Too much compression?
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New User
Joined: Apr 2003
Posts: 11
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From: Sydney, Australia
Too much compression?
Hi, i brought a 351C with what i believe are 302C heads, i can only run it at 2-4 degrees of initial timing before it pings under load, its also very flat down low and runs real hot, headers actually glow, has basically stock cam. Also runs on when i turn it off.
Does this sound like too much compression, and how can i check to make sure before pulling heads of to open them up a bit?
Actually would putting a larger overlap cam be a better way to fix this problem, its a bit out of my league this stuff.
Any help appreciated as i need to fix it soon as its my daily driver.
Cheers
Does this sound like too much compression, and how can i check to make sure before pulling heads of to open them up a bit?
Actually would putting a larger overlap cam be a better way to fix this problem, its a bit out of my league this stuff.
Any help appreciated as i need to fix it soon as its my daily driver.
Cheers
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
Too much compression?
Are they 2V heads or 4V heads? Look at the corner of the head, a 4 there means it is a 4V head.
It sounds like a lean mixture, what kind of carb do you have?
Putting more overlap will help, if you have too much compression. But first you should find out what the problem really is before you start to fix it.
You may be able to check the casting #'s to determine which heads that you have. Some heads have casting #'s under the valve cover.
It sounds like a lean mixture, what kind of carb do you have?
Putting more overlap will help, if you have too much compression. But first you should find out what the problem really is before you start to fix it.
You may be able to check the casting #'s to determine which heads that you have. Some heads have casting #'s under the valve cover.
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New User
Joined: Apr 2003
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From: Sydney, Australia
Too much compression?
Im in australia and the engine was originally a 302C, but has had 351 crank and rods fitted, the only way to check the heads is to pull them off, which i was hopeing to avoid, but will port them and open up the chambers if it turns out they r 302C heads.
I believe stock 56cc 302C heads on a 351 bottom end makes around 11.3 comp, would that work out with the symtoms my engine has? Ive been replaceing carbies, regraphed the dizzy trying to get this flat spot out, but maybe ive been looking in the wrong area?
PS, i have a good pic of exactly where to remove material from the chambers if anyone would like to see it. Takes them to around 66cc, which is around 10:1, im told is about right for a 351-302 head motor.
I believe stock 56cc 302C heads on a 351 bottom end makes around 11.3 comp, would that work out with the symtoms my engine has? Ive been replaceing carbies, regraphed the dizzy trying to get this flat spot out, but maybe ive been looking in the wrong area?
PS, i have a good pic of exactly where to remove material from the chambers if anyone would like to see it. Takes them to around 66cc, which is around 10:1, im told is about right for a 351-302 head motor.
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
Too much compression?
They get good money for those 302C 2V heads here in the states.
The high CR would account for the pinging. I assume you are using the highest octane available in Australia.
The flat performance and glowing headers mean retarded timing or lean mixture. Do you have any vacuum leaks?
Where is your dist vacuum advance connected? Check your intial timing with the dist vacuum diconnected and plugged.
The high CR would account for the pinging. I assume you are using the highest octane available in Australia.
The flat performance and glowing headers mean retarded timing or lean mixture. Do you have any vacuum leaks?
Where is your dist vacuum advance connected? Check your intial timing with the dist vacuum diconnected and plugged.
Posting Guru
Joined: Jun 2002
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Too much compression?
I have a similar problem on my engine and I think it's for the same reasons. Unfortunately there is no easy way to find out without pulling the motor apart. Even if you determine they are 302C heads, I've seen anywhere from 54 to 66cc advertised for the chamber size. To figure out the true compression, the heads need to be measured, the piston dish and valve reliefs need to be measured, the deck clearance and gasket thickness also need to be measured. I wasn't prepared to do all that so I tuned it to run the best I could with the highest octane gas available.
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New User
Joined: Apr 2003
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From: Sydney, Australia
Too much compression?
Thanks for help Danlee and Brian, would checking the static comp be any indication of comp ratio, thats with a stock type cam? Ill check it and compare to stock figure of 150psi for a 9:1 351 that my manual says.
I know the car is running real lean on and just off idle, ive tried everything except drilling idle feeds. Ive had 5 carbies on it in last 18 months and had dizzy regraphed also.
Ive got a reco 1850 on it now, from Holley themselves and winding the mixture screws all the way makes no difference at all, winding them in kills it dead though.
I spose its off with the heads. Motors only done 5,000 klm,s, since full rebuild. Oh well.
I know the car is running real lean on and just off idle, ive tried everything except drilling idle feeds. Ive had 5 carbies on it in last 18 months and had dizzy regraphed also.
Ive got a reco 1850 on it now, from Holley themselves and winding the mixture screws all the way makes no difference at all, winding them in kills it dead though.
I spose its off with the heads. Motors only done 5,000 klm,s, since full rebuild. Oh well.
Posting Guru
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Too much compression?
Possibly. but those numbers are meant for a stock duration cam. When I get a chance I'll download an article on my 351C forum that explains static vs dynamic compression. It should give you a good idea how the cam duration affects your compression.
http://autos.groups.yahoo.com/group/...1Boss351HO400/
Mine was running lean too. Same problem. I had non functioning A/F scews. I had to enlarge the idle feed restrictors guite a bit to make a difference. Older carbs didn't come with these restrictors. Ideal screw setting is about 1.5 turns out.
What else did you try? pump shot, jet size?
Don't worry, we'll figure it out.
http://autos.groups.yahoo.com/group/...1Boss351HO400/
Mine was running lean too. Same problem. I had non functioning A/F scews. I had to enlarge the idle feed restrictors guite a bit to make a difference. Older carbs didn't come with these restrictors. Ideal screw setting is about 1.5 turns out.
What else did you try? pump shot, jet size?
Don't worry, we'll figure it out.
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Postmaster
Joined: Aug 2001
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From: Delaware
Too much compression?
Measure your manifold vacuum at idle, you may have a vacuum leak. Do you have a rough idle? The idle screws should make a difference.
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Junior User
Joined: May 2003
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Too much compression?
This might help also.
Cam Timing vs. Compression Analysis
http://victorylibrary.com/mopar/cam-tech-c.htm
Note the Excel file:
http://victorylibrary.com/mopar/Formulas.xls
Cam Timing vs. Compression Analysis
http://victorylibrary.com/mopar/cam-tech-c.htm
Note the Excel file:
http://victorylibrary.com/mopar/Formulas.xls
Thread Starter
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New User
Joined: Apr 2003
Posts: 11
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From: Sydney, Australia
Too much compression?
It idles dead steady at 16" vac, but its the same from 1.5 turns out to fully out, I used a exhaust analyzer and even with screws all the way out it was still slightly on the lean side. Im looking at installing mikuni jets in the idle feeds to make it adjustable, just chasing up the info.
Ill get rid of this off idle hestitaion yet
thanks again guys.
Ill get rid of this off idle hestitaion yet
thanks again guys.
Senior User
Joined: May 2003
Posts: 150
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From: San Diego
Too much compression?
If you do find that you have to much compression ratio, you can use a thicker head gasket. CR shouldn't make your headers glow. Did you put the head gaskets on right? I think if you put them on backwards they block off 2 of the water ports. "don't quote me on that" but that could be the problem.
Now is it ping or your lifters? I assume you have a hydrolic cam.
Before I went to a solid cam, this would happen when it got hot.
Your lifters could be bleeding down as the oil thins from the heat.
My 2 cents
Now is it ping or your lifters? I assume you have a hydrolic cam.
Before I went to a solid cam, this would happen when it got hot.
Your lifters could be bleeding down as the oil thins from the heat.
My 2 cents
Posting Guru
Joined: Mar 2002
Posts: 2,070
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From: Boise, Idaho
Too much compression?
I'm wondering if you've got a vacuum leak or if your cam is not installed right.
Vacuum leak - evidently the intake manifolds on Clevelands need to be carefully torqued or you'll have trouble.
Here's a link that explains.
Or the vacuum diaphragm on the brake booster could be leaking or some other problem could be the matter.
If the engine used to have EGR and it's been taken off, you'll want to fix the distributor's curve.
If the cam is retarded, that won't help, either.
Vacuum leak - evidently the intake manifolds on Clevelands need to be carefully torqued or you'll have trouble.
Here's a link that explains.
Or the vacuum diaphragm on the brake booster could be leaking or some other problem could be the matter.
If the engine used to have EGR and it's been taken off, you'll want to fix the distributor's curve.
If the cam is retarded, that won't help, either.
Thread Starter
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New User
Joined: Apr 2003
Posts: 11
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From: Sydney, Australia
Too much compression?
Im leaning towards a vac leak under the intake, ill pull it off and redo it, do u guys use the cork end gaskets or a bead of siliastic instead?
Im looking at a cam change, so will double check cam timing when i do it. Just got to work out best cam for the job.
Im looking at a cam change, so will double check cam timing when i do it. Just got to work out best cam for the job.
Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
Too much compression?
Download this Program and read the text on this website. It will help you with the cam selection.
http://cochise.uia.net/pkelley2/DynamicCR.html
http://cochise.uia.net/pkelley2/DynamicCR.html