[hurryinhoosier62]

Originally Posted by Scarebird

***threadjack alert!!!***

Where did you get those 40 over pistons?

Common oversize. .030, .040, and .060 are the most common oversizes and are pretty much available from ANY engine parts supplier.

[footjoy]

Originally Posted by SD455DJ

Footjoy, I just dyno'd a 400 with stock small valve #16 heads (78 cc's) that is .030 over with the 2801 cam and it made 360 hp/425 lbft torque through the stock exhaust log manifolds, cast iron intake and Q-jet. The pistons are .0015" in the hole and using Felpro 1016 .039" gaskets netted .0405" quench and 9.94 static compression.

I think you can mill your #15 heads 0.036" to reduce your chambers to 81-82 cc's and use a .025" Cometic head gasket to get a reasonable .045" quench and 9.5 static compression. Enlarge the pushrod holes to accept 1.65 rocker arms and keep the cam you have. RA exhaust manifolds are nice and will help add power throughput the range. I think you should get a true 300 hp to the rear wheels with a good tune and 93 octane fuel.

Dennis

Well I do have a set of 1968 #16s large valve but I think that would be to much compression for A/C power brakes but I am not sure. I just bought them on a knee jerk reaction over the weekend.

When you mill heads do you need new shorter push rods?

[Will]

Originally Posted by SD455DJ

Footjoy, I just dyno'd a 400 with stock small valve #16 heads (78 cc's) that is .030 over with the 2801 cam and it made 360 hp/425 lbft torque through the stock exhaust log manifolds, cast iron intake and Q-jet. The pistons are .0015" in the hole and using Felpro 1016 .039" gaskets netted .0405" quench and 9.94 static compression.

I think you can mill your #15 heads 0.036" to reduce your chambers to 81-82 cc's and use a .025" Cometic head gasket to get a reasonable .045" quench and 9.5 static compression. Enlarge the pushrod holes to accept 1.65 rocker arms and keep the cam you have. RA exhaust manifolds are nice and will help add power throughput the range. I think you should get a true 300 hp to the rear wheels with a good tune and 93 octane fuel.

Dennis

Originally Posted by amcmike

Although I didn't find a .025" Cometic gasket, they do carry a .027" MLS, which is pretty much as low as you an go without o-ringing the block.

Cometic C5769-027 has the compressed thickness along with a 4.2" bore to minimize gasket volume down to 4.77cc. The .047" quench is not great, but much improvement over what you probably have right now.

What these two guys said, plus put big valves in those heads and have the machine shop use a throat cutter to add a 1.66" or so backcut under the intake valves. Also convert to screw-in studs.

[steve25]

A good rule of Thumb for a street motor with a decent flowing iron head is that Lift wise at the valve you will need.110" lift for every 100 hp your loooking for the motor to make.
All stock 1968 and up small valve Pontiac heads with 30 degree Intake seats flow 180 cfm @ .500" lift.

If you look at post 19 in this string where the motor developed 360 HP that level of power took 174 Intake cfm per cylinder to make that 45 HP per cylinder.

Big valve iron heads will generally already be flowing 200 cfm @ .450" lift, so you can see why it will always pay to step up to bigger valves with the needed valve job if you need new valves anyway, and all iron head needs new valves if they have over 45,000 miles on them so don't kid yourself!

[slowbird]

Originally Posted by steve25

A good rule of Thumb for a street motor with a decent flowing iron head is that Lift wise at the valve you will need.110" lift for every 100 hp your loooking for the motor to make.
All stock 1968 and up small valve Pontiac heads with 30 degree Intake seats flow 180 cfm @ .500" lift.

If you look at post 19 in this string where the motor developed 360 HP that level of power took 174 Intake cfm per cylinder to make that 45 HP per cylinder.

Big valve iron heads will generally already be flowing 200 cfm @ .450" lift, so you can see why it will always pay to step up to bigger valves with the needed valve job if you need new valves anyway, and all iron head needs new valves if they have over 45,000 miles on them so don't kid yourself!

Small valve heads had 30° seats? I thought they all had 45° seats

[steve25]

Most that I have come across have 30s, but yes a bunch 45 seats.

I think that more casting numbers where made with 45 degree seats before 1971, but production numbers used of heads with 45 degree seats was higher after 71.

[78w72]

Originally Posted by footjoy

Well I do have a set of 1968 #16s large valve but I think that would be to much compression for A/C power brakes but I am not sure. I just bought them on a knee jerk reaction over the weekend.

When you mill heads do you need new shorter push rods?

compression doesn't matter for a/c & power brakes, its vacuum that matters for power brakes. if you have a mild cam which it sounds like you will use, you should have plenty of vac for brakes.

yes, if you shave the heads enough that will change the geometry & may require new push rods, but those are pretty cheap, or you may still be ok if you dont mill too much... back in the 90's on my first 455 before i knew much about checking push rod lenghts, with .040-.050 shaved 6x heads the rockers still sat pretty close to centered on the valve tip & had no issues with lots of miles & abuse.

Originally Posted by Will

What these two guys said, plus put big valves in those heads and have the machine shop use a throat cutter to add a 1.66" or so backcut under the intake valves. Also convert to screw-in studs.

i said most of that before they did...

[steve25]

Please note that the smaller the total combustion chamber volume then the higher the vacuum level will be for any given Cam used.
I.e, a 400 motor with a 068 Cam and 9.5 comp will pull more vacuum then the same motor with 8.5 comp.

[SD455DJ]

Originally Posted by steve25

Most that I have come across have 30s, but yes a bunch 45 seats.

I think that more casting numbers where made with 45 degree seats before 1971, but production numbers used of heads with 45 degree seats was higher after 71.

Call me a gluten for punishment, but I always wanted to dyno the '70 small valve XV code 400 from the A-body line. This motor was built to run at the Pure Stock Drags and the heads still run the 45 degree seats (both intake & exhaust). I had the rocker studs pinned to prevent pulling out with higher pressure springs and am using 3/8" polylocks to adjust the rockers to zero lash. All 70 small valve heads ran 45 degree seats, and fairly sure most of the early small valve heads did as well. There were likely a few exceptions, but don't know them off-hand. The higher compression small valve heads have a nice small turn radius to help low lift flow and the motor, as Steve25 said, only used 174 cfm flow for the 360 horsepower (it made 380 hp corrected, btw).

Footjoy, running #16 heads (75 cc's stock chamber) will push your compression to 10.2 to 1, especially if they need a clean-up cut. You still want to run a .027" Cometic head gasket to keep quench as tight as possible, but the distributor timing curve will need to be tailored to stay out of detonation.

Dennis

[68lemans462]

Get some KRE heads and a cam matched to the combo.

[steve25]

It's all so easy to just open up someone else's wallet isn't it?

[footjoy]

Originally Posted by SD455DJ

Call me a gluten for punishment, but I always wanted to dyno the '70 small valve XV code 400 from the A-body line. This motor was built to run at the Pure Stock Drags and the heads still run the 45 degree seats (both intake & exhaust). I had the rocker studs pinned to prevent pulling out with higher pressure springs and am using 3/8" polylocks to adjust the rockers to zero lash. All 70 small valve heads ran 45 degree seats, and fairly sure most of the early small valve heads did as well. There were likely a few exceptions, but don't know them off-hand. The higher compression small valve heads have a nice small turn radius to help low lift flow and the motor, as Steve25 said, only used 174 cfm flow for the 360 horsepower (it made 380 hp corrected, btw).

Footjoy, running #16 heads (75 cc's stock chamber) will push your compression to 10.2 to 1, especially if they need a clean-up cut. You still want to run a .027" Cometic head gasket to keep quench as tight as possible, but the distributor timing curve will need to be tailored to stay out of detonation.

Dennis

Machine shop called said the heads ccd at 75

My math isn't working

bore 4.16
stroke 3.75
heads 75CC
gasket .045
Deck height .021

I come up with 9.65 SCR

Where am I going wrong and I know that head gasket is big.

Is there a camshaft that would maybe help to bring it down to 9.5?

Thanks

[78w72]

Originally Posted by footjoy

Machine shop called said the heads ccd at 75

My math isn't working

bore 4.16
stroke 3.75
heads 75CC
gasket .045
Deck height .021

I come up with 9.65 SCR

Where am I going wrong and I know that head gasket is big.

Is there a camshaft that would maybe help to bring it down to 9.5?

Thanks

a cam wont change the static compression. you can pick a cam to help with cylinder pressure which will help alleviate detonation.

using those numbers & 6.6cc piston valve relief & 4.3 gasket bore size, i get 9.61. so i think you're pretty close. & 9.5 is an excellent compression for a pump gas engine, should be able to select a good cam for that compression & make some decent power.

http://www.wallaceracing.com/cr_test2.php

[SD455DJ]

Originally Posted by 78w72

a cam wont change the static compression. you can pick a cam to help with cylinder pressure which will help alleviate detonation.

using those numbers & 6.6cc piston valve relief & 4.3 gasket bore size, i get 9.61. so i think you're pretty close. & 9.5 is an excellent compression for a pump gas engine, should be able to select a good cam for that compression & make some decent power.

http://www.wallaceracing.com/cr_test2.php

I used .027" head gasket thickness for better quench and 72 cc chambers after a clean-up cut.

Dennis

[tom s]

Use a Butler head gasket,46 thou compressed.Your close enough that even I would not worry about it.Tom

[footjoy]

Originally Posted by SD455DJ

I used .027" head gasket thickness for better quench and 72 cc chambers after a clean-up cut.

Dennis

Lamen question where does quench come into play and what does it affect HP?

comparing a .027 gasket to a .046 head gasket what will I lose by doing that?
Thanks

Greg

[steve25]

Yes, it effects HP by means of a faster burning of the air and fuel mixture with less needed timing and as a side benefit it allows for better fuel efficiency .
There's nothing fully cut in stone in regards to what the quench clearance should be as the amount of square inches of quench clearance formed by the top of the piston and the chamber shape varies with the shape of each of these.

In short if your piston deck clearance is zero and the head gasket you are running compresses to .045" your as golden as golden gets with a street / strip motor.

Also in this same vein running a deck clearance of .025" and a compressed head gasket of .027" does not provide the same mixture agitation as the above example even though there quite close number wise!

[footjoy]

In my case I am .021 deck clearance and want to use a .046 head gasket to achieve 9.6 compression.

What will I lose with this combination.? I guess that is a open question.

Thanks

Greg

[JLMounce]

I'm not certain I'd go in that direction. Although you may slightly drop the compression ratio of the engine, what little bit you gain there is offset by worsening the quench distance. You may actually be more prone to detonation than you are currently.

Do you have verifiable measurements that your pistons are .021 in the hole or are you providing that information based on a piston spec sheet? Depending on the actual deck measurement of the engine, after any remediation for twist, warp and surface finish, it's possible your pistons aren't actually that far down.

If you have a set of the common TRW/SPeedPro pistons in your 400, the listed compression height of the piston is 1.714, so you can calculate rotating assembly height by 1/2 the stroke + rod length + piston compression height. Such a rotating assembly would give a deck height of 10.214" Accounting for even just a minimal surface on the head mounting surface of the block, you'd be looking at a deck height of more like around .007-.005 in the hole.

[footjoy]

Originally Posted by JLMounce

I'm not certain I'd go in that direction. Although you may slightly drop the compression ratio of the engine, what little bit you gain there is offset by worsening the quench distance. You may actually be more prone to detonation than you are currently.

Do you have verifiable measurements that your pistons are .021 in the hole or are you providing that information based on a piston spec sheet? Depending on the actual deck measurement of the engine, after any remediation for twist, warp and surface finish, it's possible your pistons aren't actually that far down.

If you have a set of the common TRW/SPeedPro pistons in your 400, the listed compression height of the piston is 1.714, so you can calculate rotating assembly height by 1/2 the stroke + rod length + piston compression height. Such a rotating assembly would give a deck height of 10.214" Accounting for even just a minimal surface on the head mounting surface of the block, you'd be looking at a deck height of more like around .007-.005 in the hole.

Ok so that scenario put me at 10:1 compression which won't work as a driver.