Compression...

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BoredandStroked

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This should be a poll but I am going to put it on here since I have heard many different answers.What would you call streetable compression?

Wont mention names but I heard of a guy using 110 racing fuel and mixing it with 92 octane just to get it to run right and it was only 9.5 compression. Give your opinions cause this thread isnt for a build its just an opinion thread.
 
9.5 to 1 seems to be the standard.. but compression can vary on alot of things. Quench, and other variables effect it quite alot. When you have a really good quench, you can run higher compression, but if youre just trying to be safe in general, 9.5 to 1 is the standard Ive heard many times. Remember the 1 point discount for aluminum heads. Hope that helps.
Ryan
 
There should be absolutely no reason to ever need 110 octane fuel for a 9.5 to 1 compression if the timing is set right. Racing fuel burns slower and with the lower compression won't benefit you anything. I've read reports of it actually losing HP in a low compression engine.

Streetable compression has a lot of variables. The cam has alot to do with how an engine acts. A larger cam will bleed off some cylinder pressure and allow a higher compression with the same fuel. This is where the difference in dynamic and static compression shows up. Dynamic compression is what is really important.

Head material also comes into play. Aluminum heads disapate heat faster so you can run more compression with them. From everything I've read about .75-1 point higher for aluminum over iron heads.

All in all with iron heads 10-1 is a good figure to shoot for with todays junk gas with iron heads and a fair size performance cam but I've seen many guys go over that with a proper setup and still run pump gas.
 
Yup, 10 to 1 on 92 octane with iron heads, timing set at 36 and she rattles at 38-39. But remember in Canada our 92 octane is equal to 94 in the states. It's the metric over imperial difference. (that'll get em thinking).

Terry
 
We can still get 94 octane at the pump here with Sunoco Ultra. Not a lot of stations still carry it, but some still do. My duster is around 9.35:1 and I don't have a problem with the pump 93-94 gas. I haven't seen signs of detonation with a 150 shot either so far.

don
 
Well I am running 10.6:1 on a 360 with magnum heads and a comp XE268H cam, my cranking pressure is 175-185 psi.

Because of the excellent quench provided by the zero deck pistons, closed chmaber magnum heads and .039 compressed head gasket this engine runs with no audible detonation on 89 octane.

I am also running 15 degrees intial timing and 34 total (again because of the excellent chamber design less total timing is required) and it's all in at 2500 rpm.
 
My opinion..Static ratio is not what you really should go by. It's what most guys use, but it's misleading. Static doesnt take into account the rod legth or valve timing. Both of which directly effect the way the mixture is compressed in each compression stroke. To generalize, 9.5:1 static is the accepted safe limit for pump fuels and iron heads. Anything over that and you may have detonation/preignition issues on available pump fuels. For around me, I can get 93 or 94, but from november to February they add extra oxigenation, and all year we run 10% methanol. The end result is, the fuel burns hotter and faster than it ever has. That can become an issue in 40 year old engine designs. That being said, I have several street cars running closed iron chamber heads with mid sized hydraulic cams (cams that pull 7-8" of vaccum at idle in gear) that run 10.8:1 static ratio. They run on the 93 or 94 pump unleaded. Why? Well, because the cam's lobe timing is such that the cylinder pressure is not too high at low (under 1500-2000) rpms and because the heads' chambers and ports are designed to keep good atomization and get decent swirl into the cylinder and good tumble as the piston comes up. That all adds up to a mix that is very well suited to burn fast, and is stable in the compressing stage so it wont ignite early or burn uncontrolled. It also means you can run less igition lead. So total timing in those engines is from 28-32° as opposed to 32-38°. It also takes setting the curve so it doesnt come in too fast. The Pump gas dyno monsters you read about in the engine masters use all kinds of trcks, and they end up with static ratios in the 12.5 to 13.5:1 range and run "pump" 93. OK, they arent good for anything other than a few hundred dyno pulls, and that pump gas surely isnt what comes from the ground..but you get the idea. Very generally, you can run up to about 185psi on unleaded pump fuels with open chamber iron heads without running into detonation threshold. Add a closed chamber head and/or a small dish in the piston, and that threshold rises. Add aluminum heads, that rises. Polish the iron to eliminate carbon deposits and hot spots, that rises. Bring the flat of the piston to the flat of the head (quench) that rises. Run a cam that closes the intake valve later, that pressure number will fall, but the threshold of detonation rises...Coat the piston tops, valve heads and chambers, that threshold rises. How much more you can run depends on how well it's all executed. I've done 10.8:1 big blocks with iorn closed chamber heads and flat tops, and 11.2:1 small blocks with dishes and modern aluminum heads that run well on pump fuel. I've also had 9:1 big blocks with large open chambers and no mechanical issues that I couldnt get to stop pinging no matter what the tune up was..lol. The best I've had for open chambers was a true 10.4:1 383 with milled 906s that pumps 185psi and runs like a top on pump fuel. A larger bore 440 or 400 may not run that well on pump gas. The KB-Silvolite.com site has a good dynamic calculator as part of a static calculator. Just make sure you have your cam card handy, and that all your info is inputted the way the thing says to. If you play around with cam timing, rod length, and chamber size, you can see how they relate to the cranking psi/dynamic compression ratio.

PS..never ask opinion...I'm too long winded :)
 
Good morning;

At the risk of repeating some and ...well, it's to early for me to read Mopers reply. Though I'll tell anyone else that hasn't, go back.

At that ratio, and octane, I agree, theres nearly no reason to ever use anything but pump gas. His balancer must be off and/or the wrong cam is in there.

Look at DGC's combo. High comprsion ratio and pump gas. Great squeeze and head design.

Streetable compresion is something everyone can agree on @ no more than what the local pump offers. Race gas exempt. Or above whats easy to get. In New York it's 93.
And thats key for me. Your max octane is key for you.

Thats why in general, I answer the question with not more than 9.5-1.
For anybody thinking thats not enuff of a ratio to get a engine up in some serious power numbers, you'll need to do some homework. Heres your hint to set you in the right direction to start your research.
Crafstman truck series. Engines are limited to 9.0-1.

Poor chamber design or just simply open chamber design and pistons that are very low in the hole will cause problems.
 
BoredandStroked said:
This should be a poll but I am going to put it on here since I have heard many different answers.What would you call streetable compression?

Wont mention names but I heard of a guy using 110 racing fuel and mixing it with 92 octane just to get it to run right and it was only 9.5 compression. Give your opinions cause this thread isnt for a build its just an opinion thread.

My 273 is going to end up with 9.8 CR. Since the car will be street/strip I am hoping that since I had the piston tops thermal coated it may help keep detonation away when using pump gas. Of course I'll be running race fuel at the track.
 
PS..never ask opinion...I'm too long winded

I dont blame you either Moper, but I just had a itch that I wanted to scratch. I saw this guy mixing the fuel with a 9.5 compression engine and I thought, has he been smelling that gas a little to much.

From calling various parts supplier from heads, cams, intake, carb, etc I pretty much know now that I need to have the cranking pressure, exsisting compression ratio, or wanted compression ratio. So I see why you wrote so much Moper and I should have sent you a PM and told you to take it a little easy, haha. Thank you for your info though and its always greatly appericated.
 
The problem is, one thing leads to another in any build. So a brief "run 9.5:1" leaves a lot out IMO. You also want to be careful talking to most tech lines. The "response notebook" only says so much. There are other builders here and on other web sites that have all kinds of good ideas. One of my favorite haunts is Speedtalk.com . They get very heady, I'm more of a watcher there, but the tech archives in the engine section is second to none, and guys like Darren Morgan and Larry Widmer are members there. They will answer even teh most mundane question with respect. So use that resource too.
 
I had a 440 that was built using low compression TRW pistons that were rated for 8.5:1. They sat around .090" in the hole. The heads were milled a bunch and I used a thing gasket to get compression up. I would say it was under 10:1 still when I was done. The cam was a good size, 245 @ .050". It didnt like pump gas. I had to mix to keep it from detonation.
 
Moper's right on the money. One thing that hasn't been mentioned yet was the effect of altitude on compression. Here in Denver, you can get away with an additional point of compression (10.5:1) due to the lack of oxygen, keeping in mind the variables that moper has previously mentioned. The problem is, if I plan to run my vehicles @ less than 4,000, I need to back off the timing, hence the MSD in-cab timing control, which also gives me the ability to bump up the timing when mountain cruising (6,000 to 10,000+ ft.).

Oh yeah, those pump gas guys in Engine Masters are running "Rocket" brand fuel, far better stuff than the swill we get at the corner pump.
 
I called Indy Cylinder Head about my stroker kit regarding compression. Jim Hill said I might have to use double head gaskets as my compression with the Edelbrock magnum heads may end up with 11.0:1 comp. ratio. Does this sound right? It's a 360 Magnum block and I ordered their 408 stroker kit with dished pistons. He said the pistons are pretty close (I assume) to zero deck? Am I missing something here? Anybody had something similar to this combo?
 
Doubling head gaskets? wow. That's one I've only heard of in desperation or lack of understanding conversations. Indy uses thier own Wiseco pistons IIRC, and I dont know the size of the dish. But, the Magnum heads are about 7ccs smaller than the typical LA heads. SO your compression will be up in the high tens with 63cc LAs and over 11 with 58cc Magnums. You can run an MLS (Cometic) type gasket, but the heads and block's deck will need ot be milled to the proper finish. You can also enlarge the Magnum chambers if yo uhad to. Depending on the cam choice, 11:1 may be fine for high test unleaded. Depends on how good the package is put together. I've seen Indy's work. If you are buying it thru them and they built it, it's priced about 20% higher than it should be assuming they do everything right. And that's a big assumption with them.
 
I agree, Indy has alot of knowledge but its mainly for a racer, few items can be used for a budget guy thats building a high power small or big block. I do admit though that if you got the money and dont mind laying down it for the power, after a lil work done to them everything from Indy can make a killer combo. Now streetable, yes, daily street driving, hell no unless you are a millionaire lol.
 
Stacking head gaskets is nothing new, it's been done for many years in diesel engines. To get the correct amount of clearance in some Kubota diesels you stack head gaskets. They make quite of few different thicknesses to get the clearance just right. I thick Cometic is one of the companies that make stackable head gaskets.


Chuck
 
340mopar said:
Stacking head gaskets is nothing new, it's been done for many years in diesel engines. To get the correct amount of clearance in some Kubota diesels you stack head gaskets. They make quite of few different thicknesses to get the clearance just right. I thick Cometic is one of the companies that make stackable head gaskets.


Chuck
So you guys have had some bad luck with Indy?
 
I would expect that stackable gaskets are designed with that possibility. MLS (Multi Layer Steel) gasket designs are not stacked. They are fastened together when the right thickness has been reached. You cant put two on top of one another and expect them to seal. They wont. I have heard of some guys experimenting, but that's short term. If you're building an engine, you design it and build it right. Stacking gaskets is the equivolent of tying a car door shut with mechanic's wire and calling it a permanent repair. Sure, it will work..but for how long, and under what circumstances.

69- I've seen thier quality in terms of parts and labor first hand. They are simply not worth what they charge.
 
Crap. I guess I will have to wait and find out during my build then. Guess I won't be using them for my stroker Hemi in the Charger then either. Jeez, now I'm nervous about putting this thing together.
 
Most of the issues I've seen are labor related. The pricing of "special" Indy stuff is crazy high. And the execution is average if that, and you pay for premium. I've also seen a lot of engines they built have trouble making the numbers Indy claims from there benches an dynos for various packages. It's buyer beware. I'd find a trusted local shop to use thier parts.
 
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