Boost: What will it do if I raise CR?

[DiMora]

Question:

My stock (9.8:1 CR) shortblock is fed 9 PSI by my TVS.

If I put in my 298 stroker with 11.5:1 compression and make no other changes to the build (no pulley changes), what will that do to my boost pressure?

I will actually be adding:
Comp 1273509 cams w/ Livernois or Comp Limiters
Ford Racing Performance Products twin 65MM CobraJet throttle Body
JLT 127mm Carbon Fiber BIG AIR intake
VMP tuning TVS high-flow inlet elbow

I want to stay around 9 PSI with the above mods; I'm wondering if I'll need to change blower snout pulley sizes.

 

[DiMora]

Mods---please move to S197 section, not S550. Sorry.

 

[weather man]

Just had this conversation with DB Performance on the short block I am building. He is firmly in the lower compression is better camp. Definitely likes the idea of turning up boost to make power. He said if you run corn all the time, it is not as big a deal. He said it comes in when you run pump gas. The higher compression is much less forgiving of fuel quality and tune quality.

 

[DiMora]

Just had this conversation with DB Performance on the short block I am building. He is firmly in the lower compression is better camp. Definitely likes the idea of turning up boost to make power. He said if you run corn all the time, it is not as big a deal. He said it comes in when you run pump gas. The higher compression is much less forgiving of fuel quality and tune quality.

I'm torn on this. I go back-and-forth about putting lower comp pistons in or not.

My tuner says build it as-is with the 11.5:1's. He will tune it for me on pump. I *may* go E85 if I can find a station near my house. There is one right by work, so that part is good.

Coyotes run 11.0:1.

My effective CR at 9 PSI (18.54) will be the same as a guy with stock compression 9.8:1 pistons running 13 PSI or a guy with 8.5:1 compression running 17.5 PSI. Additionally, at lower boost, my IAT's should be lower.

I also have a Snow Performance stage 3 meth system (I won't tune for it, but I do run it for safety).

 

[loots06]

BruceH had a similar build with high compression and made some decent power.

http://www.s197forum.com/forum/showthread.php?t=107717

 

[lito]

With coyotes you can play with cams to drop dynamic compression, you dont have that opportunity with 3Vs, I would not suggest unless you are running alcohol.

 

[stkjock]

Aren't cyl pressure the issu, no matter how one gets there?

 

[BruceH]

Aren't cyl pressure the issu, no matter how one gets there?

I think so. I'm of the opinion that higher compression means less boost needed to obtain cylinder pressure. This calculator will approximate cylinder pressures with compression and boost changes:

http://www.wallaceracing.com/boost-compression-ratio-calc.php

Less boost means lower iat and less parasitic loss. Not too long ago something like 9.5:1 was considered dangerous with boost but as time and engine management systems move along the compression threshold seems to rise.

Shane, if you go with e85 it doesn't mean you always have to run it. Changing fuels is just about as easy as changing tunes and maybe a pulley for lower boost. Running e85 also means not being concerned with fuel being the weak link in the setup.

I think that you will probably be able to make all the power you will want with lower compression and pump gas. The real question would be how much power do you want to make? More reasonable numbers will be achievable with low boost and the potential for higher power is there with a pulley change. IMO a higher compression ratio will extend the hp limit of your TVS if that's something you might want to do in the future.

My 322 inch 12:1 motor made 585rwhp with 7-8psi and e85. It wasn't a max effort tune either, iirc max spark advance at wot was only 21 degrees. I was able to run the same combo with 92 pump gas without issue but spark was kept to 16 degrees at wot.

Compression is one way to make hp with less boost. Use e85 and you should gain at least 40rwhp and that's with a reasonable spark advance. More power with less boost and superb detonation resistance.

If you have blender pumps available you could even experiment with e30 and e50. E30 is rumored to get the same mpg as gasoline but have a much greater detonation resistance. I wish we had blender pumps to try it out.

 

[lito]

Aren't cyl pressure the issu, no matter how one gets there?

Not exactly, there is a difference on having a bigger fill than compressing it more, is not the same.

 

[Saleen304]

I just did this when moving from 9.3:1 to 10.7:1. Using same pulley my boost pressure did not change. Power on the other hand changed quite a bit![emoji106]

 

[JeremyH]

To answer the question I don't think you will see much of a change in boost level, unless you dramatically change the ve of the head/motor setup. Which based on your description doesn't look like.

As far as what compression to go with, my views are based on the purpose of the car. Max effort, track, street dd etc all plays a part in this decision. I think a lot of people these days think you need higher comp to make good power which is a misconception imho. Just a part of the puzzle. Saying that, I realize belt guys can get limited with pulleys and ribs slip efficiency windows etc. and may want higher comp to get more out of their setup.

I like the lower static compression/more boost for a street car. I'd rather put more air and fuel in only when power is needed, vice compressing more all the time. This is based on longevity and power potential. I chose 9:1 and built a setup that made over 600rwhp at 10psi on very mild 3v build (stock heads and stock displacement). Been beating on it for 5 year now. I run a e85 and 93 tune as well.

If you strictly comparing final effective compressions, the setup with lower static and higher boost levels will make more power every time over the other if final compression is the same. 1 point of compression is roughly 30hp vice 3-4psi 60-100hp depending on setup. You just can't get hung up on boost levels. Keep the boost level efficiency of your power adder in mind though. I can see why a roots/ts guy it may not work for if your already on the edge.

 

[DiMora]

Thank you all for the input.

Bruce, your previous experience and the fact that I can run two tunes is swaying me toward just going for it.

I'll get a tune for 93 and a tune for E85.

Initially, I'll just run the same pulley and confirm I am still at ~ 9 PSI. I have two snout pulleys (with a stock diameter crank damper)...I have a 3.47" and a 3.0" (88MM and 76MM, respectively). I also have a Roush P51 overdrive crank damper (180MM 7.09")...

Available drive ratios:
Stock GT damper (169MM/6.65") +88MM/3.47" snout = 1.92:1 (This is what I run today at 9 PSI)
Roush overdrive damper (180MM/7.09") + 88MM/3.47" snout = 2.04:1
Stock GT damper (169MM/6.65") + 76.2MM/3" snout = 2.21:1

Assuming I don't gain any boost, it looks like the second and third combinations above will allow me to increase boost back to 9 PSI if it drops with the high CR engine and 127350 cams. I won't go beyond 9 PSI, that way I can run pump gas if needed with conservative timing (17 degrees or so).

Bruce - I don't know how much power I want because I have never experienced "too much" yet. A lot of people who I respect have told me that there is such a thing as too much...most tell me that for a street car, around 550-RWHP is perfect and beyond that the car becomes dangerous, less enjoyable, and with power that is often unusable. My tuner (who has driven 1200 RWHP cars like twin turbo vettes) told me he likes Mustangs around 500 RWHP and that he would not go beyond 550 or so.

I want the car fun, reliable, and with all the power that I would even want without over-doing it. I am thinking 600 RWHP or so is my goal since I have tire mods (335's on rear with Whiteline watts link / Tru-Trac) and can get the power down better than others with similar builds.

I drag it at the strip a few times a year, but it is 90% street car.

Thoughts?

 

[Saleen304]

Thank you all for the input.

Bruce, your previous experience and the fact that I can run two tunes is swaying me toward just going for it.

I'll get a tune for 93 and a tune for E85.

Initially, I'll just run the same pulley and confirm I am still at ~ 9 PSI. I have two snout pulleys (with a stock diameter crank damper)...I have a 3.47" and a 3.0" (88MM and 76MM, respectively). I also have a Roush P51 overdrive crank damper (180MM 7.09")...

Available drive ratios:
Stock GT damper (169MM/6.65") +88MM/3.47" snout = 1.92:1 (This is what I run today at 9 PSI)
Roush overdrive damper (180MM/7.09") + 88MM/3.47" snout = 2.04:1
Stock GT damper (169MM/6.65") + 76.2MM/3" snout = 2.21:1

Assuming I don't gain any boost, it looks like the second and third combinations above will allow me to increase boost back to 9 PSI if it drops with the high CR engine and 127350 cams. I won't go beyond 9 PSI, that way I can run pump gas if needed with conservative timing (17 degrees or so)l

Bruce - I don't know how much power I want because I have never experienced "too much" yet. A lot of people who I respect have told me that there is such a thing as too much...most tell me that for a street car, around 550-RWHP is perfect and beyond that the car becomes dangerous, less enjoyable, and with power that is often unusable. My tuner (who has driven 1200 RWHP cars like twin turbo vettes) told me he likes Mustangs around 500 RWHP and that he would not go beyond 550 or so.

I want the car fun, reliable, and with all the power that I would even want without over-doing it. I am thinking 600 RWHP or so is my goal since I have tire mods (335's on rear with Whiteline watts link / Tru-Trac) and can get the power down better than others with similar builds. Thoughts?

That's how I am doing it too, but I found I never go back to the 93 tune since E85 is just amazing and it's readily available here. I am over 700rwhp with no traction issues. Sure I can smoke them in first but it hooks second no problem.

 

[JeremyH]

That's my threshold for enjoying the car and being able to use the power on the street 550-600rwhp. I rarely up boost to the 760rwhp on the street, I just drive around on low boost 10psi.

 

[tjm73]

I am in camp that favors the higher compression with the lowest boost needed to acheive the desired goal. Compression brings efficency and throttle responce. Higher boost brings heat that had to be managed.

 

[BruceH]

Thank you all for the input.

Bruce, your previous experience and the fact that I can run two tunes is swaying me toward just going for it.

I'll get a tune for 93 and a tune for E85.

Initially, I'll just run the same pulley and confirm I am still at ~ 9 PSI. I have two snout pulleys (with a stock diameter crank damper)...I have a 3.47" and a 3.0" (88MM and 76MM, respectively). I also have a Roush P51 overdrive crank damper (180MM 7.09")...

Available drive ratios:
Stock GT damper (169MM/6.65") +88MM/3.47" snout = 1.92:1 (This is what I run today at 9 PSI)
Roush overdrive damper (180MM/7.09") + 88MM/3.47" snout = 2.04:1
Stock GT damper (169MM/6.65") + 76.2MM/3" snout = 2.21:1

Assuming I don't gain any boost, it looks like the second and third combinations above will allow me to increase boost back to 9 PSI if it drops with the high CR engine and 127350 cams. I won't go beyond 9 PSI, that way I can run pump gas if needed with conservative timing (17 degrees or so).

Bruce - I don't know how much power I want because I have never experienced "too much" yet. A lot of people who I respect have told me that there is such a thing as too much...most tell me that for a street car, around 550-RWHP is perfect and beyond that the car becomes dangerous, less enjoyable, and with power that is often unusable. My tuner (who has driven 1200 RWHP cars like twin turbo vettes) told me he likes Mustangs around 500 RWHP and that he would not go beyond 550 or so.

I want the car fun, reliable, and with all the power that I would even want without over-doing it. I am thinking 600 RWHP or so is my goal since I have tire mods (335's on rear with Whiteline watts link / Tru-Trac) and can get the power down better than others with similar builds.

I drag it at the strip a few times a year, but it is 90% street car.

Thoughts?

My thoughts with a pd blower are that 525-550 is just about right. A centri will allow for more peak power while still being under control on the street.

You will be able to obtain your goal with lower compression pistons. Using higher compression will give you some reserve capacity just in case you want to see how far it will go bit it also increases risk if you get bad fuel, especially pump gas.

My own thoughts about the fuel choice are that if you have e85 stations readily available that's all you will probably use. More power, less heat, maximum safety. Motors run very well on it and the exhaust has a far better odor to it than gasoline exhaust. The coldest I've run it is at about 20 degrees. The motor still started and ran fine but it took quite a bit of fiddling to get the cold motor start reliable at temps that low.

 

[Pentalab]

I'm torn on this. I go back-and-forth about putting lower comp pistons in or not.

My tuner says build it as-is with the 11.5:1's. He will tune it for me on pump. I *may* go E85 if I can find a station near my house. There is one right by work, so that part is good.

Coyotes run 11.0:1.

My effective CR at 9 PSI (18.54) will be the same as a guy with stock compression 9.8:1 pistons running 13 PSI or a guy with 8.5:1 compression running 17.5 PSI. Additionally, at lower boost, my IAT's should be lower.

I also have a Snow Performance stage 3 meth system (I won't tune for it, but I do run it for safety).

8.5:1 CR + 17.5 psi is nuts. IAT's would be through the stratosphere, the timing would get pulled, and you would be back to square one...esp in warm wx, like a 70-100F day. ( roush used 8.5:1 CR + 15 psi on their P51A+B).

The difference between shane's proposed 9 psi and the other extreme, 17.5 psi is like apples + oranges. That extra 8.5 psi of boost will wreak havoc with iat's. (like another 42- 68F increase in IAT's.) Then factor in the extra HP used to achieve the extra 8.5 psi of boost..which is gonna be a bunch..which will put an even bigger load on the belt, tensioner, and front crank bearing.

Even with 93 pump gas + 11.5:1 CR, just reduce the timing to a safe level. It would still make for a good DD. If you want to go crazy, then fill up with E85, and swap tunes, add a bunch of timing etc. With DD, most of the time you are not in boost, so the 11.5:1 CR would be of benefit...vs 8.5:1 CR. I mean where do you draw the line, 6:1 CR and 20 psi boost ?

 

[eighty6gt]

I would not change the car from its current configuration. Maybe add the 127350's if you have stock cams. Leave the pullies alone.

The rule of built 3V's is the more you mess with it the more time your parts spend in the machine shop or on a bench. Or in the junkyard. Just my observation after being on here for 3-5 years.

 

[Pentalab]

Bruce, which of the 2 results that the wallace racing calculator spits out, do you use ?

 

[tigerhonaker]

I don't know how much power I want because I have never experienced "too much" yet. A lot of people who I respect have told me that there is such a thing as too much...most tell me that for a street car, around 550-RWHP is perfect and beyond that the car becomes dangerous, less enjoyable, and with power that is often unusable. My tuner (who has driven 1200 RWHP cars like twin turbo vettes) told me he likes Mustangs around 500 RWHP and that he would not go beyond 550 or so.

I want the car fun, reliable, and with all the power that I would even want without over-doing it. I am thinking 600 RWHP or so is my goal since I have tire mods (335's on rear with Whiteline watts link / Tru-Trac) and can get the power down better than others with similar builds.

I drag it at the strip a few times a year, but it is 90% street car.

Thoughts?

I can tell you from personal experience on power on the street with running (Street-Tires) not [Drag-Radials}.

When I was running the Saleen with the Super-Shaker option it was around 560 H/P.
It was Fast and easy to drive on the street.
It went 138 MPH and 10.30's 1/4 mile.
(Then it had 4:10 gears)

Now it has a lot more H/P & Torque and I can tell you it is actually not as fast as it was with the Saleen set-up on the street.

So stick with the 500 to say 600 and IMO you will have a much more Fun Machine on the street.

And also far less maintenance issues to keep it running correctly !!!

.