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Old 04-29-2019, 12:29 PM
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sbc cylinder pressure and octane rating

Hello guys I have some questions here and I did a compression cylinder check on all eight cylinders and they are all at 180 psi which after many builds is the most I have ever had in a performance engine. My question is what octane rating can I get by with minimum. In my previous 350 I had 9.1 compression and a 270/278 219/[email protected] 515/530 lift 112 lsa. Heads were dart shp 200cc 72cc chambers with 2.02 1.60 valves. and though not the ideal quench it was about .056

I got a dynamic compression ratio of 7.8 or so on the keith black site. I got a cylinder pressure of 160 psi on all eight. I ran 89 octane.

My question is with my new build that I have had for a year and I have ran 89 and 93 octane from what I can tell with no issues but would like to know if I am safe to run 89 with these results.

Engine is a dart shp 377 destroke 4.155 bore x 3.48 stroke with dart pro 1 platinum 200cc heads and 72 cc chambers that was cleaned up and ported and polished for any rough edges etc and I have my block decked at .016 and 1.561 piston height. The pistons are icon forged brand with 3.7 2 valve relief.

I use a felpro gasket that is .039 compressed for a .055 quench which is not the best but not worst. My cam specs are 276/280 228/232 @50 547/547 lift 110 lsa and its 3 degrees advanced at straight up position. My cam is a custom Mike Jones hydraulic roller and I get 9.7 compression and a dynamic of about 8.3 on the keith black site.

Is that 180 psi too much for running 89 octane. Yes I can run 93 but there are some stations that are charging a dollar more a gallon compared to 89 and when filling up my 18 gallon tank that adds up over time. So far I have not hear any pinging but I also am running two tiny square cherry bomb mufflers at the rear which are quite loud and hard to really hear things.

I have my timing at around 12-14 initial and I have vacuum advance of 14 degrees limited to that with a msd stop and I have it on full manifold which gives me about estimate 26 degrees of combined timing at idle and I have 32 degrees total mechanical by 3000 rpm and it does not start to advance mechanical wise till about 1300 rpm and I am using medium weight springs as anything lighter it will be to much to quickly.

My plugs look nice and tan and clean and very little rich reading around the outer ring area and no signs of pinging. This is in a chevy s10 with a turbo 350 and 3.42 rear gears and a 3000 stall converter.

Thanks guys for any input and if I forgot anything I apologize. Trying to calculate engine octane rating needed is a big topic and I have read a lot of different opinions on things.

I put in all my numbers in the keith black

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Old 04-29-2019, 01:20 PM
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Pretty nice build

I don't see 180 psi cranking pressure being anywhere near a problem, with the help in temp rejection from aluminum heads it would not surprise me to find it would run just fine on 87 octane, maybe running into a slight ping on only the hottest summer days of the year if at all.

Only thing I could see that could be better is an .026" thick head gasket to tighten quench up even more.
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Old 04-29-2019, 01:33 PM
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Your compression ratios SCR and DCR are actually conservative for the compnents used, this and the fact there is no sign of detonation certainly establishes that 89 octane is safe in regard to how you're using and driving this truck.

My daily driver is an 89 S15 with an LT4HOT cam, modified for standard cooling LT1 heads, had been running a 454 TBI, it always had a bit of summer hot day ping that you heard in the headers on 91 unleaded E10 when I really got on it hard. I never was able to program that out. Recently I switched out the EFI and installed an Edlebrock Thunder that runs with no header ping and delivers waaaay more power it just plain pulls very hard from off idle all the way to 6000. I dropped it back to 89 octane which while the weather is still cool it runs without the header ping what the heat of summer brings especially on the desert side of the mountains where temps into the lower hundreds is common is still to be seen.

Gearing is a 3.08 posi with a built 700R4 with 2800 stall and lockup, sitting on 245 60x16 rubber. The truck weighs 2800 with me in it.

Bogie
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Old 04-29-2019, 03:28 PM
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It will come down to the tune and heat IMO with heat being the largest contributor to an issue (unless you manage to tune it 'silly' lean). Aluminum heads help a ton, but also anything to get your intake temp to ambient temps is critical as well. This includes getting the hot air from the engine compartment away from the air cleaner and keeping fresh air circulating through the engine bay.

The challenge is in building a cooling system and intake system that will work with street driving in traffic on a hot day.

Have you dyno'd this? This should put darn near 400 to the wheels.
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Old 04-29-2019, 03:59 PM
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I have to ask this so don't shoot me - do you drive this enough to put your operating costs too high to drive due tot he difference in price between 89 and 93 octane? I went through the same issue with my son - his current build is only 8.2:1, but 16lbs of boost. Is the extra cost, or savings, worth the damage that can be caused by detonation? I told him to look at it from a different POV - is an extra $3-5 in a tank of gas the difference between driving it and it not driving it?
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Old 04-29-2019, 04:20 PM
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https://www.ebay.com/itm/Longacre-50...75.c100623.m-1

Following a full-throttle blast, chop the ignition and pull to the side of the road (or out of the way at the track if you are testing at the drag strip). DO NOT IDLE OR RUN THE MOTOR ANY MORE AFTER YOU HAVE CHOPPED IGNITION FOLLOWING THE FULL THROTTLE BLAST. Remove the plugs and store them until you can get home with them. Install another set of plugs to get you home. At home, cut the threads off the plugs with a thin stone cut-off wheel and inspect the ceramic with your Longacre magnifier viewer. The first indication of detonation will be little black specks on the insulator. This is oil that has been smashed from the top ring groove by detonation and deposited onto the spark plug ceramic. If left unfixed with better fuel or other remedy such as reduced ignition timing, the next thing you will begin seeing is little aluminum specks on the insulator. This is molten aluminum from the crown of the piston as it comes apart.
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Old 04-30-2019, 07:23 AM
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I had a '79 Datsun 280ZX back in the day with over 150k miles. That thing showed 215 PSI cranking pressure and ran on regular grade gas. Doubt it was carboned up, I ran it pretty hard...

Russ
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Old 04-30-2019, 07:34 AM
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I understand is the cost really worth the difference of the risk and I understand that part its if I don't have to run 93 then I am wasting money if I do but I also don't drive that much anyways I was just curious that for the most part as I was used to that with my old engine. As far as dyno goes no I have not had this on the dyno but when I had this built it was built off of a crate motor that had the old version of the 180cc dart pro 1 aluminum heads no work done on them and a flat tappet cam with the lower 220's @50 and 9.5 compression.

That motor was dynoed at 470 horse power and 470 torque with a dual plane intake manifold. With my dart pro 1 platinum 200cc heads which my Father ported all through them and gasket matched them along with cleaning up the combustion chamber and also ported and gasket matched my victor jr intake as well so its opened up really well and should be making more then that crate motor did. I can say it really takes off after getting to the 3000 rpm mark and yet still has a lot of throttle response on the bottom end.

I will have to try your method techinspector later on this summer when its hotter out so like that I can get a true reading. I got my timing set back a little so I have no more then 32 degrees total mechanical so as to help with no pinging risk but my engine runs very strong with that and I have added more timing and there seems to be no difference by feel anyways.

Another reason is at the current moment my income is very tight compared to what I used to have and that is a reason to for some reason some stations around me charge a dollar more between octane ratings and some stations do less but I live in a small town and some of the stations have junk gas that even the highest octane is not good as some stations the good ones that I go at are the ones who can't seem to decide on there prices and keep going up and down and it seems I always end up getting gas on the wrong days where there higher.

It used to be 20 to 30 cents difference between all the stations but BP and a few others around sometimes charge a dollar difference between the different octane ratings for some reason.
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Old 04-30-2019, 07:09 PM
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Quote:
Originally Posted by eric32 View Post
It used to be 20 to 30 cents difference between all the stations but BP and a few others around sometimes charge a dollar difference between the different octane ratings for some reason.
The reason is the ignorance of the buying public. If they'll pay the difference, BP and others will gladly charge the difference.
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Old 05-01-2019, 07:11 AM
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I did some research and they oil companies say the reason for the bigger price hike between the octane ratings which for years was only ten cents was because the higher epa standards that went into effect the last couple of years and it cost more to make the higher octane grades thus they have to pass on the cost to the people is what I read but what truth is to that I don't know but what can we do about it anyways. Its getting awful how much they get away with.
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Old 05-01-2019, 07:25 AM
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Quote:
Originally Posted by eric32 View Post
I did some research and they oil companies say the reason for the bigger price hike between the octane ratings which for years was only ten cents was because the higher epa standards that went into effect the last couple of years and it cost more to make the higher octane grades thus they have to pass on the cost to the people is what I read but what truth is to that I don't know but what can we do about it anyways. Its getting awful how much they get away with.
I retired from being a refinery operator back in 2011. The unit I operated made 50k bbls/day of high octane blending stock that was between 93 and 115 depending on cuts and severity. Cuts are what's removed and severity is the "duty cycle" of the catalysts and equipment. Blending stock, along with additives, is needed to bump up the 70 to 80 octane gasoline that comes off other units. EPA is forcing the additives (lead, MTBE, etc) out of the picture. Bottom line is, is does cost more to produce a higher octane rating. That cost is considerably higher now that the additive lines have been reduced or negated, as it's mostly/all on the life span and degradation of the catalyst and equipment...

Russ
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