How does lower CR help with boost?
How does lower CR help with boost? Higher compression = more horse on NA engines.
I would really like someone to explain how lower compression makes more power on a boosted application.
The reason im not understanding it is, when i think of lowering compression, i think of loosing power..then when you add the extra boost, it just brings you back up to where you were to begin with.
Some help please.....
I would really like someone to explain how lower compression makes more power on a boosted application.
The reason im not understanding it is, when i think of lowering compression, i think of loosing power..then when you add the extra boost, it just brings you back up to where you were to begin with.
Some help please.....
TECH Enthusiast
Joined: May 2004
Posts: 567
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From: Colorado
For having all those mods you have in your sig. I would think you would know..
Simply put..
Lowering the compression on your motor... allows you to run more Boost without the risk of detonation....
Simply put..
Lowering the compression on your motor... allows you to run more Boost without the risk of detonation....
Originally Posted by Sierra5.3
For having all those mods you have in your sig. I would think you would know..
Simply put..
Lowering the compression on your motor... allows you to run more Boost without the risk of detonation....
Simply put..
Lowering the compression on your motor... allows you to run more Boost without the risk of detonation....
Yes, airflow is the key. By droping the CR down say, one or two full points, you may loose a small percentage of power, maybe around 20hp. But, by droping down, you are now able to make more boost without detonation. The higher boost will net you way more power then the higher compression will. I was running 7 pounds max on mine until I swaped for LQ9 heads and thicker gaskets which put me around 9.6:1, I may have lost 10hp but I'm now running 10-12psi on high boost which is a good 50hp or more gain. If I had it dyno'd I could tell you exact numbers but hopefully this will clue you in better.
I believe top-fuelers run well over 12:1 compressions (I've heard up to 15:1) ratios and 55+ lbs of boost. But I think most people would rather drop the compression, have a cooler running less powerful engine off boost that will be a monster when they put their foot into it. Although higher compression ratios have a big effect on the efficiency of a motor (higher = more efficient) so who knows. It's a highly debated topic, that's for sure.
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[QUOTE=hissin'98] The higher boost will net you way more power then the higher compression will.
That is what I was unsure of. I wasnt sure if it would equal out somewhere or not. Say you put the 6.0L heads on a stock ls1 block, how much boost are you saying you could run? Whats the CR usually fall to?
That is what I was unsure of. I wasnt sure if it would equal out somewhere or not. Say you put the 6.0L heads on a stock ls1 block, how much boost are you saying you could run? Whats the CR usually fall to?
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Originally Posted by hissin'98
Yes, airflow is the key. By droping the CR down say, one or two full points, you may loose a small percentage of power, maybe around 20hp. But, by droping down, you are now able to make more boost without detonation. The higher boost will net you way more power then the higher compression will. I was running 7 pounds max on mine until I swaped for LQ9 heads and thicker gaskets which put me around 9.6:1, I may have lost 10hp but I'm now running 10-12psi on high boost which is a good 50hp or more gain. If I had it dyno'd I could tell you exact numbers but hopefully this will clue you in better.
Also remember that in some classes you are limited by blower/turbo size, in thise cases the only way to make more power after the blower/turbo is maxed out is raise the compression. Thats why you see higher compression cars running FI..
The top fuel dragster cars are torn down regularly and i thought they run straight alcohol or something like that.Dropping the compression raises the detonation threshold and makes there be less cylinder pressure.Its just like why do people add an intercooler or meth.....same overall goal just a different way going about doing it.
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Joined: Jan 2004
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From: Castro Valley, CA
My understanding, and it may be flawed, is that high compression increases effective airflow but it also vastly increases charge temp.
Lower compression with more boost is capable of increasing airflow with less heat increase, and therefore less pre-ignition/detonation.
Lower compression with more boost is capable of increasing airflow with less heat increase, and therefore less pre-ignition/detonation.
On N/A motors you have higher compression. Lets say 10.0:1 Your compressing air into a tight area, creating a high pressure. On a boosted car you run a lower compression to increase volume, and use the power adder (turbo/supercharger) to increase the pressure. Now you have more pressure AND more volume = MORE POWER
first of all, in most cases people drop the compression to make more power on PUMP (ie 93 oct) gas. For example, take an average ls1 motor running 10:1 compression, you can run around 8 psi without detenation on pump gas and make around 475rwhp. if you drop the compression to 9:1, and then safely run 12psi on PUMP gas, you would make around 575rwhp. So you are able to make more power without detenation on PUMP gas. (numbers will vary, but you get the idea)
If you start adding in other fuels, that changes everything. A higher compretion motor will make more power than a low compression motor at the same boost, it would just take a higher octane fuel to aviod detenation.
It's all relitive to what kind of car you are trying to build and what octane fuel you want to run.
Later,
Dave
If you start adding in other fuels, that changes everything. A higher compretion motor will make more power than a low compression motor at the same boost, it would just take a higher octane fuel to aviod detenation.
It's all relitive to what kind of car you are trying to build and what octane fuel you want to run.
Later,
Dave
very cool...now this has me interested in what superdave said about dropping the compression to 9:1 so I can run 12psi. I already have the heads off so this would be the time to do it. What would be the best route to get me there? spacers or can 6.0L heads drop me that far? Does anyone know if you can put spacers in with arp head studs?
Spacers are a no go, they kill your quench and aren't very reliable anyways.
Just go with some 6.0L heads with some bigger chambers. Run the numbers and aim for around 9:1 but anywhere close should be fine.
Just go with some 6.0L heads with some bigger chambers. Run the numbers and aim for around 9:1 but anywhere close should be fine.
Originally Posted by LTLHOMER
Just go with some 6.0L heads with some bigger chambers. Run the numbers and aim for around 9:1 but anywhere close should be fine.
Not quite understanding....Are you saying bigger chambers as compared to the stock heads or bigger chambers than normal 6.0L heads?
Run what numbers boost?
sorry for the confusion
Bigger than normal 6.0L or just use 6.0L's and be done with it...if I were you, I'd simply get a stock set of LQ4 heads, throw some good springs on to match your cam, and go have fun. No use dropping big coin when a stock set will do what you want and save you some cash.
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Originally Posted by LTLHOMER
I believe top-fuelers run well over 12:1 compressions (I've heard up to 15:1) ratios and 55+ lbs of boost. But I think most people would rather drop the compression, have a cooler running less powerful engine off boost that will be a monster when they put their foot into it. Although higher compression ratios have a big effect on the efficiency of a motor (higher = more efficient) so who knows. It's a highly debated topic, that's for sure.
we actually have 7.5:1 with 50 lbs of boost
on our fuel car. torn down after every mid 4 second pass.