Compression Ratio Limits and Iron Heads

[skinny z]

[quote="F-BIRD'88"]For reference: post the carb model number and how you have it set up. jets,PV, IAB's HSAB's IFR etc.[/quote]

Barry Grant 750 cfm vacuum secondary w/ annular boosters.
4 corner idle.
Pri 68
Sec 78
IAB .065
HSAB .039
IFR .031
PVCR.059
PV 10.5"

Keep in mind that I've done no WOT tuning.
Idle is at or around 14:1 depending on the heat soak and ambient temperature. Part throttle cruise is high 15 at 95 kph to high 14 at 125 kph. WOT is low 12.

[F-BIRD'88]

Just looking at this I would have not built this engine with a 10.37:1cr using this camshaft in it.
It is a bit too agressive for every day street driving. Thats the trouble with pushing the envelope on engine cr.
A bit too much cr is a LOT too much, once you cross the knock threshold of the fuel in the tank.
I have been using the same Ultra 94 octane gas as you, for over 25 years.
Knowing that you cannot always find a Sunoco/PetroCan Ultra 94 octane gas station, I built mine vortec head motor at a modest
9.80:1 cr. If the recent Ultra 94 octane is now slipping, this may have been a wiser choice than I thought.

The next best pump gas I found here, was the Pioneer Magnum 93. It was pretty good stuff. The 91 octane gas here plainly sucks for real engine knock resistance.
Try some Pioneer Magnum 93 gas.

That's just it. My old Vortec had a 10:1 SCR and the timing specs as this cam although with less lift (hence the custom grind using the XFI lobes). That thing ran just fine on 94 and the Pioneer 93 and whatever they serve at the UMI in Listowel. (that's on my way to Grand Bend).
This one just seems over the top and I may have gone too far.
I will keep at the tune for a while yet but I'm seriously thinking about taking some of the bite out of it with either reduced compression or retarding the can by 4 degrees. That may make the difference for least impact on my resources (time and money).

If it will run fine @ 36deg BTDC WOT acceleration, then the whole problem is the vacuum advance set up which is WAY TOO agressive.
Recurve the distributor as I outlined. I bet the 1/4 mile ET will pick up a bit @ Grand Bend also, once the distributor is corrected. You don;t have ner enough intial timing at idle, way too much centr advance travel
and the vacuum advance is WAY too agressive.

Why does the main jet WOT AFR need to be so rich? ( richer than 12.5:1 @ WOT) is this the only way you can keep it from knocking @ WOT? ( without vacuum advance which is way too agressive)

It should run just fine with 34-36deg timing. Go to the track and tune it (max timing) for best track MPH.
All my vortec head motors run best with 36deg BTDC. on Ultra 94 but some cars only need 34deg to run best.

[skinny z]

[quote="F-BIRD'88"]If your "64cc heads" are actually 62cc the cr is 10.60:1 If your "64cc heads" are actually 60cc the cr is 10.86:1
IMHO that is really pushing it with that cam. Especially if there is some build/carb tune/dist timing set up error.

True. I think it's pushing it with this even if the heads are 64 cc. That's why I started the thread in the first place.

You don't need to "deshroud" the combustion chamber of a SBC Vortec head.

They're not OEM Vortecs. They're RHS Pro Torker Vortec with 2.02/1.60 valves. The amount of deshrouding was minimal however it was plain to see the there were improvements to be made.

Clairify: it is knocking @ WOT with only 28deg BTDC timing @ WOT. yes/no

No. Never has been. Before or since the timing change.

it is knocking @ 31deg but not at 28deg..@ WOT.. yes/no

Still knocking despite the change.

It also knocks on part throttle to full throttle roll in from cruise yes/no

Yes

But this stops after reducing the timing to....________..

Haven't eliminated the knock yet.

All this tested with vacuum advance disconnected (for now)

is the balancer/timing tab TDC location ( still) accurate?

Yes. Verified several times with a piston stop as well as a dial indicator when degreeing the cam (also done several times).[quote]

[F-BIRD'88]

Barry Grant 750 cfm vacuum secondary w/ annular boosters.
4 corner idle.
Pri 68
Sec 78
IAB .065
HSAB .039
IFR .031
PVCR.059
PV 10.5"

What happens if after recurving the distributor mech and vac adv systems as I outlined and checking adjusting the PCV valve flow rate. (possible wrong PCV valve)
you return the carb to stock default jetting setup and try it?

[Dan Timberlake]

I’m struggling with detonation especially on the initial hit from a standing start.

1. How hard is this initial hit? Pulling away from a stop or full strip launch? If launch, what RPM?
2. What initial and total timing at what RPM?

It shows in a couple of ways. On the street (a quiet one in the country!) from cruise at 30 mph, pin it, the convertor flashes to 4000, the trans downshifts (auto) and the rattle is massive.
Part throttle acceleration from a standing start, (a full launch would be impossible), the same rattling. It's brief but loud.
Initial timing is (was) 12 degrees with another 20 mechanical all in by 2900.
All of this is above the stall of the convertor so there's no rpm less than 3000 in these tests.
Full manifold vacuum advance connected or disconnected doesn't make a difference in the detonation.

====================

So 32 total is just too much for the combo at ~ 3000/4000 rpm, even at part throttle.
What happens at full throttle over 4500?

Attached is an image from the 1959 SAE paper for Mopar's new big block V8.
A few things really caught my eye.
One was >that< engine ( so, probably some others) was much better served with an advance curve other than the "all in by XXXX rpm."
Another was being 4 to 6 degrees below optimum only cost 2% full throttle HP. (Actually similar info has appeared other places.)
Another was that 14 degrees vac advance (at whatever cruising condition resulted in 12 inches manifold vacuum) needed as much octane as WOT.
(and the text says that is quite sensitive to coolant and inlet air temp)
Another was that regardless whether the combustion chamber is "good" or not, 32 degrees total was best, and then only at the highest rpm.
Compare that to the SBC/348 comparison harvested from the equivalent SAE paper for the new 348.

[F-BIRD'88]

DIY Engine Ears knock detector headphones.
http://www.autospeed.com/cms/article.html?&A=0353

build it yourself using the Radio Shack ("The Source") Amplifier PN 33-1096
The mic clips to the side of the engine block near the frost plugs.

is there some reason I am missing why the 4000 stall converter combined with such a mild 274deg XFI camshaft?

[raceczar]

Looking at your situation...I will never understand why head makers still use that "S" plug. In Champion you have essentially 2 choices. Either the 12 or the 9. If you go to a racing plug you go all the way down to a 59 to start. I realize that some people never consider plug heat range, especially when the CR is at 11:1 or lower. But on 9.5:1 race engines we always use race plugs. You need a colder plug, but you have no choice in Champion. If the head manufacturer used a .750 reach (either the "N" or "C" series you could use a 65 and then tune from there.

[F-BIRD'88]

Looking at your situation...I will never understand why head makers still use that "S" plug. In Champion you have essentially 2 choices. Either the 12 or the 9. If you go to a racing plug you go all the way down to a 59 to start. I realize that some people never consider plug heat range, especially when the CR is at 11:1 or lower. But on 9.5:1 race engines we always use race plugs. You need a colder plug, but you have no choice in Champion. If the head manufacturer used a .750 reach (either the "N" or "C" series you could use a 65 and then tune from there.

this is a RHS vortec head. it uses the .708" reach "S" plug. I have used the 12 the 10 and the 9 trying them all out in more than 1 vortec head high perf motor. The RS12YC plugs work fine. The 10's and 9's just burn a,ittle darker in colour
I have never needed to run anything else on a Vortec head motor, than the RS12YC plugs.

This motor should not be that octane sensitive at all, on this fuel even @10.37:1 . But the over agressive vacuum advance set up (which is all wrong) is throwiing a wrench in that.

Racing plugs are way too cold for street driving and will soon foul. You spend most of your time idling on a street car.
cold racing plugds never work well on street cars.
His vacuum advance set up is all wrong. He has the vacuum advance jacked way up to compensate for the lack of base timing and incorrect mechanical advance curve set up.
It is near the opposite of what I have found works real good on vortec head motors on 94 octane gas and 10:1+cr.
( and other motors too)
its the vacuum advance set up that is the problem, not the plugs.
the whole distributor setup is out of wack for this motor.
There may be more issues too, but this is the biggy.

I got cast head BBC's here running fine on this Ultra 94 unleaded fuel without any issues at all with just under 11:1cr
its the distributor. not the plugs
and a .750" reach "C" plug is the wrong plug for this head. (I checked)

[raceczar]

Looking at your situation...I will never understand why head makers still use that "S" plug. In Champion you have essentially 2 choices. Either the 12 or the 9. If you go to a racing plug you go all the way down to a 59 to start. I realize that some people never consider plug heat range, especially when the CR is at 11:1 or lower. But on 9.5:1 race engines we always use race plugs. You need a colder plug, but you have no choice in Champion. If the head manufacturer used a .750 reach (either the "N" or "C" series you could use a 65 and then tune from there.

this is a RHS vortec head. it uses the .708" reach "S" plug. I have used the 12 the 10 and the 9 trying them all out in more than 1 vortec head high perf motor. The RS12YC plugs work fine. The 10's and 9's just burn a,ittle darker in colour
I have never needed to run anything else on a Vortec head motor, than the RS12YC plugs.

This motor should not be that octane sensitive at all, on this fuel even @10.37:1 . But the over agressive vacuum advance set up (which is all wrong) is throwiing a wrench in that.

Racing plugs are way too cold for street driving and will soon foul. You spend most of your time idling on a street car.
cold racing plugds never work well on street cars.

His vacuum advance set up is all wrong. He has the vacuum advance jacked way up to compensate for the lack of base timing and incorrect mechanical advance curve set up.
It is near the opposite of what I have found works real good on vortec head motors on 94 octane gas and 10:1+cr.
( and other motors too)
its the vacuum advance set up that is the problem, not the plugs.
the whole distributor setup is out of wack for this motor.
There may be more issues too, but this is the biggy.

I got cast head BBC's here running fine on this Ultra 94 unleaded fuel without any issues at all with just under 11:1cr
its the distributor. not the plugs
and a .750" reach "C" plug is the wrong plug for this head. (I checked)

I said he was screwed because the CYLINDER HEAD MANUFACTURER (for whatever reason) used a taper seat plug. There are just not enough plugs available in that style. The coldest in a Champion is a 9 and that is too hot.

We use racing plugs on the street all the time. If your tune up is correct they work.

The OP's combo is screaming for a colder plug. I just don't know what to tell him. I only use Champion. I would suggest to look at another brand and look in the RACING plug section.

FWIW a -7 heat range NGK street plug is hotter than a -7 NGK race plug. Be careful if you go that route.

[F-BIRD'88]

We use racing plugs on the street all the time. If your tune up is correct they work.

I disagree. If and when the tuneup is right,, you don't need or want racing spark plugs on this type of motor.

The typical S59c S63Yc .708" reach champion racing plugs are more than often WAY too cold for the strip too, even on a high compression race motor.

I have never had to use anything colder than a 8 (champion heat range) on a street strip car
even with nitrous or a blower or race gas or with very high compression.

The too cold plugs are just a crutch and will not fix this street motor. Even when it is racing hard at the track.

The race plugs will have a short service life on the street, (they soon foul) and are not necessary.
if you want o try a NGK plug (which are very good spark plugs too) try a 6 or 7 heat range NGK

NGK TR6 or NGK BP7EFS or BPR7EFS one of these NGK plugs for Vortec head high perf street motors will be just right

[raceczar]

We use racing plugs on the street all the time. If your tune up is correct they work.

I disagree. If and when the tuneup is right,, you don't need or want racing spark plugs on this type of motor.

The typical v59c V63Yc champion racing plugs are more than often too cold for the strip too, even on a high compression race motor.

I have never had to use anything colder than a 8 (champion heat range) on a street strip car
even with nitrous or a blower or race gas or with very high compression.

The too cold plugs are just a crutch and will not fix this street motor. Even when it is racing hard at the track.

The race plugs will have a short service life on the street, (they soon foul) and are not necessary.
if you want o try a NGK plug (which are good sparkplugs too) try a 6 or 7 heat range NGK

Looks like I've been doing it wrong for years then.

So all the "race" engines use "street" plugs?

Essentially, this guy has a race engine in a street car. I would use colder plugs, in a race series plug, lose the Vaccuum advance and run 38* total, all in no later than 3000.

But the problem is he can't get the correct plug, at least not a Champion.

If the fuel and timing curves are correct, and you get the correct heat range plug they won't foul. I know you are bound and determined to live with hot plugs because it is a "street" car. Nothing will change your mind. FWIW I will be driving my "street" car with a 65 Champion plug in it to town today. Hasn't fouled a plug in 7500 miles. Doubt it will today.

[F-BIRD'88]

Looks like I've been doing it wrong for years then.

So all the "race" engines use "street" plugs?

Essentially, this guy has a race engine in a street car.

Yes you have.

No, They don't all.... AND THIS ain't even close to being a Race Engine.. this ain't no race engine and many may Race engines work a whole lot better
with a street het hange or just slightly cooler than . Most times a race only cold plug is just
too cold and not nesessary either. A drag car actually has to do quite a bit of idling and low speed driving.
I GET YEARS AND Years if service out of ONE set of Champion spark plugs on a street strip motors.
YEARS not 7500miles. 7500miles they are just getting broke in.
Your race tune up cold plug method will not work on a street car. Or even most street strip bracket cars.
Not for long. I have had to redue many many cars because the owner brought me the car to fix it.
Because it won;t stay in tune and they are always under hood playing with the carb and changing spark plugs
Using cold racing spark plugs is a major fault. (Most of the time) (and all the time on a street driven car) And when re tuning these cars the cold race only spark plugs (among the other typicial racer tune stuff errors) is the first thing to go.

( A hard running circle track car might be a good example of a motor that can use a colder race plug because they run hard @ WOT for a LONG TIME.) But even these many guys get carried away needlessly on spark plug heat range selection.)

[raceczar]

Looks like I've been doing it wrong for years then.

So all the "race" engines use "street" plugs?

Essentially, this guy has a race engine in a street car.

Yes you have.

No, They don't all.... AND THIS ain't even close to being a Race Engine.. this ain't no race engine and many may Race engines work a whole lot better
with a street het hange or just slightly cooler than . Most times a race only cold plug is just
too cold and not nesessary either. A drag car actually has to do quite a bit of idling and low speed driving.
I GET YEARS AND Years if service out of ONE set of Champion spark plugs on a street strip motors.
YEARS not 7500miles. 7500miles they are just getting broke in.
Your race tune up cold plug method will not work on a street car. Or even most street strip bracket cars.
Not for long. I have had to redue many many cars because the owner brought me the car to fix it.
Because it won;t stay in tune and they are always under hood playing with the carb and changing spark plugs
Using cold racing spark plugs is a major fault. (Most of the time) (and all the time on a street driven car) And when re tuning these cars the cold race only spark plugs (among the other typicial racer tune stuff errors) is the first thing to go.

( A hard running circle track car might be a good example of a motor that can use a colder race plug because they run hard @ WOT for a LONG TIME.) But even these many guys get carried away needlessly on spark plug heat range selection.)

You win.

To the OP...I would look for a plug that is 4-5 ranges hotter (that would be the RS17YC in a Champion), use 6-8 sizes larger main jet and pull another 6-8 total out of it. That should be the correct tune up for a "street" engine, per F-BIRD.

EDIT: spark plugs cost about $5.00 each. I usually don't go over 5000 miles on a set of plugs. So instead of calling them "street" or "race" engines, I will call them "performance" engines. Seems silly to me to spend all those $$$$$ on speed stuff and skimp on plugs. The only reason my plugs have 7500 miles on the now is because a customer bet me $250.00 they would go 5000 miles. He lost. We are at double or nothing for 10,000 miles. He will lose that too.

FWIW, this customer learned the hard way about plugs. He would go 2-3 seasons on the same plugs and thought himself a hero. I taught him how after 25-50 runs a plug will not function as well as it did out of the box. He lost that bet too. He now changes his plugs around 40 runs. So keep on doing what you are doing F-Bird. If it works for you that is great. But there is more than one way to skin a cat. No offence to kittys. Street plugs in a "performance" engine??? What next??

[F-BIRD'88]

Looks like I've been doing it wrong for years then.

So all the "race" engines use "street" plugs?

Essentially, this guy has a race engine in a street car.

Yes you have.

No, They don't all.... AND THIS ain't even close to being a Race Engine.. this ain't no race engine and many may Race engines work a whole lot better
with a street het hange or just slightly cooler than . Most times a race only cold plug is just
too cold and not nesessary either. A drag car actually has to do quite a bit of idling and low speed driving.
I GET YEARS AND Years if service out of ONE set of Champion spark plugs on a street strip motors.
YEARS not 7500miles. 7500miles they are just getting broke in.
Your race tune up cold plug method will not work on a street car. Or even most street strip bracket cars.
Not for long. I have had to redue many many cars because the owner brought me the car to fix it.
Because it won;t stay in tune and they are always under hood playing with the carb and changing spark plugs
Using cold racing spark plugs is a major fault. (Most of the time) (and all the time on a street driven car) And when re tuning these cars the cold race only spark plugs (among the other typicial racer tune stuff errors) is the first thing to go.

( A hard running circle track car might be a good example of a motor that can use a colder race plug because they run hard @ WOT for a LONG TIME.) But even these many guys get carried away needlessly on spark plug heat range selection.)

You win.

To the OP...I would look for a plug that is 4-5 ranges hotter (that would be the RS17YC in a Champion), use 6-8 sizes larger main jet and pull another 6-8 total out of it. That should be the correct tune up for a "street" engine, per F-BIRD.

We disagree on correct heat range for this street motor.
But now you are clearly getting silly. if that is all you can post to support your opinion, it is clear to me
you have no clue.... I thought I was debating with someone who at least had a clue.


OP: if you are still with us,,, and get to removing the plugs to have another look or to replace, take a few pics of the tips of the plugs in order. The plugs tip appearance is one of your windows into whats going on in your motor.
Lots of info there. If it has been knocking hard they are propbabily peppered by now and may have a cracked porcelin.
A good reason to have a look.

Post a few pics of the plugs if you can.

What distributor is in this motor? needs some set up changes, to get it dialed in.

the Champion RS12YC is a "street high perf plug" for this street vortec engine. at worst case senerio it might need the RS9YC
But I bet after trying it without fixing the distributor curve first you will find that the 9's don;t fix it either nor wil a cold race plug like the Champion S59C.
By the way there is a whole series of cold champion race only plugs for this cylinder head. "S" .708" reach
But you don;t need any of them for this motor. And they wont fix the problem.

[Orr89rocz]

One was >that< engine ( so, probably some others) was much better served with an advance curve other than the "all in by XXXX rpm."
Another was being 4 to 6 degrees below optimum only cost 2% full throttle HP. (Actually similar info has appeared other places.)
Another was that 14 degrees vac advance (at whatever cruising condition resulted in 12 inches manifold vacuum) needed as much octane as WOT.
(and the text says that is quite sensitive to coolant and inlet air temp)
Another was that regardless whether the combustion chamber is "good" or not, 32 degrees total was best, and then only at the highest rpm.

This is why i love efi. Complete control over timing, and fuel, you will sometimes find different points in the rpm band want different air fuel and different timing lol

Anyway, i would think if a stock heat range for an L31 is a 4-5, i would try a 7 in this motor being on the threshold of pump gas, thats assuming pump shot and vacuum advance isnt causing the problem. I was 11:1 scr 8.3's dcr on aluminum and had no issue with 6 heat range ngk's. 7's ran fine too once i sprayed small shot. Never fouled but those arent that cold.

Iron holds chamber heat more so 8.3 scr may be more sensitive.

My turbo car is 9:1 aluminum and i run heat range 8 for high boost but driving around last year normally, although not many miles, it never fouled plugs. Cold start was abit tricky and richer than i'd like but plugs seemed ok after goood drive to clean them off.