Low vacuum on new motor - need some help.

[43tutor]

I started my new motor a few weeks ago but the motor just doesn't pull a lot of vacuum. Around 7-8 inches at idle of approx. 900 RPM.

The motor is completely fresh. 400 block bored .020" over, it has a 470 stroker kit installed, 10.3 compression. 452 heads that are ported per Mopar performance templates. Cam is a MP #P41220235 (.484 lift, 284 duration, 241 @ .050, 108 degree lobe separation) factory hydraulic flat tappets. Intake is an Edlebrock 383 performer, carb is a TQ 850 CFM (#6322).

I have checked for leaks on the intake manifold by spraying around carb cleaner around all joints, no change in RPM. I have remove the carb and made a Lexan plate to bolt on top of the intake and use a small bead of RTV as a gasket. The plate has to 1/4" NPT fittings. One for a PSI gage and the other for air inlet. I removed the rocker assemblies to make sure all valves were closed. I pressurized the intake to 5 PSI and it held pressure for several minutes with only a very slight decay in pressure.

I checked the all vacuum lines and found no leaks. Brake booster is completely rebuilt (Booster Dewy's work). No leaks at booster.

I degreed in the cam and my numbers only varied by 1 degree from the cam card specs. The cam is advanced 4 degrees.

I have tried the timing from 5 degrees BTC to around 20 BTC. No change in vacuum. Stock distributor.

I have removed the TQ carb and installed an Edlebrock that I borrowed from a friend. Same issue with vacuum, about 8 inches.

I'm wondering about the intake manifold. In the manifold directly below the carburetor there are two holes that appear to go in the EGR passage. I bought this intake used, without any installation instructions. Should those holes be plugged. The holes are not threaded. I checked on Edlebrock's website but didn't find much information. The holes are open to the EGR passage, which is the crossover passage from head to head (am I thinking right ????). The passage is open to the exhaust valve.

What are your suggestions.

 

[lewtot184]

plug the holes and re-curve the distributor.

 

[texas69bee]

I hate to brake it to ya... http://www.dodgecharger.com/forum/index.php?topic=45930.0

 

[Banzaiii67]

I have no idea why their is holes in your intake, but I'd fix that. Regarding your vacuum issues, that about what can expect with a 108 LS. Since you already purchased a new booster, all you can do is swap out that cam, 112 LS would work out well and reduced the cranking psi especially since you have iron heads. Are you running 100 octane? Surprised if your not pinging like crazy at moment.

 

[cdr]

if you dont have adjustable rockers,the pusrods could be to long

 

[Mebsuta]

I don't think those holes connect to the heat crossover. I don't think they go anywhere as long as the EGR stuff is not hooked up, which I think would go where those two pipe plugs are. I could be wrong, but I think that is the deal with Performer 383. I used one for a long time. Manifold vacuum did not change when I switched to another intake manifold.

 

[mopar 3 B]

Sound like a case of bad planing. Get rid of the power brakes and the loss of vacuum will not be a problem.

 

[lewtot184]

the holes are for exhaust gas recirculation. those manifolds came with a pair of block off threaded plugs. block the holes and recurve the distributor.

 

[Matrix2]

If vacuum for power brakes is what you are looking for, check out an electric vacuum pump. http://www.summitracing.com/search/product-line/comp-cams-electric-vacuum-pump-kits. These work well, are easy to install, the main issue being noise.

 

[Meep-Meep]

That's about right for that cam. Personally, I'd pull it out and run something else.

 

[Rocketman]

I started my new motor a few weeks ago but the motor just doesn't pull a lot of vacuum. Around 7-8 inches at idle of approx. 900 RPM.

The motor is completely fresh. 400 block bored .020" over, it has a 470 stroker kit installed, 10.3 compression. 452 heads that are ported per Mopar performance templates. Cam is a MP #P41220235 (.484 lift, 284 duration, 241 @ .050, 108 degree lobe separation) factory hydraulic flat tappets. Intake is an Edlebrock 383 performer, carb is a TQ 850 CFM (#6322).

I have checked for leaks on the intake manifold by spraying around carb cleaner around all joints, no change in RPM. I have remove the carb and made a Lexan plate to bolt on top of the intake and use a small bead of RTV as a gasket. The plate has to 1/4" NPT fittings. One for a PSI gage and the other for air inlet. I removed the rocker assemblies to make sure all valves were closed. I pressurized the intake to 5 PSI and it held pressure for several minutes with only a very slight decay in pressure.

I checked the all vacuum lines and found no leaks. Brake booster is completely rebuilt (Booster Dewy's work). No leaks at booster.

I degreed in the cam and my numbers only varied by 1 degree from the cam card specs. The cam is advanced 4 degrees.

I have tried the timing from 5 degrees BTC to around 20 BTC. No change in vacuum. Stock distributor.

I have removed the TQ carb and installed an Edlebrock that I borrowed from a friend. Same issue with vacuum, about 8 inches.

I'm wondering about the intake manifold. In the manifold directly below the carburetor there are two holes that appear to go in the EGR passage. I bought this intake used, without any installation instructions. Should those holes be plugged. The holes are not threaded. I checked on Edlebrock's website but didn't find much information. The holes are open to the EGR passage, which is the crossover passage from head to head (am I thinking right ????). The passage is open to the exhaust valve.

What are your suggestions.

I run a .545 lift cam and will have a CVR vacuum pump installed by the end of the day, I will let you know how it goes. I don't necessarily like where I am going to have to put it due to space but with it between the radiator and the front bumper, I will probably not hear it as much.

Keep the cam, nothing beats that sound.

 

[43tutor]

Thanks for the responses. I plugged the EGR holes in the intake and it did not make any difference on the vacuum. I have the 850 TQ on the car and now have the car so it will idle at approximately 750 RPM. Vacuum is only 5 at inches. Timing is set at about 10 degree initial and approx. 35 total, I will continue to work on timing adjustments once I get the car road so I can drive it. So far all adjustments are with the car setting in the shop. Car has not been driving in 2 1/2 years.

I did make a mistake on my cam specs in the original post. My cam has 114 degree lobe separation and not 108 degree (I looked at the wrong line on the cam chart).

Rocketman - let me know how the vacuum pump works out.

 

[67Satty]

That does not seem like much initial timing to me. I would think you would want it to be at least above 15, I bet vacuum would come up if you raise the initial.

 

[Meep-Meep]

114 LSA is a different story and much more palatable on the street. Still, agreed with above comment regarding timing. Crank it up to 15 initial but make sure you limit the total to 38 deg and all in by about 2500.

 

[GearAddict]

That's about right for that cam. Personally, I'd pull it out and run something else.

X2
I only get about 5-6Hg out of my .509 MP cam... I'm planning on pulling it.