Increasing compression....effects?
Thread Starter
Melting Slicks
Joined: Oct 2005
Posts: 2,056
Likes: 8
From: Redruth Cornwall
Increasing compression....effects?
This is a two part question really...
In the quest for knowledge and of course more HP, was wondering about the following...
After market heads provide a good HP increase, but what is the max HP a set of stock L48 heads on a 75 Vette can handle?
Secondly, would changing pistons be a cheap way to increase the compression by installing a set different from stock?
Ready to be flamed on this but...still a n00b and learning... What effect would putting in a set of dished pistons in be on a stock'ish engine?
TIA
In the quest for knowledge and of course more HP, was wondering about the following...
After market heads provide a good HP increase, but what is the max HP a set of stock L48 heads on a 75 Vette can handle?
Secondly, would changing pistons be a cheap way to increase the compression by installing a set different from stock?
Ready to be flamed on this but...still a n00b and learning... What effect would putting in a set of dished pistons in be on a stock'ish engine?
TIA
Racer
Joined: May 2005
Posts: 470
Likes: 1
From: Amelia Ohio
The heads are the problem. Large combustion chamber and small ports and valves. Dish pistons would compound the issue lowering the combustion ratio. The best thing to do is change the heads, most of the after markets have 64 cc chambers raising ration to the 10:1 range and ports and valves are much larger for better flow.
Chris
Chris
Thread Starter
Melting Slicks
Joined: Oct 2005
Posts: 2,056
Likes: 8
From: Redruth Cornwall
Chris, Thanks for that. I know the stock heads are strangling the beast within....I just dont have the ££ at he mo to change them even tho the £ is strong against the $ at present. That is of course unless someone has a set of old/used/new heads that are just taking up space and would like to send me...I will pay shipping! 
Just wondered what the max HP that could be expected with stock heads and what difference to the compression rating pistons would make...would a piston change make negligible compression difference?
Thanks again.
Just wondered what the max HP that could be expected with stock heads and what difference to the compression rating pistons would make...would a piston change make negligible compression difference?
Thanks again.
Thread Starter
Melting Slicks
Joined: Oct 2005
Posts: 2,056
Likes: 8
From: Redruth Cornwall
Have a different cam, has headers and a dual chambered 2 1/2 inch exhaust...timing is set at 36deg all in by 2800rpm and original rebuil Qjet soon to be installed with Edelbrock 2101 intake.....
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Your L48 came with dished pistons. Cheap way to more comp is install pistons with a small dome (like speedpro H618CP)... along with a thin head gasket (like victor 5746) ... those will take you from a questionable quench & no more than 8:1scr to ... a good 0.051" quench and right at 9.6:1scr. H618CP coupled w/ an even thinner felpro 1094 gasket will drop the quench to a great 0.040" and raise comp further to right at 9.9:1.
-edit-piston swap is cheap if YOU do the work.
-edit-piston swap is cheap if YOU do the work.
This is a two part question really...
In the quest for knowledge and of course more HP, was wondering about the following...
After market heads provide a good HP increase, but what is the max HP a set of stock L48 heads on a 75 Vette can handle?
Secondly, would changing pistons be a cheap way to increase the compression by installing a set different from stock?
Ready to be flamed on this but...still a n00b and learning... What effect would putting in a set of dished pistons in be on a stock'ish engine?
TIA
In the quest for knowledge and of course more HP, was wondering about the following...
After market heads provide a good HP increase, but what is the max HP a set of stock L48 heads on a 75 Vette can handle?
Secondly, would changing pistons be a cheap way to increase the compression by installing a set different from stock?
Ready to be flamed on this but...still a n00b and learning... What effect would putting in a set of dished pistons in be on a stock'ish engine?
TIA
Last edited by jackson; Feb 15, 2007 at 04:40 PM.
Melting Slicks
Joined: Jul 2005
Posts: 2,060
Likes: 46
From: winter haven florida
St. Jude Donor '12
the stock heads are terrible on the 75. i have pics of mine after i took them off i will try to post tomorow. if you do not have the cash to change them then perhaps you might try port matching them to your intake. what are you cam specs?
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Thread Starter
Melting Slicks
Joined: Oct 2005
Posts: 2,056
Likes: 8
From: Redruth Cornwall
Comp Cams Part # 12-210-2
Grind number : CS 268H-10 High Energy.
Gross Valve Lift: .460 .460
Duration at .006 Tappet Lift 268 deg 268 deg
Does that sound correct? Look forward to the pics.
Your L48 came with dished pistons. Cheap way to more comp is install pistons with a small dome (like speedpro H618CP)... along with a thin head gasket (like victor 5746) ... those will take you from a questionable quench & no more than 8:1scr to ... a good 0.051" quench and right at 9.6:1scr. H618CP coupled w/ an even thinner felpro 1094 gasket will drop the quench to a great 0.040" and raise comp further to right at 9.9:1.
-edit-piston swap is cheap if YOU do the work.
-edit-piston swap is cheap if YOU do the work.
Last edited by jotto; Feb 15, 2007 at 05:47 PM.
Le Mans Master
Joined: Mar 2004
Posts: 7,424
Likes: 1,458
From: Little Rock AR
Not much good with numbers but...
Comp Cams Part # 12-210-2
Grind number : CS 268H-10 High Energy.
Gross Valve Lift: .460 .460
Duration at .006 Tappet Lift 268 deg 268 deg
Does that sound correct? Look forward to the pics.
I have no probs doing the work myself....If for arguments sake Iwas to swap the pistons and get a 9.9:1 Comp ratio, what kind of power increase would it yield?
Comp Cams Part # 12-210-2
Grind number : CS 268H-10 High Energy.
Gross Valve Lift: .460 .460
Duration at .006 Tappet Lift 268 deg 268 deg
Does that sound correct? Look forward to the pics.
I have no probs doing the work myself....If for arguments sake Iwas to swap the pistons and get a 9.9:1 Comp ratio, what kind of power increase would it yield?
If you are short on cash and want to upgrade the current setup inexpensively I would advise you to sell that 2101 intake manifold and install an Edelbrock Performer RPM. The 2101 intake IS holding you back even at this level. You can sell it and buy a better intake with the proceeds and a little more cash. Might need to go to a square bore carb though so keep that in mind.
-Mark.
Melting Slicks
Joined: Jul 2005
Posts: 2,060
Likes: 46
From: winter haven florida
St. Jude Donor '12
i had a cam very similar to that one with 1.6 roller rockers on the intakes and it ran ok with ported 461 heads. it was a huge improvement over the stock set up. it was not crazy fast but it was still a fun set up for the cost. the amount of power you will have will depend on the money you have to spend.
if you could score a set of used vortec heads and add a good vortec manifold you would see a great improvement on your set up. this could be done fairly cheap.
if you could score a set of used vortec heads and add a good vortec manifold you would see a great improvement on your set up. this could be done fairly cheap.
Drifting
Joined: Jul 2005
Posts: 1,622
Likes: 29
From: The Netherlands
Hey Jotto, good to see your at it still 
Want cheap heads which are better? Buy 'camel hump' heads like these:
http://cgi.ebay.com/ebaymotors/CORVE...QQcmdZViewItem
Some info (there probably is better information out there somewhere) on camel humps:
http://www.jalopyjournal.com/forum/s...d.php?t=119815
http://www.hotrod.com/techarticles/3...l_block_build/
Want cheap heads which are better? Buy 'camel hump' heads like these:
http://cgi.ebay.com/ebaymotors/CORVE...QQcmdZViewItem
Some info (there probably is better information out there somewhere) on camel humps:
http://www.jalopyjournal.com/forum/s...d.php?t=119815
http://www.hotrod.com/techarticles/3...l_block_build/
Safety Car
Joined: May 2001
Posts: 3,758
Likes: 3
From: Surrey
Hi Jotto,
I'm in the same boat: good exchange rate, no cash
I've decided to hold out until I can get some decent heads due to previous experience. I've got an engine that I've fitted all the "correct" parts in, but haven't been able to find the, now rare, late model head (that isn't knackered). My wife has exactly the same engine, but with the sought after head on it, & the performance difference is like night & day. Raising the compression in mine did improve the performance (& it sounded a lot nicer
), but nothing like fitting a head with decent size/shape ports & big valves. I'm assuming the same will apply to the smog heads on the Vette, so would rather save the money on a set of pistons & put it towards a set of heads.
I'm in the same boat: good exchange rate, no cash
I've decided to hold out until I can get some decent heads due to previous experience. I've got an engine that I've fitted all the "correct" parts in, but haven't been able to find the, now rare, late model head (that isn't knackered). My wife has exactly the same engine, but with the sought after head on it, & the performance difference is like night & day. Raising the compression in mine did improve the performance (& it sounded a lot nicer
), but nothing like fitting a head with decent size/shape ports & big valves. I'm assuming the same will apply to the smog heads on the Vette, so would rather save the money on a set of pistons & put it towards a set of heads.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Jotto:
Your HE268 cam, your freshened OE heads and H618CP pistons should make an honest 310-325hp on engine dyno w/ open headers & a good tune. Your OE heads will easily support 325fwhp. The pistons & rings & gaskets can be had on feebay for under $300 ... pistons item # 300080099704 .
Maybe it's just me ... but if you're willing & able to do the work ... that seems like pretty dang cheap compression & hp to me.
http://cgi.ebay.com/ebaymotors/SBC-C...QQcmdZViewItem
Your HE268 cam, your freshened OE heads and H618CP pistons should make an honest 310-325hp on engine dyno w/ open headers & a good tune. Your OE heads will easily support 325fwhp. The pistons & rings & gaskets can be had on feebay for under $300 ... pistons item # 300080099704 .
Maybe it's just me ... but if you're willing & able to do the work ... that seems like pretty dang cheap compression & hp to me.
http://cgi.ebay.com/ebaymotors/SBC-C...QQcmdZViewItem
