Advice on stroker build with cracked cylinder

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moparodbuster said:
Ok maybe I'm not clear on it then. The pump unleaded here is 98 octane so I felt safe with a higher compression motor. Its mainly gonna be towed to the track and maybe a few shows
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Your octane ratings there are so called 'research octane numbers' (RON). Our octane numbers are the average of RON and MON (motor octane number), and that average number ends up typically 4-5 point lower. So your 98 octane is like out 93 octane over here.

If you are going to tow, go, and show, then you might consider and be able to afford a race fuel as your standard fuel and move up on DCR. But then you are boxed into a corner on fuel type.
 
It's fun to compare the DCR of various camshafts, but to be honest I would advise not to get hung up on them. DCR rises as the rpms rise and you'll never know exactly where it reaches. I think it could be calculated but you'd need to be on an engine dyno with the sensors. Anyway as long as the efficiency is rising, the DCR is too. IMO getting to the ragged edge of usable in a package that is for a multi-use (street & strip) is asking for more problems. If you have 8:1 at cranking rpm that will peak higher, and if you're running high test from any Uncle Willy's corner gas pump the quality of fuel might be a problem fill-to-fill.
Plus, you won't have a "lazy" low rpm engine using a 4" stroker crank that ends up in the high 7s for DCR when calculated. I've built a bunch. 430-550hp they ain't lazy.
 
If I may... DCR is a fixed computation. I'd call what moper speaks about as 'effective CR' or 'effective DCR' which adds in the effects of volumetric efficiency (like which varies with RPM's), any intake pressure from turbo-or-super-charging, and altitude effects (negative turbo-charging).

If you don't start with knowing your DCR, then you don't have any chance of knowing where your 'effective DCR' will end up, unless you:
  • Build the same thing as before
  • Just randomly experiment
Just looking for thoughts/opinions here... Isn't the VE at cranking speeds assumed as 100% in the DCR computations? Since it only considers the intake effects, and ignores any exhaust effects (like reversion), then it seems to be so. If that is true, then 'effective DCR' would typically peak only a few % above computed DCR. But I am not 100% sure on that aspect, so would welcome comments.

"They ain't lazy"? The suggestion was made that he lay in some $$ for tires a few posts back LOL
 
not 100% if you are getting backflow as the ling rod piston travels up before the valve closes
at some point rpm and intake velocity my get you more VE till you run out of flow (cam, heads)
it also varies with compression
Vizard writes about this- I'll see if I can find it

Because of the delayed intake closure, there is considerable piston motion up the bore from BDC before the intake actually closes. This, at low rpm, pushes some of the mixture back into the intake manifold. This means the volumetric efficiency (breathing efficiency), and thus the effective displacement of the cylinder, is well below 100 percent. In other words, a 100-cc cylinder with a static CR of 10:1 may only trap 75 cc of air. This means the dynamic CR, at about 8.5:1, has dropped well below the static CR of 10:1. The bigger the cam, the more this effect comes into play.
 
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nm9stheham said:
If I may... DCR is a fixed computation. I'd call what moper speaks about as 'effective CR' or 'effective DCR' which adds in the effects of volumetric efficiency (like which varies with RPM's), any intake pressure from turbo-or-super-charging, and altitude effects (negative turbo-charging).

If you don't start with knowing your DCR, then you don't have any chance of knowing where your 'effective DCR' will end up, unless you:
  • Build the same thing as before
  • Just randomly experiment
Just looking for thoughts/opinions here... Isn't the VE at cranking speeds assumed as 100% in the DCR computations? Since it only considers the intake effects, and ignores any exhaust effects (like reversion), then it seems to be so. If that is true, then 'effective DCR' would typically peak only a few % above computed DCR. But I am not 100% sure on that aspect, so would welcome comments.

"They ain't lazy"? The suggestion was made that he lay in some $$ for tires a few posts back LOL
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I was thinking about this while puttering around the house before and you're right NM - I had pressure and ratio mixed up. Ratio won't change. Pressures will. I thinking about it (I didn't go look it up) that VE at cranking will always be 100%. It has to be because there's no actual dilution, no power stroke, and no blowdown. Meaning all the dynamic things are not present at that low an rpm. But a good street engine wil go 110%+, and a good race engine go 115%+. So the pressures will change as the physics of the running engine come into play.

Edit - My position of "it's too much overthinking" is still unchanged however...lol. Unless this is a max effort in every part choice and a specific performance window to hit, it's not really relevant beyond setting up for the static ratio and estimating the dynamic ratio in regard to cam choice.
 
Wyrmrider said:
not 100% if you are getting backflow as the ling rod piston travels up before the valve closes
at some point rpm and intake velocity my get you more VE till you run out of flow (cam, heads)
it also varies with compression
Vizard writes about this- I'll see if I can find it

Because of the delayed intake closure, there is considerable piston motion up the bore from BDC before the intake actually closes. This, at low rpm, pushes some of the mixture back into the intake manifold. This means the volumetric efficiency (breathing efficiency), and thus the effective displacement of the cylinder, is well below 100 percent. In other words, a 100-cc cylinder with a static CR of 10:1 may only trap 75 cc of air. This means the dynamic CR, at about 8.5:1, has dropped well below the static CR of 10:1. The bigger the cam, the more this effect comes into play.
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Again just thinking out loud... I agree Wyrm - but the "100%" I'm talking about is simply the intake charge from my perspective. Yes, there is a moment where the piston coming up will push the mix up the intake - because there's no intake inertia present at 150rpm. So I guess it really comes down to "percentage of what?" are we talking about. I say 100% simply because there's a full volume above the piston when the intake valve closes but you're right in that the content of that 100% might be a little messed up. On a running engine at idle it would be dilution from the leftover exhaust gasses. At 3Krpm it's not going to have that dilution, and at 5250 it will be more than 100% filled by burn-able mix. At least I think so anyway. Back to the shop for now...
 
nm9stheham said:
If I may... DCR is a fixed computation. I'd call what moper speaks about as 'effective CR' or 'effective DCR' which adds in the effects of volumetric efficiency (like which varies with RPM's), any intake pressure from turbo-or-super-charging, and altitude effects (negative turbo-charging).

If you don't start with knowing your DCR, then you don't have any chance of knowing where your 'effective DCR' will end up, unless you:
  • Build the same thing as before
  • Just randomly experiment
Just looking for thoughts/opinions here... Isn't the VE at cranking speeds assumed as 100% in the DCR computations? Since it only considers the intake effects, and ignores any exhaust effects (like reversion), then it seems to be so. If that is true, then 'effective DCR' would typically peak only a few % above computed DCR. But I am not 100% sure on that aspect, so would welcome comments.

"They ain't lazy"? The suggestion was made that he lay in some $$ for tires a few posts back LOL
Click to expand...


I don't know my DCR. I don't even care. And it's easy to explain why.

Because I've had engines that have low cranking pressures, and a low DCR and they were virtually untuable.

I've had the opposite and they don't rattle unless the tune up is stupid.

That's how I can run 11.01:1 on pump gas with zero issues. I didn't figure my DCR. I just don't care. It's irrelevant to me.
 
There was another post where poster was running 11:1 with a 256 comp cam
I think his cranking was over 190 maybe 205
Better at his tuneups than I am
I have had 440's with low cranking and low dcr that were virtually untunable- all they did was make heat and would fail half way to Vegas on a hot day
I quit doing those stock type rebulds a long time ago because failure is always my fault
 
yellow rose said:
That's how I can run 11.01:1 on pump gas with zero issues. I didn't figure my DCR. I just don't care. It's irrelevant to me.
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You don't need to know as you hit a combo, probably through loooong experience, that was not too high or too low for your application; you 'just know'. Without that, folks can read something like the Crane cams catalog and follow their SCR recommendations with particular cams. That is the same idea, keeping DCR in the best range, just without the computations. I just started down this road with the OP since he mentioned a high SCR, and wanted to see if he was getting into a dangerous territory, detonation-wise.

For an entirely different engine, or starting from scratch with less experience, or going for certain applications with no catalog info, one had better compute it IMHO. I'll never build another N-A rally car engine without knowing where I am heading for DCR; a mid-high 7's DCR minimum will make the torque curve wider and that is very critical for a rally car engine.... unless you have a factory team 6 or 7 speed trannie with electronic paddle shifting! I've done the high 5's DCR type, that struggled to have a 2:1 usable RPM torque range, and also a low 7's DCR engine, with a solid 2.75:1 usable RPM torque range; the higher DCR 1.6L would outperform the low DCR 1.9L in that application (both with 4 speeds manual trannies, 3.73 or 3.91 rear gear, no OD).

On the street cruising scene and driving scene, low-mid RPM torque is important and that comes with good DCR. But, with drag racing, low DCR can be overcome to some degree with high stall
TC's and clutch slipping. And I've come to think more and more that circle track applications will be mostly track-dependent; you had better have a good 2:1 RPM range at Martinsville, no time to shift gears. But at Pocano, top end speed near 200 mph is key, so you sacrifice RPM range to get higher peak torque/HP and then use the gearbox in the corners to make up for a narrower RPM range.

As for the OP, if he is emphasizing drag racing, then a high DCR is perhaps less important, as with that engine size, maximizing DCR and low-mid RPM torque just makes the traction problem harder. If he CAN solve the traction challenges off the line, then more torque will be better but if you can't use it, then it may be a waste of effort. So that is another reason IMHO that fretting over a higher than 8:1 DCR may not be all that useful.
 
nm9stheham said:
You don't need to know as you hit a combo, probably through loooong experience, that was not too high or too low for your application; you 'just know'. Without that, folks can read something like the Crane cams catalog and follow their SCR recommendations with particular cams. That is the same idea, keeping DCR in the best range, just without the computations. I just started down this road with the OP since he mentioned a high SCR, and wanted to see if he was getting into a dangerous territory, detonation-wise.

For an entirely different engine, or starting from scratch with less experience, or going for certain applications with no catalog info, one had better compute it IMHO. I'll never build another N-A rally car engine without knowing where I am heading for DCR; a mid-high 7's DCR minimum will make the torque curve wider and that is very critical for a rally car engine.... unless you have a factory team 6 or 7 speed trannie with electronic paddle shifting! I've done the high 5's DCR type, that struggled to have a 2:1 usable RPM torque range, and also a low 7's DCR engine, with a solid 2.75:1 usable RPM torque range; the higher DCR 1.6L would outperform the low DCR 1.9L in that application (both with 4 speeds manual trannies, 3.73 or 3.91 rear gear, no OD).

On the street cruising scene and driving scene, low-mid RPM torque is important and that comes with good DCR. But, with drag racing, low DCR can be overcome to some degree with high stall
TC's and clutch slipping. And I've come to think more and more that circle track applications will be mostly track-dependent; you had better have a good 2:1 RPM range at Martinsville, no time to shift gears. But at Pocano, top end speed near 200 mph is key, so you sacrifice RPM range to get higher peak torque/HP and then use the gearbox in the corners to make up for a narrower RPM range.

As for the OP, if he is emphasizing drag racing, then a high DCR is perhaps less important, as with that engine size, maximizing DCR and low-mid RPM torque just makes the traction problem harder. If he CAN solve the traction challenges off the line, then more torque will be better but if you can't use it, then it may be a waste of effort. So that is another reason IMHO that fretting over a higher than 8:1 DCR may not be all that useful.
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Nope. I've never worried about DCR or cranking compression. IMO it's a waste of time. Like I've said, I've seen built around DCR and all they did was detonate themselves to death. I've also seen the opposite. Engines that should have been a rattling machine had a tune up window a mile wide.
 
TBH, I've never heard the term DCR before it was mentioned here. All I can say is thanks for the tip. My combo is for the most part a fingers crossed, best guess. That's why I'm putting it out here for advice. Tuning is not something I want to be sweating after my build so yes, a high or even borderline high DCR is something I'd like to stay away from ideally. It's going to be 90% trailored, 10% street driven so I don't want to fall down with either. I've had the reverse manual 904 built so long ago that I've forgotten the stall but remember it was somewhere between 3000-3800 so I'm not sure if this will help/hurt my proposed combo... especially the DCR.
 
moparodbuster said:
TBH, I've never heard the term DCR before it was mentioned here. All I can say is thanks for the tip. My combo is for the most part a fingers crossed, best guess. That's why I'm putting it out here for advice. Tuning is not something I want to be sweating after my build so yes, a high or even borderline high DCR is something I'd like to stay away from ideally. It's going to be 90% trailored, 10% street driven so I don't want to fall down with either. I've had the reverse manual 904 built so long ago that I've forgotten the stall but remember it was somewhere between 3000-3800 so I'm not sure if this will help/hurt my proposed combo... especially the DCR.
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Like I've already posted...you can have a low DCR and it will rattle its brains out and have a tune up window the width of a BCH.

I have never seen a correlation between DCR and a tuneable engine. Or a rattler.
 
@EL5DEMON340 runs a high compression ratio successfully. Maybe he will chime in if he hasn't already. I ain't readin back four pages.
 
moparodbuster said:
TBH, I've never heard the term DCR before it was mentioned here. All I can say is thanks for the tip. My combo is for the most part a fingers crossed, best guess. That's why I'm putting it out here for advice. Tuning is not something I want to be sweating after my build so yes, a high or even borderline high DCR is something I'd like to stay away from ideally. It's going to be 90% trailored, 10% street driven so I don't want to fall down with either. I've had the reverse manual 904 built so long ago that I've forgotten the stall but remember it was somewhere between 3000-3800 so I'm not sure if this will help/hurt my proposed combo... especially the DCR.
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OK, then I'd personally stick where you are and not push CR up any more; that was the whole point in having this discussion. Higher RPM's typically keep you away from the worst detonation probabilities, and if you get a decent quench, and with the AL heads, then you'll be in a good spot IMHO. You still need to not be sloppy/lazy with tuning and ignition timing.

FWIW, we have an 8.0-8.1 DCR 340 here with quench and AL heads: "Effective DCR" is at 7.8-7.9, since we average around 1500' elevation. 15 degrees initial and mid 30's total ignition timing. Carb is even leaned out at light loading for better cruise fuel economy. It has not knocked a peep ever. Niiiice torquey engine, for street driving and roaring around on the local mountain roads.
 
nm9stheham said:
OK, then I'd personally stick where you are and not push CR up any more; that was the whole point in having this discussion. Higher RPM's typically keep you away from the worst detonation probabilities, and if you get a decent quench, and with the AL heads, then you'll be in a good spot IMHO. You still need to not be sloppy/lazy with tuning and ignition timing.

FWIW, we have an 8.0-8.1 DCR 340 here with quench and AL heads: "Effective DCR" is at 7.8-7.9, since we average around 1500' elevation. 15 degrees initial and mid 30's total ignition timing. Carb is even leaned out at light loading for better cruise fuel economy. It has not knocked a peep ever. Niiiice torquey engine, for street driving and roaring around on the local mountain roads.
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Thanks. Is your 340 a stroker? What rpm are you shifting at? My cam lists power btwn 3500-7500 but its my understanding that because its stroked, it will actually build power at lower rpm... what's a safe high rpm shift point IYO?
 
No, not a stroker. Shifted above 6k RPM but not like what you are doing. Where the engine makes torque/power is heavily dominated by the cam choice; what you chose is definitely a high RPM cam. But the inherent low RPM torque of a stroker, and with that DCR, it would be expected to start the 'usable torque' band at a lower RPM. IMHO, the heads, even ported, are going to be the thing that stops the upper end of the RPM band before that cam stops; that engine is gonna want a lot of air.

The upper RPM range is quite variable and depends on so many things (strength of rods, rod bolts, crank, pistons, valve train parts), and as I never have built your combo, I could make only a pure guess that would have no useful meaning. I would be a lot better to ask those who have built combo's like yours on that, and list the parts you plan to use in detail. (And, don't hold anyone to their opinions, as they won't control the build.)
 
nm9stheham said:
No, not a stroker. Shifted above 6k RPM but not like what you are doing. Where the engine makes torque/power is heavily dominated by the cam choice; what you chose is definitely a high RPM cam. But the inherent low RPM torque of a stroker, and with that DCR, it would be expected to start the 'usable torque' band at a lower RPM. IMHO, the heads, even ported, are going to be the thing that stops the upper end of the RPM band before that cam stops; that engine is gonna want a lot of air.

The upper RPM range is quite variable and depends on so many things (strength of rods, rod bolts, crank, pistons, valve train parts), and as I never have built your combo, I could make only a pure guess that would have no useful meaning. I would be a lot better to ask those who have built combo's like yours on that, and list the parts you plan to use in detail. (And, don't hold anyone to their opinions, as they won't control the build.)
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Sure. The devil is in the details. Thanks!
 
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