I just finished building my first TPI motor. It is a 383 with a stock TPI induction except for a ported base. I have Edelbrock alum heads, 9.6:1 comp 204/214@050 112 LSA cam. I built this for a truck application with emphasis on low end and strong midrange. The motor runs fine, has excellent throttle response and has a VERY smooth idle. I have finished breaking in the motor and it pulls only 16" of vacuum at 750 rpm idle. This seems low to me for a mild cammed 383. What do you think this motor should pull for vacuum?? If some of you have similar type setups, could you tell me how much vacuum you have?? Thanks

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Dave Zelinka

 

I'm running a carbed 383 with the same heads and compression ratio and a 224/230@050, 110 degree LSA, 470/480 gross lift cam, and it pulls about 10 or 12 inches at 750 rpm with 14 degrees static timing and lean-best-idle mixture. I have doubts about the accuracy of my gauge though.

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1984 Silver Z28, 383 cid 4 bolt, ARP Fasteners, 2 1/2" Cowl Induction Hood, 600 cfm Performer Carburetor, Torker II Intake, Performer RPM Heads, XE268H Cam, Magnum Roller Tip Rockers, MSD6AL Ignition, Blaster2 Coil, Recurved HEI Distributor, 700R4, TransGo SK, 2.73:1 Limited Slip Dana 44, Terminator Headers, Custom 3" TIG Welded Stainless Exhaust, Flo-Pro Twister Muffler, 4 Wheel Disc Brakes, Oil Cooler

 

Anyone else?????

 

350, Sportsman II heads, Accel base and ltr's, BBK 52 tb, GM LT4 hot, pulls 13" at idle speed around 700 rpm.

 

Yelo,

I'd tend to agree with you. The relatively short duration and reasonable LSA of that cam on a long-stroke engine should probably exhibit more vacuum at idle. Unless you've advanced the lobe centerline more than 4-6° over stock, the vacuum sounds like it is a little low. Nothing drastic, just not what I would expect, either.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Vader:

Thanks, the cam is a summit TPI grind and it had 5 degrees advance "built in". that is how I installed it. Do you think this is wrong? some others have said in a TPI motor most cams should be installed "straight up" Maybe I should retard the cam.

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Dave Zelinka

 

Yelo,

As I mentioned earlier, 16" Hg is not so low that you shoudl have serious problems unless you're using a MAP sensor. I've noticed problems with MAP starting at 4° advance on the cam installation. Since your engine is a larger displacement, it should be a little more tolerant of the advance, although it would be a little better if the extra cubes were made by bore rather than stroke increase.

What particular problems are you having with the installation, if any? You've mentioned you have an acceptable idle, good throttle response, and I'd expect reasonable power at the lower RPM ranges with that cam profile. If the only "problem" you're having is with the aparent low vacuum, I wouldn't be too concerned.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

I'm pulling 18" with my set up at 700 rpm. I've run and installed a boat load of cams and I'd think that small a cam in a 383 with 9.6:1 compression should be in the 22+ range. However, I'm with Vader, I wouldn't loose any sleep over it. Plus,, unless you know for sure,,, your guage could be wrong.

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86 IROC - 9.8:1 - 355, TFS twisted wedge heads, 218/228-110, fully ported GM base and plenum, SLP runners, 52mm BBK, 24# SVO, 1 5/8" Headman headers, 3" Dyno-max, TH350, B&M Megashifter, 3.23 peg leg, 245/45ZR17s on 212 Eagles

 

I am running a MAP speed density system. The motor does run fine in my Jeep, but I dont really have a good point of rference to compare it to, it had an AMC 304 before....it runs MUCH better now. I was told by someone that piston ring sealing has a big effect on vacuum, especially end gap. I am running KB hypereutectic with the KB specified .024" top ring and .018 second ring end gap. Some stuff I read says this is ALOT of end gap. Could this make a difference? I checked my gauge with an autometer in dash vac guage and they read the same.
I REALLY appreciate the help guys!!! This is my first TPI motor I have built.

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Dave Zelinka