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too much cranking compression? #946901 03/09/11 03:41 AM 03/09/11 03:41 AM
Joined: Aug 2010 Posts: 164 mesa arizona moturbopar OP member
moturbopar OP member Joined: Aug 2010 Posts: 164 mesa arizona	Hi all the 360 Im building will have 9.4 to 1 mechanical, and I will be running a comp cams xe268. The heads are regular open chamber small block heads the chambers deburred and some polishing. I have been told it might have too much cranking compression to run on pump gas!!?? If so do you guys think that the comp xe274 would be a better choice to bleed of some cylinder pressure? Thanks

Re: too much cranking compression? [Re: moturbopar] #946902 03/09/11 03:49 AM 03/09/11 03:49 AM
Joined: Jan 2003 Posts: 9,826 las vegas 70AARcuda master
70AARcuda master Joined: Jan 2003 Posts: 9,826 las vegas	how thick is your head gasket? Tony 70 AARCuda Vitamin C 71 Dart Swinger 360 10.318 @ 128.22(10-04-14 Bakersfield) 71 Demon 360 10.666 @122.41 (01-29-17 @ Las Vegas) 71 Duster 408 (10.29 @ 127.86 3/16/19 Las Vegas)

Re: too much cranking compression? [Re: 70AARcuda] #946903 03/09/11 03:54 AM 03/09/11 03:54 AM
Joined: Aug 2010 Posts: 164 mesa arizona moturbopar OP member
moturbopar OP member Joined: Aug 2010 Posts: 164 mesa arizona	Its .041

Re: too much cranking compression? [Re: moturbopar] #946904 03/09/11 03:58 AM 03/09/11 03:58 AM
Joined: Feb 2010 Posts: 18,493 Granite Bay CA Kern Dog Striving for excellence
Kern Dog Striving for excellence Joined: Feb 2010 Posts: 18,493 Granite Bay CA	A long duration cam tends to actually bleed off cylinder pressure. I have read that a rough guideline for the pump gas limits are 200-230 psi cranking pressure. When I built the 440/493, I advanced the '509 cam by 4 degrees. It had cranking numbers in the 230 range. Later when I returned the cam to a straight up position, it was down in the 190s. The advanced position closed the valves earlier in the cycle, so MORE pressure was evident. Last edited by Frankenduster; 03/09/11 04:04 AM.

Re: too much cranking compression? [Re: Kern Dog] #946905 03/09/11 04:26 AM 03/09/11 04:26 AM
Joined: Mar 2007 Posts: 3,281 West Coast, USA jbc426 master
jbc426 master Joined: Mar 2007 Posts: 3,281 West Coast, USA	I heard the limit was closer to 175psi in a well-sorted and highly tuned motor. Mine's got 225psi, and I have to run 100 octane unleaded. 1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's 1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)

Re: too much cranking compression? [Re: moturbopar] #946906 03/09/11 04:28 AM 03/09/11 04:28 AM
Joined: May 2008 Posts: 5,399 Aurora, Colorado 451Mopar master
451Mopar master Joined: May 2008 Posts: 5,399 Aurora, Colorado	Quote: Hi all the 360 Im building will have 9.4 to 1 mechanical, and I will be running a comp cams xe268. The heads are regular open chamber small block heads the chambers deburred and some polishing. I have been told it might have too much cranking compression to run on pump gas!!?? If so do you guys think that the comp xe274 would be a better choice to bleed of some cylinder pressure? Thanks You should be fine with the comp XE268 cam. The cam (if installed per spec) closes the intake valve 60 degrees after bottom dead center (ABDC.) If you plug the info into the Wallace Racing Calculator for Dynamic compression/cranking pressure you get 146 cranking PSI at your altitude of 1,241 feet. Even at sea-level (0 Feet altitude) the cranking pressure would only be around 153 PSI. Links: http://www.wallaceracing.com/dynamic-cr.php http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=627&sb=0

Re: too much cranking compression? [Re: 451Mopar] #946907 03/09/11 08:55 AM 03/09/11 08:55 AM
Joined: Jan 2005 Posts: 4,862 the frozen wastes... Pale_Roader Swears too much
Pale_Roader Swears too much Joined: Jan 2005 Posts: 4,862 the frozen wastes...	My 68 Caddy's 472 cranks around 185 average, and hauls a 6000lb rig around with no issues on 94. Friends ov mine have gotten away with 195ish on pump gas, no issues while running it, but one guy did have a few ugly surprises when he finally tore it down. On the other hand, my bone stock 76 440 with its 3:1 compression wont run on anything less than 91 octane without detonation you can hear, and runs WAY better on 94 than 91. WAY too many variables to pin an exact number, even on a forum that deals largely with a LOT ov very similar combos. For what its worth, and completely apples to oranges i realize... but the N/A 2L VTec S2000 4-popper cranks about 230 average on pump gas... thats 11:1. They also make a 11.7:1 version that runs well on pump gas as well. (!!!)

Re: too much cranking compression? [Re: jbc426] #946908 03/09/11 11:31 AM 03/09/11 11:31 AM
Joined: May 2006 Posts: 2,046 ky. K kenworth_goose top fuel
kenworth_goose top fuel K Joined: May 2006 Posts: 2,046 ky.	The last 360 I built was 10 to 1 and it had 240psi cranking. Also had a 508 purple shaft. Never had any issues running 97. It ran 12.40's in a 68 dart with a stock convertor and 3.91's.

Re: too much cranking compression? [Re: kenworth_goose] #946909 03/09/11 11:39 AM 03/09/11 11:39 AM
Joined: Jan 2003 Posts: 4,723 Moved to N.E. Tennessee GomangoCuda master
GomangoCuda master Joined: Jan 2003 Posts: 4,723 Moved to N.E. Tennessee	Quote: The last 360 I built was 10 to 1 and it had 240psi cranking. Also had a 508 purple shaft. Never had any issues running 97. It ran 12.40's in a 68 dart with a stock convertor and 3.91's. Where do you get 97? Or did you mean 87? In theory, there is no difference between theory and practice. In practice, there is.

Re: too much cranking compression? [Re: moturbopar] #946910 03/10/11 12:13 AM 03/10/11 12:13 AM
Joined: Jan 2003 Posts: 923 Missouri C convertriple super gas
convertriple super gas C Joined: Jan 2003 Posts: 923 Missouri	My 340 has 9.5:1 static compression using TRW's old 10.5 flattops. I use an 80's version of the 280/.474 MP purple shaft cam, which I read waaay back when, builds compression. This same motor ran awesome in the 80's, but fuel quality is so poor now , I ping at 32 degrees total on 93 octane. My cranking PSI is around 190. To be fair, I haven't messed with the timing curve yet, and have to experiment with jets more, hopefully one of those can help me.

Re: too much cranking compression? [Re: moturbopar] #946911 03/10/11 12:17 AM 03/10/11 12:17 AM
Joined: Nov 2003 Posts: 36,040 Lincoln Nebraska R RapidRobert Circle Track
RapidRobert Circle Track R Joined: Nov 2003 Posts: 36,040 Lincoln Nebraska	Quote: the 360 Im building will have 9.4 to 1 mechanical, Is this a measured 9.4 (CC's deck height etc). Some nominal specs vary widely live every 24 hour block of time like it's your last day on earth

Re: too much cranking compression? [Re: RapidRobert] #946912 03/10/11 01:10 AM 03/10/11 01:10 AM
Joined: Jan 2007 Posts: 7,664 IN A ahy master
ahy master A Joined: Jan 2007 Posts: 7,664 IN	The 9.4 is right up against the "rule of thumb" of 9.5 for iron heads. As posted there are a lot of variables. It doesn't sound like you have quench so the 9.5 is really an upper limit - max under ideal circumstances. I think you would be better off with the bigger cam to limit detonation potential. That may allow you to run a more aggressive timing curve which is more valuable than the last couple .10's in CR.

Re: too much cranking compression? [Re: ahy] #946913 03/10/11 01:18 AM 03/10/11 01:18 AM
Joined: Nov 2003 Posts: 36,040 Lincoln Nebraska R RapidRobert Circle Track
RapidRobert Circle Track R Joined: Nov 2003 Posts: 36,040 Lincoln Nebraska	Quote: run a more aggressive timing curve which is more valuable than the last couple .10's in CR. live every 24 hour block of time like it's your last day on earth

Re: too much cranking compression? [Re: RapidRobert] #946914 03/10/11 01:33 AM 03/10/11 01:33 AM
Joined: Aug 2010 Posts: 164 mesa arizona moturbopar OP member
moturbopar OP member Joined: Aug 2010 Posts: 164 mesa arizona	So what cam would you guys recommend? The car is 69 barracuda has 3.55 gears, 2800 stall. I have a .484 cam available but would like something a little newer. I would like to stay with the springs I have now, comp cams 901-16. I was thinking xe274 but I dont know how well it would work with my springs.

Re: too much cranking compression? [Re: moturbopar] #946915 03/10/11 05:09 AM 03/10/11 05:09 AM
Joined: May 2008 Posts: 5,399 Aurora, Colorado 451Mopar master
451Mopar master Joined: May 2008 Posts: 5,399 Aurora, Colorado	I think the XE268 would be fine, but if you really want cam suggestions we really need more information. How did you figure the 9.4:1 compression? Which pistons (type, valve reliefs, dish/dome?), cylinder head volume size?, head gaskets? What valve sizes? any head porting? Intake manifold/carb, exhaust system, tire size,etc.

Re: too much cranking compression? [Re: 451Mopar] #946916 03/10/11 01:10 PM 03/10/11 01:10 PM
Joined: Aug 2010 Posts: 164 mesa arizona moturbopar OP member
moturbopar OP member Joined: Aug 2010 Posts: 164 mesa arizona	360 h116cp pistons -5cc volume releifs .015 below deck height measured gasket 4.1 bore .041 thick heads are 587 castings, 2.02 - 1.60 valves mild portwork not sure of flow number anymore, lost the sheet. I remeber they flowed at .500 was 239 cfm @28 on the intake, so not much help there. averagecombustion CCs are 72 1 5/8 header 2.5 exhaust Intake is a single plane m1, but will probably me changed to rpm airgap. gears 3.55 2800 stall 904 trans 255/50-16 drag radials car weighs around 3400 lbs

Re: too much cranking compression? [Re: 451Mopar] #946917 03/10/11 01:46 PM 03/10/11 01:46 PM
Joined: Jan 2003 Posts: 16,123 Grand Haven, MI patrick I Live Here
patrick I Live Here Joined: Jan 2003 Posts: 16,123 Grand Haven, MI	Quote: Quote: Hi all the 360 Im building will have 9.4 to 1 mechanical, and I will be running a comp cams xe268. The heads are regular open chamber small block heads the chambers deburred and some polishing. I have been told it might have too much cranking compression to run on pump gas!!?? If so do you guys think that the comp xe274 would be a better choice to bleed of some cylinder pressure? Thanks You should be fine with the comp XE268 cam. The cam (if installed per spec) closes the intake valve 60 degrees after bottom dead center (ABDC.) If you plug the info into the Wallace Racing Calculator for Dynamic compression/cranking pressure you get 146 cranking PSI at your altitude of 1,241 feet. Even at sea-level (0 Feet altitude) the cranking pressure would only be around 153 PSI. Links: http://www.wallaceracing.com/dynamic-cr.php http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=627&sb=0 my 318 with magnum heads, stock pistons .055 in the hole (no quench) and a reground roller cam runs at 155-160 PSI on a compression test, and I have no problems running 87 octane...you'll be fine 1976 Spinnaker White Plymouth Duster, /6 A833OD 1986 Silver/Twilight Blue Chrysler 5th Ave HotRod SOLD!* 2011 Toxic Orange Dodge Charger R/T 2017 Grand Cherokee Overland 2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)

Re: too much cranking compression? [Re: GomangoCuda] #946918 03/10/11 01:50 PM 03/10/11 01:50 PM
Joined: Jan 2003 Posts: 16,123 Grand Haven, MI patrick I Live Here
patrick I Live Here Joined: Jan 2003 Posts: 16,123 Grand Haven, MI	Quote: Quote: The last 360 I built was 10 to 1 and it had 240psi cranking. Also had a 508 purple shaft. Never had any issues running 97. It ran 12.40's in a 68 dart with a stock convertor and 3.91's. Where do you get 97? Or did you mean 87? based on their profile location listing, I'm guessing they're in canada. I think their gas is rated by MON only, instead of the US standard of RON+MON/2 average.... 1976 Spinnaker White Plymouth Duster, /6 A833OD 1986 Silver/Twilight Blue Chrysler 5th Ave HotRod SOLD!* 2011 Toxic Orange Dodge Charger R/T 2017 Grand Cherokee Overland 2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)

Re: too much cranking compression? [Re: moturbopar] #946919 03/11/11 12:30 AM 03/11/11 12:30 AM
Joined: May 2008 Posts: 5,399 Aurora, Colorado 451Mopar master
451Mopar master Joined: May 2008 Posts: 5,399 Aurora, Colorado	I like those pistons. I used the KB232 D-dish with quench pad and they added alot of extra machine work to get them set up correctly. I checked the numbers you posted and they look good, I got 9.41:1 compression? Your tire height looks to be about 26", so with the 3.55 gears 65MPH would be about 3,000 RPM. With your car weight, converter stall, and gear/tires size you could go with the XE274 cam. On the dyno simulation, it does not show much difference between the XE268 and XE274 cams. I think the simulation may be limiting upper end power because of the exhaust flow numbers I used (147cfm@0.500".) When the cams were installed per spec at 106 intake cl, the xe274 gives up about 4hp at 2,600-2,800 RPM (your stall speed), and only gains about 3HP at peak HP around 4,400 RPM. The xe274 stays around peak power longer before dropping off. Because the simulation thinks the exhaust is a bit restrictive, advancing the installed CL of the cams (102 degrees looks good) improved the power output of both cams. With the xe274 cam installed at 102 degrees I got peak HP of 317@4,400RPM, and 410 ft/lbs @ 3,400 RPM. I then entered the flow numbers of the Edelbrock heads which have pretty good exhaust ports, and with only that change the output went to 367 HP@5,200 RPM, and 420 ft/lbs @ 4,000 RPM. Last edited by 451Mopar; 03/11/11 12:40 AM.

Re: too much cranking compression? [Re: 451Mopar] #946920 03/11/11 01:11 AM 03/11/11 01:11 AM
Joined: Aug 2010 Posts: 164 mesa arizona moturbopar OP member
moturbopar OP member Joined: Aug 2010 Posts: 164 mesa arizona	thanks for that info, I did forget that i will be running a 100 shot of nitrous. I might just keep the xe268 I think it would make for a nice fun streetable combo. I dont know, would the trade off in vaccuum be worth the few extra hp? AAHHHHHH what to do???

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