Crazy 408 in the works....Compression Ratio question

[Black1987Stang]

So what turned out to be doing a simple 351 buildup Im now picking up a fully forged 4340 Eagle 408 in a 69 block from a friend , converted to 4 bolt main with billet main caps, with block fill, ARP hardware throughout, canton pan, custom cam, custom pistons for boost etc etc... built by Pat Musi....it should be able to hold a wee bit of power

With 58cc heads it supposedly will have 8.6:1 compression.....Can I run it n/a without feeling like it's missing some power with that?

 

[blackened88lx]

I'm not really sure what you're asking. No, it's not going to be slow as balls but yes, you are giving up a decent amount of power.

 

[Wallzy]

Not sure, but i think that you are a bit off with your compression calculation, i'm comming up with 12.77CR.

408ci/8cyl = 6685.9cc/8cyl = 835.74cc/cyl

Say that your gasket is 12cc-13cc and your chamber is 58cc then:

(835.74 + 13 + 58)/(13 + 58) = 906.74/71 = 12.77

Walz

 

[347HO]

Not sure, but i think that you are a bit off with your compression calculation, i'm comming up with 12.77CR.

408ci/8cyl = 6685.9cc/8cyl = 835.74cc/cyl

Say that your gasket is 12cc-13cc and your chamber is 58cc then:

(835.74 + 13 + 58)/(13 + 58) = 906.74/71 = 12.77

Walz

How do you calculate CR without the compression height of the piston, depression of the piston top area and ring position?

You need to check your cylinder psi.
If your camshaft is in fact for boost, and you run it N/A and true pump gas... you'll have serious issues.

I hesitate to comment on a "converted" 4 bolt main. Even if it's an early block.

 

[Stopsign32v]

How do you convert a 2 bolt to a 4 bolt? ???

 

[parrish5o]

How do you convert a 2 bolt to a 4 bolt? ???

Thats what I was wondering, I know I am far from an expert but I have never heard of this before.

 

[90lxcoupe]

How do you convert a 2 bolt to a 4 bolt? ???

i have heard of this before, and ive seen pictures of billet 4 bolt caps being mocked up on a block that was on a machine table. I think(dont quote me on this) that they drill and tap the block to accept a splayed 4 bolt cap. This has to be a pretty involed procees seeing that the 4 bolt caps bolts dont go straight in so the hoes have to be machine on an angle.

I think that the picture i had seen was in Car Craft in a recent issue. The block had the cap torqued down and they were milling the side bolts with an tool in what looked like a Brideport head that was angled.

As for the motor, i dont think any 408 is going to be lazy if it is built right, 8.6 compression seems to be pretty low, how big is the dish in the piston? and do you know the compression height? What blower do you plan on using? I hope its big!

 

[Wallzy]

2 - 4 bolts can be done, the cap is usually match drilled to thru the block at an angle, have actually seen this done very successfully a few times.

As for CR, I assumed flat top pistons with no valve relief. I also assued the ring gland is in the noise for most calcualtions unless you are trying to get as close as possible to the upper limit of a CR rule in a race class. I just don't think that there is an extra 39cc's hiding in valve relief's and ring glands - just think that the math is off, but i could be wrong too. Have a good one!

Walz

 

[90lxcoupe]

3 - 4 bolts can be done, the cap is usually match drilled to thru the block at an angle, have actually seen this done very successfully a few times.

Walz

fixed

i didnt mention it in the previous post, but Walz is correct, the 4 bolt mains are added to the 2,3, and 4 main caps. If your not planning on pushing a 1000 hp ragged edge tune that is a good foundation for a powerful street motor. I'd be comfortable with that block making 800 hp assuming the tune is good.

 

[Black1987Stang]

Yeah basically just wanted to know if there would be any ill effects of running such low CR, and if theres anything I can do to bump it up.....I still need to find out some more info to see if I can run a little thinner head gasket without any piston to valve clearance, I havent gotten the motor yet. It does have a custom cam for a carb, boost, and the original guys combo but I'll be getting a new one ground for my combo.

Here's a pic....http://i39.photobucket.com/albums/e197/Mercs6/file003.jpg

 

[347HO]

I'm interested in what the average cylinder psi is?

A custom camshaft is definitely in order, and should help a bunch but you're still leaving a lot on the table with those pistons.

 

[Wallzy]

90LX - My bad, yes it is 3 - 4bolts can be done.

347Ho - Average Cyl Pressure is often misleading because it is highly dependant on the overlap of the camshaft, that's not to say that CR has absolutly nothing to do with it, but same compression ration and different camshafts can have dramatic effects on cyl pressure.


The pistons that are in there, are they full dish pistons, or do they have a quench area? This is a more important feature as you want to create eddy's in your cyl charge and burn all the fuel as quickly and efficiently as possible.

Walz

 

[347HO]

90LX - My bad, yes it is 3 - 4bolts can be done.

347Ho - Average Cyl Pressure is often misleading because it is highly dependant on the overlap of the camshaft, that's not to say that CR has absolutly nothing to do with it, but same compression ration and different camshafts can have dramatic effects on cyl pressure.

Walz

BINGO!
The OP stated his CR is 8.xx:1
He also stated he will be using the camshaft designed for boost.
In my experience, you are incorrect... cylinder pressure is extremely important and is never misleading. You can dial in a camshaft using cylinder pressure, you cannot using CR.
Because of the low CR and no boost, getting the cylinder pressure right is crucial.
In my opinion, even when tuned and perfect will leave power on the table because the valve events will not be "on".

90LX - My bad, yes it is 3 - 4bolts can be done.
The pistons that are in there, are they full dish pistons, or do they have a quench area? This is a more important feature as you want to create eddy's in your cyl charge and burn all the fuel as quickly and efficiently as possible.

Walz

What are you saying here?
I agree piston design is important to promote efficient "flame front", but I will say a full dish piston, when used in a well thought out combo will make power more efficiently than a piston with a squish pad.
Let me/us know what you're leading into with this?

Thanks
Dana

 

[Wallzy]

347HO - "In my experience, you are incorrect... cylinder pressure is extremely important and is never misleading. You can dial in a camshaft using cylinder pressure, you cannot using CR."

What i am saying is that cylinder pressure is a result of camshaft duration and overlap as well as a component of CR - there has been a lot published that goes into depth on this topic - thus i'm not sure i understand what you are saying about dialing in a camshaft with cyl pressures.

"I agree piston design is important to promote efficient "flame front", but I will say a full dish piston, when used in a well thought out combo will make power more efficiently than a piston with a squish pad."

This assumption is not correct, there has also been alot published on this topic as well. Being able to ignite the flame and make it burn fully will always give you more power. If you have a chance, check out John Heywood, he was a brilliant man and did a lot of research on these topics. Have a good one!

Walz

 

[347HO]

What i am saying is that cylinder pressure is a result of camshaft duration and overlap as well as a component of CR - there has been a lot published that goes into depth on this topic - thus i'm not sure i understand what you are saying about dialing in a camshaft with cyl pressures.
Walz

How many camshafts have you degreed in?
how many of these same camshafts have you measured cylinder pressure?
Then, using the same setup, how many times have you moved the camshaft timing fwd or rev to increase engine output... then measured the cylinder pressures to see what's happening?
You can read books all you want, but when you start doing these things and experimenting... you'll start to really learn.

This assumption is not correct, there has also been alot published on this topic as well. Being able to ignite the flame and make it burn fully will always give you more power. If you have a chance, check out John Heywood, he was a brilliant man and did a lot of research on these topics. Have a good one!

Walz

I'm not up to speed with John Heywood's research, for that I'm ignorant... however, I'm inclined to believe the applications you and he are referring to are probably not true pump gas 91 octane setups.
Not everyone will use a true custom setup, ie pistons, camshaft, crank and rods. But those who do and make outstanding hp to ci ratio in N/A form, are probably using a form of dished pistons.

 

[Wallzy]

347HO,

I'm not here to start an argument with you, was trying to provide insight into cause and effect. I can't count the amount of camshafts that i have degreed on chevy, dodge, or ford nor is that really important to me. I have spent my time behind a dyno, flow bench, spintron and smoketron device optimizing HP engines, so I have learned a few things over the years. I am sorry that you aren't familiar with John Heywood, that wasn't an attack at you if you infered it, it puts things into persepective answering the question of "why did that happen, oh because i did that..." As for the 91 octane and cars with high NA hp to ci ratios, those theory's from books are being practiced in today's auto industry and on average have produced much higher hp to ci ratios than have been done in the past. Well, it's time for an afternoon beer, cheers, have a good day.

Walz

 

[347HO]

347HO,

I'm not here to start an argument with you, was trying to provide insight into cause and effect. I can't count the amount of camshafts that i have degreed on chevy, dodge, or ford nor is that really important to me. I have spent my time behind a dyno, flow bench, spintron and smoketron device optimizing HP engines, so I have learned a few things over the years. I am sorry that you aren't familiar with John Heywood, that wasn't an attack at you if you infered it, it puts things into persepective answering the question of "why did that happen, oh because i did that..." As for the 91 octane and cars with high NA hp to ci ratios, those theory's from books are being practiced in today's auto industry and on average have produced much higher hp to ci ratios than have been done in the past. Well, it's time for an afternoon beer, cheers, have a good day.

Walz

No attack or ill-feelings inferred nor expressed Walz, just wanted some expansion on your version and insight to the OPs post.

I admit I haven't "done it all" nor claim to... but I have tinkered a ton in this field.
I receive great tech advice and knowledge from my camshaft maker, and through the years talking with people much like yourself have taken their knowledge and applied it to the 1320'.

 

[Black1987Stang]

ok well if this is all true....8.6 to 1 CR with 58cc heads....then I have to do something with my heads. I have twisted wedge heads which are 65cc (not stock), can they be decked to make them 58cc or smaller? I've never seen production heads any smaller than 58cc so could being any smaller hurt the quench area or anything? Anyone know how much would have to be decked off to get em down that far? or anything else I can do besides changing the pistons which I'm not messing with?

 

[Rick 91GT]

I never see where you listed what cc the piston is?? Typically a 22cc dish on a 408 with a 58cc head is in the 10.5:1 range. What piston are you using?? What is the cc of the dish. Usually if it is a flat top around 4cc you'll need a 72cc chamber in the head for a 10.x:1

Having a TW milled down to 58cc is no problem, you can go further the large concern is deck thickness and integrity under boost... Are your pistons made for the rotated valve? If not you may not be able to run them at all...

 

[Black1987Stang]

They're custom Ross blower/turbo pistons...they costed the guy a grand.

It was getting used with a custom hydraulic roller cam with .681 lift...His setup was fully worked highports, intake, carb, and a HUGE turbo..I'm taking the cam out and selling it since it's way too big for me. Mine will probably be no more than .600, I want a very very mild one since I need driveability with good vacuum....then again I'm no cam grinder, I plan on getting in contact with Ed Curtis for a cam. From the pics there are very very shallow valve reliefs that were put in for that .681 cam, so I would assume none would be needed for a much smaller cam...and I could use twisted wedge heads without them either.....but it would be clay checked since there's way too many variables like hg thickness, rr ratio, cam timing and etc to just assume. Here's some pics. Let me know what you think


http://i39.photobucket.com/albums/e197/Mercs6/file000.jpg

http://i39.photobucket.com/albums/e197/Mercs6/file003.jpg

http://i39.photobucket.com/albums/e197/Mercs6/file001.jpg