Compression Readings All over..... What did I do :-(

[gbeltran]

Re: Compression Readings All over..... What did I do :-(

You need more cam or less compression. That cam is kind of small for the compression you are running. You could make it work with some good fuel and close attention to the timing, but I'll bet in the end you have to take enough of the advance out to keep it from running at it's peak. You need new pistons anyway, I'd pick up a set that lowers it to about 8.5 or 9 to 1, the reverse dome type, not just dished. You can put a thicker head gasket on it, but that's kind of a band aid approach, it takes away some of the squish and can make the engine more prone to detonation. Call comp and ask them what the recommeded compression ratio is for that cam.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

Running a dished piston on a 64cc head will be almost the same as running a flat top piston on the 70 cc head. Personally, I'd run the dished piston on the 70 cc head and get my compression down to about 9.2 - 1 for safety.

The aluminum heads will help with that also. Because they will dissapate heat better, I usually tell my customers in car applications that they can run about 1/2 of a compression point higher with no problems.

Also, the KB134 piston will require a little larger ring gap on the top ring.

Thats one of the things I was thinking, just decking it to .010 - .013. The deck is straight but I want to clean it up. This will allow me to adjust my CR.

The KB134 is out of the running, I have tried all different combo's when you start getting into.051 and higher gaskets the cost goes through the roof for them, I might as well buy new pistons. I just started a search for something around a 12cc that would give me the perfect area to work with a 70cc head I can adjust the compression with the deck

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

Call comp and ask them what the recommeded compression ratio is for that cam.

Hi gbeltran,
Thanks for the input and advice.
Just got off the phone with them, the tech said that cams range is 9-10:1. I was told 9.5:1 is optimal for her. I guess that is what I am going to dial her in for.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

I think I have it figured
Here is where I am at, please tell me what you think before I order

Keith Black/KB Pistons 9912HC-030

Edelbrock E-Street 70cc heads

By using the Edelbrock head gasket the math will go as follows:

Bore 4.030
stroke 3.75
Piston +13
gasket bore 4.155
Compressed .031
deck .012

Calculated CR 9.48:1
Displacement Vol 92.39cc

I can save $250 if I use the ProComp 210 heads BUT it don't flow as good up to 400 lift and the edelbrock is a USA product

 

[coheej]

Re: Compression Readings All over..... What did I do :-(

I think I have it figured
Here is where I am at, please tell me what you think before I order

Keith Black/KB Pistons 9912HC-030

Edelbrock E-Street 70cc heads

By using the Edelbrock head gasket the math will go as follows:

Bore 4.030
stroke 3.75
Piston +13
gasket bore 4.155
Compressed .031
deck .012

Calculated CR 9.48:1
Displacement Vol 92.39cc

I can save $250 if I use the ProComp 210 heads BUT it don't flow as good up to 400 lift and the edelbrock is a USA product

Your CR calculation appears to be accurate.

Your compressed gasket thickness of .031" and your deck of .012" gives you a piston to head clearance of .043". While .043" is ok, I wouldn't won't that to go below .040". I would not do my decking until I had the pistons. I would do a trial assembly and measure each piston deck height before I decked the block.

 

[Aloysius]

Re: Compression Readings All over..... What did I do :-(

The pistons with the D dish are the ticket. A dished piston reduces turbulence because there is no quench area left. Vortec heads are more compression friendly because of the turbulence induced..they also require less total ignition advance.

Obviously you've never seen parts travel from one cylinder to another..it's common in engine failures.

 

[gbeltran]

Re: Compression Readings All over..... What did I do :-(

I think that piston and head combo will serve you well. Here is some reading for you.
http://www.empirenet.com/pkelley2/DynamicCR.html

Looking at your current build I come up with a DCR somewhere in the 9 to 1 range, way to high for pump gas. With the parts you picked I show it closer to 8 to 1, a good number. Keep an eye on the timing, although with that head 32-34 degrees total should put it in the sweet spot and keep detonation away.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

The pistons with the D dish are the ticket. A dished piston reduces turbulence because there is no quench area left. Vortec heads are more compression friendly because of the turbulence induced..they also require less total ignition advance.

Obviously you've never seen parts travel from one cylinder to another..it's common in engine failures.

THe only piston I come up with in my price rage is this one Keith Black/KB Pistons KB142-030 Unless I go to a 350 piston and use a 5.565 rod
With it being at +18 I would have to switch to the 64cc head to get the cr around 9.5

BTW, yep I have seen parts go from one to another but only in my race engines, and then it usually did a lot more damage. Its been lots of years since I have built engines, last time was in 98. I am finding it amazing & real frightening how much I have forgotten about



Hey gbeltran<
That was some real food reading thanks for the lesson

 

[coheej]

Re: Compression Readings All over..... What did I do :-(

THe only piston I come up with in my price rage is this one Keith Black/KB Pistons KB142-030 Unless I go to a 350 piston and use a 5.565 rod
With it being at +18 I would have to switch to the 64cc head to get the cr around 9.5

BTW, yep I have seen parts go from one to another but only in my race engines, and then it usually did a lot more damage. Its been lots of years since I have built engines, last time was in 98. I am finding it amazing & real frightening how much I have forgotten about



Hey gbeltran<
That was some real food reading thanks for the lesson

The KB142 is a 350 piston. If you use the 5.565 rod, you're going to be at zero deck without any more decking on the block. Also, I wouldn't use the short rod.

 

[Aloysius]

Re: Compression Readings All over..... What did I do :-(

Do NOT use that short 400 rod...the longer the rod length, the more torque..thus hp..you'll get, with less side load on the pistons.

That's a clean engine, btw..congrats. Consider a more free flowing flame arrestor also.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

Do NOT use that short 400 rod...the longer the rod length, the more torque..thus hp..you'll get, with less side load on the pistons.

That's a clean engine, btw..congrats. Consider a more free flowing flame arrestor also.

No worries, I am going to use the 5.7 I don't want to bind her up . guess I grabbed the wrong one, back to searching.

I plan on getting I think its a a 9" K&N arrestor for her.

Thanks on her being clean, what you see on the inside is glyptal, I'm an old racer from years gone by lol. I can deal with the extra heat for the better oil flow

No telling from my questions now, but I used to build engines from back in 80 then built and raced dirt track modified from I 83-98 .

I had an accident in the spring of 98 at the world trade center where I busted up my legs and ankles real bad. At that point my wife forbid me to race again ( said I don't have many lives left LOL )