Engine vacuum and timing for cammed 400 - Pontiac GTO Forum
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post #1 of 10 (permalink) Old 10-25-2012, 10:44 AM Thread Starter
 
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Engine vacuum and timing for cammed 400

I have read up on the topic of engine vacuum and idle speed on several Pontiac forums and have a few questions regarding timing and engine vacuum on a 67 400 with an aftermarket cam:

Comp Cams 51-224-4 Extreme Energy
- Hydraulic Flat Tappet
- 1,800-6,000 rpm range
- 230/236 duration @ .050" lift (274/286 advertised)
- 0.488 intake/0.491 exhaust
- LSA 110*

Lars Tuning secrets sticky states that on "a performance engine, I like to see initial timing in the 16 18 degree range. Mild-cammed engines can get by with a little less. Big cammed engines like 20 to 24 degrees initial. But set total to 36 as a starting point."

My questions: Would my cam be considered mild or big? How much engine vacuum at idle (auto in park) should I expect to see? Idle speed is around 950 rpm in order to keep it running.

Other:
Holley 750 DP, no choke, heat crossover in intake is blocked, phenolic 1" open spacer to keep the fuel from boiling, Edelbrock intake with water crossover separated

I ask because my engine seems to like about 12* initial according to the timing light (vac adv unplugged and capped). Any more and I have trouble getting it to start, especially if the engine is warm. I'm sure if either my balancer slipped or the cam timing were slightly off I could better explain the quirky nature of this thing. I'm not sure how to check the balancer but will check the cam when I replace the timing cover (broken stud for water pump). Per Lars' article I will be pulling the HEI distributor to fix the shaft end play as soon as I can find a shim kit. I recall it felt like it need it the last time it was out. I want to get the little things worked out of the ignition system, idle, and fueling so I can get busy enjoying the drive while restoring it.


Any thoughts would be appreciated.
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post #2 of 10 (permalink) Old 10-25-2012, 12:29 PM
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That cam is their XE274H grind, which is something I'd say on a 400 would be past "mild", into the "moderate" range, and starting to sneak up on "pretty rowdy". On a 400 I'd expect it to make somewhere in the general neighborhood of 12" to 14" of idle vacuum when everything is tuned right. A bit light for running things like power brakes and other vacuum accessories, in my opinion. Which heads are you running and how much compression? Cylinder heads (material and chamber design) have more effect on how much total advance an engine is going to "like" than any other single factor.

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post #3 of 10 (permalink) Old 10-25-2012, 12:51 PM Thread Starter
 
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The engine is from a 67 Catalina (325/333HP) and the heads are stock iron 143 "big car" heads. The have a fresh 5 angle valve job and stainless 1.92/1.64 valves with stock compression in the neighborhood of 10.5 to 1. I run only 93 octane fuel usually 100% with no ethanol additives. The heads have not been ported to my knowledge.
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post #4 of 10 (permalink) Old 10-25-2012, 05:17 PM
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Ok, if those 143's have factory nominal chamber volumes of 72 cc's and everything else about your engine is also "the usual" for Pontiacs (meaning - standard bore, .045 compressed head gasket thickness, pistons .020 "down", 6 cc's in the valve pockets) your actual compression ratio is right at 9.9:1. If it's been bored +0.030, then it's around 10.0:1 Changes in any of those assumptions is going to have a corresponding change in compression ratio. It's good that you have that cam. Less intake duration than that would make the intake close event earlier and build more cylinder pressure and make it more prone to detonation. Is it pinging now? Have you taken a good hard close look at your plugs to make sure there aren't any "shiny specs" on the center insulator? Do you happen to know if the cam was degreed when installed and if so, where it was set?

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post #5 of 10 (permalink) Old 10-25-2012, 06:30 PM
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Pontiacs like a total advance right where Lars said they do. With your set up, I would run as much "initial" timing advance as your high compression will allow. Likely, it won't allow very much. If it's not pinging at 10-16 degrees initial, great. I have a big old school Sig Erson cam in a stock compression 389 and it requires race gas added and lower initial timing to run without pinging, but where I live, only 91 octane fuel is available. My base is set at 6 to 8 degrees, and my total is 36.
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post #6 of 10 (permalink) Old 10-28-2012, 09:35 AM Thread Starter
 
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I have changed the plugs in the last few weeks and didn't notice any shiny specs. I did get a small amount of pinging during a test drive after a timing adjustment. I have since adjusted it back a bit and it seems to be pulling good with no pings. The cam was installed when the engine was rebuilt (2000 miles ago) when I bought it. I have put around 800 miles on it cruising to local shows and cruise nights. I had some issues early on with hard starts and overheating that have since been corrected with timing adjustments and a Flowkooler 8 bolt pump (for good measure).
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post #7 of 10 (permalink) Old 10-28-2012, 10:49 AM
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I just noticed your location I have a cousin who lives just down the road from you in Elberta.

Do you happen to know how much total advance you have to run in order to get it to stop pinging? It sounds like you're right on the edge. I'd recommend periodic, close inspection of the plugs every so often, maybe with a magnifying glass, to make sure they're not getting shiny specs on them. There's such a thing as detonation you can't hear. Those shiny specs will be bits from the piston tops - not good.

I don't recall you mentioning what kinds of transmission and rear gear you're running?

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post #8 of 10 (permalink) Old 10-29-2012, 09:03 AM Thread Starter
 
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Elberta is 10 or so miles from me - you've mentioned that before. You have helped me out with my car before both here and the Pontiac Street Performance forum.

I currently have a TH400 installed and 3.73 rear. I hope to change over to a 200-4r in the next year or so for the overdrive and direct bolt on without an adapter plate. I'm running a Summit HEI currently with a standard vacuum advance (probably 20* total in it) that was installed when I bought it. I am saving up for a Pertronix with an Ignitor III and external coil to match. It will have to wait a little longer as my next big purchase will be a new wiring harness to compliment the new gauges I have collected over the last few months. I have an extra instrument cluster that I've modified for the gauges, new switches, stereo, etc. for a more up to date resto mod dash look. I 'm keeping my original pieces and not cutting any sheetmetal in the dash in case I want an all stock 67 Tempest in 20 years. Once again, thanks for the help.
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post #9 of 10 (permalink) Old 10-29-2012, 09:33 AM
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Quote:
Originally Posted by jday240 View Post
Elberta is 10 or so miles from me - you've mentioned that before. You have helped me out with my car before both here and the Pontiac Street Performance forum.
I didn't remember mentioning that - I guess I'm getting senile in my old age She's all excited right now because her son (middle of 3 kids) will be getting home from the Marine Corps soon.

Quote:
I currently have a TH400 installed and 3.73 rear. I hope to change over to a 200-4r in the next year or so for the overdrive and direct bolt on without an adapter plate.
I've thought about going O.D., but if I do it'll probably be with a Gear Vendors box. I'm concerned about the torque-handling capacity of the other O.D. transmissions. Heck, my TH400 is getting rebuilt right now due to a rolled intermediate sprag..

Quote:
I'm running a Summit HEI currently with a standard vacuum advance (probably 20* total in it) that was installed when I bought it. I am saving up for a Pertronix with an Ignitor III and external coil to match.
Lars and GeeTee might chime in on this too, but if you're wanting to make that change because you expect it to make a performance difference you'll be able to feel - you'll probably be disappointed. There have been tests that show zero performance difference between even the standard points-type ignition systems and various aftermarket electronic types, at least in the rpm ranges where our Pontiacs are happy. Ifn' it wuz me and if that HEI was otherwise in good shape, I'd just have it recurved instead of replacing it. That'll save cash for other fun things.

Quote:
It will have to wait a little longer as my next big purchase will be a new wiring harness to compliment the new gauges I have collected over the last few months. I have an extra instrument cluster that I've modified for the gauges, new switches, stereo, etc. for a more up to date resto mod dash look. I 'm keeping my original pieces and not cutting any sheetmetal in the dash in case I want an all stock 67 Tempest in 20 years. Once again, thanks for the help.
I'd sure like to see pictures of what you're doing...

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post #10 of 10 (permalink) Old 10-29-2012, 05:08 PM Thread Starter
 
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This is the "template" I'm using. These are pictures sent by a fellow Pontiac guy of his gauge cluster. I am doing something similar. I removed the climate controls and plan to put my CD player there. The aftermarket A/C i"m looking into has a small climate control plate that will mount inside the hole of the original radio so I will not have to cut any sheet metal.





As for the Pertronix, I'm hoping for a quicker start and cleaner burn mostly. I want the benefits of a MSD box without the box. The Ignitor III has multiple sparks throughout the rev range. I don't run the car very hard and plan to have the 200-4R freshened up after I find one. This is my toy/hobby/cruiser. The 05 in my avatar pic is the fast car in my garage.

Last edited by jday240; 10-29-2012 at 05:20 PM.
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