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too much cranking compression?

Posted By: moturbopar

too much cranking compression? - 03/09/11 07:41 AM

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
Posted By: 70AARcuda

Re: too much cranking compression? - 03/09/11 07:49 AM

how thick is your head gasket?
Posted By: moturbopar

Re: too much cranking compression? - 03/09/11 07:54 AM

Its .041
Posted By: Kern Dog

Re: too much cranking compression? - 03/09/11 07:58 AM

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.
Posted By: jbc426

Re: too much cranking compression? - 03/09/11 08:26 AM

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.
Posted By: 451Mopar

Re: too much cranking compression? - 03/09/11 08:28 AM

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
Posted By: Pale_Roader

Re: too much cranking compression? - 03/09/11 12:55 PM


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. (!!!)
Posted By: kenworth_goose

Re: too much cranking compression? - 03/09/11 03:31 PM

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.
Posted By: GomangoCuda

Re: too much cranking compression? - 03/09/11 03:39 PM

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?
Posted By: convertriple

Re: too much cranking compression? - 03/10/11 04:13 AM

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.
Posted By: RapidRobert

Re: too much cranking compression? - 03/10/11 04:17 AM

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
Posted By: ahy

Re: too much cranking compression? - 03/10/11 05:10 AM

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.
Posted By: RapidRobert

Re: too much cranking compression? - 03/10/11 05:18 AM

Quote:

run a more aggressive timing curve which is more valuable than the last couple .10's in CR.

Posted By: moturbopar

Re: too much cranking compression? - 03/10/11 05:33 AM

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.
Posted By: 451Mopar

Re: too much cranking compression? - 03/10/11 09:09 AM

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.
Posted By: moturbopar

Re: too much cranking compression? - 03/10/11 05:10 PM

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
Posted By: patrick

Re: too much cranking compression? - 03/10/11 05:46 PM

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
Posted By: patrick

Re: too much cranking compression? - 03/10/11 05:50 PM

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....
Posted By: 451Mopar

Re: too much cranking compression? - 03/11/11 04:30 AM

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.
Posted By: moturbopar

Re: too much cranking compression? - 03/11/11 05:11 AM

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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