Head Gasket Opinions ??
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Well, what is being done with the thinner head gasket is to increase CR by reducing the fixed volume:
CR = (Vs + Vf) / Vf
Vs = swept volume
Vf = fixed volume, where cc of the heads is one part.
For a 350cid SBC each cylinder has 716cc swept volume. If the CR is 9.5:1 the fixed volume is:
Vf = 716 / 8.5 = 84cc
With a 58cc head there is then 26cc of "dead space" due to deck height, piston dishes, volume to compression ring and gasket compressed thickness.
While it is true that using a thinner gasket does not actually decrease change head volume it can be thought of as doing so. The thinner head gasket reduces fixed volume just as would the equivalent reduction in head volume.
Of course the benefit in enlarging the head volume while reducing gasket thickness and/or deck height is that the quench volume is reduced. This reduces octane sensitivity.
CR = (Vs + Vf) / Vf
Vs = swept volume
Vf = fixed volume, where cc of the heads is one part.
For a 350cid SBC each cylinder has 716cc swept volume. If the CR is 9.5:1 the fixed volume is:
Vf = 716 / 8.5 = 84cc
With a 58cc head there is then 26cc of "dead space" due to deck height, piston dishes, volume to compression ring and gasket compressed thickness.
While it is true that using a thinner gasket does not actually decrease change head volume it can be thought of as doing so. The thinner head gasket reduces fixed volume just as would the equivalent reduction in head volume.
Of course the benefit in enlarging the head volume while reducing gasket thickness and/or deck height is that the quench volume is reduced. This reduces octane sensitivity.
Race Director
Joined: May 2003
Posts: 10,749
Likes: 41
Originally Posted by 65Z01
Well, what is being done with the thinner head gasket is to increase CR by reducing the fixed volume:
CR = (Vs + Vf) / Vf
Vs = swept volume
Vf = fixed volume, where cc of the heads is one part.
For a 350cid SBC each cylinder has 716cc swept volume. If the CR is 9.5:1 the fixed volume is:
Vf = 716 / 8.5 = 84cc
With a 58cc head there is then 26cc of "dead space" due to deck height, piston dishes, volume to compression ring and gasket compressed thickness.
While it is true that using a thinner gasket does not actually decrease change head volume it can be thought of as doing so. The thinner head gasket reduces fixed volume just as would the equivalent reduction in head volume.
Of course the benefit in enlarging the head volume while reducing gasket thickness and/or deck height is that the quench volume is reduced. This reduces octane sensitivity.
CR = (Vs + Vf) / Vf
Vs = swept volume
Vf = fixed volume, where cc of the heads is one part.
For a 350cid SBC each cylinder has 716cc swept volume. If the CR is 9.5:1 the fixed volume is:
Vf = 716 / 8.5 = 84cc
With a 58cc head there is then 26cc of "dead space" due to deck height, piston dishes, volume to compression ring and gasket compressed thickness.
While it is true that using a thinner gasket does not actually decrease change head volume it can be thought of as doing so. The thinner head gasket reduces fixed volume just as would the equivalent reduction in head volume.
Of course the benefit in enlarging the head volume while reducing gasket thickness and/or deck height is that the quench volume is reduced. This reduces octane sensitivity.
ahh! thanks jim! u always have a way of explaining things.....
Thread Starter
Racer
Joined: Oct 1999
Posts: 270
Likes: 0
From: Phoenix, AZ, USA
Wow, Glad to see this thread had some educational use...
While I could not explain it as eliquently as 65Z01. I'm glad my assumption of recovery of cc's held true.
Doing the math for this...
For the original factory compression ratio of 10:1
9 = (716 + Vf) / Vf
Vf = 79.6
of which the cyl head chamber was 58cc's
leaving 21.6cc's for deck height and gasket
now that the head is 63 cc's, with a similar gasket
the Vf goes to 84.6, dropping the compression to 9.46:1
recovering almost 6 cc's with the .015 gasket...
the effective Vf is 78.6 resulting in a compression ratio of 10.11:1
Just what I wanted
If I may bring it back to my original question...
Does anyone feel there is a substantial risk to reliability, for a street driven, 10.1:1 compression (stock bottom end) 350 that will not see 6000 RPM, when using the .015 gasket?
Or should I just live with the half point loss in compression...
Anyone want to guess at the power loss ?
While I could not explain it as eliquently as 65Z01. I'm glad my assumption of recovery of cc's held true.
Doing the math for this...
For the original factory compression ratio of 10:1
9 = (716 + Vf) / Vf
Vf = 79.6
of which the cyl head chamber was 58cc's
leaving 21.6cc's for deck height and gasket
now that the head is 63 cc's, with a similar gasket
the Vf goes to 84.6, dropping the compression to 9.46:1
recovering almost 6 cc's with the .015 gasket...
the effective Vf is 78.6 resulting in a compression ratio of 10.11:1
Just what I wanted
If I may bring it back to my original question...
Does anyone feel there is a substantial risk to reliability, for a street driven, 10.1:1 compression (stock bottom end) 350 that will not see 6000 RPM, when using the .015 gasket?
Or should I just live with the half point loss in compression...
Anyone want to guess at the power loss ?
Instructor
Joined: Jan 2002
Posts: 225
Likes: 0
From: Regina Sask
ok here is my guess at the power loss, it will be around Nil or even a slight negative power loss, you cleaned up the heads to flow better and give a better burn increasing power, you knocked compression down half a point slightly hurting power i will bet it will even out.
even if you lose a bit of HP from it, its nothing that cant be overcome with a supercharger or Nitrous.
anyway thats my guess, if i am wrong in my thinking, please guys correct me.
even if you lose a bit of HP from it, its nothing that cant be overcome with a supercharger or Nitrous.
anyway thats my guess, if i am wrong in my thinking, please guys correct me.
Thread Starter
Racer
Joined: Oct 1999
Posts: 270
Likes: 0
From: Phoenix, AZ, USA
Delta: 65Z01: Thanks for both your replies, but I think you missed my point... all things being equal... a 9.46:1 engine and a 10.1:1 engine with all the same go fast goodies are going to make different HP. I wanted a guess as to that difference.
I know I'm going to come out ahead with the cam and head change over stock configuration (even with a drop in Comp ratio). But if the above suggests 25 CHP are lost due to the compression change... I'll take my chances with the thin gasket... for 5 CHP I wouldn't.
The .1 comp ratio over stock with the improved quench is a nice benefit, but I wasn't concerned about that HP gain... I think we agree the result should be less prone to knocking.
I know I'm going to come out ahead with the cam and head change over stock configuration (even with a drop in Comp ratio). But if the above suggests 25 CHP are lost due to the compression change... I'll take my chances with the thin gasket... for 5 CHP I wouldn't.
The .1 comp ratio over stock with the improved quench is a nice benefit, but I wasn't concerned about that HP gain... I think we agree the result should be less prone to knocking.
Last edited by Roadster89; Dec 16, 2004 at 05:34 PM.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by 65Z01
Well, what is being done with the thinner head gasket is to increase CR by reducing the fixed volume:
CR = (Vs + Vf) / Vf
Vs = swept volume
Vf = fixed volume, where cc of the heads is one part....
CR = (Vs + Vf) / Vf
Vs = swept volume
Vf = fixed volume, where cc of the heads is one part....
Originally Posted by Roadster89
Wow, Glad to see this thread had some educational use...
While I could not explain it as eliquently as 65Z01. I'm glad my assumption of recovery of cc's held true.
Doing the math for this...
For the original factory compression ratio of 10:1
9 = (716 + Vf) / Vf
Vf = 79.6
of which the cyl head chamber was 58cc's
leaving 21.6cc's for deck height and gasket...
While I could not explain it as eliquently as 65Z01. I'm glad my assumption of recovery of cc's held true.
Doing the math for this...
For the original factory compression ratio of 10:1
9 = (716 + Vf) / Vf
Vf = 79.6
of which the cyl head chamber was 58cc's
leaving 21.6cc's for deck height and gasket...
RACE ON!!!
Thread Starter
Racer
Joined: Oct 1999
Posts: 270
Likes: 0
From: Phoenix, AZ, USA
CFI: Thanks for the encouragement...
I just read that the stock gasket was a .051 compressed (as Felpro 1044)... thats 11.2 cc's in area... the .015 compressed gasket (Felpro 1094) comes in at 3.2 cc's, so the resultant Vf drops to 76.6 cc's and the compression ratio would actually improve to 10.35:1... a rather significant .9 points of compression over the stock gasket with the 63cc heads.
One last thought on my understanding of the data that I'd like someone to confirm...
A stock .051 gasket's 11.2 cc volume would suggest that the remaining Vf after accounting for the 58cc cyl head volume is only 10.4 cc's, which needs to be divided among the piston contures, valve reliefs, and piston to deck height. This doen't suggest a whole lot of clearance room.... Seems to me the pistons come right to the top of the block
I just read that the stock gasket was a .051 compressed (as Felpro 1044)... thats 11.2 cc's in area... the .015 compressed gasket (Felpro 1094) comes in at 3.2 cc's, so the resultant Vf drops to 76.6 cc's and the compression ratio would actually improve to 10.35:1... a rather significant .9 points of compression over the stock gasket with the 63cc heads.
One last thought on my understanding of the data that I'd like someone to confirm...
A stock .051 gasket's 11.2 cc volume would suggest that the remaining Vf after accounting for the 58cc cyl head volume is only 10.4 cc's, which needs to be divided among the piston contures, valve reliefs, and piston to deck height. This doen't suggest a whole lot of clearance room.... Seems to me the pistons come right to the top of the block
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Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by Roadster89
CFI: A stock .051 gasket's 11.2 cc volume would suggest that the remaining Vf after accounting for the 58cc cyl head volume is only 10.4 cc's, which needs to be divided among the piston contures, valve reliefs, and piston to deck height. This doen't suggest a whole lot of clearance room.... Seems to me the pistons come right to the top of the block
RACE ON!!!
Melting Slicks
Joined: Jan 2002
Posts: 2,238
Likes: 4
From: Milan
About the .025" deck during a rebuilt:
This is a tipic value with STOCk piston and UNSHAVED block.
Many, many aftermarket pistons are available only with understroked compression height. (the distance from the axle of the wrist pin and the top flat surface of the piston). On a stock piston this value is a 1.560", but some aftermarket pistons are 1.540" (.020" LESS!)
So, the aforementioned value of 0.25" be now 0.045"!!
If you are not resurfacing the block an aftermarket piston set with the stock compression height is a good way to do not loose compression.
NOT always this data are available on all aftermarket piston sets.
Hope this helps
-Beppe-
This is a tipic value with STOCk piston and UNSHAVED block.
Many, many aftermarket pistons are available only with understroked compression height. (the distance from the axle of the wrist pin and the top flat surface of the piston). On a stock piston this value is a 1.560", but some aftermarket pistons are 1.540" (.020" LESS!)
So, the aforementioned value of 0.25" be now 0.045"!!
If you are not resurfacing the block an aftermarket piston set with the stock compression height is a good way to do not loose compression.
NOT always this data are available on all aftermarket piston sets.
Hope this helps
-Beppe-
Melting Slicks
Joined: Jan 2002
Posts: 2,238
Likes: 4
From: Milan
.So the next question will be:
Which head gasket (for alum heads) with a thickness good to achieve the correct deck height (near .035" - .040") that promote the good effect of the squish?
Where to buy a .015"-.020" gasket good for alum gaskets?
-Beppe-
Which head gasket (for alum heads) with a thickness good to achieve the correct deck height (near .035" - .040") that promote the good effect of the squish?
Where to buy a .015"-.020" gasket good for alum gaskets?
-Beppe-
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by conv90
About the .025" deck during a rebuilt:
This is a tipic value with STOCk piston and UNSHAVED block.
Many, many aftermarket pistons are available only with understroked compression height. (the distance from the axle of the wrist pin and the top flat surface of the piston). On a stock piston this value is a 1.560", but some aftermarket pistons are 1.540" (.020" LESS!)
So, the aforementioned value of 0.25" be now 0.045"!!
NOT always this data are available on all aftermarket piston sets.
Hope this helps
-Beppe-
This is a tipic value with STOCk piston and UNSHAVED block.
Many, many aftermarket pistons are available only with understroked compression height. (the distance from the axle of the wrist pin and the top flat surface of the piston). On a stock piston this value is a 1.560", but some aftermarket pistons are 1.540" (.020" LESS!)
So, the aforementioned value of 0.25" be now 0.045"!!
NOT always this data are available on all aftermarket piston sets.
Hope this helps
-Beppe-
The various on-line catalogs may not show the compression height of a piston, but the paper catalogs almost always have specs in the back of the book. This is part of engine building. Check the specs on EVERYTHING, and plan ahead.
RACE ON!!!
