new 355 motor, 9:1 flat top pistons, cam is a : .050 duration, int. 223 ex 223
valve lift .447 int/.447 ex

Would a cam with that grind result in a low vacuum reading of 10 ?
The idle is pretty bad.
Or should I look for a vacuum leak else where on the engine ?
I read a few posts here about where to hook up the vacuum gauge, but most of the locations were different. Is the intake manifold the best place to get a better reading?


Thanks, just trying to rule some things out before exploring........

 

Yes, 10" of vacuum is less than acceptable for that cam.
There are so many things that you could check it's difficult to pick a starting point.
For a vacuum reading, the intake manifold is a good choice.
Any other details on your build?
Carb or EFI?
Have you done a compression test?
Is it computer controlled?
What is the base/initial timing?

 

the carb is an Edelbrock 1406, no computer,
base timing is set at 12 degrees, per what many members here quoted me.

did not do a compression test only because the Hedman headers are so tight near the plugs.

I will be checking this morning for intake manifold leaks. I ruled out the power brake booster cause that runs off a vacuum pump.

I cant hear a vacuum leak at all.

could a tight valve adjustment cause low vacuum?
Again, this was built from the knowledge of the guys from here.

 

That old "151" cam that you have a copy of there, is well and widely known to give low vacuum; has been since about 1964. That's the problem right there.

Apart from that, any place on the intake manifold is fine for a gauge.

Yes excessively tight valve adjustment can cause low vacuum; but it would have to be SO TIGHT that it's got valves hanging open. You can try re-adjusting em (I'd suggest using the "engine running" method to avoid ambiguity) but don't expect a miracle.

Long-term, you need to be looking at getting a better cam, perhaps designed within the last 50 years or so, to go with that low-compression motor. It's a major mismatch.

 

Sofa hit it on the head. Not a good cam to start with, especially with only 9-1 compression. You can up the idle timing to help it out, but in the end it just mismatched parts. I would try and give it at least 20 degrees at idle and learn how to properly adjust the transfer slot on the carb.

just realized you have an edelbrock carb, so forget the transfer slot thing and all you can do is up the idle timing.

 

Yeah. What they said.

 

What grind cam would you suggest?

I searched that entire engine for a vacuum leak, found nothing.

20 degrees at idle? that wouldn't be to much?

Right now it starts right up without hesitation.

thanks for the suggestions !

 

Flat tappet or roller?
No. It'll undoubtedly be suggested here soon enough but a search of vacuum advance will turn up some interesting observations. The recent posts are the most relevant here.

https://www.thirdgen.org/forums/carb...m-vs-full.html

My Gen 1 SBC has 12 degrees base timing with an additional 22 degrees from full manifold vacuum advance for 34 total idle timing. Idles better, idles cooler, better in town mileage too. That's with a hydraulic roller cam, 224/230 @ .050" , 10:1 + compression ratio and it pulls 11+" of vacuum at 800 rpm.

 

No it is not to much. Keep raising the timing at idle until the rpm stops going up. That is how much timing your engine likes at idle!
20 degrees will not make it start any harder. You have to go a lot higher than that to have hard starting. Keep in mind though if you do not have an aftermarket distributor that has an adjustable advance you will wind up with to much total timing.
In most cases in order to get that much timing at idle you either need an adjustable distributor or a vacuum advance hooked up to full manifold vacuum or both.

As far as a cam, what heads do you have? Auto or manual and if auto what converter? What rear gears?

 

Im running the stock cast iron heads, rear gears are 2:73's.............(I know)

700r4 with a 1700 stall

 

I would do something like a lunati 60101 or 60102 with the stock heads. Although i don't know which stock heads they are so i'm just going conservative.
As far as what you have right now, extra timing down low will make a big difference. What distributor do you have?

 

the iron heads were off a 1973 Corvette, casting number 624 76cc chamber "2.02" heads......the machine shop took a .001 off the surface, and a 3 angle valve job.

 

Ouch. People should be ashamed of doing stuff like this to newcomers to our hobby.

Some of the worst heads there are... poor flow, typical smogger chambers from deeeeeep in the earliest and crappiest part of the crappy early smog era, basically impossible to make run good. One of the last choices for a "performance" engine. Just jamming 2.02" intake valves into them, which is something anybody can do to any heads at any time, and putting a stock valve job on em, doesn't alter their basic inferiority.

Definitely would be a good idea to upgrade those.

In addition to all of that, the compression ratio calculation is as follows:

Flat-top pistons w 6cc valve reliefs; .045" deck clearance (probably a little optimistic, as that tends to be about the minimum found in a non-racing rebuilt engine); .039" head gasket (typical composition gasket); 76cc chambers:

8.3:1



To get the "9:1" compression claimed by those pistons, you need 64cc chambers. A couple of ways to get there: you could go the Vortec route (96 - 02 350 trucks), or aftermarket such as Dart for example.

The Voodoo cams are a good match for any of those heads; they have aggressive lobes, MODERN profiles, and a larger exhaust lobe than intake, as most heads tend to require (due to the relative flow of the intake side vs the exhaust). The Comp XE line is another product with similar properties.

How many miles does this short block have on it? Who is the piston mfr?

 

Hi, its been a few years since I did an engine job. I knew this engine from the previous owner, it had 66k miles on it before I decided to rebuild it. I took the block and heads to machine shop in Hudson Fl. Told the owner what horsepower range I wanted to be in, and he did the machine work, and picked up all the parts for it. I assembled the motor. I guess he really didn't do a good job of explaining that that year engine wouldn't be good candidate. I was looking for 350 horsepower.