454 Compression help
Thread Starter
Racer
Joined: Mar 2002
Posts: 468
Likes: 36
From: OKC OK
454 Compression help
I want a little more power than the stock 72 454. When I bought the car someone had recently rebuilt the motor. It had a set of 840's which I was planning on using on another engine and I would replace with a set of 781's. I think they may be a little milder and more daily driver friendly.
Here's what I found:
I pulled the heads and found a L2377-30 piston. These pistons have the following characteristics:
Compression with 100.9cc heads is approx. 8.72 to 1, with 106.9cc heads is approx. 8.36 to 1, with 109.4cc heads is approx. 8.22.
The 781's are more like 118cc, so I am thinking this will not be a good match running very high 7.x or low 8.x compression.
How will that low compression affect me? I know I will change the cam to around 500.
To me this sounds like more of a "mis-match" or not the best combination. Any suggestions on how I might improve or will this be fine for a daily driver?
Here's what I found:
I pulled the heads and found a L2377-30 piston. These pistons have the following characteristics:
Compression with 100.9cc heads is approx. 8.72 to 1, with 106.9cc heads is approx. 8.36 to 1, with 109.4cc heads is approx. 8.22.
The 781's are more like 118cc, so I am thinking this will not be a good match running very high 7.x or low 8.x compression.
How will that low compression affect me? I know I will change the cam to around 500.
To me this sounds like more of a "mis-match" or not the best combination. Any suggestions on how I might improve or will this be fine for a daily driver?
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
You'll be going the wrong direction, as new heads with a lower CR and a bigger cam don't add up to performance gains. Rather, you'll actually loose power.
If you're intent on installing different heads and re-camming, it's back to the drawing board time for a complete build-up. If that's out of the question, you should consider pocket porting and angle milling the old heads. That would improve the flow a bit and increase your CR. Then re-cam accordingly, watching your dynamic CR to stay in the sweet spot.
If you're intent on installing different heads and re-camming, it's back to the drawing board time for a complete build-up. If that's out of the question, you should consider pocket porting and angle milling the old heads. That would improve the flow a bit and increase your CR. Then re-cam accordingly, watching your dynamic CR to stay in the sweet spot.
Thread Starter
Racer
Joined: Mar 2002
Posts: 468
Likes: 36
From: OKC OK
You'll be going the wrong direction, as new heads with a lower CR and a bigger cam don't add up to performance gains. Rather, you'll actually loose power.
If you're intent on installing different heads and re-camming, it's back to the drawing board time for a complete build-up. If that's out of the question, you should consider pocket porting and angle milling the old heads. That would improve the flow a bit and increase your CR. Then re-cam accordingly, watching your dynamic CR to stay in the sweet spot.
If you're intent on installing different heads and re-camming, it's back to the drawing board time for a complete build-up. If that's out of the question, you should consider pocket porting and angle milling the old heads. That would improve the flow a bit and increase your CR. Then re-cam accordingly, watching your dynamic CR to stay in the sweet spot.
I think the problem before that existed was to much cam and heads. I think it was around high 8.
However, I think you are right - I may be going bac for a complete build up and replace the pistons.
I was hoping my goal could be was to save the short block, replace the cam, heads and intake.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Relative to the particular items you're looking at, I believe you'd be better off with "too much" flow with good CR and well selected cam than the "right" heads and way too little CR.
Bear in mind that with those better ports, you can "under-cam" to get/keep your DCR up and easily maintain adequate cylinder filling due to better flow, while improving port velocity.
With smaller heads, a bigger cam would be needed to flow the same volume, which would even further reduce your DCR.
Bear in mind that with those better ports, you can "under-cam" to get/keep your DCR up and easily maintain adequate cylinder filling due to better flow, while improving port velocity.
With smaller heads, a bigger cam would be needed to flow the same volume, which would even further reduce your DCR.
Thread Starter
Racer
Joined: Mar 2002
Posts: 468
Likes: 36
From: OKC OK
Relative to the particular items you're looking at, I believe you'd be better off with "too much" flow with good CR and well selected cam than the "right" heads and way too little CR.
Bear in mind that with those better ports, you can "under-cam" to get/keep your DCR up and easily maintain adequate cylinder filling due to better flow, while improving port velocity.
With smaller heads, a bigger cam would be needed to flow the same volume, which would even further reduce your DCR.
Bear in mind that with those better ports, you can "under-cam" to get/keep your DCR up and easily maintain adequate cylinder filling due to better flow, while improving port velocity.
With smaller heads, a bigger cam would be needed to flow the same volume, which would even further reduce your DCR.
Help me here, so I may be able to reduce the cam size to make a better balance of what I have now?
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
I'll clarify that better. Without changing to pistons with larger domes to get your CR up, you don't wnat the larger combustion chambers. With the right CR, there's nothing "wrong" with the 781's on a mild street motor. However, with the existing pistons the loss of CR would be the greater factor than any flow issues.
I'm just saying that between the two, I'd rather have a good CR with the "wrong" head than a poor CR with the "right" ones. With the former, I can correct somewhat with a cam. With the latter, no cam selection is going to bring it back.
I'm just saying that between the two, I'd rather have a good CR with the "wrong" head than a poor CR with the "right" ones. With the former, I can correct somewhat with a cam. With the latter, no cam selection is going to bring it back.
Thread Starter
Racer
Joined: Mar 2002
Posts: 468
Likes: 36
From: OKC OK
I'll clarify that better. Without changing to pistons with larger domes to get your CR up, you don't wnat the larger combustion chambers. With the right CR, there's nothing "wrong" with the 781's on a mild street motor. However, with the existing pistons the loss of CR would be the greater factor than any flow issues.
I'm just saying that between the two, I'd rather have a good CR with the "wrong" head than a poor CR with the "right" ones. With the former, I can correct somewhat with a cam. With the latter, no cam selection is going to bring it back.
I'm just saying that between the two, I'd rather have a good CR with the "wrong" head than a poor CR with the "right" ones. With the former, I can correct somewhat with a cam. With the latter, no cam selection is going to bring it back.
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Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Hey, doing your homework takes time. Better than making the wrong snap decision.
Post up when you get ready to nail down your cam, and you'll get plenty of advice on here.
Post up when you get ready to nail down your cam, and you'll get plenty of advice on here.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Try a short duration single pattern cam first.
If u have a low c.r. motor wether sb or BB u want to run a short single pattern cam (<270* full duratiuon). What cam do u have now? U may already have a small cam now but u need to pull the timing cover to read the cam numbers to find out. What a short single pattern cam does is close the intake vlv sooner on the compression stroke and trapping more A/F mixture/better cylinder filling than a longer duration cam would.
U won't need the longer duration on the exh as with the dual pattern cam for increased exh blow-down - it will only lower your vacuum.
Well i'm not familiar with BB heads but u want a tight chamber to keep what compression u have - any larger chamber will only reduce compression.
If your on the learning curve here (as most of us) then my advice is u should try a cam swap first for the experience before u spend big $$$ on a full overhaul - maybe your first try and too much to learn. Nothing ruins your enthusiasm more than to spend $2K to overhaul and many hours and have the engine eat itself from bad clearances or bad parts.
Hope this will help.
cardo0
U won't need the longer duration on the exh as with the dual pattern cam for increased exh blow-down - it will only lower your vacuum. Well i'm not familiar with BB heads but u want a tight chamber to keep what compression u have - any larger chamber will only reduce compression.
If your on the learning curve here (as most of us) then my advice is u should try a cam swap first for the experience before u spend big $$$ on a full overhaul - maybe your first try and too much to learn. Nothing ruins your enthusiasm more than to spend $2K to overhaul and many hours and have the engine eat itself from bad clearances or bad parts.
Hope this will help.
cardo0
Drifting
Joined: May 2005
Posts: 1,909
Likes: 6
From: Lake Arrowhead - Georgia > 72 Base Coupe & 74 BB Roadster
I just finished a 74 LS4 with flat tops / .30 over / CompXE268 flat tappett / stock manifolds and carb. Has a slight rough lope but make crazy smooth tire smoking power through 5000 .... then it spits up and wants a nap.
Thread Starter
Racer
Joined: Mar 2002
Posts: 468
Likes: 36
From: OKC OK
If u have a low c.r. motor wether sb or BB u want to run a short single pattern cam (<270* full duratiuon). What cam do u have now? U may already have a small cam now but u need to pull the timing cover to read the cam numbers to find out. What a short single pattern cam does is close the intake vlv sooner on the compression stroke and trapping more A/F mixture/better cylinder filling than a longer duration cam would. U won't need the longer duration on the exh as with the dual pattern cam for increased exh blow-down - it will only lower your vacuum.
Well i'm not familiar with BB heads but u want a tight chamber to keep what compression u have - any larger chamber will only reduce compression.
If your on the learning curve here (as most of us) then my advice is u should try a cam swap first for the experience before u spend big $$$ on a full overhaul - maybe your first try and too much to learn. Nothing ruins your enthusiasm more than to spend $2K to overhaul and many hours and have the engine eat itself from bad clearances or bad parts.
Hope this will help.
cardo0
U won't need the longer duration on the exh as with the dual pattern cam for increased exh blow-down - it will only lower your vacuum. Well i'm not familiar with BB heads but u want a tight chamber to keep what compression u have - any larger chamber will only reduce compression.
If your on the learning curve here (as most of us) then my advice is u should try a cam swap first for the experience before u spend big $$$ on a full overhaul - maybe your first try and too much to learn. Nothing ruins your enthusiasm more than to spend $2K to overhaul and many hours and have the engine eat itself from bad clearances or bad parts.
Hope this will help.
cardo0
Thanks
John
