350/290 GMPP Engine Build need advice(cam, intake etc) - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-25-2010, 05:56 PM
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350/290 GMPP Engine Build need advice(cam, intake etc)

Hey guys, its been a while since ive posted last but now I finally have a new scheme

I recently purchased a good set of 041X heads for $25.00 Right now they are disassembled and in the process of being cleaned. Being that these are 64cc chamber heads and will be decked to remove a slight (about .003") dimple in one of the heads it will be a bit different compression wise from the 76cc heads that are on the motor now.

So before I get ahead of myself let me tell you what i am working with. Currently I have the GMPP 350/290hp crate motor. For those of you who arent familiar with this set up it is a 4bolt main, 350, with 76cc chamber 1.94/1.50 heads, the rods are Powdered metal, and the pistons are "dished"(no matter how much searching I have done i cannot find the actual amount of dish they have...) so out of the box the Compression is advertised at 8.5:1 although i have talked to a few guys and they said the actual CR is closer to 8:1. The cam is a .450"/.460" Lift 222*Intake 222* exhaust @ .050" duration with a 114* centerline.

The Heads that I purchased are a pair of 3947041X Heads, my plans for them are new valves, screw in studs and guideplates, new springs, hardened seats, bronze guides, decked, and 1.6:1 roller rockers. When i put the new heads on I would also like to change the cam, intake, and carb to utilize the modifications.

Seeing as I have never fully rebuilt an engine this way before I figured i could ask for some guidance and advice from more experienced builders/members. I would like it to have a nice lopey idle and crisp throttle response. But keep in mind that this is a budget build and it will not be run on the track very often if at all.

Thanks for bearing with me on this long post.

Andrew

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Old 08-25-2010, 07:24 PM
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You're going to want to make sure the heads can handle the extra lift that a larger cam and 1.6 rockers will provide. Also make sure you have strong enough springs so you dont get valve float with the extra lift.
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Old 08-25-2010, 11:20 PM
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Being a new guy to engine building how do I go about checking to make sure the heads can handle the extra lift?

But as for the springs and such summit sells a kit that includesall the necessary stuff to install a bigger cam(springs, retainers, locks, etc) the kit is said to be good up to a .550 lift.

Also would someone be able to give me an idea ofwhat to expect CR wise?
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Old 08-26-2010, 07:18 AM
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Need to know piston volume, deck height, head gasket thickness to calculate cr.
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Old 08-26-2010, 11:32 AM
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i can give you a rough estimate on what the compression will be, and yes from my experiance with the GMPP 350/290 motors they are acually inbetween 7.9:1 and 8.2:1, so taking 8.1:1 (avg) with 76cc heads and lowing them down to "64cc" which these heads are typically about 66cc and then milling them .003" which will only take off about 0.5cc (.006 = about 1cc) you will be working with about a reduction of 76-65.5 = 10.5cc and for about every 8cc's you will gain about a full 1 point in compression. so based on 8.1:1 i would say you would end up with around 9.3:1

as for a cam i would stick to one that is similar to the one that was in there, about 220-225 duration @ 050, 270-278 advertised duration, 110LSA, and closer to a 106centerline, lift around .460-.480
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Old 08-26-2010, 12:32 PM
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Whats stoping you from actually measuring/calculating the dish volume of your pistons?
If you really want to determine the compression ratio of your motor, you need to do this.

Based on a -15cc dish volume and .025" piston/deck clearance 76cc heads and a 4.166x .040" composite gasket the stock cr is 7.8:1


If you were to finish the 041 heads to a verified 58cc and install with a .015" shim gasket the cr would be 9.8:1.

If you don't want to mill the heads to 58cc and use a .015" shim gasket , you'll have to replace the pistons with flat tops with a 1.56" pin height and 0 deck the block based on a verified 64cc head.

As stated do not assume your 041 heads are 64cc.

Cam: you can degree in the 350/290hp cam you have and advance it 7-8 degrees to a 107 intake C/L. or...

You can re-cam it based on what you are putting the motor in and the trans converter and rear gear ratio.

A Isky #201281 280 Mega cam combined with some 3.73-4.10 gears and a 3000-3500 stall would make for a fun street motor.
1.6 rockers: high valve lift

Use Isky 235D springs installed at 1.75" or Lunati 73943 @1.70-1.71"
When machining the 041 heads valve guide bosses for modern posi seals shorten the valve guide boss height to accommodate the extra lift and seal thickness. (shorten approx .200") Measure twice, cut once.

Watch the spring pockets if you go for larger than stock valve springs on these heads.
I'd stick a stock diameter 1.26" type spring for this one.

Use a Performer RPM manifold and 750cfm carb and 1.625" long tube headers.

Last edited by F-BIRD'88; 08-26-2010 at 12:41 PM.
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