Compression vs. Octane

[2fast4mom]

Does anyone have a look-up table (or perhaps this is too simplistic) for what octane fuel is required to prevent detonation at specific compression numbers in psi?

For example something along the lines of:

135 psi = 92 octane
185 psi = 110 octane

(not that those numbers are necessarily correct, just looking for such information.)

Thanks to anyone who can help.

 

[xb03fs]

I don't know but I am thinking about cutting my heads supposedly u have 130ishun cut on my 200, 140-145 is supposed to need 93 but I only have 87-89 or 91

 

[whipper]

Jay Smith says 145+ 93 min bellow that 91 min 40-1. off course when you go way up to that 185 things change. When Jay and his buddy did the aces test they ran a 200psi motor on 91 with aces at wot for over a mile multiple times with egt readings and tore down the motor after and everything. It lived and actually saw faster speeds than splashed with 110!! Theres a thread on S&F about that from a few years ago. Ive never ran anything over 91 in my promax with 33cc heads and 7500+ limiter for 9+ years now on the same build and she still runs like a champ! Used to run aces for a couple of years. need to get some more. Other wise just gas and oil side pinned.

 

[RBT]

I doubt such a list exists.
Ports, head design, rpm and load are some of the factors that effect your motor.
That "test" isn't something I would use with any accuracy. Put a big wheel on that motor and it wouldn't have made a full pull without going Palestinian. Add, they used a stock a63 fuel map which is extremely fat (read coolant).

 

[whipper]

Haha i love that quote!! Without going Palestinian...

Yep just saying it must work pretty good for consumer motors if that one actually even lived. Was good regardless of prop without getting to into load and how to make it go Palestinian. As you said there isn't really a list and there are tons of variables. I wouldn't hesitate to run 91 up to 140. i wouldn't mind a splash of aces running at that level probably? After that I would be running 93 min 145-155. Im not sure after that. Most guys run a splash of av gas after that dont they in the 160+ range? This is just my idea and some others Ive read about and some builders say also. As far as Aces goes thats a whole other thread and usually a long and heated one every time.

 

[xb03fs]

So does a 200 98 block better porting lead itself to more or less safe
A5 200 promax raised limited mor or less safe
Ported 4 petal reed cages guessing less safe
SVS I am guessing less safe


So how much compression should I be able to run on 91? 91 and aces?

 

[whipper]

I would run 91 and aces to 145 without hesitation. The highest compression I've ever had was 140 and always ran 91 with and without aces but always 40/1. 96 225 33 cc heads brucato acu.
I don't know how much svs would change the need for higher octain over compression? But I wouldn't run less than 91 that's for sure 120 to 145 ish. Not 100% sure but I seem to recall 150 and up requires higher than 91 for sure. Aces well again that's up to the believers? It seems to have good results added to 91 up to around that 150 range. I'm not sure if I would risk it or not. I would probably go 94 at 150 with or without aces. Maybe guys with 160+ will say what they burn.

 

[xb03fs]

I've always ran 87 and aces and have had the best luck in the current configuration of aces and 87 at half the recommended dosage. When out of aces I run 89 and seafoam

 

[whipper]

Hear the 91 boat gas is cheeper than the 87 pump. So i always go 91 anyway. I have tryed 87 with aces once but chickened out and next time went with the normal 91. Whats your Comp with that motor?

 

[chad202]

I know a few that run 190ish on 93 octane. They feed it volume rather than octane. I run 170ish on 93 octane.

 

[xb03fs]

Honestly not 100percent sure last year I asked and the guy told me his good guage broke but it was nice and even ln his backup.

Years ago I had an issue and they checked it because they we having trouble finding the problem and I can't remeber if it was 120-124 across all 6 or 130-134 across all 6?

If I send them to be cut they cut to a cc
Correct no worries of getting to much compression???

 

[whipper]

Wow Chad 170 sweet! I would have guessed 93 or 94 at 170 but I never knew 190ish would take pump gas. i would have thought at least a splash of sunco. i understand the Volume part. Jay talked to me about that when he cut my 33CC heads. He said when you have more compression you need to feed the motor more fuel to get the benefits of higher compression.

 

[whipper]

I would think 91 and aces at 145 would be perfectly safe. One of the things with cut heads is ya gotta feed them to get the benefit. With my brucato acu I was able to increase fuel at any level from 1500 rpm up and every were in between. So after every run I would add fuel until I could feel the difference. Go past were great was then dial back to great again. Hole shot and mid range are were you feel the tighter heads. Your elevation will have a factor on how tight you can go safely. Jay and Tony had a table and a calculation to determine psi at elevation and how many cc on a given motor you can go based on what fuel you have available. I would call them and find out what they have to say.

For instance. I believe stock 225 heads were 38 or 37 cc. Sea level comp is 125 to 130 stock. Some a little higher ive heard but not more than 135 stock. Mine fresh build was 127 across the board with 130 in two holes. At 2300 ft above sea level comp was 120. There was a noticeable diff in performance from sea level. Since I live inland most of my boating was at the higher elevation. Jay took all this information and determined to get back to stock comp we needed to go 33 cc. That would get me just a little over stock but not so much that I needed to change fuel octain. Heads on thick gasket gave me 135 on a not so fresh 3 years after rebuild motor comp at 2300 ft with one 137. So I gained basicly 5 over stock at sea level 3 years after the motor was fresh.
Ran boat not a big difference. Started playing with fuel demands and as I increased demand performance started to become noticeable. The lowest I have ran is 1500 ft above. The comp would have been around 140 there. So the first time running there I had to increase fuel even more. I have it dialed so when I boat at that lower lake I know my settings. That's were I hit 97 ish a couple of times. At my home lake I can't go 95. So I loose hp and comp as elevation increases. I wish I had taken a comp read at the lower elevation but I didn't. I just knew it would be higher. 33 is pretty tight at sea level for a Promax. So take this into consideration when you cut heads and know the elevation were you boat. One thing is that every internal combustion engine at the lake is in the same boat. 34 or 35 is lots at sea level.

Another quick thing i found out is the oct requirements are less at higher elevation. The higher ya go the more you can get away with 89 over 91 for instance. i was told this long drawn out explanation why but wont get into it. Thinner air is the main thing but pressure is another. Snow Mobile racers run into this all the time as they climb 8000+ ft and im in the heart of snow mobile country so lots of good builders hear for sleds.