Took a look at my 1970 Owners manual. They don't mention octane rating in the fuel page, just an engine list and recommended fuel grade of regular and premium. Regular for the 250-6 cyl, 350-2, and 350-4. Premium for the 350-4 Hi Comp, and 455. As a 16 year old kid in 1972, I remember my job pumping gas in a Shell station. The premium pumps had a sticker saying 96 octane, regular was 91. These were Research octane numbers.
I've heard you talk many times to others about dynamic compression ratio........just curious what you feel the octane should be for a motor with dynamic just under 9
I can tell you from what I have read in 3 different books. Anything over 8.5 dynamic they suggested racing fuel. We worked hard to keep my current combination at a 8.25 and I still keep a mixture of 110 and non-ethanol with avgas for the street. At the track is 100% 110. That dynamic yield me an 11.2.1 static measure with a Whistler meter. Fernando
That might not be a simple question to answer. It's pretty much a package deal with lots of considerations. I found this on Yellowbullet.com, https://www.yellowbullet.com/thread...-static-compression-for-octane-choice.472656/ They talk about volumetric efficiency like it affects Dynamic Compression, but it doesn't. Dynamic compression is calculated from the Dynamic Stroke Length, the stroke left after the intake valve closes. Once an engine is built, static and dynamic compression doesn't change. Only an engine with variable valve timing would have changing DCR. From this article, http://cochise.uia.net/pkelley2/DynamicCR.html "Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine." 8.4:1 DCR is what the article recommends for pump gas, but he mentions staying below 8.25:1 is probably best for "trouble free" motoring. I think Jim Weise would be able to provide a better answer than I. I know this much, he used straight 110 leaded fuel when my 10.54:1 engine was on the Dyno. Of course the Dyno loads an engine more than anything you can do to it on the street. When my engine was delivered, Jim told me that it would likely be fine on 93 pump gas, BUT, if I took it to the track, he recommended I mix in some Race fuel. That is exactly what I have done for 9 years now. That being said, street driving is different then holding the engine at WOT under high load through a 1/4 mile. Do you drive this car on the street? If it was me, I would not trust the aviation fuel for the track, but it might be OK for the street depending on how you drive it. At the track, I would want at least what I use, 50/50 93/110.
No your correct... but Larry's motor is also 10.5-1 SCR , so a 50/50 mix of leaded race fuel with his premo, gives him a good safety margin. I put no stock in DCR calculations when it comes to detonation control.. for the exact reasons they lay out in Larry's post above. Too many variables that are out of the equation. JW
The only street time my car has seen in the last 20 years was just either small test, for driving after an alinement, or to help heat and break in a new set of gear or rings etc. This has been a good and informative talk for sure
Yes, it's comparable, it just doesn't have the lead. Oil companies use other anti knock compounds. The research octane of today's 93 is up around 98.
It also has alcohol so it runs leaner. Compression isn't the only factor. How hot motor is running makes a big difference. I leave the line at 155-160 degrees. 185-190 would require more octane. And there is timing. You could hurt an 8.5 motor on 93 octane with 36-38 degrees timing. Whereas a 10-1 70 motor with 28-30 deg be OK. And then there is carbon buildup. I had a 70 motor that would rattle like crazy on 93 with 30 deg timing. Did a Seafoam treatment (2 cans, neighbor was going to call fire dept)) and it was a happy camper.
Jim, Here is my Cam card from Bullet. I am guessing I dont have the figures for Intake valve closing ABDC to calculate Dynamic Compression. Just curious. Here are the specs 4.5" bore 4.36" stroke 7.1" rod length 13.5:1 Static I run this engine on our highest octane here in Australia which wouldn't be as good as your 93. I have to run an octane booster to slow the burn down, as it runs on when hot. Otherwise, no problems at all. When I race it, I use VP MS-109 as all leaded based race fuels have been outlawed here
Odd it shows that at .020....not closer to off the seat. So give the at .020 your 9.6 it will be slightly lower....bets its close to 9:1 mine is at. Isn't VP MS-109 only about 104/105 octane
Ahhh my mistake, just been gliding through the thread...Bullet's advertised duration is indeed right at what the cam card presented states. They don't give a lift spec on their website for their advertised duration.........I use AvGas like Ben, myself. Never an issue over 30 years despite being told there would be.
At the track? What compression? I know lots of guys who use AV gas. On the street, it might be OK depending on the application, but for sustained WOT, maybe not.
Older brother and pops both used to run it on BBC's in '66 and '67 Chevelles at 12.2:1 and 12.5:1. Mach 1 is 13.5:1 now, the old nitrous engine was 11.7:1 and it ran for years on it. We used to just get our fuel from the tank on the farm for gramp's spray plane. Now that I'm in the city I get it at the airport....We didn't have access to race gas "back in the day" of the 80s and 90s.
Yeah, my point is there is a big difference between occasional WOT throttle street fun, and sustained WOT track runs with traction.
