HI ALL, after tuning and dynoing my engine I think I've found the problem. The long block was assembled when I got the car and I was told the compression was about 9.5 to 1. But the car just didn't seem to run like it should. After finding the original machine shop paper work, I went down to the shop and started asking questions. It appears the '71 402 is 30 over with stock type pistons and the heads were rebuilt complete stock. This would mean it's 8.5 to 1.
I'm thinking of changing the heads and cam. This is the set-up I have:

70 velle ss 396
TH400 2800 stall converter
402BBC 30 over 8.5 to 1 compression
stock heads rebuilt, 113cc oval port closed chamber
comp cams roller rockers
edelbrock rpm cam
edelbrock air gap intake
holley 770 avenger carb
msd ignition
dynomax 1 7/8" headers
flowmaster 2 1/2" exhaust pipes
dynomax ultra flo mufflers
12 bolt 3.73 new clutches moser axles
boxed lower control arms w/polygraphite bushings

I'm on a buget and thinking something like 063 heads to raise the compression. I want to change the cam to something more friendly on the street. A hydrolic roller would be nice but not in the buget unless I can get the heads cheap. I'm not sure how to choose a cam that will work with my existing stall converter/rear gears and still have a fairly smooth idle, but maintain as much torque and horsepower as possible.
Any suggestions on heads/cam would be appriciated.

oops...forget something there Glenn???

well you forgot to add your cam specs, but depending on how big the cam is, changing to a closed chambered head could help you out alot. I know alot of people dont think to much of the edelbrock cams though....

The current cam is the edelbrock rpm cam: dur @0.050" 240/246, lift .560/.573.

The RPM cam wants 11:1 but will run pretty well with 10.5:1. If you can get the CR to about 9.5 and put a cam around 268-270º in, I think you'd like it much better. The pistons are an issue because stock type cast pistons are usually .020 short on pin height. This makes getting the proper quench distance nearly impossible and raising the CR difficult. Can't help with the heads. I'm a SB guy.

Should I look at changing the pistons and keeping the heads that I have? It would not be much more work since I'll be pulling the engine out. I could get a domed piston (forged or hypereutectic?) to raise the compression. The heads I have now are casting number 3993820. Will these heads work well in my combination if I get the compression upto (at least) 9.5 to 1?

If you suggest changing the pistons. How big a dome piston should I use?

I use the 820 heads on my combo,just make sure you pick a combo you'll be happy with,daily driver,weekend cruise,with a little drag racing on the weekends! graemlins/waving.gif

I want a weekend cruiser that I can take the family for a ride in and enjoy but also spank a stock '87 Buick GN at the strip once a year. smile.gif

Stock type 402/396 pistons have a .180 dome..get a pair of 063's or 215's and your compression will be about as true to 9.5 to 1 as you can get.
A smaller cam is in order too..something that will build cylinder pressure..do a search for the Lunati/Ultradyne 280/288..I think it would be a great choice with your gear/convertor.

Glenn,

I respectfully suggest that you don't have quite enough information to determine what direction you should go.

1) Find out exactly what pistons are in it (brand and PN).
2) Find out what the current pistion to deck height is (also see #4).
3) Try to determine if the existing heads have been milled, or have one of the combustion chambers CC'd (CC'ing them is preferable).
4) Try to find out if the block decks have been milled (also see #2).

None of the above is costly at all. Once you have a base line you'll then be better able to figure out what needs changing.

Dan

Your absolutely right Dan. Untill I get this thing apart and measure I won't know for sure. The machine shop said the heads and decks were cleaned up just enough to make them flat. I can't wait to get this thing apart.

Yes, more info is always wanted to help out..

But imho, I think Pat hit the nail right on the head with a good street motor cam for a 9.5 to 1 CR. pump-gas friendly motor..

I will come right out and say you may not be able to spank that GN like you want at this power level b/c some of them suckers get right up and jump up in the air and go!!!

pdq67

Yeah Glenn, I did this dance a couple of months ago when I decided to re-cam my 408 (.060 over 396). I had a bit more information because I knew what pistons were in it and I know the decks have not ever been cut.

Nevertheless - because I did not want to pull the heads, I still had to make educated guesses with some of the rest of the measurements; actual combustion chamber CC's, piston to deck heights, head gasket thickness. It seems to run pretty well with the cam I installed but there's still that nagging voice that reminds me that "you STILL don't really know". But that's just me. graemlins/sad.gif

I'm hoping to yank the engine this fall. Good luck, let us know what you find in there.

Dan

O.K. I got the driver side head off. Is there a way (do it myself)that I can measure the heads combustion chamber and the cylinder/piston volume in the block?

Just get a cheap, Drug Store/Nursing Home/Hospital 50 cc syringe and a big dull tipped needle and a piece of thick glass or clear plastic sheet and make up some liquid and have at it.

Set your head up level with the plate over to one side so you can get a big needle down into the chamber and fill it up and measure.

Do the same with the piston, but put it down a true measued amount so you can calculate the full cylinder length you are filling and then subtract out the piston's dome and such.

Slightly lube everything so nothing leaks out and usually the liquid is water with some alcohol andfood coloring in it to make it wet out better and so you can see it..

Then have at it.

pdq67

You have the same set up as mine. '71 402 block .030 over except I am running 290 heads with 2.19 intakes, port match and some bowl work and a lot of polishing and blending in the exhaust ports. I even installed a simple Performer 2-O (also port matched). If the block has not been decked and you definitely have stock TRW pistons, those babies are way down in the hole [ .020 or better]. We decked the block to square it up and still wound up with the piston down .014. I had installed the .022 shims to bring the comp up but wound up using the .041 Felpros when I put it back together after debacle (?) of thinking I had bent a couple of valves while trying out a roller setup. Bottum line is, my 290's [listed as 101CC] actually CC'd to 105 CC [w/ .008 clean up cut and polished chambers].

The moral of this is twofold.
One - you really have no true idea of what you have unless you take measurements. Piston/deck and Comb chamber volume. It is possible for you to be down around 8.25/1 CR as your motor sits.

Two - on a street driven 408 keep the cam timing shorter (no more than 224 - 226 @ .050 on the intake side, exhaust timing could/should be 10 - 12 degrees longer, maybe longer if you can get it ) with a motor that has around 9.3 - 9.6 CR and it will make a lot of power right where you want it. No detonation or tuning issues at all. I can vouch for this first hand.

Joe,good to know your combo is running well :cool: graemlins/thumbsup.gif

Just curious if anyone knows what the top of a stock 402 piston looks like. Photo would be nice, just to compare with the look of mine.

Well I took the head off and cc'd two of the combustion chambers. I used a piece of 1/4" thick plexiglass and a 60ml suringe. The two chambers I did came out to be 118cc. I took some digital photos of the piston top to post here. Could someone explain how to make photos show up in a post.

Can I use clay to make a mold of the piston top and use this to find the cc? Once I find out the piston cc and I know the combustion chamber cc and the thickness of the head gasket how do I calculate the compression ratio? Anyone know the formula?

Glen,

The stock 402 piston has a dome ht of about 3/16" (.188"). There are few choices for piston domes for these. Typically the 3/16" dome on the hydraulic cam motors or a 3/8" dome on the solid cam motors. The aftermarket pistons are basically identical in spec for the 396/402's. If you have the correct 3/16" dome these are 18 - 19 CC (net domes). Another thing to check is piston/deck height. Rotate the crank to bring the piston to TDC. Use a dial gauge (if you have one) or lay a straightedge across the top of the block and use feeler gauges to measure the piston/deck ht. I can almost predict your outcome though, unless the block has been decked significantly, the piston/deck is arouund .020. These motors were originally built with .022 steel head gaskets. From the sound of things, your motor is around 8.4-8.5 / 1 CR. You would have to go to a 100 CC head to make it worth while. With a 105CC head like I am running the CR only jumps to 9.3 - 9.4 / 1. Think about having your open chamber heads cut to 110CC and use the GM .022 steel gasket to put it back together. That should get your CR to about 9.0 - 9.1. Then cam the motor with something in the 220 @ .050 range (intake). This is a prime example of where the UD 280/288 fits in. The difference will be like night and day over what you took apart. The path to correct this is a very inexpensive yet effective fix.

Chris runs this same cam in a 454 as do others, they could comment on this and give you some insight. And yes Chris, the 408 combo has a ton of power right on the converter (2800). With the 3.73's it feels more like a 450 incher than a bored out 402. I was pleasantly surprised. I haven't ran it past about 4k yet but it feels like it is right on the the powerband at 4k. I had figured I needed power to 5800-5900 to make it work. Should easily run there.

Finally! This is what I found out:
</font> 402 bored .030
stock heads #820, cc'd at 118
stock pistons 3/16" dome, approx. 18cc
deck height to piston .038" in the hole
.045" gasket thickness
1.72 comp roller tip rockers
cam: int. lift @ lifter .328"
cam: ext. lift @ lifer .338"</font> This is what I suppose to do: </font> Change heads to either 063's or 215's
Install a .022" steel head gasket
Install a UD 280/288 cam</font>
If anyone has a better head cam suggestion please speak up. I don't mind a slightly rough idle, but I want to be able to walk out to the car, fire it up and take the family for a nice cruise. BUT also be able to spank a stock '87 GN.
Is 500hp and 500 ft lbs possible?
Where can I get spec's on UD 280/288 cam and buy it?

Glen,

You may be off a little on your piston/deck, two things to watch when doing this are the accuracy of finding TDC and how straight your straight edge is. Also, did you clean the top of the block well before checking. If you didn't do it, check several cylinders. .038 down is pretty severe. The only explanation is that the rods were shortened too much when they were reconditioned.

Do a search for Pat Kelly and download his Compression ratio and dynamic comp calculator, it works real well.

Based on the numbers you are reporting back, your motor only has 8.06/1 CR. Installing a 100 CC head and .022 gasket will get it to 9.76/1. Cutting your head to 110 CC and .022 gasket will get it to right around 9.0/1. A good option for you may be to look at the Edelbrock TBI 100CC heads. These are the 290CC intake port roval heads but are angles milled from the manufacturer to reduce CC volume. Those heads would make a lot of power with a good cam like the 280/288. You could also locate some 063's or 215's or maybe 290's, but these wil need some professional work to accomplish what you are after.

Hope this info helps.

Where can I get the spec's for UD 280/288 cam?
Is there a place online I can purchase this cam?

You can probably search it on here in "engine" or "performance". What are you thinking regarding raising the compression?

The Lunati PN for the UD 280/288 is 302A7LUN.

The specs are 280 / 288 degrees advertised, 223 / 231 @ .050", .525" / .550" lift ground on a 112 lobe sep.

http://www.holley.com/HiOctn/ProdLine/Products/IEC/IECC/Chevy7.html

GRN69CHV, I'm going to recheck the deck height this afternoon. I'm thinking of a pair of #215's that I saw on ebay here (http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7903109786&sspagename=STRK%3AMEWA%3AIT) . The one problem is I don't know if the springs on the heads will work with the UD 280/288 cam.

Hey guys, what's your thoughts on my combination if I go with the comp cams XE274H insted of the UD 280/288? It seams I would have a better dynamic compression (guessed 106 C/L), but would drivability suffer. (I used .020 deck height, .022 gasket, 101cc chamber).

As the guy if the springs aren't GM 502, BB "take-off" stock springs like Competition Products sells b/c if they are I think they should be OK..

As for whether the CC cam is better for your application??? I figure it's up to you to decide how much to let the dreaded, "more power, Scottie" bug bite you!! He, He!!!

The darn sucker bit me twice!! Once when I went from a stock, shade-tree rebuild for my 454 core motor to a .100" over 475" motor and then again when I finally settled on a 496" motor!!!!

pdq67