10:2:1 CR to low for 396?
06-01-2006, 08:41 PM
#21
I am running 11.5 on a stock shortblock with my AI combo. Daily driving it too and I am at about 700ft or so. I had not intended to daily drive it but did tell them I planned on some LONG trips to race, at this point though the Caprice drive so well I can't bring myself to park it and drive the basically stock Roady wagon.
Oldschool guys are amazed at the compression newer engines can get away with and it isn't just the cooling, the injection plays in as well as do a lot of other variables. Any chance you can educate him and get the builder you want?? If not find another builder. The iron head LT1 straight from GM was 10:1 and tuned for 87 octane with 200psi cranking compression and that is the iron head version. There are guys with 355-396s running 12:1 on pump gas at low altitude.
Far as OldSStrokers comments on port volume he is Bret Bauer's(as in LE's cam guy) father so he is a bit biased. AI manages to make good power with "small" ports and that is the "theory" I have long believed best and why they got my money, Lloyd uses a good bit more port volume and his customers make good power too. Comes down to more than one way to skin a cat. They both have happy customers going fast enough to turn heads at the local track and daily drivers beating trailered "race" cars.
Oldschool guys are amazed at the compression newer engines can get away with and it isn't just the cooling, the injection plays in as well as do a lot of other variables. Any chance you can educate him and get the builder you want?? If not find another builder. The iron head LT1 straight from GM was 10:1 and tuned for 87 octane with 200psi cranking compression and that is the iron head version. There are guys with 355-396s running 12:1 on pump gas at low altitude.
Far as OldSStrokers comments on port volume he is Bret Bauer's(as in LE's cam guy) father so he is a bit biased. AI manages to make good power with "small" ports and that is the "theory" I have long believed best and why they got my money, Lloyd uses a good bit more port volume and his customers make good power too. Comes down to more than one way to skin a cat. They both have happy customers going fast enough to turn heads at the local track and daily drivers beating trailered "race" cars.
06-01-2006, 09:32 PM
#23
You want the build to be sucessful too and there is a lot of room left above 10.2 and below risky. Seriously see if your builder is open to learning a little about the LT1, if he is not I would not want him building anything for me. Then again maybe he will come around and he can be the next big builder to hit the boards.
06-01-2006, 09:37 PM
#25
Its not about the heat, its about the Altidtude.
I think this is were "I" get confused.
I would think that a higher DA (density altitude) there is more air/oxygen per volume of space and thus, the air is thicker. With thicker air, compression is already done a little bit for you, so you don't need to run as high of a CR to make the same HP at a lower DA.
This seems to make sense to me.......
I think this is were "I" get confused.
I would think that a higher DA (density altitude) there is more air/oxygen per volume of space and thus, the air is thicker. With thicker air, compression is already done a little bit for you, so you don't need to run as high of a CR to make the same HP at a lower DA.
This seems to make sense to me.......
Last edited by Speed Density; 06-01-2006 at 09:49 PM.
06-01-2006, 09:55 PM
#26
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Here's a helpful idea.....
What cam size do you plan on using (I know this is real general) but, it would be nice to know if your going to be using something in the 260's, 270's, 280's, 290's BASE duration...In each one of those catagories you can advance or retard the intake ABDC to get the desired DCR. Example..260 (maybe 210@.050 duration) would only need 10.5 compression depending on ABDC on intake closing point. If your worried about to much DCR then stay between 8.5-8.8. Those have been proven easy numbers for our engines to tolerate. (8.5 DCR is around stock, by the way) Remember DCR is what important, not SCR. Also, ve. plays in to how much actual timing the engine wants or needs, along with gearing, heat/cooling weight, quench, altitude ect...A real good safe number with OK quench say .05 (.034-.040 is better) would be 8.5 DCR (no less though)...I ran right at 9.1 DCR on my engine and did not require any different timing and or fuel, however I did get a load of bad gas and on one of my dyno runs it knocked a little...So, I backed it down to 8.77 DCR and have never seen or heard a rattle with anyone's gas...
So, depending on cam choice and ABDC of intake lobe you can get a lot better idea for what SCR you should set your engine at...
If I were going with say something in the 270-280 base duration (very good streetability on a 396) I would look for atleast 11 to 1 compression SCR.....
What cam size do you plan on using (I know this is real general) but, it would be nice to know if your going to be using something in the 260's, 270's, 280's, 290's BASE duration...In each one of those catagories you can advance or retard the intake ABDC to get the desired DCR. Example..260 (maybe 210@.050 duration) would only need 10.5 compression depending on ABDC on intake closing point. If your worried about to much DCR then stay between 8.5-8.8. Those have been proven easy numbers for our engines to tolerate. (8.5 DCR is around stock, by the way) Remember DCR is what important, not SCR. Also, ve. plays in to how much actual timing the engine wants or needs, along with gearing, heat/cooling weight, quench, altitude ect...A real good safe number with OK quench say .05 (.034-.040 is better) would be 8.5 DCR (no less though)...I ran right at 9.1 DCR on my engine and did not require any different timing and or fuel, however I did get a load of bad gas and on one of my dyno runs it knocked a little...So, I backed it down to 8.77 DCR and have never seen or heard a rattle with anyone's gas...
So, depending on cam choice and ABDC of intake lobe you can get a lot better idea for what SCR you should set your engine at... If I were going with say something in the 270-280 base duration (very good streetability on a 396) I would look for atleast 11 to 1 compression SCR.....
06-01-2006, 10:04 PM
#29
Originally Posted by Speed Density
Its not about the heat, its about the Altidtude.
I think this is were "I" get confused.
I would think that a higher DA (density altitude) there is more air/oxygen per volume of space and thus, the air is thicker. With thicker air, compression is already done a little bit for you, so you don't need to run as high of a CR to make the same HP at a lower DA.
This seems to make sense to me.......
I think this is were "I" get confused.
I would think that a higher DA (density altitude) there is more air/oxygen per volume of space and thus, the air is thicker. With thicker air, compression is already done a little bit for you, so you don't need to run as high of a CR to make the same HP at a lower DA.
This seems to make sense to me.......
06-01-2006, 10:07 PM
#31
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Originally Posted by Speed Density
Its not about the heat, its about the Altidtude.
I think this is were "I" get confused.
I would think that a higher DA (density altitude) there is more air/oxygen per volume of space and thus, the air is thicker. With thicker air, compression is already done a little bit for you, so you don't need to run as high of a CR to make the same HP at a lower DA.
This seems to make sense to me just backwards
.......
I think this is were "I" get confused.
I would think that a higher DA (density altitude) there is more air/oxygen per volume of space and thus, the air is thicker. With thicker air, compression is already done a little bit for you, so you don't need to run as high of a CR to make the same HP at a lower DA.
This seems to make sense to me just backwards
.......
Simple...less air in per stroke=less dense cyl with compression...If I were building only a high altitude set up, I would think a DCR of 9.5 to one would be good to go..."IF you have the same octane available as lower altitude area....SAY 92..." If only 90 octane your back to the same DCR requirement at sea level....
06-01-2006, 10:16 PM
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Originally Posted by Speed Density
Then he goes on about tunning and how its going to be a pain without a Standalone system.
Originally Posted by Speed Density
how im going to tune this thing to run.......
Get a Moates AutoProm and Tunercat RT. http://www.moates.net You've got a '93. You can make changes and tune on the fly. You're looking at about $400 to get set up with that. You can tune in real time. Travelling to a lower altitude? No problem. Tweak it.
Will he consider 10.8-11:1 if you get the real time tuner?
06-01-2006, 10:32 PM
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Look for Pat Kelley's and Blair Legate's "DCR calculator" it's on line and free...You will then start to understand the relationship of SCR and what info is needed for that, and DCR, when you put different cams in it....A word of warning using it...Start from Compression Ratio Calculator then Cam timing and Overlap Calculator then move on to Dynamic Stroke Lenght Calculator...then go back to Compression Ratio Calculator and check the box that says "Check box for dynamic stroke"...then hit calculate at bottom again...This well help with your decision on compression for that combo...
06-02-2006, 01:15 AM
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Originally Posted by Camaroholic
The issue becomes needing a high altitude tune and a low altitude tune. I lived in Amarillo - which is 2700 ft, typically 4000-5000 ft DA's. I could run 36-37 degrees of WOT timing up there, and my LT1 just loved it. Crazy stuff. Take that down to sea level on 90 octane (which is what most pumps had as "high octane" in the area), and it'd sure break things. I only went to low altitude once with that setup, and I retuned the car for low altitude before going down the dragstrip.
I can understand his point. It's his *** in the sling as far as motor warranty goes - and he's covering it this way. You tune a car to run at high altitude, then take it to a lower altitude, it *could* cause harm, especially if you're "on the edge".
Sounds like he actually does know what's going on.
I can understand his point. It's his *** in the sling as far as motor warranty goes - and he's covering it this way. You tune a car to run at high altitude, then take it to a lower altitude, it *could* cause harm, especially if you're "on the edge".
Sounds like he actually does know what's going on.
the TA club of amarillo hangs out there on weekends!
06-02-2006, 10:51 AM
#40
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Speed,
You are asking the right questions here and elsewhere. Educate yourself about DCR, DA as well as valve events and have a heart-to-heart with your engine builder. Along that line, the current issue of Popular Hot Rodding (with yellow/black GTO on cover) has a good article on cams by David Vizard. There is some good stuff in there which might make things more clear.
Head flow basically determines max horsepower. IOW, a 250 cfm @ .600 head is going to support a given amount of hp (let's say 500 fwhp just as an example). The larger the engine displacement for a given head, the lower the power peak rpm, so a 355 might make the power at 6700 or so but a 396 might make about the same power closer to 6000. The 396 will make more torque across the board, but if you are looking for horsepower, you need head flow more than displacement. 396's cost considerably more to build correctly than 355s or even 383s.
FWIW, if you have a hp target, "better" heads with more flow will need less valve timing ("smaller" cam) to produce the power. You can see this demonstrated all the time when people have to throw 250-260 or more .050 duration at small-headed engines to get power, when "bigger" heads get the same or more power, and a much healthier mid to high torque curve with lots less duration. That usually equates to better driveability.
You really might consider rethinking your engine size vs. your budget. A very strong, very driveable 396 LT1 is going to be very expensive. Depending on your performance goals, a ~450 RWhp 383 might be enough. It can be done for a LOT less money.
Keep up the dialog with your engine builder. If you hear too many thangs that raise a red flag (some of which you've already said), you might look elsewhere.
You are asking the right questions here and elsewhere. Educate yourself about DCR, DA as well as valve events and have a heart-to-heart with your engine builder. Along that line, the current issue of Popular Hot Rodding (with yellow/black GTO on cover) has a good article on cams by David Vizard. There is some good stuff in there which might make things more clear.
Head flow basically determines max horsepower. IOW, a 250 cfm @ .600 head is going to support a given amount of hp (let's say 500 fwhp just as an example). The larger the engine displacement for a given head, the lower the power peak rpm, so a 355 might make the power at 6700 or so but a 396 might make about the same power closer to 6000. The 396 will make more torque across the board, but if you are looking for horsepower, you need head flow more than displacement. 396's cost considerably more to build correctly than 355s or even 383s.
FWIW, if you have a hp target, "better" heads with more flow will need less valve timing ("smaller" cam) to produce the power. You can see this demonstrated all the time when people have to throw 250-260 or more .050 duration at small-headed engines to get power, when "bigger" heads get the same or more power, and a much healthier mid to high torque curve with lots less duration. That usually equates to better driveability.
You really might consider rethinking your engine size vs. your budget. A very strong, very driveable 396 LT1 is going to be very expensive. Depending on your performance goals, a ~450 RWhp 383 might be enough. It can be done for a LOT less money.
Keep up the dialog with your engine builder. If you hear too many thangs that raise a red flag (some of which you've already said), you might look elsewhere.