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Discussion Starter · #1 ·
Alright Guys, I have a topic to get ur brains workin. Or maybe I just need come clarification. The topic on my mind is boost pressure VS comp. ratio. I have a few examples to help.

Say you have a SBC 350, 9:1, street driveable. static cylinder pressure should be arond 160 or 170. I dont know what the calcualtion would be for running pressures. If anybody has a way to calculate that, let me know. Lets say that you put 15 psi, 1 bar, into the same engine, which should put twice the air in the cylinder (i know it sounds hard to believe that it will make 15psi during cranking, just hear me out). the technically shouldnt the pressure inside the cylinder be around 320-340?

Ok so next we have a SBC 350 with 15:1 compression ratio. Race gas only!! so cranking pressure would (i assuse since ive never been able to test it myself) be maybe 300 psi?

So assuming that both a 15:1 engine, as well as an engine running around 12 psi of boost should have the same cranking pressure. So now we bring in the running condition. If we cant run pump gas on a 15:1 engine, why can we run pump gas in a 9:1 engine running 15 pounds of boost.

Like IF a 7:1 engine would run well, and a 14:1 engine were put to the test, 15 psi would double the compression ratio on the 7:1 engine, basically bringing it up to 14:1 WHEN RUNNING. So what i say, WHERE ARE THE DIFFERENCES?? At WOT, how much different are they, i dont care about manifolds or anything, im just talking about inside the combustion chamber.

I know cranking pressure is a LOT different then actual running pressures, but seeing guys trying to squeeze 800 HP out of a 15:1 small block NA having to run race gas, compared to guys runnin 9:1 with 15 pounds of boost making that if not more on pump gas, where is the discrepancy.

The reason im wondering this is, I was going to build a TT motor for my Camaro, but i was wondering if a high compression motor, with Rhodes variable roller lifters (or flat tappet), with a well tuned EFI system, would be a contender.

PLEASE!!! any thoughts or theories or knowledge would be awesome!!
 

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You're running into a volume vs pressure difference. I think to get the same cyl. pressure, the compression ratio would be up over 22:1 or something rediculous like that.
 

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Check out wallace racings web site its pontiac based but has a compression to boost calculator over there.
So an 8 to 1 motor at 15 psi = this compression ratio
type pf a thing.
Its a very useful site as far as clculators go.
 

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Discussion Starter · #6 ·
thanks for the reply guys. i will check out that calculator when i get a few more free minutes. i was thinking more about this when i was at work today. another example would be, a 15:1 motor, vs a 9:1 motor, but at ?? boost, took in the SAME amount of air/fuel, which im sure would be possible to test (although in sure difficult). with the same ignition timing, intake air temp, ect.... wouldnt it be almost identical? any ideas?
 

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Discussion Starter · #7 ·
well i went to wallace racing and their site must be under maintenence. so i had a few more questions for some of you. for Rodrigues, if the volume of the air/fuel is the same for both engines at BDC, when that volume is compressed into the same area, wouldnt pressure be the same? and for gasguzzler, if ur saying that a 22:1 engine and a 9:1 engine at 15 psi are around the same cylinder pressure, why cant we run pump gas int the 22:1 engine, becasue we can in the 9:1 engine?
 

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The two hypotheticals you are comparing have significant differences. The foremost being the mass of fuel each is consuming. Even though they may have similar cranking cylinder pressures the boosted motor will have a drastically higher fuel content, which in itself changes everything such as evaporative cooling, knock sensitivity, etc.
 

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Discussion Starter · #11 ·
alrighty. so wallace racing is back up and running. And I messed around with the calculators for a while. Seems like a 9:1 engine on 15psi of boost is around 18:1 static ratio. so lets think of this another way. lets cut out the fuel for now. lets say we are running on air (wouldnt that save some money at the pump!). Ok so lets say we have a 400 SBC, just to make it easy. lets say that on a good day, one cylinder get a good fill of 50 cubic inches of air. now ur gonna squeeze that down to lets say 85cc total chamber, including deck and gasket, with a 10cc dish. gives us roughly 9:1. So now we take that same amount of air, with a 55cc head, with a 5cc dome, gives us 15:1. so by my understanding, pressure has risen, and volume has remained the same (as you guys have stated).

ok so now, lets say we take 1bar, and force it into the 9:1 engine, which (i think!) would double air density (correct me if im wrong guys) so we would have 100cc of air, and would make our compression ratio 18:1.

Now we add the fuel into the mix. so lets say it takes X amount of fuel to run the 9:1 engine at 100% VE. and it takes 1.7X to run the 15:1 at 100%VE. Now with the boosted engine, running at 15 psi, which has a static ratio of 18:1, needs 2X. It seems that might run a bit lean but I have no way of checking. Im assuming that double the volume of air needs double the volume of fuel (which makes sense to me).

So if we CAN run a 9:1 motor on 15 psi on 91 octane, with 18:1 static ratio, over 100% VE under boost, why cant we run 15:1, at maybe 80% VE, with much less cylinder pressure, on 91 octane. I know that AFR has a lot to do with detonation prevention, But in my mind this is all it comes down to: cant we just put more fuel into the 15:1 engine? the piston doesnt know where the air comes from above it, if its pushed in by a turbo, or by the atmosphere. just give it enough fuel to get a good AFR (12:1 under moderate load maybe?) and things schould be alright?

Jeeebus im rambling again. Thanks for hearing me out everybody. This is just a topic that ive been dieing to get some answers on.
 

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The answer has something to do with compression, and perhaps heat?

My Formula is 7.9/8.0:1 and I can run 16 psi on pump gas, which might be like 650-700rwhp. But if my motor was 15:1 I'd have to run 110/112 and I'd make nowhere near the same power level.
 

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It's becasue of the compressed volume of the charge. The turbo's chamber is still large enough to have a controlled flamefront and such with pumpgas...just keep ignition advance in check.

The high-comp motor is so tightly packed...even pulling timing back doesn't stop the pre-ignition. Simple?
 

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Discussion Starter · #14 ·
Matt, thanks for the answer. That does make sense. I still have my theories that I want to get some answers to, but Ill have to figure out how to explain them first! Thanks Guys!
 

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wouldn't the pressure spike from ingition travel faster in the high comp piston having a greater greater effect on ignition timing,fuel octane needed and the larger boosted chamber have a slower pressure front?
 

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this might help

I had a similar question a couple of years back. I got more than I bargained for in response. In particular, note the responses of "Ford-Swap." His name is Rob Lupell and he is an internal combustion engineer, and I can assure you, even though he is younger than me (I'm 45) he's forgotten more than most of us will ever know about engines and what makes them work.


http://www.theturboforums.com/smf/index.php?topic=73688.0


Scott Russell
 

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I had a similar question a couple of years back. I got more than I bargained for in response. In particular, note the responses of "Ford-Swap." His name is Rob Lupell and he is an internal combustion engineer, and I can assure you, even though he is younger than me (I'm 45) he's forgotten more than most of us will ever know about engines and what makes them work.


http://www.theturboforums.com/smf/index.php?topic=73688.0


Scott Russell

Cool post lol I find it funny I know rob personally and used to talk to him quite a bit before he moved down to cali.
 

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Cool post lol I find it funny I know rob personally and used to talk to him quite a bit before he moved down to cali.
He's no dummie. He and Don Bailey were both really great contributors on the turbo forums. Considering Don gets paid for what he knows (as a tuner) he was very helpful and has some really interesting posts over there. Rob can get so deep he can give most people a mental melt down lol. Smart guy to say the least.
 

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He's no dummie. He and Don Bailey were both really great contributors on the turbo forums. Considering Don gets paid for what he knows (as a tuner) he was very helpful and has some really interesting posts over there. Rob can get so deep he can give most people a mental melt down lol. Smart guy to say the least.
He is a very smart guy indeed ... I know when his turn comes to put together a project it will be 110% bad ass
 

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Check out wallace racings web site its pontiac based but has a compression to boost calculator over there.
So an 8 to 1 motor at 15 psi = this compression ratio
type pf a thing.
Its a very useful site as far as clculators go.

8:1 cr 28psi boost and pump gas = The Boosted Compression Ratio from your Boost of 28 psi is 23.24 : 1 :-Daw

the engine is a turbo charded iron headded pushrod.

I dont tink that kinde of calkulators work, thank god for that:)
 
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