Thread: 70 Monte 454 Fuelie Bad Combo Questions:(

Tweet

Originally Posted by ssvolvo

My issue is a terrible spark knock that starts around 2200 RPM.

Pat McCarthy and Jerry Clayton, among others, have way more experience building big block Chevies than I do, but I enjoy sitting down and figuring out what's going on and how to fix it. In the end, every motor out there is just a big ole air pump that responds to pretty much the same parts and procedures as the next one.

First thing I see is that the squish isn't tightened up as much as it could be. Also, that big lump of dome on the piston is gettin' in the way of flame propogation. And the static compression ratio is too high for pump gas and the cam you're using.

The piston you have used has a compression height of 1.640". Add a 6.135" rod and a 2.000" stroke radius and you have a stack of 9.775" in a ~9.800" block deck height, leaving you with a 0.025" piston deck height. Add a nominal 0.040" head gasket to that and the squish measures 0.065". You can't use a thinner gasket because of the aluminum heads, so you have to do whatever else you can to fix the situation.

If we were to cut the block decks 0.010", registering off the centerline of the main bearing bore and use pistons with a taller compression height, like SRP forged #141635 that measure 1.645", you could improve the squish by 0.015" and arrive at a ~0.050" squish. The other plus side of using this piston is that the dome is 14 cc's instead of the 25.7 cc dome you have in the motor now, so flame propogation across the chamber should work a lot better.
Static compression ratio should end up at 9.72:1 and I'd bet that would work pretty well on pump gas with the Lunati cam you're using.

The SRP pistons are designed to work with the Edelbrock Recti-Oval heads you have. Here is a blurb about them....

"SRP big block Chevy small dome profile pistons are designed to fit Edelbrock 110cc and GM Signature series heads. They're 4032 low-expansion, high-silicon aluminum alloy that's heat-treated to SRP specifications. CNC-machined domes with radiused valve reliefs provide optimum flame travel. They include pin fitting, double Spiro locks, and wrist pin."
http://www.competitionproducts.com/p...?number=141635

Here are the volumes I used in computing the SCR....
4.280" x 4.000" = 943 cc's
Chambers +/- 110 cc's
Pistons 14 cc's
Piston deck height (block deck height 9.790", piston deck height 0.010") 2.4 cc's
0.039"/0.040" composition gasket for aluminum heads 9.7 cc's.

Deduct the dome from the chamber. 110 less 14 = 96 cc's. This is what we use for chamber volume. Piston volume is now zero.

Figured thusly....
943 (cylinder) + 96 (chamber) + 0 (piston) + 2.4 (piston deck height) + 9.7 (head gasket) = 1051.1 cc's.

96 + 0 + 2.4 + 9.7 = 108.1

Divide 1051.1 by 108.1 and find 9.723:1 static compression ratio.

Oh, and by the way, your present static compression ratio is 10.55:1

Also, I ran into a blurb somewhere that said the valve guides are semi-finished as produced. Check with whomever finished the guides. That may be where the oiling is coming from. Just a thought. Also, when you pull the motor apart, look closely at the intake gaskets to see if they are pinched all the way around. If you were pulling crankcase air into the ports from the bottom side because of a mis-match at the sealing surfaces, the motor could have been going lean and you'd never be able to find it.

You spent a bunch of time on this for me.
Thank-You.


John

Originally Posted by techinspector1

Pat McCarthy and Jerry Clayton, among others, have way more experience building big block Chevies than I do, but I enjoy sitting down and figuring out what's going on and how to fix it. In the end, every motor out there is just a big ole air pump that responds to pretty much the same parts and procedures as the next one.

First thing I see is that the squish isn't tightened up as much as it could be. Also, that big lump of dome on the piston is gettin' in the way of flame propogation. And the static compression ratio is too high for pump gas and the cam you're using.

The piston you have used has a compression height of 1.640". Add a 6.135" rod and a 2.000" stroke radius and you have a stack of 9.775" in a ~9.800" block deck height, leaving you with a 0.025" piston deck height. Add a nominal 0.040" head gasket to that and the squish measures 0.065". You can't use a thinner gasket because of the aluminum heads, so you have to do whatever else you can to fix the situation.

If we were to cut the block decks 0.010", registering off the centerline of the main bearing bore and use pistons with a taller compression height, like SRP forged #141635 that measure 1.645", you could improve the squish by 0.015" and arrive at a ~0.050" squish. The other plus side of using this piston is that the dome is 14 cc's instead of the 25.7 cc dome you have in the motor now, so flame propogation across the chamber should work a lot better.
Static compression ratio should end up at 9.72:1 and I'd bet that would work pretty well on pump gas with the Lunati cam you're using.

The SRP pistons are designed to work with the Edelbrock Recti-Oval heads you have. Here is a blurb about them....

"SRP big block Chevy small dome profile pistons are designed to fit Edelbrock 110cc and GM Signature series heads. They're 4032 low-expansion, high-silicon aluminum alloy that's heat-treated to SRP specifications. CNC-machined domes with radiused valve reliefs provide optimum flame travel. They include pin fitting, double Spiro locks, and wrist pin."
SRP, Racing Pistons, Chev BB Small Dome (Closed), 4.280 Bore-Competition Products

Here are the volumes I used in computing the SCR....
4.280" x 4.000" = 943 cc's
Chambers +/- 110 cc's
Pistons 14 cc's
Piston deck height (block deck height 9.790", piston deck height 0.010") 2.4 cc's
0.039"/0.040" composition gasket for aluminum heads 9.7 cc's.

Deduct the dome from the chamber. 110 less 14 = 96 cc's. This is what we use for chamber volume. Piston volume is now zero.

Figured thusly....
943 (cylinder) + 96 (chamber) + 0 (piston) + 2.4 (piston deck height) + 9.7 (head gasket) = 1051.1 cc's.

96 + 0 + 2.4 + 9.7 = 108.1

Divide 1051.1 by 108.1 and find 9.723:1 static compression ratio.

Oh, and by the way, your present static compression ratio is 10.55:1

Also, I ran into a blurb somewhere that said the valve guides are semi-finished as produced. Check with whomever finished the guides. That may be where the oiling is coming from. Just a thought. Also, when you pull the motor apart, look closely at the intake gaskets to see if they are pinched all the way around. If you were pulling crankcase air into the ports from the bottom side because of a mis-match at the sealing surfaces, the motor could have been going lean and you'd never be able to find it.

That's quite the calculation Tech.

I checked the dynamic and came up with 9.5.
Would this have more to do with pinging than the static of 10.55:1 ?

I'm curious about this as I'm planning a similar project (EFI) with 10.45:1 static and 8.5:1 dynamic using the same aluminum heads on a stroker.