mark at 12 oclock and then the cam gear mark at 6 cause I thought they were supposed to be together but then realized both need to be 12 oclock thus the "straight up" reference, 12 and 6 is #6 cylinder TDC

eric32 09-04-2010 07:31 PM

Yeah I just put mine dot to dot 12 noon on crank and 6 on the cam gear and I put my balancer and cover on and lined it up and verified top dead center and all was good. I have never really used a degree wheel and for basic builds I don't feel you need one but that is my opinion. My Dad has built hundreds of engines over the years and never used one. And no one ever cam back and complained. O well I will have to make a few carburetor adjustments and re check everything and see how it does. I just installed a new brake booster cause it was going bad. Take care guys
Eric
Eric

ssmonty 09-05-2010 06:49 AM

Just FYI,
I've got an old Comp 260HE(212*@ 0.050" I&E) in a 350 with a stock stall convertor, 600 Holley, Performer intake, tri y headers, 2-1/2" duals w/turbo mufflers, 14* initial timing, and @ 630rpm in drive I get 15" hg.
FWIW
ssmonty

eric32 09-05-2010 07:49 AM

Thanks ssmonty I guess that is normal for my size of cam then and I guess this post is done here :) Thanks for all of you guys who posted seeing different cams and sizes and vacuum reading at idle helps me get a better idea on how much vacuum to expect. I guess things is a ok on my end then I was just a little worried something was wrong but everything seems to work just fine. Thanks guys for all of your responses it really helps me out to gain knowledge and experience.
Take care
Eric

SDLuck 09-05-2010 09:28 AM

If neither cam was degreed you are not comparing properly.


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