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Mechanical timing adv issue on high compression 327

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Dosperado

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Discussion starter · #1 ·
Hi all, new owner of a 67 Camaro, rebuilt by previous owner. I know it was built with high compression in mind, 11:1 claimed SCR, and I tried to determine the DCR but got lost... It's got 57.5 cc camel hump heads, .100 popup forged pistons, and from the looks of it the block was decked down to, and below, the partial vin pad (grrr) and the cam was just swapped to a more reasonable summit cam part # SUM-K00172 duration @50 is 218 with a .450 lift, so it's probably inherently ping prone to start with... however, here is my issue/question:

When he swapped in the more streetable cam, he also swapped in a summit racing blueprinted HEI dist, and I'm trying to figure out why that distributer has mech adv that maxes out at 10 degrees at 1500? (quoted from instructions at bottom of post) Also I found the vac adv, when backed out entirely, provides 16 deg of adv starting at 7" hg and all in at ~14", or 20 deg starting at ~5" when adjusted the other way entirely (using vac pump and with rpm < mech adv point of initialization)

I've read up extensively on recomendations, and almost all agree on total advance of 32-36 at 3500, which usually doesn't include vac adv, yet this distributer seems to rely on vac adv for some of the total (ported vac) and so WOT, where vac is essentially nil, would only see 22 degrees total advance, correct? And to make things worse, partial throttle gets more advance at 1500 rpm and ends up overly advanced at 30+ resulting in detonation (I set initial at 10, which seems to be safer, and it still pings at partial throttle)

Some research suggests I limit the vac adv to 10 deg and re-jigger the mech adv to come on at 1k and provide closer to 22 deg total by 3k-3.5k, whereas currently it's out of the equation at 1.5k with 10 degrees.
That should allow more initial timing to help at idle, less of a jump initially at off idle, and smoother+predictable timing adv curve from 1k-3.5k. Or am I just confused??

thanks in advance guys, I do love working on a car again that doesn't require a PhD and a laptop! I still can't believe I found my old "Made in America" Sears analyzer and timing light...:beers:

From the Summit Racing distributor instruction sheet:
Set initial spark timing advanced two to four degrees from the factory
recommended setting. This will usually provide improved performance and fuel
economy. However, timing advanced too far beyond factory specifications may
result in detonation, which can cause engine damage. The distributor’s
mechanical advance is 6° @ 900 RPM, 8° to 10° @ 1200 RPM and reaches a
maximum of 10° to 11°@ 1500 RPM The vacuum advance provides 10° of advance.
Click to expand...
 
#3 ·
Re: mechanical timing adv issue on high compression 327

The crankshaft spins twice as fast as the distributor, so if the numbers in the instructions are in distributor degrees you would double them for crankshaft degrees. So 10 to 11 deg at 1500 is really 20 to 22 deg at 3000. That cam is not gonna work with 11:1 and pump gas. You're gonna have to make some changes to that combination. You might want to open the motor up just to see if it's a tired old race motor put out to pasture. Were the valve springs changed back to suit that cam? Bearings still alive? Any oil restrictors you need to remove? A compression test might be interesting.
 
Discussion starter · #4 · (Edited)
Re: mechanical timing adv issue on high compression 327

I had actually come to the conclusion on my own, by measurement, on the mech advance limitations before actually reading it in Summits instructions, both matched, 10-11 at 1500, nothing more, confirmed.

I doubt the valve springs were changed from the prior cam, which was more aggressive, longer duration, not certain what exactly though. Roller rockers if that matters? Previously had Rhodes lifters as well. Not a race motor, at least not since it's last full build, but the last owner got sucked in to that old "bigger is better" trap, I can relate~ He regretted it almost immediately, and so the motor has low mileage since the build, probably under 5k, just a guess though.

This was the compression test with the old cam (it ended up having two flat lobes, different cylinders, one exh one intake, not sure which cyl's though) otherwise the cam looked great. Weird, I had just read about zddp and flat lobes in PM last month, then saw a great example.

#1 was 180
#2 was 170
#3 was 180
#4 was 165
#5 was 170
#6 was 170
#7 was170
#8 was165

I was/am concerned about the compatibility of the high compression with that cam, but wanted to deal with what seems like the obvious before getting into it. My options for compression ratio are probably back to more aggressive cam, or swap heads, maybe aluminum? But the car runs decent and pulls very strong, even with timing retarded. Running 94 octane and 30 degree weather probably help, but come summer....?

this is probably no help, but this is the car running with the old cam... including flat lobes
 
Discussion starter · #5 ·
Re: mechanical timing adv issue on high compression 327

Just spoke with one of the techs at Summit Racing and he said that's not at all what should be designed to be happening with the mech advance, and in fact the "big book" they have with detailed specs doesn't even list the 850001-1, but does list an 850001-R, and that one is spec'd differently, and correctly, at 22 degree's mech adv at 3300 RPM.
His initial reaction was must be talking about dist degrees, but I pointed out, in addition to having measured 10-11 max by 1500, if the instructions are indicating dist degrees then they are saying 20-22 max adv occuring at 1500 rpm, which wouldn't be right either. So I guess now that I've confirmed the dist is acting "as described" but clearly not rightfor my application, I should just buy a morosso 72310 recurve kit for $10, and if that fails to sort things out find a shop to recurve the distributer, or replace it...
 
#6 ·
Re: mechanical timing adv issue on high compression 327

so it's probably inherently ping prone to start with...
Click to expand...
he swapped in the more streetable cam,
Click to expand...
You have one hell of a compression CR, putting a milder cam in then increases the DCR even further.

and almost all agree on total advance of 32-36 at 3500
Click to expand...
And this is quailfied many times, higher the CR the less advance, and higher the octane more advance....
In the ball park for a mild cam with CR around 11:1 running 105 octane fuel on is looking around 28 to 32 degress intial+ cent all in around 2800 to 3000, and no more than a all up total around 38 to 40 degs.( 7 to 10 deg VA)
The curve usually comes off farly quick, then rounds off about 1800 to 2200 rpms and slower advance out to full in.
The VA usually works at fairly high Hg" so drops advance at higher cruise rpms under lighter loads...so generally drops out around the 12 " mark.

These are just numbers to get u into the ball park so from their can fine tune.

and if that fails to sort things out find a shop to recurve the distributer, or replace it...
Click to expand...
Bottom line m8....dump those bloody heads before you end up banging your head again a brick wall trying to make something designed to run on Alocohol run on pump gas.
In the long run, and Im assuming you want a reliable , cruiser that can have a little fun with on occassions???
Now Im not saying you will not be able to get it run well, just to do so , by the time mess with dizzies, then you will need a couple long sestions on dyno...blah blah blah...

Swap the bloody heads with bigger chambers, get you below the 10:1 CR.
Do it once do it right and dont patch fix things up.
 
Discussion starter · #7 ·
Re: mechanical timing adv issue on high compression 327

Thanks, I had all the theory in the back of my mind re: CR/tiiming tradeoffs, but the numbers you give will be a help with my reality check.

I'm not planing to patch fix, but do want to set things right that would need to be right regardless of the heads/cam, and since it's a brand new dizzy, and a supposed to be a performance oriented one, I was confused as to why it was only supplying 10-11 deg mech adv, as measured at the crankshaft, all in at 1500. That, plus over adv from vac adv at part throttle, force me to pull back initial timing, but it goes like a bat out of hell, considering only seeing 20 deg adv at WOT, though from your post I'm guessing I'm not far from pinging with 20....

Assuming zero decked block, and .100 popup forged pistons, what would be a good chamber size? Alum heads overkill?

Steptoe said:
You have one hell of a compression CR, putting a milder cam in then increases the DCR even further.



And this is quailfied many times, higher the CR the less advance, and higher the octane more advance....
In the ball park for a mild cam with CR around 11:1 running 105 octane fuel on is looking around 28 to 32 degress intial+ cent all in around 2800 to 3000, and no more than a all up total around 38 to 40 degs.( 7 to 10 deg VA)
The curve usually comes off farly quick, then rounds off about 1800 to 2200 rpms and slower advance out to full in.
The VA usually works at fairly high Hg" so drops advance at higher cruise rpms under lighter loads...so generally drops out around the 12 " mark.

These are just numbers to get u into the ball park so from their can fine tune.



Bottom line m8....dump those bloody heads before you end up banging your head again a brick wall trying to make something designed to run on Alocohol run on pump gas.
In the long run, and Im assuming you want a reliable , cruiser that can have a little fun with on occassions???
Now Im not saying you will not be able to get it run well, just to do so , by the time mess with dizzies, then you will need a couple long sestions on dyno...blah blah blah...

Swap the bloody heads with bigger chambers, get you below the 10:1 CR.
Do it once do it right and dont patch fix things up.
Click to expand...
 
#8 ·
Re: mechanical timing adv issue on high compression 327

Assuming zero decked block, and .100 popup forged pistons, what would be a good chamber size? Alum heads overkill?
Click to expand...
1st thing "assuming" assume nothing, assume = a55 of U and me...assume will always come back and bite u in the a55.

you know that the CR is way too high, so will be looking at bigger chamber heads....have the money in the bank and take them off and see if being decked , how much
Form there you WILL KNOW what heads will be more suitable for the fuel u want to use with accurate calculations.

"Alum heads overkill?"
Well Al heads by nature of the material and technoloy thats not 40 yrs old ate far more toternate to detonation.....better performance
The questions then are
1/ Do you need that performace
2/Do the suit what you are going to use the car for
3/ I have no idea what your budget is or how much heads , Al /cast etc are in the States.
 
#9 ·
Re: mechanical timing adv issue on high compression 327

I can't get that combo to come up with less than 12:1 compression if the block has been decked. The 11:1 spec on those pistons was most likely with a 64cc head and composite gasket.

If you want to run that cam on the street with pump gas and no issue with those pistons your best bet would be to move to a 76cc head. It is essentially a Comp 268H High Energy with a 60 degree intake valve closing point. A 76cc head will still give you about 10:1 but will drop your dynamic below 8.5.

Even if I fudge all the numbers to get you down to 11:1 static, the dynamic with that cam is over 9:1.
 
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Discussion starter · #10 ·
Re: mechanical timing adv issue on high compression 327

ouch, maybe I can try some Sunoco GT100 I guess, couple miles from home, although pricey... http://www.sunocoinc.com/site/Consumer/RaceFuels/260GT100Locations/
Steiner said:
I can't get that combo to come up with less than 12:1 compression if the block has been decked. The 11:1 spec on those pistons was most likely with a 64cc head and composite gasket.

If you want to run that cam on the street with pump gas and no issue with those pistons your best bet would be to move to a 76cc head. It is essentially a Comp 268H High Energy with a 60 degree intake valve closing point. A 76cc head will still give you about 10:1 but will drop your dynamic below 8.5.

Even if I fudge all the numbers to get you down to 11:1 static, the dynamic with that cam is over 9:1.
Click to expand...
 
#11 ·
Re: mechanical timing adv issue on high compression 327

I can't get that combo to come up with less than 12:1 compression
Click to expand...
Nore did I and that was just a bit off mental calculation when I read the 57 cc heads.
That sort of thing is for running CNG (methane or pure Alchol.)
Running pump gas with those compression could be compared to running Kero (think u guys callit parrafin??) in a modern car....u can do it in old vintage engines around 5 or 6:!, so long as the engine is held at WoT...like a massy fergerson tractor plowing a feild all day...but cruise home in (on) it and stuffs the engine.
 
#12 ·
Re: mechanical timing adv issue on high compression 327

About the mechanical advance in the distributor, if you look at the advance springs there is usually an oversize bushing that restricts the total movement of the springs.
If you remove or change the bushing to a smaller one, you should get more advance.
 
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Discussion starter · #13 ·
Re: mechanical timing adv issue on high compression 327

Just an update, original owner finally replied, as quoted below, not zero decked, though as you say, need to measure. And the heads are double hump, which I thought were 64cc, no? Gotta get the numbers still...

"Tom, Don't know exactly. Decks were cleaned up aproximately 10 thousandths off not sure if they had been done before. heads also. My calculations when I did the engine came out to 11.5:1. and I went with 50 thousandths head gasket "

Oh, and I also found a manifold vac fitting which had been wide open, a lean condition would contribute to detonation, not sure how much, but I will have to wait and see once the seasons change.




Steptoe said:
Nore did I and that was just a bit off mental calculation when I read the 57 cc heads.
That sort of thing is for running CNG (methane or pure Alchol.)
Running pump gas with those compression could be compared to running Kero (think u guys callit parrafin??) in a modern car....u can do it in old vintage engines around 5 or 6:!, so long as the engine is held at WoT...like a massy fergerson tractor plowing a feild all day...but cruise home in (on) it and stuffs the engine.
Click to expand...
 
#14 ·
With only .010" off you should still be able to see the numbers, mine was decked .010" and I can see the numbers, that is assuming it wasn't decked previously?
Also, there should be .015" of the deck remaining, and with a .060" head gasket, you have a quench of .075" which is massive and even more prone to detonation with the smaller cam, despite it lowering the compression.

I roughly calculated your compression, and I think what alot of people forget in the real world is, you cannot assume compression ratios with smaller stroke motors, because everything from most cyl. head companies and people are thinking "64cc head, flat tops, etc... equals 10:1" whereas on a 302, 327 it'd be a lot lower.

You should have under 11:1 compression, the calculator said 10.26:1 and with rough guesstimates, it shouldn't be off much unless you've got a negative deck, a head smaller than 57cc's and a big honking slug in there.

BTW... My 327 is 10.8:1 a DCR of 7.99:1 (totally street able) and I run the CC286H-10 I can lope around @30mph in 4th doing 1,500rpm... That is street ability and a solid 420hp.

I'd tare down your current setup and see whats up, gaskets and bearings are cheap, no need for work unless it really needs new stuff.
 
#15 ·
Pretty good advice there from John

The Humper heads are generally 64 cc though earlier castings may have less volume, extra shaving, etc. the only way to tell is to remove one cyl head....easy project. Then you can see what your deck height is. Simple to clean the deck surface, lay the edge of a machinist's ruler across the bore and run a feeler gauge, measuring the non-dome part of the piston with the piston at TDC.
You want to run .040-.045" Quench, (deck height plus gasket thickness)
While you are inside the engine...maybe have a more serious cam standing by. Some more duration at .050" would certainly help reduce pinging as well as running a tight quench.
 
#16 ·
If I remember my 327 production years correctly (1962-1965) they had 62cc chambers. In 1966 the chambers were enlarged to I believe 68cc. This is the reason the early 327's using these heads had an advertised compression ratio of 11.25 to 1 with the forged TRW pop-up pistons that you probably have. Your cam choice is aggravating your problem. Widen your cam lobe centers and increase your cam overlap. I run a 327/365 HP on premium fuel with the stock pistons and heads without a problem. My cam is the stock 30-30 from GM, you need to bleed off some of that compression. As others have said, you cannot assume anything. When someone else has been into an engine everything needs to be checked so you know what you are starting with; only then can you make an accurate assessment to solve the problem.

There are many 327's with 300, 350, 365, 375 horsepower being successfully driven on todays fuels, the key is always knowing what kind of parts are inside the engine.
Bill
 
Discussion starter · #17 · (Edited)
ace's68 said:
With only .010" off you should still be able to see the numbers, mine was decked .010" and I can see the numbers, that is assuming it wasn't decked previously?
Also, there should be .015" of the deck remaining, and with a .060" head gasket, you have a quench of .075" which is massive and even more prone to detonation with the smaller cam, despite it lowering the compression.
Click to expand...
Thanks guys, that is very helpful info, I tried reading up on quench, but need to reread a few more times.
As for the sum total decking, no idea, but if he had it decked .010" and that resulted in numbers being decked off, it may have had at least one prior decking. Also, the head gasket is .050",. and the head was cleaned up as well, .010" I assume, so all totalled would put me at no more than .045"-.050", is that right?

After more thought, I guess the head being taken down bit won't affect quench, just CR via volume reduction... and do the popup pistons affect quench?

And I see I assumed again, sorry bout that, habit :) I'll start with ID'ing the heads, go down from there.

I'm buried in snow currently and my garage is damn cold, so head removal may be a ways off, trying to grasp my options in the meantime. Not sure how much the vac leak was contributing to a pinging problem, but it was substantial. Cam was put in right before I bought it, too bad.

Hey, here's a question, can comparison between compression test psi with old cam vs psi with new cam provide me with any useful info, since it's a relative change in psi and not absolute, which I've read is useless, to determine DCR as affected by current cam? Or would I need to call up Hawking to get anything from it?


Well, thanks for your help, I really do enjoy the process when there are so many people in the know willing to offer info.

Cheers,
Tom
 
Discussion starter · #18 ·
sbcBill said:
Your cam choice is aggravating your problem. Widen your cam lobe centers and increase your cam overlap. I run a 327/365 HP on premium fuel with the stock pistons and heads without a problem. My cam is the stock 30-30 from GM, you need to bleed off some of that compression.
Click to expand...
Just out of curiosity, this 327 with the current cam is pretty streetable and pulls great through the rpm's, and I actually did drive the car with the long duration cam a month before buying it , which can be heard in the above video, but he had since swapped to the current cam which Summit racing recommended #SUM-K00172, and I really didn't care for the amount of clutch slip it took to get her rolling with the old longer duration cam. If that's the case, I'm guessing steptoes suggestion about swapping heads makes more sense then swapping cams, as long as I find out exactly what I have before choosing, and since the heads need to come off anyway to determine deck height...
If I like the characteristics of the current cam, and want to make the most of what is otherwise a solid bottom end (we shall see), what heads make sense with this cam, .100 forged popups and a near-zero decked block, so I can start researching that option? Any thoughts on what kind of HP gains might I get by switching to more modern heads? (anecdotal comments welcome) I doubt I would want to drop more than 1k on heads, and used is fine with me.

Thanks,
Tom
 
#19 ·
as for heads you would want? well, I am a fan of spending a little more on the more modern heads... specifically for the AL heads and the weight savings, but not completely necessary. But, I would take a head off and check your piston distance in the hole so you can figure out what head gasket you need to be in the total .035-.045 quench, and then see what combo of head cc's will give you a more manageable compression ratio. I would do that and report back so we can give you an accurate cc head you should purchase.
 
Discussion starter · #20 · (Edited)
Warm weather for once, so I pulled the valve covers and checked the valve lash, also got the numbers off the heads, 3917291, so they are 64cc heads, 275-350 hp 1.94's or 2.02's, and I saw at least chart list them as the "Turbo" heads" whatever that's worth.

Would the 64cc volume change substantially from shaving .010 off them to square the surface? (oops, meant to say .010 off the heads,sorry bout that)

One things for certain, you could eat off them, and I know he had them substantially worked on, hardened seats and the usual upgrades, probably less than a few thousand miles ago when it was all built.
Hate to ditch them, seems like a well put together package, maybe I'll reconsider that cam first...
 
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Steptoe
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