i'm helping my buddy pick a wicked n/a cam and i believe i'm pushing the limits of the motor and the vacuum reading is going to be very low even with a canister.

So does anyone know about at what vacuum reading you need to switch?

i'm thinking about 5" or so

The vacuum can won't have anything to do with it, as it just stores vacuum for power accessories. I think it has a lot to do with the tuning capabilities of the tuner. A good tuner can tune a low vacuum engine to run speed density.

I do think your 5" Hg is a pretty good cutoff though. The best thing is to try speed density and see how well it will work. I always prefer it to alpha-N if possible, especially in street cars. You can tune Alpha-N to run well though too.

IMO, the biggest problem with Alpha-N is if you run a BIG throttle body that has a lot of airflow change with minimal TPS change. Then it is hard or impossible to tune well, unless you can mess with the TPS scaling.



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Doug
72 388 EFI Nova
Chevytalk.com EFI Moderator

I don't know what type of cam you're planning on but maybe sharing the specs might help. As an example I had a 377 with a circle track cam that made 7" at 900 rpm and made peak power of around 500 hp at 6500 rpm. I now have a 400 that makes 10" at 900 rpm with a new X Comp cam that makes the same power @ rpm (obviously average power and torque is much greater) and either engine I believe could easily be managed with a SD system.

Back to your question, maybe Electromotive has an answer. A long time ago when I did some research they had a software that went from alpha-n to SD (or MAF depending on what you have) at a user specified rpm. Much more useful than alpha-n alone it it works as advertised.

Just my 2 cents worth but vacuum cans are useless if you *seriously* need more vacuum. Also, a high idle helps a lot. At about 1200 rpm I have about 15". Reportedly, and not that I have any direct knowledge, pro stock cars idle fairly well. I guess as well as any 500" 10K rpm 1500 HP engine can so as to not foul plugs and overheat in the pre-200 mph blast. Not a direct comparison between the types of engines we're talking about but relative enough I guess.

What are the cam specs and displacement. Another thing that really helps is running a lot of idle timing with a big cam. I assume you have programmble timing control. I run 25+ with big cams.

I run a 242/248 @ .050 with a 110 lc solid roller cam in my 388 and run around 56-60 KPa at 900 rpm. It idles and drives very well. I could idle lower, but want some oil splash for the roller lifters if it matters.

Car is going to be a 377 because a steel 400 crank isn't cheap and i can get a 350 steel crank for cheap through GM that my friend works for a dealer

Cam will be a solid roller and specs should be in the 240/250 range and about 620/620 range with a lsa of about 110.

Thanks for the opinions.

So you are saying that an easy way to get around it is to just raise the idle to create more vaccum?

I bought my Scat 400 4340 crank for $600 from Scoggin Dickey. Lots of folks say these are junk but I'm just going to have to wait and see. BTW, the factory 400 nodular crank is pretty damn tough. I just thought I would put the extra money in it as I just had the thing blow up and didn't want to go through it again. One other thing to consider is the bearing issue. The bearings I used for my 377 were extra thick and required no spacer but were expensive. I think in the $200 range.

Sounds a whole lot like the Comp XR286 cam I have. Like I said before, 10" @ 900 rpm and 15" @ 1200 rpm. I wouldn't think it would be that much different on a 377. I just cranked it up today after doing some fuel system mods and the readings were like I quoted before.

Not the best way, but yeah.

Thanks for the replies.

My GM Nitrided steel crank i'm getting for $275 Gotta love the discounts

Also, i have many friends who have built 377's and some that have run in the 8s on bearing spacers.

they are tougher than most people think. Just have to make sure your machinist knows what he is doing

Thanks again

Hey Jim.
Find noslo1 on nfraracing.com UBB. He was at BG and had a 400 SBC with DFI and was running Alpha N mode because he had low vacuum. He ran 11.21 @ 125 on motor so I dont question his tune.

Why he doesnt show up on results because he broke his slave cylinder in practice. Runs aftermarket 6 speed. Its a 3rd gen formula that looks like Tory's.

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-86 IROC
11.28 @ 120.6 mph 1.59 60'
-=ICON Motorsports=-