Compression Ratio Limits and Iron Heads

[skinny z]

[quote="skinny z"][quote="wil8115"]Or the 11:1 352 sbc Hot Rod did and ran on 87 pump gas np. had dcr in mid 8's.
http://www.airflowresearch.com/articles ... /A3-P1.htm (June 97)[/quote]

That one I haven't see. I'll check it out later.
Thanks.[/quote]

Gave it a read. Could use a little more data before evaluating that one.
6.2" rods. Aluminum heads. That's not my engine that's for sure. I think the consensus here is that with aluminum heads, I wouldn't be having any issues.
Not sure where your DCR of mid 8's came from. I might have missed it in the article. However, that cam installed straight up with those rods and 11:1 SCR results in a DCR of 8.9:1 !! Mind you, that cam peaks around 5100 rpm so perhaps they laid in back 4 degrees or so. Still that's around 8.6:1.
Add the controlled environment of the dyno cell and I guess it works. Then again, it's a magazine build but an interesting one for sure.

[skinny z]

That stock calibration is NOT very rich at all. Put the carb back to stock and retest it.
it is now obvious you don;t know how to tune a carb at all. You don't get it.
it is obvious you do not know how the size of the High speed air bleed effects the response of the circuit.
This time of year when density altitude is low near seal level and temps are low the stock calibration will not be too rich at all.

You got the cruise circuit ( primary main jet) way too lean and the power circuit jacked up (PVCR) and the response of the main circuit slowed down all to hell. That is why it knocks when you get on it and then stops.
Put the tiiming back to 34-36deg BTDC too.

70-78 .070 .070 .031 .031

You never did test the QFT 680 properly with its stock jet and air bleed calibration.
You have completely missed or ignore the critical relevance of the HSAB size to the response of the main metering system
to throttle changes.

"stock" calibration for this carb I believe is 70 pri 78sec jets. .070" pri and sec idle air bleeds and .031" primary and secondary HSAB's.
Return it to stock (including the stock out of the box IFR size and primary IAB size and primary PVCR size) and re test it.

That stock calibration is NOT very rich at all. Put the carb back to stock and retest it.
it is now obvious you don;t know how to tune a carb at all. You don't get it.
it is obvious you do not know how the size of the High speed air bleed effects the response of the circuit.
This time of year when density altitude is low near seal level and temps are low the stock calibration will not be too rich at all.

You got the cruise circuit ( primary main jet) way too lean and the power circuit jacked up (PVCR) and the response of the main circuit slowed down all to hell. That is why it knocks when you get on it and then stops.
Put the tiiming back to 34-36deg BTDC too.

70-78 .070 .070 .031 .031

Look. It's obvious this isn't going to work. None of your suggestions will make a dent in the issue at hand. I know that because I've already been there. With this engine and with others.
Please don't insult my abilites. I've been kind up to this point. Remember, you're a guest in MY thread so be nice. If you make a suggestion and it's politely declined, then accept it.

[BigBro74]

I hate to try to suggest easy things to do at this point, and I know that theoretically they should be plenty cold and all BUT...

I will not use a champion spark plug in a general motors product-ever. try a different brand in a cooler range.

It can make night and day difference.

I like NGK and have used quite a few, and in GM's AC plugs are also always a good bet. (ive been using autolite in fords and mopars)

I have seen people get rid of whole engine projects because of trying multiple sets of champion plugs more than once, and not just on Chevy's.

you sometimes even have to get more than 1 or two for lawnmowers to get a good one.

they are terrible IMHO

I won't even put them in my Mopar.

Sorry!, this is the closest thing to a rant I've had in awhile

Just my experience though, Jason

[My427stang]

A couple of comments here, only because I hate to talk in absolutes and it seems like there is an either or discussion here.

1 - Typically if you are a bit rich, it will less likely to ping
2 - Typically, an 8.4 DCR, when calculated using programs I have experience with will be a bit much for the street regardless of mixture, and that assumes that you aren't seeing extreme SCR/DCR splits ( some guys try to kill compression with a cam), which you aren't. If you were, then even a lower DCR may get destructive when the engine starts really breathing at higher RPM
3 - Tight quench helps
4 - I personally don't think alum vs iron matters much assuming the iron chamber doesn't have sharp edges that can act like a glow plug

So

In my experience, you should do the following

1 - Either rock the cam back to change IVC or go with a bigger cam, either will work well, but if you like your cam now, just rock it back. With PKelley's calculator, a 10-ish SCR car at 8 flat - 8.1 ish is a good bet to take, I think yours is the same calculator, it looks exactly the same as the QBasic program I have
2 - Pick total timing based on use and engine family chamber performance, guys can help here
3 - Tune it carefully after changes to make sure its not lean anywhere

I personally think you have a bit too early IVC for the compression you have combined with a lean condition. Fix both and truck on

[skinny z]

[quote="My427stang"]A couple of comments here, only because I hate to talk in absolutes and it seems like there is an either or discussion here.

1 - Typically if you are a bit rich, it will less likely to ping
2 - Typically, an 8.4 DCR, when calculated using programs I have experience with will be a bit much for the street regardless of mixture, and that assumes that you aren't seeing extreme SCR/DCR splits ( some guys try to kill compression with a cam), which you aren't. If you were, then even a lower DCR may get destructive when the engine starts really breathing at higher RPM
3 - Tight quench helps
4 - I personally don't think alum vs iron matters much assuming the iron chamber doesn't have sharp edges that can act like a glow plug

So

In my experience, you should do the following

1 - Either rock the cam back to change IVC or go with a bigger cam, either will work well, but if you like your cam now, just rock it back. With PKelley's calculator, a 10-ish SCR car at 8 flat - 8.1 ish is a good bet to take, I think yours is the same calculator, it looks exactly the same as the QBasic program I have
2 - Pick total timing based on use and engine family chamber performance, guys can help here
3 - Tune it carefully after changes to make sure its not lean anywhere

I personally think you have a bit too early IVC for the compression you have combined with a lean condition. Fix both and truck on [/quote]

I agree with what you have to say as it follows my thinking. This isn't to say the input from others doesn't have any merit, it's just in THIS particular case, I feel that thee's just too much dynamic compression.
Considering I have the compliment of parts to easily make a cam timing change it seems the best move available. What's more it allows to keep the combination I've selected and with some tweaking, I should be able to achieve my performance goals in both torque and fuel economy.

[raceczar]

I hate to try to suggest easy things to do at this point, and I know that theoretically they should be plenty cold and all BUT...

I will not use a champion spark plug in a general motors product-ever. try a different brand in a cooler range.

It can make night and day difference.

I like NGK and have used quite a few, and in GM's AC plugs are also always a good bet. (ive been using autolite in fords and mopars)

I have seen people get rid of whole engine projects because of trying multiple sets of champion plugs more than once, and not just on Chevy's.

you sometimes even have to get more than 1 or two for lawnmowers to get a good one.

they are terrible IMHO

I won't even put them in my Mopar.

Sorry!, this is the closest thing to a rant I've had in awhile

Just my experience though, Jason

So all of us who use Champion plugs are getting lucky?

Maybe we are just don't know?

The OP can get a colder plug. It would help. Won't fix it. He has at least come to the conclusion that a reduced timing curve, for whatever reason isn't good.

OP if you want a colder Champion you could use a 8401 (Powersports plug but it won't know it's not in a quad) or, you could use the S63HX.

BUT if you don't keep the timing up it wil foul them.

[BigBro74]

"BUT if you don't keep the timing up it will foul them.[/quote]

in the past fuel fouling has been the actual problem I have run into with champion, and they seem MUCH less tolerant than other brands in my experience, your results may vary.

I recently was reading an article which David Vizard wrote many years ago in which he and another builder were flogging a 4cyl car on a chassis dyno (don't remember now which car or what year) the crux of the story was that it was way down on power and they couldn't figure out why, they eventually changed out the plugs and the engine performed as was expected. They then went back to a new set of spark plug a (or should I say "C") and lost all the power again, ABAB type test. heat range equivalent. interesting. his problem didn't seem to be one of wetting or fuel type fouling. Just interesting stuff. I'm not saying that you are misguided, Just that this is now my practice (and why). Thanks ,Jason

[skinny z]

I had been using AC Delco R44LTS in the Vortec headed 353 that preceeded this one. I never had a problem with them.
I found that Delco PN has been discontinued and replaced with an Iridium plug. I don't have any experience with those other than in my service truck.
That's how I ended up with the Champions. After the first run through last season, they didn't look too good after a few thousand miles however it was at that point that the valve guides were quickly being pounded to pieces because of a screw up in valve train geometry. (Long story short is that the new shortblock had a significantly lower deck height and the push rods ended up being too short. The valve tip pattern wasn't checked in the rebuild).
I could never get a fix on the heat range and although I know my tune was reasonable enough, the plugs were turning an odd colour. Sort of yellow/green, the likes of which I hadn't seen before. The engine was sucking in a ton of oil.
Anyway, this season sees the same plug and they look fine although I didn't have a lot of run time on them when I checked last.
As I get this compression thing sorted out, I'll be determining if they're suitable or not. As of right now, they don't appear to be running hot based on what I know about reading plugs.

[raceczar]

I had been using AC Delco R44LTS in the Vortec headed 353 that preceeded this one. I never had a problem with them.
I found that Delco PN has been discontinued and replaced with an Iridium plug. I don't have any experience with those other than in my service truck.
That's how I ended up with the Champions. After the first run through last season, they didn't look too good after a few thousand miles however it was at that point that the valve guides were quickly being pounded to pieces because of a screw up in valve train geometry. (Long story short is that the new shortblock had a significantly lower deck height and the push rods ended up being too short. The valve tip pattern wasn't checked in the rebuild).
I could never get a fix on the heat range and although I know my tune was reasonable enough, the plugs were turning an odd colour. Sort of yellow/green, the likes of which I hadn't seen before. The engine was sucking in a ton of oil.
Anyway, this season sees the same plug and they look fine although I didn't have a lot of run time on them when I checked last.
As I get this compression thing sorted out, I'll be determining if they're suitable or not. As of right now, they don't appear to be running hot based on what I know about reading plugs.

That odd yellow/green color is an indication the timing/heat range is off. I don't use NGK because I have never seen an NGK show this. And several other reasons, such as NGK say correct heat range is taking the cad off 3 threads down. IMO that is way too hot. But, if you use an NGK and try to go colder than 3 threads down the plug isn't happy. The Champions are far easier to read. Also, I have never had good luck trying to use heat range cross reference charts. They are never quite right. So if you decide to try a plug, always go by the plug manufacture's starting plug, not using the cross reference.

[raceczar]

"BUT if you don't keep the timing up it will foul them.

in the past fuel fouling has been the actual problem I have run into with champion, and they seem MUCH less tolerant than other brands in my experience, your results may vary.

I recently was reading an article which David Vizard wrote many years ago in which he and another builder were flogging a 4cyl car on a chassis dyno (don't remember now which car or what year) the crux of the story was that it was way down on power and they couldn't figure out why, they eventually changed out the plugs and the engine performed as was expected. They then went back to a new set of spark plug a (or should I say "C") and lost all the power again, ABAB type test. heat range equivalent. interesting. his problem didn't seem to be one of wetting or fuel type fouling. Just interesting stuff. I'm not saying that you are misguided, Just that this is now my practice (and why). Thanks ,Jason[/quote]

So back in 1995 when I got lost on a tune up for a circle car and I...oh never mind.

Lots of big name racers use Champion.

The OP can use whatever he wants. But to sit here and say that Champions are "much less tolerant" etc just tells me you didn't start with the correct plug. Not going to argue about plugs because it's like rolling with a pig in crap. Gets me nowhere and the pig likes it.

[BigBro74]

"BUT if you don't keep the timing up it will foul them.

in the past fuel fouling has been the actual problem I have run into with champion, and they seem MUCH less tolerant than other brands in my experience, your results may vary.

I recently was reading an article which David Vizard wrote many years ago in which he and another builder were flogging a 4cyl car on a chassis dyno (don't remember now which car or what year) the crux of the story was that it was way down on power and they couldn't figure out why, they eventually changed out the plugs and the engine performed as was expected. They then went back to a new set of spark plug a (or should I say "C") and lost all the power again, ABAB type test. heat range equivalent. interesting. his problem didn't seem to be one of wetting or fuel type fouling. Just interesting stuff. I'm not saying that you are misguided, Just that this is now my practice (and why). Thanks ,Jason

So back in 1995 when I got lost on a tune up for a circle car and I...oh never mind.

Lots of big name racers use Champion.

The OP can use whatever he wants. But to sit here and say that Champions are "much less tolerant" etc just tells me you didn't start with the correct plug. Not going to argue about plugs because it's like rolling with a pig in crap. Gets me nowhere and the pig likes it.[/quote]

Okay man,
wish you all the best. May your floats never stick and your choke flap be always open



BTW- never worked on a circle track car before 2001.

[skinny z]

I can't imagine anyone will follow up and re-read 13 pages of postings but here it is in a nutshell.
Two seasons ago I decided to try and built as efficient an engine as I could regarding compresson ratio (SCR and DCR) using a short cam and my new iron RHS Vortec heads. The result was a SCR of 10.4:1, a DCR of 8.5:1, cranking pressure averagaing 205 psi, quench of .040" and a detonation problem that despite taking out a ton of timing and using a blend of 110 race fuel and 91 octane pump gas, still rattled.
This problem was compounded by the ultra lean cruise mixture I tune for and I wasn't interested in flooding the engine with fuel (which I did in an effort to combat the pinging) on a full time basis.
The decison was made to take out some of the compression ratio. Worked the chambers to 66 cc from 64 cc and bumped up the head gasket from my favouirte .026 Victor Reinz to a Fel-Pro .039". Quench suffered as well.
The results are a SCR of 9.8 and a DCR of 7.9. Cranking pressure is averaging 185 psi.
While I'm still working through the timing curve and fuel, preliminary testing is showing that this engine likes a typical timing curve during WOT events. The hit is as expected. There's a slight knock when the throttle is opened quickly from a rolling start although I can attribute some of that to the lean mixture at that throttle position. Standing start launches haven't really been possible as this testing has been on the street.
Anyone, that's it. Had I had aluminum heads to begin with, I'd have been much further ahead. I may still make the switch if I can find a decent flowing small intake runner head at a decent price and go back to 10.4:1.
Thanks for all of the input and attempts to help. It just didn't seem like it was workable.