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Old 03-07-2011, 05:14 PM   #1 (permalink)
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Default Max comp for small turbo on pump?

What is about the maximum compression ratio you can run on pump gas, on a small turbo (max 250whp), safely?
I've seen several 10:1 and a few 10.5:1 builds, but as people approach 11:1, they all seem to switch to E85/race fuel.

Chamber would be comprised of Vitara pistons (~15cc) and a milled Y8 Head (~25cc).

What is the limit and what issues do you run into?
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Old 03-08-2011, 04:11 AM   #2 (permalink)
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It's different for every combination. Most people who switch to E85 do it for the wrong reasons and end up successful because of factors they really don't take into consideration.

There is no exact answer. Every engine will have a maximum cylinder pressure it will be able to deal with before all the factors that prevent detonation come up short and you start the pinging.

I've seen a few builds on "normal" pump gas that have gone over 400WHP with over 10:1 compression, but that was not on a D. I think that it is even more likely that it can happen on a D because you don't need as much timing to get complete burn because of the smaller cylinder bore.

Milling a Y8 head that much is another story. How to you plan to take off more than 7cc from the head without running into the valve seats? (I honestly have no idea how far the seats are from the deck, but, it seems unlikely that taking that much off the Y8 will be easy.)
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Old 03-08-2011, 05:17 AM   #3 (permalink)
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It's different for every combination. Most people who switch to E85 do it for the wrong reasons and end up successful because of factors they really don't take into consideration.

There is no exact answer. Every engine will have a maximum cylinder pressure it will be able to deal with before all the factors that prevent detonation come up short and you start the pinging.

I've seen a few builds on "normal" pump gas that have gone over 400WHP with over 10:1 compression, but that was not on a D. I think that it is even more likely that it can happen on a D because you don't need as much timing to get complete burn because of the smaller cylinder bore.

Milling a Y8 head that much is another story. How to you plan to take off more than 7cc from the head without running into the valve seats? (I honestly have no idea how far the seats are from the deck, but, it seems unlikely that taking that much off the Y8 will be easy.)
91civichillclimber managed it, so I figured it wasn't too hard.
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Old 03-08-2011, 09:09 AM   #4 (permalink)
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He's been on E85. He's not on pump, nor do I think he's ever been on pump gas with his setups. So, it's different. It's absolutely possibly to do "high" compression on pump gas, though, but, there is a lot to consider before doing it.
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Old 03-08-2011, 09:21 AM   #5 (permalink)
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there are reasons they switch to E85 when they go HIGH COMP.
A few include:
Its cheaper than pump
you can go high CR with E85 on boost
You can use more timing in tuning
you WILL make more power
Burns cooler than pump

You will use a little more fuel, approx 30%, but Fuel spending vs. consumption balance out well with the decreased cost.
Depending on if you go boost, or Just NA, fuel consumption will vary somewhat

But high comp, and turbo=E85 if its available to you. If you dont.....well, appearantly logic and reason dont come easy then lol
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Old 03-08-2011, 09:49 AM   #6 (permalink)
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there are reasons they switch to E85 when they go HIGH COMP.
A few include:
Its cheaper than pump
Not really, unless it is 30% cheaper, which many times it isn't.

you can go high CR with E85 on boost
Troof

You can use more timing in tuning
Burn rate is slower, so you have to in order to hit MBT. The additional advance isn't really an "advantage".

you WILL make more power
You have the potential to, but gasoline is actually more energy dense.

Burns cooler than pump
Troof

You will use a little more fuel, approx 30%, but Fuel spending vs. consumption balance out well with the decreased cost.
See first response.

Depending on if you go boost, or Just NA, fuel consumption will vary somewhat

But high comp, and turbo=E85 if its available to you. If you dont.....well, appearantly logic and reason dont come easy then lol
Not trying to be a dick. At all. Just lots of mis-info and misunderstanding regarding e85.
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Old 03-08-2011, 09:58 AM   #7 (permalink)
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Not trying to be a dick. At all. Just lots of mis-info and misunderstanding regarding e85.
Its all good man

E85 is over a dollar a gallon cheaper here

premium is 3.85
times that by .3=1.15

E85 is 2.64 gal

3.85-1.15= 2.70 at least here the difference compensates cost wise

You will make more power than on gasoline, plus it will help out your local farmers

and, I also said the consumption vs. cost balance out well. Didnt say he would be SPending $$$$$$$$$ less on E85.

I wouldnt call that Mis-info
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Old 03-08-2011, 10:09 AM   #8 (permalink)
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Yeah, I didn't think about the fact that you are in a corn state. I'm in Houston, so gas is almost always below the national average. Premium is $3.35/gal and e85 is $2.63 for only 21.5% variance. I guess it works out in your area. Either way, I'll be going e85 when I push above 300hp. It's like a wooby for your ride.
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Old 03-08-2011, 10:12 AM   #9 (permalink)
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Yeah, I didn't think about the fact that you are in a corn state. I'm in Houston, so gas is almost always below the national average. Premium is $3.35/gal and e85 is $2.63 for only 21.5% variance. I guess it works out in your area. Either way, I'll be going e85 when I push above 300hp. It's like a wooby for your ride.
Yup, Im runnin E85 and lovin it so far.
More and more people are switching to it, especially around here lol
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Old 03-08-2011, 10:18 AM   #10 (permalink)
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It depends on the cams and how much power youre trying to make.
There is no fundamental reason why you couldt boost on top of 11:1, but there wont be a ton of safety margin in tuning, especially in low cam.
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Old 03-08-2011, 10:34 AM   #11 (permalink)
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my SRP A6 pistons with a Z6 head and .027 HG put me at about 9.7:1 CR. not far off from 10:1.

i made 450whp on pump gas, 94oct (R+M)/2 chevron.

it blew up right after that. LOL.


i think u can pull off 250whp easily if u can keep your IAT's down with a small compressor. 11:1 shouldnt be a problem. id do it...
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Old 03-08-2011, 11:03 AM   #12 (permalink)
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but there wont be a ton of safety margin in tuning, especially in low cam.
I understand that the CR reduces the safety margin, but why is it more so on low cam? I would think lower flow rates mean lower real cylinder pressures, with timing and afr being as appropriate as on high cam. Not arguing, just trying to learn why my logic is wrong.
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Old 03-08-2011, 11:25 AM   #13 (permalink)
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because it can't bleed off the static comp as good as a high cam. At least thats my understanding. someone can be more specific.
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Old 03-08-2011, 11:36 AM   #14 (permalink)
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Through valve overlap?
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Old 03-08-2011, 12:43 PM   #15 (permalink)
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Through valve overlap?
bigger cams especially turbo cams can have alot of overlap to bleed off the compression which is important with increased cylinder pressures
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Old 03-08-2011, 12:51 PM   #16 (permalink)
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bigger cams especially turbo cams can have alot of overlap to bleed off the compression which is important with increased cylinder pressures
That doesn't really make a whole lot of sense to me. Overlap is designed into cams to clear the cylinder of exhaust gases and to help cool the combustion chamber. Are you saying this is what allows for higher dynamic CR before det on a high cam vs. low?
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Old 03-08-2011, 12:56 PM   #17 (permalink)
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I need to let someone who knows more about cams chime in at this point cuz I could be wrong. If I am then my bad. someone on here will have the proper info. but you are right about the overlap (sorry for the n00b moment)
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Old 03-08-2011, 01:11 PM   #18 (permalink)
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But high comp, and turbo=E85 if its available to you. If you dont.....well, appearantly logic and reason dont come easy then lol
I fully understand why people switch to E85.
What I'm wondering what is the limit for compression in a small turbo, relatively low (peak) power engine.
And more importantly, why it is that limit presents itself.

Like, ~400whp seems to be the rough limit on pump gas for D16's, but quite a bit higher has been managed. Is cylinder pressure the reason people don't go higher? If so, at what compression would you run into a similar cylinder pressure as a high HP (and lower comp) build?

If it is the case that high cylinder pressures cause the limit for power on pump gas, what is it that?


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bigger cams especially turbo cams can have alot of overlap to bleed off the compression which is important with increased cylinder pressures
*noob*

I though it was the otherway around. NA engines run overlap to take advantage of exhaust scavenging (and other things) and FI engines use high lift, wide duration, but low overlap cams?
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Old 03-08-2011, 01:25 PM   #19 (permalink)
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I fully understand why people switch to E85.
What I'm wondering what is the limit for compression in a small turbo, relatively low (peak) power engine.
And more importantly, why it is that limit presents itself.

Like, ~400whp seems to be the rough limit on pump gas for D16's, but quite a bit higher has been managed. Is cylinder pressure the reason people don't go higher? If so, at what compression would you run into a similar cylinder pressure as a high HP (and lower comp) build?

If it is the case that high cylinder pressures cause the limit for power on pump gas, what is it that?



You're on the right track. Pump gasoline is rated by Octane. Octane is basically the fuels resistance to detonation/burning (in lamens terms)
THe higher your compression goes, compounded with boost, the farther you are pushing the limit of the gasoline, and your ability to control it ,before chemically and physically the fuel cant be kept from blowing up/burning.THat in turn can lead to problems with *engine* detonation and that fun basket of problems. Higher comp also raises the Heat inside the combustion chamber, that needs to be mitigated via more fuel, methanol, nitrous etc...to cool it to avoid detonation as well.
E85 is Ethanol based, and burns cooler. Its similar to running a Higher octane fuel, allowing more compression before it detonates vs. pump fuel. Like I said before E85 also burns cooler, so it helps keep cyl temps down somewhat to aid in avoiding detonation, and giving more of a safety margin.


Does this help? I tried keeping it pretty simple, Hope it answered your questions!


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Old 03-08-2011, 02:06 PM   #20 (permalink)
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Turbo cams usually dont have a lot of overlap due to the poor exhaust scavenging you get due to the back pressure of the turbo.
NA cams thrive on overlap as the scavenging is one of the most fundamental ways of improving breathing at high RPM.

With a cam with more overlap you can get away with more static compression as the dynamic compression is decreased by the overlap. Yet, to much overlap may also allow boost to be lost through the exhaust or even exhaust reversion, which will hurt power.
My comment about "low cam" was basically hinting at the fact that in low cam overlap is less and dynamic compression ratio is higher. If youre going to have detonation issues, this is where its going to happen (unless youre not hitting peak boost until you get into high cam, but thats not typical)

You have to find the right balance between cam, turbo, and compression.

The tradition way is to keep compression low, overlap/duration relatively low, lift high, and throw as much boost at it as you can (Kinda sounds like a Vitara built D16, doesnt it?)

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