Compression vs Boost

[05stroker]

Since some of you seem good at running the numbers. With a car that was making 940rwhp with 9.01:1 and then changed to 12.0:1pistons with all other things the same at 21 psi what would be the gain?

 

[Unreal]

Maybe I'm just use to LS motors. Mine is 9.7:1 which is on the low end. 18psi made 1050rwhp on 91 octane. Friends did 1012 on 10.5:1 Novi 2200 build. Just saw a vette make 890rwhp on pump and 11.5:1 compression (stock c7, direct injection which helps). I guess it depends on what you consider high and low compression. For a centi car I like to see 10.3-10.5. Same for turbos. PD car can do fine with 9.5-10.0 because they don't have to worry about the low end/etc. That is where I would put a motor for pump 91-93 octane. E85 I would go up at least 1/2 point. High compression in LS land is 12+ which gets tricky to run any boost on.

To me there is no point in running what a lot of people consider low (8-9:1) compression on modern engines.

 

[JeremyH]

Probably around 60rwhp on gas I would think Billy. Rough rule of thumb is 20rwhp per point of compression iirc similar to adding 1 lb of boost. Now throw in the higher compression with some e85 and gains would be more I would expect. The higher compression really helps with lowend to mid though since there is more heat and cylinder pressure at lower rpms which will then carry through out the rpm range. I run 11.9-12.0 afr with my 9.1:1 compression, if it was higher I would need to richen that up a bit.

The lower compression makes for a nice safe street car when running high boost on the edge on 93 octane. As long as the tune is good and fuel system is up to par it can be done just as easily on higher compression. Just depends on what your comfortable with and the goals of the build, on the street when you hit it your already at 2-3k rpms most of the time and boost comes in quick. I have no problem with the lowend tq/response on my lower compression turbo build. With a properly setup kit and good turbo, boost response is instant when you get on the throttle.

 

[05stroker]

Probably around 60rwhp on gas I would think Billy. Rough rule of thumb is 20rwhp per point of compression iirc.

On an N/A motor correct? I am expecting three times that on mine. All the numbers I have ran shows the gain should be like going from 21 psi to 32-34 psi. That should be good for 150+ rwhp I would think.

 

[gray1622]

4 percent per point of compression is what is listed in the article .

 

[BruceH]

4 percent per point of compression is what is listed in the article .

The actual numbers of 380 with 8.5 and 440 with 10 reported by Justin comes out to a little over 10% per compression point. The formula laden post on modular fords wasn't even close in theory when compared to actual numbers.

 

[05stroker]

The actual numbers of 380 with 8.5 and 440 with 10 reported by Justin comes out to a little over 10% per compression point. The formula laden post on modular fords wasn't even close in theory when compared to actual numbers.

How is that 10% per point effected by boost then? In my case that would come up to roughly 270 rwhp since I was at 9.0 and went to 12.0. I cant see that happening at all.

 

[stkjock]

comparing dyno numbers is just a circle jerk

 

[05stroker]

Looks like they gained 215 rwhp on there set up at 25 psi going from the low comp to high. I will take anything close to that!

Stock, Im just trying to get an idea of where I am going to be at. I have a goal of 1100 rwhp this round. LOL

 

[BruceH]

How is that 10% per point effected by boost then? In my case that would come up to roughly 270 rwhp since I was at 9.0 and went to 12.0. I cant see that happening at all.

That was with 7 psi. He had a car with the same exact setup except for short blocks. Same car both times, same tune.

10% was his result but it's not going to be linear with other motors imo. Each setup will be different.

At one time Marc S had a motor with 8:1 iirc and it was a dog. He switched the pistons out to 9.8 and gained a lot of hp with the same setup. I want to say it was over 150 but I don't remember the details too well. What had happened was he had a shop put together his motor with existing parts. The piston compression height was for a stroker so his pistons were in the hole an extra bit.

 

[Department Of Boost]

What blower will make 30psi on a 4.6 or 5.0?

Whipple 3.4 or 4.0L.

 

[05stroker]

That was with 7 psi. He had a car with the same exact setup except for short blocks. Same car both times, same tune.

10% was his result but it's not going to be linear with other motors imo. Each setup will be different.

At one time Marc S had a motor with 8:1 iirc and it was a dog. He switched the pistons out to 9.8 and gained a lot of hp with the same setup. I want to say it was over 150 but I don't remember the details too well. What had happened was he had a shop put together his motor with existing parts. The piston compression height was for a stroker so his pistons were in the hole an extra bit.

That's good news, I am hoping to gain around 200 or so with the 3 points of CR and head work.

 

[BruceH]

That's good news, I am hoping to gain around 200 or so with the 3 points of CR and head work.

When is it going together and when do you think it will be back on the dyno?

 

[stkjock]

Stock, Im just trying to get an idea of where I am going to be at. I have a goal of 1100 rwhp this round. LOL

Billy - I mean between different cars, different dynos, different conditions.

 

[JeremyH]

I think 150rwhp gain isn't unreasonable with the all the changes you are making. Look forward to seeing the results!

 

[05stroker]

When is it going together and when do you think it will be back on the dyno?

Maybe 3-4 weeks.

Billy - I mean between different cars, different dynos, different conditions.

Understood.

I think 150rwhp gain isn't unreasonable with the all the changes you are making. Look forward to seeing the results!

That is about what I am looking for.

 

[eighty6gt]

My freakin' eyes!

Read this post among others this afternoon. Looks like Bruce was running 10-12 psi @ 11:1 CR on 92. This is very interesting to me... because of where I live and the variability in the economy/dollar/how hard I get slammed when I go to buy parts in a year, I've got a few different builds in my spreadsheet, resulting in 9-11:1 CR's.

One is using flat top pistons, my boss rods, and a stock crank for 11:1 cr with my 3V heads. I think with my small 1.9 TVS I will probably stop at around 10-11 psi. Was reading up on other's experiences with IAT's on Muscle Car Mod Father! Heh...

The gas here is bad @ 91 octane, but I might look into a snow kit - I can get washer fluid that's good for -40F here which is 45% methanol, run the pump out of my factory tank and theoretically not get harassed at the track.

Be an interesting experiment.

 

[Pentalab]

My freakin' eyes!

Read this post among others this afternoon. Looks like Bruce was running 10-12 psi @ 11:1 CR on 92. This is very interesting to me... because of where I live and the variability in the economy/dollar/how hard I get slammed when I go to buy parts in a year, I've got a few different builds in my spreadsheet, resulting in 9-11:1 CR's.

One is using flat top pistons, my boss rods, and a stock crank for 11:1 cr with my 3V heads. I think with my small 1.9 TVS I will probably stop at around 10-11 psi. Was reading up on other's experiences with IAT's on Muscle Car Mod Father! Heh...

The gas here is bad @ 91 octane, but I might look into a snow kit - I can get washer fluid that's good for -40F here which is 45% methanol, run the pump out of my factory tank and theoretically not get harassed at the track.

Be an interesting experiment.

11:1 CR.... go for it. Then tweak the timing for whatever gas you use....and more if a snow kit is used. Carmen used a green-yellow-red led setup to warn of the water-meth levels. You can also get multi coloured leds, like 3 colours if you want to use just one led. The snow kit might be more practical vs your limited radius of obtaining 94 E15 fuel..esp way outa town.

Forget how big those windshield washer fluid containers are ( just checked one in the basement....they are 3.78 litres..or 1 us gallon). I think you might be better off carrying 1-3 of those 3.78 litre containers in the trunk.... vs 10-20 litres of 94 octane in the trunk. A lot safer.

Just thought of something. Isn't there already a low level warning light / indicator for the windshield washer fluid ??

Dunno how long the full oem windshield washer container would last...depends on blower useage ...+ how much u squirt on the windshield. Think carmen said he could get a 30 min track session done..barely. One of the fellows in the local club uses the snow kit. I believe he said it lasted aprx 1 month. 91 is typ 3 cents per litre cheaper than 94, so it would come out a wash anyway.

Then u could use just abt any amount of timing you desired..and with 11:1 CR and the tvs-1900... and 9-11 psi boost, make loads of pwr. You would also make loads of hp/tq in NA mode..blower off..and low rpm. Combined with a good exhaust /LT's etc, it would be a damned good setup.

 

[eighty6gt]

Yeah, I'm glad I looked into it - just because the 5.3 block was coming up on $10,000 after buying mounts and head gaskets - and more as the Canadian dollar heads down to $.62.

I'll polish up the combustion chambers on the stock heads to get a cc or 2, actual cr will be 10.8:1 or so, also run the stock cams 10 degrees retarded which will lower cylinder pressure a bit and help out up top.