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Old 08-11-2008, 02:24 PM
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Low manifold vacuum

Background: I have a freshly rebuilt SBC 350 with 76cc (1.94-1.5) heads a Weiand dual plane intake Holley 600 cfm with vacuum secondaries and a mild flat tappet hydraulic cam: 224 @.050 duration with .465 lift, the LSA is 112 degrees. I have the initial timing set at 14 BTDC.

Problem: I only get 14 hg of manifold vacuum at idle (800 rpm), further it won't idle smooth without the HEI vacuum advance being connected to full manifold vacuum port, lastly the idle mixture screws are out 2 1/4 turns.

Things I've done: Had the carb checked on a mule motor, pressure checked the manifold for vacuum leaks and have adjusted the valves (1.52 comp cams roller tip rockers)using both the "spin the push rod" and "jiggle the push rod" techniques...
any ideas

additionally. all other vacuum readings are normal, to include at 2000 rpm the vacuum is 20hg and at 3000 rpm 21hg; also, I can't drive the car as I'm in the middle of a frame off and the body isn't on the frame...

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Old 08-11-2008, 02:47 PM
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Did you plug up ALL the holes in the manifold? How much of a turn on the tappet adjustment screws did you make?
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Old 08-11-2008, 03:02 PM
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You didn't give piston specs, but if we assume a stock Chevy type rebuilder pistons with 4 valve reliefs and a slight dish, static compression ratio could be as low as 7.38:1 with the 76cc heads. Waaaayyyyy too much cam. This c.r. would hardly support a bone-stock cam, much less a 224* cam. The late intake closing point is pushing the fuel/air slug back up the intake tract before the intake valve closes, resulting in a false signal to the venturis and low manifold vacuum reading. Also, the closing point on the intake is failing to trap enough mixture to make any significant cylinder pressure. The carb is reading this as low manifold pressure and thinks the motor is under load, so it is pulling the metering rods out of the jets to richen up the mixture. Once the r's come up and the motor gets up on the cam a little, things improve due to the improved efficiency. This may be one of the worst camshaft mis-match combinations I have ever read about.

Now, swallow real hard, step up to the plate and answer back concerning the details on the pistons so we can guide you in the right direction.

Last edited by techinspector1; 08-11-2008 at 04:16 PM.
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Old 08-11-2008, 05:21 PM
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Come on now Mr. Senior Curmudgeon dude.....tell us what you REALLY think!!! ROFLMAO.....

Does sound like a pretty bad miss-match though.
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Old 08-11-2008, 06:48 PM
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re: Low manifold vacuum

thanks guys

alittle1: I used 1/2 turn past zero lash

techinspector1: ouch, but I appreciate your candor: flat top cast pistons...my engine guy said this should give me close to 8.5:1, but I didn't do the math

what's the best solution other than buying a 383 crate motor? I want mild street performance and reliability without spending a ton o cash...I thought this combo would get me close to 290 hp, but sounds like I (and the machine shop I took my engine to) were way out in left field, perhaps in the parking lot...I didn't [obviously] do enough home work
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Old 08-11-2008, 07:15 PM
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I was figuring a worst case scenario. You may indeed be at around 8.5, depending on volume in the gasket, piston crown and the piston deck height (distance from the crown of the piston to the flat surface of the block deck with the piston at TDC). But nevertheless, it's still too much cam for 8.5. That c.r. would call for a cam with around 205* duration and an earlier intake closing point. If you can provide the volume in the piston, piston deck height and compressed head gasket thickness, I can run the numbers for you and nail down the c.r.

Without knowing anything else, my first inclination would be to yank the lifters and replace them with Rhoads variable duration lifters. I think that would be the simplest and cheapest route to taming down the cam that's in there now.
http://www.jegs.com/p/Rhoads+Lifters...10002/-1/10213

Of course, at $98 plus shipping, it may be just as easy to change cam and lifters as a set.
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Old 08-11-2008, 07:26 PM
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Low manifold vacuum

Thanks again, I'll try to get that info tomorrow. Before I overhauled the engine it had the same combo minus the cam and roller tip rockers (and .020 over)...I felt it ran strong (10 years ago when it last ran) so I figured I'd freshin' everything up, keep close to the same combo plus the addition of a 700r4 (vs turbo 350) and go to 3.55 gears in my 12-bolt vice the 3.31s, but unfortunately I couldn't cross reference the numbers on the cam to find any specs, however I compared my new cam and old cam and could see a difference in both lobe height and thickness (duration); new cam = more. I have the old cam in the basement, anything the average knucklehead could figure out with a micrometer?
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Old 08-11-2008, 07:46 PM
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Low manifold vacuum

The lifters don't sound like a bad idea, but I've already broken in my cam (got about 3 hours of time on the engine, no load) will adding new lifters cause premature wear on my cam?

Would milling the heads to increase the CR be effective?

I've read good things about the production cast iron Vortec heads, but I imagine they would need reworked to up the CR as my pistons are the limiting factor...I would have to get a different intake as well...
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Old 08-12-2008, 10:22 AM
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Low manifold vacuum

here's what I've got:
I didn't check the deck height , however I'll assume it's stock as the block has never been decked (and the heads have not been milled): deck ht. between .020 -.040; the valve reliefs in stock cast piston are suppose to be between 4 - 6 cc's; the felpro gasket compressed thickness is .041; so when I play with the numbers (on the Performance Trends calculator) I come up with somewhere between 8.3:1 (.040 deck ht/ 6cc pistons) and 8.8:1 (.020 deck ht/4cc pistons) if I got a thinner felpro gasket (.024) I should be able to get at least 8.6:1
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Old 08-12-2008, 08:33 PM
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OK, in order to change gasket thickness, you really should know the piston deck height. The block may have been surfaced by a previous owner and if you use a thin gasket, you may get into trouble. I doubt it, but it could happen.

You have two choices. You can either put up with it the way it is, or you can pull the intake and heads to measure the piston deck height. The piston deck height added to the gasket thickness equals the squish. If you don't have measuring tools, you can easily check piston deck height by bringing the piston to TDC, placing a steel rule on edge at the edge of the bore and inserting feeler gauge blades until you get a snug fit between the edge of the steel rule and the top edge of the piston at the crown.
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Old 08-14-2008, 07:24 PM
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low manifol vacuum

Thanks again for the advise.
I noticed the cam in the GM 290hp/350 crate motor is VERY similar to the cam in my '57, further it has the same 1.94/1.50 76cc heads and low compression; they advertise the vacuum to be 10-12hg; I'm surprised GM would put together such a "mis-match combination"...
GM recommends a 670 Holley for their 290hp/350, given the similarities between the GM crate and my abomination its no wonder my little 600 Holley, which has a "somewhat lean idle circuit" can't supply sufficient fuel to run at light throttle due to the low manifold vacuum. So, I understand why I have the low manifold vacuum and the idle mixture screws tweaked way rich, however still not sure about the massive timing advance needed at idle, but to keep things simple, within budget and to my desired performance level, I'm ditchin' the "mild cam" recommended by my machine shop guy and going with something like the Lunati 270/280:
Duration @ .050 (Int/Exh): 204/214
Gross Valve Lift (Int/Exh): .420/.442
LSA/ICL: 112/108
RPM Range: 900-4800
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Old 11-03-2008, 01:24 PM
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Help needed, I have the same problem more or less. Engine (351 Cleveland in DeTomaso) runs fine except rough idle. It idles at 13InHg, when in Drive 10InHg. Rev in neutral to 2000 rpm produces 18InHg. Engine is (I think) stock 1978 2V, I've added Holley 600 vacuum and Summit distributor w/o vacuum advance.

Timing: The earlier timing, the higher idle vacuum. I now have the timing so high that a few degrees more makes the starter struggle to turn it when warm. Balancer reads 10 degrees, but I don't trust that.

Carb: Seems to idle a little better the more the idle screws are turned out, I now have them at 3+ revoultions out. Main jets are 64, PV 6.5

The plugs are light, indicating a lean situation. I've pressure tested the manifold w/o carb, and I've tried spraying around the carb base, no leaks. I've also taken one ignition wire off at a time, all 8 cylinders contribute almost equally. Compression is OK.

Note that there are 2 natural vacuum leaks, the PCV and the hot air choke. I've increased the hole for the hot air choke to get it to work properly, but the rough idle problem was there before.

I have another DeTomaso, a Pantera, with same engine and carb. It idles beautifully at 18InHg, idle screws around 1 revolution out and plugs a little rich. Jets are 58 and PV 6.5. I've tried switching the carbs, no change. How can two similar engines be so different?

Any suggestions?
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Old 11-03-2008, 03:08 PM
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Quote:
Originally Posted by belair57
Background: I have a freshly rebuilt SBC 350 with 76cc (1.94-1.5) heads a Weiand dual plane intake Holley 600 cfm with vacuum secondaries and a mild flat tappet hydraulic cam: 224 @.050 duration with .465 lift, the LSA is 112 degrees. I have the initial timing set at 14 BTDC.

Problem: I only get 14 hg of manifold vacuum at idle (800 rpm), further it won't idle smooth without the HEI vacuum advance being connected to full manifold vacuum port, lastly the idle mixture screws are out 2 1/4 turns.

Things I've done: Had the carb checked on a mule motor, pressure checked the manifold for vacuum leaks and have adjusted the valves (1.52 comp cams roller tip rockers)using both the "spin the push rod" and "jiggle the push rod" techniques...
any ideas

additionally. all other vacuum readings are normal, to include at 2000 rpm the vacuum is 20hg and at 3000 rpm 21hg; also, I can't drive the car as I'm in the middle of a frame off and the body isn't on the frame...
224 degrees at .050 inch lift falls outside the definition of a mild cam, 14 inches at idle sounds like what I'd expect. Seeing the vacuum pick up with RPM is normal with no load on the engine because with out "working" the engine it will develop a lot of RPM with almost no throttle opening. In this instance the manifold vacuum climbs up, it would not do that if you loaded the engine down and had to use a lot of throttle opening to get those RPMs.

I think you're sweating a problem that isn't really there.

It won't idle smooth because there's more cam there than you realize, plus there are other factors that affect running beside the cam, in your case big chamber heads, these don't like much cam timing and inherently don't idle well. Idle is dependent upon getting a burn thru the available mixture when the air density is low because of the nearly closed throttle position, mixture dilution with un-expelled exhaust gases, a lazy low turbulent chamber that has the sparkplug stuck over on one side. Such a set up results in poor mixing of fuel and air molecules, and pollution with exhaust this is a recipe where it's hard to get the flame to travel thru the mixture. This results in the rumpty-rump sound of the idle with the occasional, or more frequent, miss-fire thrown in. The addition of vacuum advance helps because it provides more time for the flame front to get thru the mixture. I'd ditch the heads and the pistons as well if they are not a flat top or D dish design. What you want for a top end is a tight chamber, where the squish/quench deck is both large and closes very closely at .045-.050 inch. Compression is maintained with a D-dish located under the valve pocket at a max of about 9.5 with iron heads and 10.5 with aluminum. The spark plug needs to be as close to the center of the bore diameter as possible with a skew toward the exhaust valve. This improves the prime in the plug as it's in the swirl of the incoming mixture flow. Located close to center knocks several important milli-seconds off the burn time so the engine develops more power with less advance. This reduces pressure spikes that want to drive the piston backwards and reduces the tendency of the mixture to detonate. The high degree of squish/quench builds what's called mechanical octane into the engine. This is where the side opposite the spark plug closes closely together between the piston and head. This first ejects mixture from the far side of the chamber toward the spark plug with great force. This stirs the mixture breaking up fuel droplets and associating them close together with the air and pushes everything toward the spark plug so that ignition is more likely to occur and that the initial burn proceeds rapidly. As the burn progresses, temperatures and pressures increase significantly, if the temperature is not suppressed to some point the mixture will become so hot the unburnt products ahead of the flame front explode (detonation). The quench function reduces the detonation tendency as it's a zone where there is a lot of surface area to volume. This provides a heat sink to quench the temperature preventing spontaneous combustion from occurring to the un-reacted mixture of the end burn.

Bogie
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Old 11-03-2008, 06:08 PM
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Thanks Bogie,
I changed the cam to a milder voodoo grind that gave it a smooth idle (within timing specs) and 18 hg of manifold pressure.
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Old 11-03-2008, 06:25 PM
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Quote:
Originally Posted by belair57
thanks guys



what's the best solution other than buying a 383 crate motor?

I want mild street performance and reliability without spending a ton o cash...

I thought this combo would get me close to 290 hp, but sounds like I (and the machine shop I took my engine to) were way out in left field, perhaps in the parking lot...I didn't [obviously] do enough home work
JMO

I'd spring for a single pattern cam of 208 to a max 212* @ .050 on a 108* LSA for good scavenging, a little idle sound, and strong mid range torque with good snappy throttle reponse. Installed at a 102-103* ILC for better cylinder pressure with your low compression.
LSA and ILC is very important. Don't get sucked into that longer exhaust duration and wider LSA that just lessens low speed torque and throttle reponse for a little more zip above 5500 rpm. It is worthless hype that millions have bought into these recent years.

and up the compression with a thin gasket or minor milling if you want to pull the heads.
Initial timing 14* and vacuum advance can hooked to manifold vacuum, using a vacuum can that starts advancing at 8" and full in by 12" with about 20* full advance.

Most SBC pistons have poor squish rings regardless of proximety to the head, so don't get too excited about making big changes. If you have true flattops, excellent.

An Edelbrock 600 AVS carb would be a better choice, but not necessary.

290 hp should be realistic with this setup and your 8.5 compression.
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