Boost vs. back pressure

[Alky V6]

I haven't read beyond the first page yet and I jumped from the start of page 2 to the most recent posts. I just thought I'd throw in my own experiences. The thread was started with the question on exhaust back pressure vs. intake boost pressure ratios. What ratios were common, which were desireable and which were impossible to obtain. Was crossover really possible?

I've obtained the mystical tip ratio of better than 1:1. Somewhere between .85-.95:1. That being done with a small cube V6, small heads, a camshaft with too much duration, too much overlap, low lobe separation angle, long tube headers, a turbo that is way too big, a short 3.06" stroke, and a 9.25 CR that is very low for a blown alcohol application.
The tradeoff for this level of efficiency is that I have to use a 1-2 second shot of nitrous to spool the turbo. And, I wouldn't have it any other way. :biggrin:

 

[bison]

Interesting thread. I'm sorry I found it so late.
From what I understand about cam design, if you have to retard the cam below straight up, you have the wrong cam and you are bandaiding a problem with your cam choice. When people retard a cam beyond straight up it's usually to control a fuel detonation problem in the mid range, or you're lowering power in the midrange due to a traction problem with a particular track. You're cutting cylinder pressure at that point in the rpm band, and a byproduct of that would naturally be longer spool up time.
If you're retarding the cam to try and gain more top end, then you simply have the wrong camshaft for what you're trying to do.

Your right. What ill be band aiding is the turbo. Its still a 3 bolt application. I havent seen actual cam specs from a hard running TSM engine but id bet they are running oddball grinds that favor retarded ex valve timing because of the big backpressure they have since they are forced to run the small 3 bolt housing. Look at the mph these guys are hitting with their engines. 1000hp through a POS 3 bolt housing.

 

[bison]

Did you read the link to the other thread? I think it was where the info was but It's been a while since I've read it. If you get good results please post the info. I'm still leaning toward a 2 degree retard on the cam myself but I'm not sure whether it would be worth it.

Of course i read it. I wouldnt lie. They have some 400hp examples where they did comparisons on different days. Also some data from a desktop dyno sort of program. If that program doesnt take into consideration the pressure between the turbocharger and engine its not going to be of any use in evaluating something like i have. That data wont help me since im at a much further performance level and dealing with a lot more exhaust pressure than they are.

 

[charlief1]

I didn't think it would matter that much but you're right about the different power levels. Now that Donnie is in on this I think you may get some good feedback about how to accomplish what you want. He's been doing some other things and I've been keeping up with him for the most part. I should've sent him a link so he could see it earlier but I just found it myself.

 

[bison]

I didn't think it would matter that much but you're right about the different power levels. Now that Donnie is in on this I think you may get some good feedback about how to accomplish what you want. He's been doing some other things and I've been keeping up with him for the most part. I should've sent him a link so he could see it earlier but I just found it myself.

He is in the exact opposite situation as i am with his ex housing wheel combo. He can open the ex valve a lot sooner than me and not lose power. The cam specs seem to remain secret once guys cross over the 140mph barrier with a 3 bolt. Probably for good reason too.

 

[charlief1]

He is in the exact opposite situation as i am with his ex housing wheel combo. He can open the ex valve a lot sooner than me and not lose power. The cam specs seem to remain secret once guys cross over the 140mph barrier with a 3 bolt. Probably for good reason too.

If you haven't read Donnies thread called "advancement of fuel" (paraphrased) in the Alky section you might want to Brian. Some good info but it will take you a while to read it all.

 

[Alky V6]

He is in the exact opposite situation as i am with his ex housing wheel combo. He can open the ex valve a lot sooner than me and not lose power. The cam specs seem to remain secret once guys cross over the 140mph barrier with a 3 bolt. Probably for good reason too.

OK. You don't know what the cam specs are. What other specs about these cars do you know?

 

[bison]

OK. You don't know what the cam specs are. What other specs about these cars do you know?

I know what mine are.

 

[Alky V6]

I know what mine are.

Well, I can pretty much guarantee they're not doing that with a 6,000 rpm shift point.

What class are you building this for and what are the restrictions?

 

[bison]

Well, I can pretty much guarantee they're not doing that with a 6,000 rpm shift point.

No there not. Most are much higher. Ive seen the datalogs from some of them. There is one in particular that told me flat out his TSM engine made peak hp below 6000rpm though. I have no doubt he is telling the truth. Keep in mind i have a hydraulic in mine so it narrows the power band that much more. Im looking for maximum cylinder pressure between 5500 and 6300. My old peak torque was at 5500. I am pretty limited to comp 3300 series lobes if running a hydraulic. Anything more aggressive will just prevent the valves from closing at higher rpm narrowing the power band even more. I only need to gain 200-300rpm breathing room and run 28-30psi boost vs the 26psi i was limited to before and i should be at a really good power level with some reliability. If this was a solid cammed 4 bolt setup (which it should be) it would be a whole different cam spec sharing very little with what i have. Im not wasting my time and $ on this since its a stock block. I have the S2 that i can play with. It is solid cammed and the profile isnt even close to the one in this stock block. Neither are being built for any class although the stock block one im referring to is legal in TAI and would be in THS if i changed the compressor housing to an E.

 

[Alky V6]

No there not. Most are much higher. Ive seen the datalogs from some of them. There is one in particular that told me flat out his TSM engine made peak hp below 6000rpm though. I have no doubt he is telling the truth. Keep in mind i have a hydraulic in mine so it narrows the power band that much more. Im looking for maximum cylinder pressure between 5500 and 6300. My old peak torque was at 5500. I am pretty limited to comp 3300 series lobes if running a hydraulic. Anything more aggressive will just prevent the valves from closing at higher rpm narrowing the power band even more. I only need to gain 200-300rpm breathing room and run 28-30psi boost vs the 26psi i was limited to before and i should be at a really good power level with some reliability. If this was a solid cammed 4 bolt setup (which it should be) it would be a whole different cam spec sharing very little with what i have. Im not wasting my time and $ on this since its a stock block. I have the S2 that i can play with. It is solid cammed and the profile isnt even close to the one in this stock block. Neither are being built for any class although the stock block one im referring to is legal in TAI and would be in THS if i changed the compressor housing to an E.

If you're looking at a torque peak of say 5900 rpm, your hp peak is going to be well out of the realm of hydraulic lifters. About 6700 to 6900 rpm, and could carry on well past that. You're creating a very narrow powerband for yourself with these limiting components. The overall power level after you get it spooled up will be higher, but narrow due to the lifters forcing an early shift point. Zero lash them? You should have a good pull going on well below your 5900 rpm peak though.

 

[tom j]

Where is the best spot to take a back pressure reading? I have a port just past my DP would that work? Or does it have to be before the turbo? Happy Turkey Day!

 

[Jerryl]

Where is the best spot to take a back pressure reading? I have a port just past my DP would that work? Or does it have to be before the turbo? Happy Turkey Day!

Tom,
While both pre- and post-turbo pressures are important measures, this thread is really discussing pre-turbo pressures, so before the turbo.

What I would really like to see are 5 pressure measures, and temps . . . LOL!
Pre-Compressor
Post Turbo/Pre-IC
Post-IC (MAP)
Pre-Turbine
Post-Turbine

I'll keep dreaming . . . .
Back to Boost VS Back pressure . . . .

 

[bison]

If you're looking at a torque peak of say 5900 rpm, your hp peak is going to be well out of the realm of hydraulic lifters. About 6700 to 6900 rpm, and could carry on well past that. You're creating a very narrow powerband for yourself with these limiting components. The overall power level after you get it spooled up will be higher, but narrow due to the lifters forcing an early shift point. Zero lash them? You should have a good pull going on well below your 5900 rpm peak though.

Highest g-forces are at the flash stall speed which is 5500@26psi. It will probably gain a couple hundred rpm at 28-30psi. That will become the new peak torque. Lifters are zero lashed. By the time they the engine is in 3rd they may actually have a tiny bit of lash from the heat expansion. I can hear them after making a pass. I plan on shifting at 6100-6200 and crossing at 6300 or so depending on the tire. I had 275/60-15 DR's which measure 27.3" installed but they dont offer any growth. Im going to try a slick that will grow to 29".

 

[Alky V6]

Highest g-forces are at the flash stall speed which is 5500@26psi. It will probably gain a couple hundred rpm at 28-30psi. That will become the new peak torque. Lifters are zero lashed. By the time they the engine is in 3rd they may actually have a tiny bit of lash from the heat expansion. I can hear them after making a pass. I plan on shifting at 6100-6200 and crossing at 6300 or so depending on the tire. I had 275/60-15 DR's which measure 27.3" installed but they dont offer any growth. Im going to try a slick that will grow to 29".

Trying to cross the finish at 145 mph at 6300 rpm is going to demand some gear and/or tire, a tight T/C, and to pull that gear a lot of cylinder pressure. On a stock block? Yikes.
Pushing the point of peak cylinder pressure up higher is a good move as far as durability. Did you say that you're going to be using nos to spool up? What size is the compressor side?

Realize that a tight T/C is going to give you a rpm drop between shifts of close to 1,000 rpm.

 

[bison]

Trying to cross the finish at 145 mph at 6300 rpm is going to demand some gear and/or tire, a tight T/C, and to pull that gear a lot of cylinder pressure. On a stock block? Yikes.
Pushing the point of peak cylinder pressure up higher is a good move as far as durability. Did you say that you're going to be using nos to spool up? What size is the compressor side?

Realize that a tight T/C is going to give you a rpm drop between shifts of close to 1,000 rpm.

With the 9.5" converter i have in there now it drops to 5500 at 26psi regardless of shift rpm. It actually hovers for a couple seconds at that rpm in 3rd then climbs. With the 27.3" tire height and 3.42 gear im at 6100 at 140mph. So 3.5% slip. With the new slick growing to 29" im speculating about 5-6% converter slip at the same speed from the added loading. So at 143mph id be at about 5.9% slip and 6000rpm. No matter what the converter will always flash to 5500 at the same boost pressure with the cam the way it was advanced 2*. Im thinking it will go to at least 5600 @ 28psi but it will cause the converter to couple a little later. I just want to hit the trap at under 6300rpm. Im thinking the tires and retarding the cam from where i had it previously should put it right where it needs to be. I am using N2O for spooling. Only had a 50 shot on before and it worked way to well so i started leaving the bottle pressure down to make it more controllable. If i have to i can always up it. Having a dry system with a table on the XFI makes it about a 2 minute change between the jet and the added fueling. The car was hitting over .8 g's at 70mph if it hooked. Only a 66 compressor but that isnt showing as a problem because the inlet air temp is 11-12 above ambient at the top of 3rd. Yeah it sucks its a stock block. Stock stroke too. Im sure id split it in half really fast if i 4 bolted and solid cammed it.

 

[Walt03]

Does anyone have the cam specs of the popular cams that they could list
stock cam, 204-214Flat Tappet:Roller Tappet Comp 206-206. 212-206, 212-212, 218-212, 218-218 or others for comaprison. FT 210-215, etc.

Include these cam specs for comparison
Intake Opening degs, BTDC or ATDC,
Intake Closing degs, BBDC or ABDC
Exhaust Opening degs, BBDC or ABDC
Exhaust Closing degs, BTDC or ATDC
Lobe separation angle
Intake lift, duration
Exhaust lift, duration

This is a lot of information but it would sure help evaluate our cams as related to this post.
Anyone have a spreadsheet already set up they could attach?

 

[bison]

Does anyone have the cam specs of the popular cams that they could list
stock cam, 204-214Flat Tappet:Roller Tappet Comp 206-206. 212-206, 212-212, 218-212, 218-218 or others for comaprison. FT 210-215, etc.

Include these cam specs for comparison
Intake Opening degs, BTDC or ATDC,
Intake Closing degs, BBDC or ABDC
Exhaust Opening degs, BBDC or ABDC
Exhaust Closing degs, BTDC or ATDC
Lobe separation angle
Intake lift, duration
Exhaust lift, duration

This is a lot of information but it would sure help evaluate our cams as related to this post.
Anyone have a spreadsheet already set up they could attach?

You need to evaluate on an individual basis. The lobe seperation could vary for any given set of lobes altering the performance at different rpm levels. Its easier to post up the lobes and LS to determine how it will react. Another thing is that very few have been able to optimize or come close to optimization for any given combo. Without data logs any info provided could be inaccurate.

 

[Boost231]

ttt for a good read

 

[Boost231]

bump again, running into this problem now