In general, how much compression can safely be run on 93 octane? I plan on a 408 build and trying to decide on pistons. Looking at around 11.0:1. Later down the road, I possibly plan on a 200 shot of nitrous to the equation. Although nitrous doesn't really raise the compression. Engine will be carbed with aluminum heads.
Brian
The design of the camshaft can blead of cylinder pressure altering the *dynamic* compression ratio. The efficiency of the combustion chamber also plays a role in how much compression can be ran with pump gas.
I believe there is more to the answer to the question you posed than just blurting out some number because someone's uncle's nephew ran this much compression in his "motor".
2011 Mustang GT, MT-82, 3.73's :
BONE STOCK 12.223@115.18
ive seen many 10.5 and 10.8 to 1 cars running around i dont see 11 to 1 being a big problem. alot of it is combustion chamber design but i think u will be alright just keep the timing back a little when out cruisin
Points taken. Although neither of the answers I can give unfortunately. I would hate to go out and choose a 9.1:1 and leave alot on the table than if I would have chosen a high 10, low 11 ratio. Although I DON'T want to get stuck with having to use a race/gas mix all the time.Originally Posted by Pure Stock
10.5 is always safe, but i have had 12:1 on pump gas, cam over lap is a big factor, but i have personally seen that on a n2o motor with lower static compression and hefty shot like a 200hp shot, they seem to react better to the spray better than higher comp motors, this is kinda hard to explain, i hope this makes sense... But i would say you would be plenty happy with a mid 10:1 motor and later add some juicey juice... A motor like that will easily propel you into the 9s, of course with the right susp, tranny, etc etc..
2015 A6 Mustang GT that so far has gone 9.48@147 but still the fastest prochargerd S550 in the world aside from Tim Essick's!
www.Rodeheavershotrod.com
That's alot of juice to run with pump on 11. anything. Like previuosly stated combustion chamber design is the most important part of how much compression you can run on any octane. As far as not wanting to have to mix fuel, you can always take timing out, to run straight pump gas. On a pushrod motor it's so quick and easy to do that it seems like your best bet.
Plus is it going to be driven on the street because the higher the compression the less streetable it becomes 10.5 is ideal I think.
Last edited by Balaska347; 02-19-2008 at 10:02 PM.
What exactly do you mean by "streetable"?
Plus is it going to be driven on the street because the higher the compression the less streetable it becomes 10.5 is ideal I think.
Streetable as in driveability on how much you want to drive your car on the street and how far. Yeah a lot of people say their cars are street legal and they can drive them on the street, but how many of those cars can make a trip to through the mountains or a several hour trip to tracks in other states and sit in traffic with no problem.
Factory muscle cars use to run high compression but no longer do because the gas we use today is nothing like it use to be and it continues to get worse. Now if you want to run E85 it is actually around 105 octane. High compression causes more heat and is harder on parts.
It doesn't matter to me what compression is in the car. I was just giving my opinion to maybe help with the decision. I look at it like this 10.5 you know you will be fine and will be able to make tons of hp no problem. With 11:1 you MIGHT be able to make more hp on pump gas and it MIGHT be safe. If you run race gas 11:1 will definitely make more hp. Look at some of the big name companies building high hp crate engines on pump gas I don't think youll find them using 11:1.
This is my unprofessional opinion I build my own engines thats it so take it for what it is worth. My opinion is if you want to only run pump gas why chance it for the little hp you might gain from 11:1. If you use 10.5:1 and you still have room turn the timing up.
I agree
2015 A6 Mustang GT that so far has gone 9.48@147 but still the fastest prochargerd S550 in the world aside from Tim Essick's!
www.Rodeheavershotrod.com
i dunno much about diffrent compression for which circumstance but i guess you just have to look at what your gunna use your car for, strictly track and show, daily, cruising every week or so. i have 10:5:1 i dont drive my car daily but in the summer i take it out alot it uses a decent amount of gas just depends how you drive it. my buddy has a 347 cobra his compression is 12 to something an he uses his everyday even in the winter its his only car. so i guess just look on your routes each day if your going to drive it daily. etc. dunno if that made sense or helped but i hope it did.
Nitrous is forced induction. The compression will raise while on the spray.
Seems like 10-10.5:1 is a nice safe number (considering a 200 shot) without worrying about being on the border of detonation while still utilizing enough timing for max power.
A cam in the 240+ dur. @ .050 would be a good idea. Don't forget a tight quench and a piston that is as high as the deck or a little out of the hole.
FWIW I'm running 10.5:1 on factory iron heads and a 230 dur cam..all 1968 stock stuff.
-JOHN
Carburetors and SAE wrenches.................
Buy Made in the USA - It Matters.
Chevy's LS7 is 11:1 so it's obviously possible to have a very streetable 11:1 motor with the right combination of parts. And that motor had to be designed for 91 octane too.
Last edited by Stangman701; 02-20-2008 at 05:12 PM.
2015 Charger Hellcat
2.4" upper pulley, ID1300 injectors, BAP, E85 tune, Cat Delete Pipes, One Piece Drive Shaft, Diff Brace, 305/35/20 555R's for the street and 305/45/18 MT ET Street R's for the track. 9.97@142 Best ET
85 GT
Heads and Cam plus other bolt ons
73 F250
Lifted, 35's, 460 4spd
Real name = Ray
Well since were throwing some numbers out there, a member of this site has a 302 based stroker that has a static compression ratio of approx. 11.29:1, give or take a couple of hundreths. Ingests a steady diet of Sunoco 93 octane. No audible detonation can be detected. Of course, a top-of-the-line cylinder head is employed that has a super-efficient combustion chamber. This is a naturally aspirated example.
2011 Mustang GT, MT-82, 3.73's :
BONE STOCK 12.223@115.18
Tim, you're in luck cause you're going to get to assemble the heads, cam and valvetrain for the 2nd time when Brian brings this up to you. Funny, both engine builds will be for a guy named Brian. Only diference will be one was a 393 and this one will be a 408.
My old GSXR 1000 had 12.?:1 compression and ran on pump 93.
Rob
1 Monte SS. White.
Banned dates
11/27/08
4/25/11
3/28/12
12/1/12
There are more but that's all that was logged!!!
I always wondered this too. I know these sportbikes have really clean innards. As far as their castings go. My cbr had stupid compression and redlined at something like 13k!
I appreciate all the great feedback!
Yeah, they do rev.I always wondered this too. I know these sportbikes have really clean innards. As far as their castings go. My cbr had stupid compression and redlined at something like 13k!
I appreciate all the great feedback!
Rob
1 Monte SS. White.
Banned dates
11/27/08
4/25/11
3/28/12
12/1/12
There are more but that's all that was logged!!!
I agree with most of what you said, but i think your mistaken on a few things. 11.1 compression is no harder on parts than 10.1 compression. Higher compression becomes harder on parts when it is to high for combustion chamber design, or not tuned correctly. I just think it's allitle foolish to throw out numbers that someone should run, when there is no idea what the motor build will be. I guess if you were talking about a stock ford motor people may be able to say what compression numbers should be. I think otherwise your kicking tires. I guess i may be guilty of that by saying 11's and pump was to high for Nitrous in a driven street car, but i think that was an educated guess
No need to pluck hairs over a half point of compression. Play it safe @ 10.5:1It's not an Engine Master's build off engine and it's not some sort of heads up class racing bullet that need every advantage possible.
Furthermore, it's all about picking matched parts and peripherals. Do the research on 408's. There is one floating around with out-of-the-box T.W's that runs 10.2's N/A on pump gas. Check it out B.
3176 lbs.
408 na, FTI, TW head unported, Spyder, 4 hole TB, AEM, c4, Coan 8", 3.73, BBK 1 3/4, 3" x pipe
10.242 130.13 1.38 60'
2011 Mustang GT, MT-82, 3.73's :
BONE STOCK 12.223@115.18
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