[Frisco]

This is somewhat lengthy, but I wanted you to understand what I am working with and want to achieve. My question is at the bottom of this post. Thanks to all in advance.

I've got a GM 350 4 bolt main replacement engine. The advertised compression ratio was 8.5-1. I immediately pulled the stock heads off and installed new AFR 190cc aluminum heads with 68cc chambers and straight plugs. The fellow I spoke with at AFR said my compression would be 9.6-1 with that engine and heads. The pistons are the stock dished pistons. I have been advised that my compression ratio is more like 9.1-1. I am running a Comp Cam 280H magnum .480/.480 lift. Static compression is 130 psi with the cam advanced 4 degrees and 120 psi with the cam on the '0' mark. All cylinders are within 2 psi of each other. Vacuum at idle of 750 RPM in gear is only 7 in. The vacuum goes up to 15 in. at 1000 RPM and will stabilize at that when cruising. Drops to zero on hard acceleration (I expect that) and will momentarily jump up to around 25 in. when I let off. There are no vacuum leaks that I can find. Engine runs very strong. Engine timing is 18 degrees initial and 38 degrees total all in at 2600 RPM with the vacuum advance hose disconnected and plugged. The vacuum advance hose is connected to the full vacuum port. Their is absolutely no ping at any time and runs great on 87 octane. The torque converter is a TCI Saturday Night Special with the stall rated at 1900-2100 RPM. I think I need a higher stall as my little truck only weighs 2660 pounds which allows the converter to begin 'working' at or very near the lower RPM figure. If I hold the brakes and rev up the engine in gear; I will begin to push the front tires around 2000 RPM. The TH350 trans has a B&M transpak that I installed and shifts very hard from 1st to second and hard again from 2nd to high. The rear gears are 3.80-1 and the rear tire diameter is 28". I am currently running a 670 cfm Holley Street Avenger. I previously ran a 770 cfm Holley Street Avenger that seemed to run a bit stronger. I switched to try to get the idle vacuum higher. I am unable to get any adjustment from the air / idle screws with either carb. I'm fairly certain this is due to the low vacuum signal at idle.

I was advised that by increasing the static compression up to around 170 psi the vacuum signal would greatly improve.

As I stated, this engine runs great and is very strong. My concern came about when I decided to adjust the air / idle screws while using a vacuum gauge. With no positive results there, I began to check further. All this has led to my question.

The engine combo has just over 1000 miles on it and naturally is stock bore & stroke.

Now for my question: What pistons would any of you suggest to achieve this. What are your reasons for that suggestion?

[DoubleVision]

I`d suggest the KB pistons that have a 12cc D - Dish, this keeps Quench active, and you`ll gain a slight compression ratio increase. If the pistons you have are the full dish or concave type, there is no Quench, and Quench is important to resist detonation and it helps with throttle response. a flat top from Sealed Power with coated skirts would likely suffice also since your running a cam that bleeds off some cylinder pressure. there all over Ebay at a good price. stay clear of the flat top pistons that are cheap, under 100, these have .020 cut off the tops. after you get the pistons you want, check how far they are in the bore and set your Quench distance, don`t go under .040 and don`t go over .060. even with the D dish pistons, a good Quench will help power.

[BOBCRMAN@aol.com]

I have run a very similar setup in several of my vehicles, you can improve the idle response just by drilling 1/16" holes in the backside of the primary butterflies of the carb. This will also improve your vac. readings. I use this particular cam and engine setup because it has a great idle sound and broad torque and hp. curve. I usually use a Holley 750 vac secondary carb.

[Frisco]

Originally posted by DoubleVision
I`d suggest the KB pistons that have a 12cc D - Dish, this keeps Quench active, and you`ll gain a slight compression ratio increase. If the pistons you have are the full dish or concave type, there is no Quench, and Quench is important to resist detonation and it helps with throttle response. a flat top from Sealed Power with coated skirts would likely suffice also since your running a cam that bleeds off some cylinder pressure. there all over Ebay at a good price. stay clear of the flat top pistons that are cheap, under 100, these have .020 cut off the tops. after you get the pistons you want, check how far they are in the bore and set your Quench distance, don`t go under .040 and don`t go over .060. even with the D dish pistons, a good Quench will help power.

Thanks for your input! I will check out the specs on those pistons. I've checked the different pistons in the Jeg's and Summit catalogs and was a bit overwhelmed by the selections to choose from. This is why I posted my question. Thanks again.

Originally posted by BOBCRMAN@aol.com
I have run a very similar setup in several of my vehicles, you can improve the idle response just by drilling 1/16" holes in the backside of the primary butterflies of the carb. This will also improve your vac. readings. I use this particular cam and engine setup because it has a great idle sound and broad torque and hp. curve. I usually use a Holley 750 vac secondary carb.

Thanks for the tip Bob! I tried all that and more with the 770cfm to no avail. That's when I put the 670 cfm on. I need to get new butterflies for the 770 now or silver solder the holes back up and start over. I like that carb better because it definately pulls stronger with it. Just very rich at idle and as I stated low idle vacuum. Hopefully changing the pistons to flat tops will raise the static compression enough to increase the idle vacuum. I could change the cam but don't want to. I also considered getting the AFR heads milled to around 64cc chambers, but decided not to alter them at this time. Thanks again.

[1BAD80]

Ditto with DV and Bob, I have run the same pistons and set-ups.
You can get different types of dished pistons/ head gaskets for compression.
On flat tops just make sure they have the valve relief size's to the valve size and lift your using.