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12.5 : 1 compression question

 
Old 04-09-2002, 10:58 PM
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BDAZVET
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Default 12.5 : 1 compression question

I do not think it wise to drive a carburated, iron headed, 454 with this much compression on pump gas alone. Outside of racing fuel and octane boosters, how would you tame this beast for the street on pump gas? I am thinking aluminium heads and fuel injection would make it pump/street friendly. With this set up I would keep my foot off the gas unless I mixed in some race gas. What think you?
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Old 04-10-2002, 12:08 AM
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Default Re: 12.5 : 1 compression question (BDAZVET)

Man, that's still alot of compression even for alu. heads and efi. I would think you would want to drop that to 11:1 or less to keep it in one piece. Unless you have a HUGE cam to drop cylendar pressure......
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Old 04-10-2002, 12:42 AM
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Default Re: 12.5 : 1 compression question (compuvette)

This engine is not mine yet and I`m still trying to verify components, C/R, etc. I`m pretty sure there is a close quench hight and it does have a Lunati mechanical roller cam;specs unknown for now. Also has big Manley valves and a competition 5 angle valve job. Anyway, with that tight of a quench hight, to lower compression a full point, I would need to change pistons because thicker head gaskets only would lower C/R but detonation would still be a problem. Am I on the correct path? I read a passing comment that there are those (them :D ) that are running over 12 without detonation.
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Old 04-10-2002, 12:45 AM
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Default Re: 12.5 : 1 compression question (BDAZVET)

thats just too much compression, you gotta get it below 12
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Old 04-10-2002, 04:40 AM
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Default Re: 12.5 : 1 compression question (vader86)

Would you say a half point for aluminium heads:12.5:1-.5=12:1 . Then. a half point for fuel injection:12:1-.5=11.5:1. and we still have not touched the "rocket fuel" :D . Depending on the cam duration and cylinder pressure, I gotta believe this might work. :crazy:
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Old 04-10-2002, 09:39 AM
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Default Re: 12.5 : 1 compression question (BDAZVET)

You have a few other options:
1. Install a electronic timing retard box.
2. Run a 50/50 mix of 100 octane low lead Aviation fuel (around $2.50 a gallon) and 92 octane motor fuel
3. Run a 5%-10% mix of Toulene (sp?) which can be purchased at a paint store...Toulene is the main ingrediant in those expensive octane boosters.
4. Convert to running methenol (over 108 octane).

Also I've never heard before that running fuel injection helps to control detonation??? Without doing at least one of the above I think your going to have problems unless you get the compression down to 10.5
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Old 04-10-2002, 11:11 AM
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Default Re: 12.5 : 1 compression question (DeenHylton)

DeenHylton: If I have to,I can run racing gas with premium or toulene with premium. Aviation fuel can be a problem since it is made for low oxygen atmospheres. And too much alchohol can eat up all kinds of stuff. I`m sure this is what you mean by "convert". I think fuel injection has more to do with engine managenent computers , such as knock sensor/timing retard, than with any mechanical advantage. :) But, yeah, ultimately the compression may have to come down.


[Modified by BDAZVET, 8:14 AM 4/10/2002]
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Old 04-10-2002, 04:19 PM
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Default Re: 12.5 : 1 compression question (BDAZVET)

, how would you tame this beast for the street on pump gas? What think you?
Before I sold it of course, I had built to specs an LS-7 in my 70 coupe. Even with timing slightly retarded she'd knock if I got on it a little bit (with pump gas). With octane boost I was able to play but not overdue it. We did race the car once (1/8 mile) with this setup, then I ran 110 octane and believe me - it liked it.

This car was a toy so I didn't mind it, but if your using the car for anything more then a weekend thing or occasional cruise, lower the compression to more user friendly levels.

Just my .02
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Old 04-10-2002, 10:55 PM
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Default Re: 12.5 : 1 compression question (70PurpleLS7)

Thanks guys, for all of your input. This car is a toy, but out here I would have to stay close to "home" or lug around home brew :D for a nice day cruise. I knew from the begining that this is essentially a factory race engine, but I was wondering if anybody had tamed such high ccompression beast without the fuel hassels. I could live with race gas and have some fun, but then that limits some of the intended purpose of the car: all purpose fun! I`m in denial and still in the mind candy phase. :crazy: :confused: ;)
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Old 04-11-2002, 08:45 AM
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Default Re: 12.5 : 1 compression question (BDAZVET)

One last thought...for what your going to have to spend to convert to an aftermarket FI kit plus the cost of swapping to aluminum heads you could easly swap in some flat tops to drop the compression.
I also noticed you live in Tempe, nice area but very hot most of the year...the higher the ambient air temperature the more prone your engine will be to detonation. Good luck with your toy! Deen
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Old 04-11-2002, 11:28 AM
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Default Re: 12.5 : 1 compression question (DeenHylton)

Deen: I`ve already started to look into different pistons if I decide to lower compression. For what fuel injection and aluminiun heads cost, I could buy alot of racing fuel. The goo is starting filter out of my head now :crazy: . I still want to go with F/I and AL/heads, but I could push these down the road. I have not made a hand shake on this deal yet so I still have room to manuver. I set my sights on a Merlin 540 some time ago until this offer came around and I could not pass up the possibilities of saving some cash and using it elsewhere. You guys know how that goes :yesnod: .You are also right about the summer heat, but last summer I could idle the LS4 in traffic and it would not go above 210-215. However, working on it in a mostly closed area and the temps would soar.
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Old 04-11-2002, 11:56 AM
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Default Re: 12.5 : 1 compression question (BDAZVET)

If your planning on swapping out the heads to Al, can't you just get bigger cc heads and not worry about the pistons?
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Old 04-11-2002, 09:24 PM
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Default Re: 12.5 : 1 compression question (Chris A)

Chris A : The heads on the engine now are 118 or 119 cc and are rectangle port, ported, polished, compitition valve job. I`m not sure there are aluminium heads, off the shelf anyway, that would enlarge the combustion area enough to make a difference. If your right, that would be great as long as the quench area remains unaffected, if that`s possible. If I`m lucky, the existing pistons are domed and can be cut down. Still talking 10 or 11 to1. :cool:
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Old 04-12-2002, 01:27 AM
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Default Re: 12.5 : 1 compression question (BDAZVET)

Anything over 10:1 is going to require good gas even 10:1 is marginal in a big block if its a TRUE 10:1 motor blue printed and checked for cc's in head and piston in block, remember published cc figures for heads etc are nominal they are usually bigger. Going to E.F.I. and aluminum heads plus 11:1 is going to cost alot of money and won't make as much H.P as a 12:1 plus motor and might need good gas any way so whats the point?? You have to leave a safety margin on the street , So how do guy with original high compression cars (427/435 etc.) run them on the street ? if assembled without blueprinting it probably has no more than 9:5 to 10:1 true compression. As an example I have been involved NHRA legal stockers for over 20 years and a 1979 305 small block a friend built made over 360hp on the dyno with the following 8:5 compression LESS than .400 inch lift at the valve cast iron intake Q-jet intake etc. you could not run this motor on anything less than vp race fuel otherwise you'd risk self destuction. There isn't a cam big enough to bleed off 12.5:1, 11:1 maybe but the motor would become lazy and not be fun to drive typical of overcammed motors. on pump gas. A good street motor would be 9.5 to10.1 true compression ratio 240 to 245 @.50 hydralic cam a little less for a solid, degreed in properly not just stuck in and a 750 cfm carb.HOWEVER if you want to use race gas the motor you have with the solid roller will run REAL nice a roller really smoothes out the idle but you might have to get a deep rear gear to make it work nice 4:10's at least plus a hi-stall torque converter if you have an auto plus an 850 double pumper and an electric pump a big motor will run out of fuel at the top of the gear with just a mechanical pump.

I hope i covered some stuff for you and that it made sense .
:cheers:
Ken
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Old 04-12-2002, 02:07 AM
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Default Re: 12.5 : 1 compression question (BDAZVET)

I don't know anything about BBC heads, but a quick search shows the ones I've found are in the 119 - 121 cc range. Maybe the chaber sizes in BBC heads doesn't vary like the SBC counterparts. Oh well.
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Old 04-12-2002, 03:25 AM
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Default Re: 12.5 : 1 compression question (ken markusa)

Ken: I will admit that only most of what you wrote made sense as I have much to learn. The engine was balanced and blueprinted but I do not have the information regarding cc in heads or block, so I do not know what the real C/R is. I`m still trying to learn static compression ratio vs. dynamic compression ratio in determining cam and piston choices. Throw in rod length, angle, stroke, and it`s fascinating, but also confusing and complicated. That`s why good engine builders make big dollars. Anyway, the 12.5:1 was the number given. I know that lowering the compression ratio will also rob some power, hopefully only in the higher RPM ranges. I`ve read that to count on a loss of about 4% HP for every half inch drop in C/R. Or was that every point drop? The roller cam that is in it is a Crower 1485 and I guess it`s not too wild. It would be fun to just run this thing on the street, as is, on race gas, but I know that I would not want to do that for very long. Our good gas here is about 91 or 92 octane. I am told that if I do not want to fool with additives, I`ve got to lower the C/R to about 9:1 or 9.5:1; unless I run AL heads and F/I. Then I could between 10:1 to 11:1. As I stated earlier, I`ve "heard" of running over 12:1 on the street with pump gas with AL heads and F/I, but I know nothing first hand. 11:to 1 may be a stretch unless I boost the octane or drive it very carefully. You posted some good stuff to think about, which is what I`m doing. :cheers:


[Modified by BDAZVET, 12:35 AM 4/12/2002]
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Old 12-13-2018, 11:55 PM
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jojo DaCosta
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Default

Originally Posted by vader86 View Post
thats just too much compression, you gotta get it below 12
12? Dude the 70 Chevelle SS had 12.5 to 1, ZL had the same, you know nothing kid.
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Old 12-14-2018, 12:05 AM
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Default 2 things.......

Look into E 85. That might work depending on cylinder pressure effected by camshaft overlap. Second option. Brodix used to make a BB 2 head CNC with 127 cc chambers. That would cut your compression a bunch. Just a thought.
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Old 12-14-2018, 03:41 AM
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16 year old thread gentlemen.
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Old 12-14-2018, 09:25 PM
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Originally Posted by ddawson View Post
16 year old thread gentlemen.
Yes it is...wonder what happened to this situation, though?
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