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#1
10-08-2023, 08:10 AM
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RA IV vacuum
My stock 1970 RA IV is producing 11" of vacuum at 850 RPM idle, dwell 30, initial advance 15. That vacuum seems a little low to me but the engine does make a nice lopey sound at idle. Is that manifold vacuum acceptable? I'm not able to drive the car yet.
Thanks, Jimbo |
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#2
10-08-2023, 08:00 PM
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i'm not an expert on RAIV, but i think that is normal for factory 400 RAIV. That is a big cam!
i'm running a crower solid 60311 in a 428 at 10.6:1 compression and only see ~6-7 hg. i think of this as the solid version of the RAIV cam.
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1979 Firebird Trans Am 301/4spd (Now 428) 1977 Firebird Formula 400/Auto 2007 Grand Prix GXP 5.3L |
| The Following User Says Thank You to 74Grandville For This Useful Post: | ||
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#3
10-09-2023, 08:17 AM
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11" in gear cold is pretty good, warmed-up should be a tad better.
Seems low, or not so good in neutral. Adj idle mixture screws for highest vacuum (or rpm incease, usually done in gear with two people For safety).
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12.24/111.6MPH/1.76 60'/28"/3.54:1/SP-TH400/469 R96A/236-244-112LC/1050&TorkerI//3850Lbs//15MPG/89oct Sold 2003: 12.00/112MPH/1.61 60'/26"x3.31:1/10"/469 #48/245-255-110LSA/Q-Jet-Torker/3650Lbs//18MPG 94oct Sold 1994: 11.00/123MPH/1.50 60'/29.5"x4.10:1/10"/469 #48/245-255-110LSA/Dual600s-Wenzler/3250Lbs//94oct |
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#4
10-09-2023, 08:52 AM
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How many miles are on the motor?
How’s level of compression balance between all 8 cylinders? Is the compression still stock, as in pistons used? Has a valve job been done to the heads?
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#5
10-09-2023, 09:52 AM
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Assuming all the above is in good health, that vacuum reading is a little low. However, as has been stated, big cam, relatively small engine= low idle vacuum. What is the vacuum reading at 1000 RPM's and 1200 RPM. I would expect around 13" @ 1000. Did you degree the cam when it was installed? Is the needle on the vacuum gauge steady or jumping around? (possible valve adjusted too tight) Are you sure your vacuum gauge is accurate. Do you have another known good gauge to compare it to? If it seems to be running OK, I wouldn't be too concerned at this point if it comes up at 1000-1500 RPM's.
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#6
10-09-2023, 10:58 AM
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11" of vacuum at those settings is about right. My original runs about 11"-12" at 750 to 800 rpm! That 87 degree overlap 041 cam is a lot and the vacuum will be low. I've seen cars back in the day with non-stock performance cams so wild they hardly activate the power brakes near idle. I had a stock RAIII car with the 744 cam years back that had relatively low vacuum at idle, but it wasn't quite as low as the RAIV engine. Very few original RAIV engines exist today and the ones that do have been worked on, bored, modified, etc. many times over which will affect the idle vacuum. However, your figures are normal for a stock RAIV engine!
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#7
10-09-2023, 01:10 PM
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Thanks for all the replies. Zero miles on this engine, all stock, machined and blueprinted by a Pontiac veteran machine shop, cam degreed when installed, steady reading on top end vacuum gauge, stock compression, vacuum 14" at 1100 RPM. The only worry was the valve springs, as this motor was assembled by me 15 years ago. I left the rockers very loose for all those years and turned the engine over by hand every few months.
I still have some fiddling to do with the timing and idle mixture. I'll report to the forum if any improvement. |
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#8
10-09-2023, 01:33 PM
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14" HG@ 1100 RPM's is normal for that camshaft. Enjoy the pull starting at 3000-3200 RPM's!
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#9
10-10-2023, 09:05 AM
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Quote:
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