Compression ratio concerns

[Cranky]

How about the weight on the new pistons. Probably not the same as the ones you have now.....

 

[RRSweden]

ooh! Did'nt think about that. Guess the engine shop will need them for the balancing of the crank?

 

[Garys1969RR]

I've found a set of nice used pistons with 1.900 compression distance. That takes me up to 0.028 deck clearance!
The hunt for a set of 915's keeps going. A guy in Norway said he had a couple, I will hear from him soon..

I would buy those pistons if they are in good shape. Check your cyl bore first to make sure the pistons are the same size or larger. Get some closed chamber heads at 79 cc, use a .020" steel shim head gasket, and you will be at 9.7 to 1. Go to 77 cc head volume, deck the block .010, then you will be at 10.14 to 1. That's what I would shoot for. You will then have .037" of squish which is perfect! Reduces pinging chances, and gives the motor really crisp throttle response. Cutting the block .010 isn't going to change port or bolt alignment enough to cause any problems. You will have a good strong running motor with good throttle response. Set the timing at 20 degrees initial, 38 total. I recommend a Street Dominator intake, or a Performer RPM. A Torker or similar manifold may work good too.

 

[RRSweden]

I would buy those pistons if they are in good shape. Check your cyl bore first to make sure the pistons are the same size or larger. Get some closed chamber heads at 79 cc, use a .020" steel shim head gasket, and you will be at 9.7 to 1. Go to 77 cc head volume, deck the block .010, then you will be at 10.14 to 1. That's what I would shoot for. You will then have .037" of squish which is perfect! Reduces pinging chances, and gives the motor really crisp throttle response. Cutting the block .010 isn't going to change port or bolt alignment enough to cause any problems. You will have a good strong running motor with good throttle response. Set the timing at 20 degrees initial, 38 total. I recommend a Street Dominator intake, or a Performer RPM. A Torker or similar manifold may work good too.

I was thinking of a 2-plane manifold. Mostly for driveability on the lower rpm. Looking at a Edelbrock DP4B

 

[Cranky]

I would buy those pistons if they are in good shape. Check your cyl bore first to make sure the pistons are the same size or larger. Get some closed chamber heads at 79 cc, use a .020" steel shim head gasket, and you will be at 9.7 to 1. Go to 77 cc head volume, deck the block .010, then you will be at 10.14 to 1. That's what I would shoot for. You will then have .037" of squish which is perfect! Reduces pinging chances, and gives the motor really crisp throttle response. Cutting the block .010 isn't going to change port or bolt alignment enough to cause any problems. You will have a good strong running motor with good throttle response. Set the timing at 20 degrees initial, 38 total. I recommend a Street Dominator intake, or a Performer RPM. A Torker or similar manifold may work good too.

I still recommend doing a mock up at the very least. I've had intakes that didn't fit all that well with stock stuff....especially aftermarket ones.

 

[Garys1969RR]

A 2 plane is good. That is an older intake from the 70s, I believe. Good place to start. You can always upgrade at a later date.

 

[Garys1969RR]

Don't know what the pistons weigh that you are looking at, but the KB 400s weigh 956 grams with pin. You can always have your machine shop weigh a piston/pin to see where you need to be. There is alot of good info on the United Engine and Machine website. As well as a C/R calculator. It's worth looking at. They make the KB Pistons. (Keith Black)

 

[RRSweden]

I still recommend doing a mock up at the very least. I've had intakes that didn't fit all that well with stock stuff....especially aftermarket ones.

Don't think I'll touch the decks.. With the taller pistons, and new gaskets (any recommendations? Would like to come close to .020) I'll improve the C/R quite a bit. 9.01:1 due to the calculator. That's a number I'm quite happy with.

And if I should find a set of affordable 915 heads the C/R will be 9.91:1 (counted 78cc in the chambers) But I don't think my budget will allow new heads...

 

[miller]

I've found a set of nice used pistons with 1.900 compression distance. That takes me up to 0.028 deck clearance!

You might think about, at the very least, get the machine shop to check the block deck surface. Unless it's real good, chance it needs a wipe to make it true. I only needed .005" on both my deck surfaces, and heads, to get them flat and true centerline.
Once that's done, with those pistons, you can set the CR you want by the head gasket thickness...up or down.

 

[RRSweden]

You might think about, at the very least, get the machine shop to check the block deck surface. Unless it's real good, chance it needs a wipe to make it true. I only needed .005" on both my deck surfaces, and heads, to get them flat and true centerline.
Once that's done, with those pistons, you can set the CR you want by the head gasket thickness...up or down.

Yes, maybe I should ask them to check the decks at least..

 

[Garys1969RR]

I would use the beaded steel head gasket from Mopar Perf, or Mr Gasket. Thickness is .020", and I've never had a problem with one. Part # P4286754. (Mopar Performance #) Only $24.97/pair from Summit.

 

[RRSweden]

I would use the beaded steel head gasket from Mopar Perf, or Mr Gasket. Thickness is .020", and I've never had a problem with one. Part # P4286754. (Mopar Performance #) Only $24.97/pair from Summit.

I guess there gaskets puts higher demands on the deck/head surfaces... I should talk to my machine shop about this..

 

[Cranky]

Thinner gaskets raise the CR some but the demand isn't anymore than others. These engines came from the factory with the steel shim gaskets...

 

[Garys1969RR]

I guess there gaskets puts higher demands on the deck/head surfaces... I should talk to my machine shop about this..

These motors have very few (if any) head gasket probs when assembled correctly. This includes running a tap thru the threads on the block, and a few drops of oil on the bolt threads helps. Also I start at 30 ft lbs of torque, then go thru the tightening sequence, raising the torque 10 lbs each time until I get to the recommended torque of 70 ft lbs for iron heads. Not sure what alum heads get, I think it's 60 ft lbs.

 

[Garys1969RR]

I found out the Eddy Perf RPM heads also need 70 ft lbs of torque. Start at 40, then 55, final torque of 70 ft lbs. So what did you finally decide on as far as a cyl head and gasket goes?

 

[RRSweden]

I found out the Eddy Perf RPM heads also need 70 ft lbs of torque. Start at 40, then 55, final torque of 70 ft lbs. So what did you finally decide on as far as a cyl head and gasket goes?

I found i guy pretty close to Gothenburg that had a couple of 0.020 gaskets. I still think my budget won't allow a set of new heads at this time. So I think I have to settle with 9.01:1 CR. Although I really would like to try and port my 906 heads a little bit. Starting with the intake-windows. I have to do some research on how to go on from there... Don't wanna ruin everything..

 

[RRSweden]

I could use some really basic walk-through's on that subject

And again, I wanna thank you all for all the help you're giving!
I'm kinda nervous as this is my first engine build, and parts are about double the price over here, so there's limited room for f**k-ups ;)

 

[Garys1969RR]

I would start by ordering some Mopar porting templates from Summit. Also read all the info you can on porting big block heads. Head Porting is a science in itself, and very interesting.

 

[IQ52]

Porting bb Mopar cylinder heads as a beginner? This is about the best you're going to get online........

http://www.hotrod.com/articles/cylinder-heads/
http://www.hotrod.com/articles/cylinder-heads-ii/
http://www.hotrod.com/articles/cylinder-heads-iii/

 

[Garys1969RR]

Those articles were very helpful when I was porting some 915 heads a few years ago. Lots of good info in there. Thanks for posting those links, I was just trying to locate them.