When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
So building my champcar motor, I'm leaving a lot of HP on the table through the stock camshaft. I planned on doing some SCCA events as well, and wanted a little more power for the tracks I planned with that; for such, I plan on swapping cams b/w races. I've done what I can to the stock exhaust manifolds which includes, welding the exterior to the flange, opening up the weld that chokes the interior of the runner, and removing the air injection obstruction crap.
Current build:
Heads CC'd at 58cc
3.48" stroke
4.030" bore
0.038" thk head gasket
4.125" dia head gasket
5cc pistons
deck should be around 0.020" per machinist
Using this calculator: The static and dynamic compression ratio calculator
I'm at 10.6:1 static which is ideal.
Stock cam has a dynamic compression ratio of under 6:1. . . which is horrible, but that's a problem for me during champ races.
I go back to the original calculator, and there's an Intake Valve Closing Point calculator. Plugging in the above cam specs, I get an IVC of 37.
Plugging that back into the original calculator and the dynamic compression ratio flies up to ~9.2:1. This is too high for pump gas which is ideally what I want to run.
Got to be honest and this is gonna ruffle the internet feathers but Dynamic Compression is a bunch of crap. It means absolutely nothing.
I looked at and tracked it for a couple of years in my builds and even went back and figured it for some previous builds trying to draw conclusions and in the end none of it mattered.... fact is most builds are in the 7 to 8 DCR range.
I used the Pat Kelly calculator because it made sense to me, to be calculating based off an intake valve closing point deprived from the seat timing.... not the 050 timing.... because believe it or not but there can be significant differences in the seat number on cams with the same 050 numbers.... and herein lies the problem with DCR.... Relying on a number to base your build decisions on that isn't even calculated the same way by different calculators. Your not even comparing the same thing if you calculated the same combination thru 5 different DCR calculators... let alone comparing your combination to other random combinations on the internet because who knows what calculator they used to get their DCR.
This doesn't even take into account different camshaft seat timing event measuring points.... did you know that most hyd cams use .006 for the advertised duration and mechanical cams use .020? Neither one are the actual point the valve hits the seat... and that a problem all to itself... but I want to go back and stress the word "most" in my statement because not everyone uses .006 and. 020... I know for a fact Jones Cam Designs uses the actual seat timing events for his advertised duration and those event calculations.
Just for fun I inputted the 440 in my 84 into both calculators you linked... now it's legit 13.5 to 1 static with a 69.5 IVC at 050 and 95.5 IVC seat. #1 = 8.97 DCR.... #2 = 7.77 DCR. Now if I was deciding my pump gas compatibility off these numbers, I'd be screwed.... and so will you. In real life I have pump gas 11.6 and less compression ratio engines with more DCR than my 440 drag engine and they run 93 octane fine and are great combinations... a few were documented builds in this forum.... my personal 383 in my 92 had 11.66 scr and 9.1 DCR and ran on 93 no problem, made nearly 500 to the tire and no being a reverse cooled LT didn't make it happen because I have numerous GEN1 based builds just like it also running fine.
DCR is just a number and a pretty worthless one at that. I'm just one engine builder. No serious engine builder I know thinks about it and most all openly scoff at the notion. Back yrs ago when I delved into DCR and sought information from people smarter than me, they told me it's a joke and when I persisted, a couple really well known people asked me to keep them informed of what I find, document it and after I felt I knew something we'd take time to go over it.... after a year and I think 35 documented engines from street cars to race cars, I understood why they scoffed and talking with the guys that wanted to talk with me about what information I had.... I then learned they went down the same road and had much of the same info... Street builds that fit fine in the DCR box, street builds that didn't fit and a few race builds that supposedly fit in the box - but they knew better.... as did I - but one guy admitted to while one of these race builds was on his dyno, to running it with 93 octane in it and he said that it knocked so bad on load in trying a warm up pull he thought he turned it into a Diesel for few seconds before he stopped the pull.
Im not beating up on you about this. I want to give you what I know for a fact about DCR. You can't base any decision about your engine off it. For the various reasons I covered above. I can go into what I believe these DCR calculators are actually (trying) to calculate but it's a little beyond the scope of my post here... Basically I believe it's trying to derive a value (ratio) for the amount of trapped charge in the cylinder once the intake valve closes... but in actuality it's just measuring the volume of the cylinder at that (static) point because it can't take into account actual dynamic events like volumetric efficiency, ram effect, how much charge is lost during overlap at the beginning of intake cycle.... so many things that vary from engine combo to engine combo and these important dynamic things change even by rpm.
Now that said, you'll be completely fine putting that Comp cam in your L98... unless it eats itself (like a large majority of Comp SADI cores do) but that won't be a product of the DCR... ie spend the extra $60 and make them grind your cam on a billet core.
Will
Beyond your question, but it does analyze compression and dynamic compression very well, look into Performance Trends Engine Analyzer. It also provides WOT timing and shows where timing may be knock limited.
Wallace Racing has pretty reputable calculators, from my understanding. they're a little cumbersome going back & forth one at a time, but they seem to work well so long as you know & input the correct values.
Got to be honest and this is gonna ruffle the internet feathers but Dynamic Compression is a bunch of crap. It means absolutely nothing.
I looked at and tracked it for a couple of years in my builds and even went back and figured it for some previous builds trying to draw conclusions and in the end none of it mattered.... fact is most builds are in the 7 to 8 DCR range.
I used the Pat Kelly calculator because it made sense to me, to be calculating based off an intake valve closing point deprived from the seat timing.... not the 050 timing.... because believe it or not but there can be significant differences in the seat number on cams with the same 050 numbers.... and herein lies the problem with DCR.... Relying on a number to base your build decisions on that isn't even calculated the same way by different calculators. Your not even comparing the same thing if you calculated the same combination thru 5 different DCR calculators... let alone comparing your combination to other random combinations on the internet because who knows what calculator they used to get their DCR.
This doesn't even take into account different camshaft seat timing event measuring points.... did you know that most hyd cams use .006 for the advertised duration and mechanical cams use .020? Neither one are the actual point the valve hits the seat... and that a problem all to itself... but I want to go back and stress the word "most" in my statement because not everyone uses .006 and. 020... I know for a fact Jones Cam Designs uses the actual seat timing events for his advertised duration and those event calculations.
Just for fun I inputted the 440 in my 84 into both calculators you linked... now it's legit 13.5 to 1 static with a 69.5 IVC at 050 and 95.5 IVC seat. #1 = 8.97 DCR.... #2 = 7.77 DCR. Now if I was deciding my pump gas compatibility off these numbers, I'd be screwed.... and so will you. In real life I have pump gas 11.6 and less compression ratio engines with more DCR than my 440 drag engine and they run 93 octane fine and are great combinations... a few were documented builds in this forum.... my personal 383 in my 92 had 11.66 scr and 9.1 DCR and ran on 93 no problem, made nearly 500 to the tire and no being a reverse cooled LT didn't make it happen because I have numerous GEN1 based builds just like it also running fine.
DCR is just a number and a pretty worthless one at that. I'm just one engine builder. No serious engine builder I know thinks about it and most all openly scoff at the notion. Back yrs ago when I delved into DCR and sought information from people smarter than me, they told me it's a joke and when I persisted, a couple really well known people asked me to keep them informed of what I find, document it and after I felt I knew something we'd take time to go over it.... after a year and I think 35 documented engines from street cars to race cars, I understood why they scoffed and talking with the guys that wanted to talk with me about what information I had.... I then learned they went down the same road and had much of the same info... Street builds that fit fine in the DCR box, street builds that didn't fit and a few race builds that supposedly fit in the box - but they knew better.... as did I - but one guy admitted to while one of these race builds was on his dyno, to running it with 93 octane in it and he said that it knocked so bad on load in trying a warm up pull he thought he turned it into a Diesel for few seconds before he stopped the pull.
Im not beating up on you about this. I want to give you what I know for a fact about DCR. You can't base any decision about your engine off it. For the various reasons I covered above. I can go into what I believe these DCR calculators are actually (trying) to calculate but it's a little beyond the scope of my post here... Basically I believe it's trying to derive a value (ratio) for the amount of trapped charge in the cylinder once the intake valve closes... but in actuality it's just measuring the volume of the cylinder at that (static) point because it can't take into account actual dynamic events like volumetric efficiency, ram effect, how much charge is lost during overlap at the beginning of intake cycle.... so many things that vary from engine combo to engine combo and these important dynamic things change even by rpm.
Now that said, you'll be completely fine putting that Comp cam in your L98... unless it eats itself (like a large majority of Comp SADI cores do) but that won't be a product of the DCR... ie spend the extra $60 and make them grind your cam on a billet core.
Will
Got to be honest and this is gonna ruffle the internet feathers but Dynamic Compression is a bunch of crap. It means absolutely nothing.
I looked at and tracked it for a couple of years in my builds and even went back and figured it for some previous builds trying to draw conclusions and in the end none of it mattered.... fact is most builds are in the 7 to 8 DCR range.
I used the Pat Kelly calculator because it made sense to me, to be calculating based off an intake valve closing point deprived from the seat timing.... not the 050 timing.... because believe it or not but there can be significant differences in the seat number on cams with the same 050 numbers.... and herein lies the problem with DCR.... Relying on a number to base your build decisions on that isn't even calculated the same way by different calculators. Your not even comparing the same thing if you calculated the same combination thru 5 different DCR calculators... let alone comparing your combination to other random combinations on the internet because who knows what calculator they used to get their DCR.
This doesn't even take into account different camshaft seat timing event measuring points.... did you know that most hyd cams use .006 for the advertised duration and mechanical cams use .020? Neither one are the actual point the valve hits the seat... and that a problem all to itself... but I want to go back and stress the word "most" in my statement because not everyone uses .006 and. 020... I know for a fact Jones Cam Designs uses the actual seat timing events for his advertised duration and those event calculations.
Just for fun I inputted the 440 in my 84 into both calculators you linked... now it's legit 13.5 to 1 static with a 69.5 IVC at 050 and 95.5 IVC seat. #1 = 8.97 DCR.... #2 = 7.77 DCR. Now if I was deciding my pump gas compatibility off these numbers, I'd be screwed.... and so will you. In real life I have pump gas 11.6 and less compression ratio engines with more DCR than my 440 drag engine and they run 93 octane fine and are great combinations... a few were documented builds in this forum.... my personal 383 in my 92 had 11.66 scr and 9.1 DCR and ran on 93 no problem, made nearly 500 to the tire and no being a reverse cooled LT didn't make it happen because I have numerous GEN1 based builds just like it also running fine.
DCR is just a number and a pretty worthless one at that. I'm just one engine builder. No serious engine builder I know thinks about it and most all openly scoff at the notion. Back yrs ago when I delved into DCR and sought information from people smarter than me, they told me it's a joke and when I persisted, a couple really well known people asked me to keep them informed of what I find, document it and after I felt I knew something we'd take time to go over it.... after a year and I think 35 documented engines from street cars to race cars, I understood why they scoffed and talking with the guys that wanted to talk with me about what information I had.... I then learned they went down the same road and had much of the same info... Street builds that fit fine in the DCR box, street builds that didn't fit and a few race builds that supposedly fit in the box - but they knew better.... as did I - but one guy admitted to while one of these race builds was on his dyno, to running it with 93 octane in it and he said that it knocked so bad on load in trying a warm up pull he thought he turned it into a Diesel for few seconds before he stopped the pull.
Im not beating up on you about this. I want to give you what I know for a fact about DCR. You can't base any decision about your engine off it. For the various reasons I covered above. I can go into what I believe these DCR calculators are actually (trying) to calculate but it's a little beyond the scope of my post here... Basically I believe it's trying to derive a value (ratio) for the amount of trapped charge in the cylinder once the intake valve closes... but in actuality it's just measuring the volume of the cylinder at that (static) point because it can't take into account actual dynamic events like volumetric efficiency, ram effect, how much charge is lost during overlap at the beginning of intake cycle.... so many things that vary from engine combo to engine combo and these important dynamic things change even by rpm.
Now that said, you'll be completely fine putting that Comp cam in your L98... unless it eats itself (like a large majority of Comp SADI cores do) but that won't be a product of the DCR... ie spend the extra $60 and make them grind your cam on a billet core.
Will
I sincerely appreciate your feedback. Yeah, something didn't quite pass the giggle test with it. I think I've set myself up for as efficient as possible. eg: Running some mahle powerpack pistons . . . seriously, what idiot pays $900+ on pistons to make 275hp? (answer: me).
Thanks for the heads up on the core material, another rabbit hole I get to dive into
So building my champcar motor, I'm leaving a lot of HP on the table through the stock camshaft. I planned on doing some SCCA events as well, and wanted a little more power for the tracks I planned with that; for such, I plan on swapping cams b/w races. I've done what I can to the stock exhaust manifolds which includes, welding the exterior to the flange, opening up the weld that chokes the interior of the runner, and removing the air injection obstruction crap.
Current build:
Heads CC'd at 58cc
3.48" stroke
4.030" bore
0.038" thk head gasket
4.125" dia head gasket
5cc pistons
deck should be around 0.020" per machinist
Using this calculator: The static and dynamic compression ratio calculator
I'm at 10.6:1 static which is ideal.
Stock cam has a dynamic compression ratio of under 6:1. . . which is horrible, but that's a problem for me during champ races.
I go back to the original calculator, and there's an Intake Valve Closing Point calculator. Plugging in the above cam specs, I get an IVC of 37.
Plugging that back into the original calculator and the dynamic compression ratio flies up to ~9.2:1. This is too high for pump gas which is ideally what I want to run.
I have to agree that the DCR numbers are to be taken with a grain of salt. I have 10.9:1 static compression in my LS swapped 84. With my cam the DCR was something like 8.5:1. I do have a flex fuel sensor on the car and run E85 sometimes but most of the time I just run 93 with no issues. I've read lots of posts about guys with a DCR over 9:1 having no issues running on pump gas. Listen to the cam manufacturers recommendations. Also don't rule out the Summit cams. I've run a few different Summit cams over the years and have been very happy with every one. Their techs are pretty knowledgeable too if you need help picking.
Like everyone else it seems, agree that Dynamic Compression Ratio in itself is trivial information, and agree with Will and all his experience/knowledge. But a similar and useful calculation is Effective Compression Ratio (efCR) as used but engine analyses software. Formula was also defined by Patric Hale, from the original ENGINE Pro (quarter jr). Basically the efCR takes into account the intake pumping, intake ramming, and the cam numbers are part of that ratio. Basically everything that determines pressure at maximum compression at different RPM's, and the Hence the suggestion earlier to use engine software to develop something more meaningful.
As a general rule for cam selection, more duration is beneficial for higher compression, longer stroke, higher RPM... and wider lobe separation also for higher RPM.
In designing a camshaft with Mike Jones, the static compression ratio is a big part of his selection for the cam, he's one of the top guys doing this stuff for pushrod V8's and motorsports. He does race cams, usually stays away from low performance stuff, and the 11.5:1 engine on pump gas will get a longer duration cam from Mike than the 10.5 engine, but there are so many other components of course. Plug it all into engine analyses software, and you can see the effect, you can optimize for goals, etc. and he's usually right on the money.
Just like everything else, it's a tool. And probably only accurate at exactly 1 point in a running engine if that.... Have I used it on the couple engines I've built to make sure nothing was stupid out of range? yeah, but using everything else leading up to it I already pretty much knew it'd be fine so what really did it matter. I didn't have to calculate it but it was another data point if needed I guess.... I'm not planning on using it with my 388. I told jones what I was running and he designed a cam for it so I could run 91 or 93...
Cam Swap and Dynamic Compression
