370 not enough turbo..custom cam?
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Teching In
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370 not enough turbo..custom cam? Im a long time member but I cant log into my old email to reset my PW. Used to be a HCI nitrous f-body and now going turbo. Im dead set on a 370 and WAS dead set on a 7875 from turbonetics but seems everyone screams back pressure issues. My goal is not 1000hp more like 700s thru my th400. Can some one explain to me how the back pressure issue comes into play? i thought the 7875 was a decently sized turbo. Can some of the back pressure issue be combated with a cam specd for a smaller turbo/large CI set up? Ive tried getting ahold of Martin at SRD but I havent heard back. Ive also looked thru liljohns motorsports cam listings and it seems my needs are in between some of his shelf cams.
Yes, ive searched...ALOT...lol...is running the 7875 gonna limit me to peak HP at 5700rpm? Thats where im seeing most of these cars fall off at. Im not wanting to turn 7200rpm but a nice 6500rpm shift would be where i wanna be.
Im in the process of dropping the old motor right now so its kind of crunch time for me to decide on the final set up. Any help or input is much appreciated.
Yes, ive searched...ALOT...lol...is running the 7875 gonna limit me to peak HP at 5700rpm? Thats where im seeing most of these cars fall off at. Im not wanting to turn 7200rpm but a nice 6500rpm shift would be where i wanna be.
Im in the process of dropping the old motor right now so its kind of crunch time for me to decide on the final set up. Any help or input is much appreciated.
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Joined: Oct 2015
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From: Pittsboro, IN
The back pressure issues come up because you are trying to cram all the exhaust from a 370" motor through a 75mm turbine exducer and a T4 0.96 A/R housing. I'm sure that turbo will make 700hp on your motor but they are better choices if you are wanting to make more power. Cam will also come into play on the back pressure. You could get a cast wheel s480 for cheaper than a turbonetics unit and have room to grow.
If your dead set on a 78/75 turbo run twin 50mm waste gates. That would be a band aid for the back pressure issue, but would work well imo. If it was me I would go with something in the mid to upper 80mm range on the exhaust side and get a custom cam to fit your needs.
Thread Starter
Teching In
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I already have the huron speed turbo kit so spacing becomes the issue for me and them BW are huge...lol...seems kinda counter intuitive that the turbo would work better on a smaller motor...my head tells me a 347 @ 15psi makes 650 so throw a bigger motor ie 370 and @ 15psi it makes 720...(just random numbers)...coming from an ex-nitrous user where 150 shot on a 408 makes more power than 150 shot on a 346...turbos are a mothertrucker
Don't over think it. Smallish zero overlap cam @ .050 on that engine will easily make 700's with that turbo. JFR 218/218 would work great IMO.
You don't need 2 50mm WG's! I've got 2 38mm on a fully divided 370 (T4 turbo) and I can drop down to 7lbs easy. Could go less if I had lighter springs in the gates. A single 44mm gate would easily regulate boost if it was installed/positioned correctly.
You don't need 2 50mm WG's! I've got 2 38mm on a fully divided 370 (T4 turbo) and I can drop down to 7lbs easy. Could go less if I had lighter springs in the gates. A single 44mm gate would easily regulate boost if it was installed/positioned correctly.
I already have the huron speed turbo kit so spacing becomes the issue for me and them BW are huge...lol...seems kinda counter intuitive that the turbo would work better on a smaller motor...my head tells me a 347 @ 15psi makes 650 so throw a bigger motor ie 370 and @ 15psi it makes 720...(just random numbers)...coming from an ex-nitrous user where 150 shot on a 408 makes more power than 150 shot on a 346...turbos are a mothertrucker
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From: Portland, Oregon
Like previously mentioned, running twin wastegates would help when running a mild sized single turbo. And with a custom cam, I am sure you could make the 78/75 work pretty well.
Personally, I would look into something larger, like an 87mm S400, and get a custom cam for it.
Or... run TWO 78/75's on your 370 with a custom cam. That would solve any and all backpressure issues immediately. And you would have plenty of room to grow.
Personally, I would look into something larger, like an 87mm S400, and get a custom cam for it.
Or... run TWO 78/75's on your 370 with a custom cam. That would solve any and all backpressure issues immediately. And you would have plenty of room to grow.
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Denmah made 700whp with the On3 7875 and a 6.0 through an unlocked 80e, so its definitely possible.
Turbos are complicated. Ever seen a compressor map?
They can flow a certain amount of CFM. They can also make a certain amount of pressure. Usually at the lower and upper ranges of their CFM capability, they aren't very good at making lots of boost efficiently. Put a turbo on a large engine with high CFM demand and it won't be able to make a lot of boost. Put it on a smaller engine that requires less CFM, and it will be able to make a ton of boost, and more efficiently.
Turbines are similar. Take a large engine and a small turbine and put them together. The turbine doesn't flow enough for the large engine to be efficient, so it spins the turbo at X RPM. Well at X RPM, the compressor isn't able to build much boost, or at least not efficiently. A smaller engine will be able to build boost with the wheel spinning at the same RPM as the larger engine, and do so before the turbine side is out of its efficiency range.
There are a TON of other factors, but long story short, a given turbo is typically more efficient on a smaller engine (especially when it comes to V8s).
Want to make that 6.0 perform good on a 7875? Small cam, no overlap. You'll reduce the CFM demand of the engine with a small cam as compared to a larger one, and the negative overlap will prevent the backpressure from causing any issues building boost.
I already have the huron speed turbo kit so spacing becomes the issue for me and them BW are huge...lol...seems kinda counter intuitive that the turbo would work better on a smaller motor...my head tells me a 347 @ 15psi makes 650 so throw a bigger motor ie 370 and @ 15psi it makes 720...(just random numbers)...coming from an ex-nitrous user where 150 shot on a 408 makes more power than 150 shot on a 346...turbos are a mothertrucker
They can flow a certain amount of CFM. They can also make a certain amount of pressure. Usually at the lower and upper ranges of their CFM capability, they aren't very good at making lots of boost efficiently. Put a turbo on a large engine with high CFM demand and it won't be able to make a lot of boost. Put it on a smaller engine that requires less CFM, and it will be able to make a ton of boost, and more efficiently.
Turbines are similar. Take a large engine and a small turbine and put them together. The turbine doesn't flow enough for the large engine to be efficient, so it spins the turbo at X RPM. Well at X RPM, the compressor isn't able to build much boost, or at least not efficiently. A smaller engine will be able to build boost with the wheel spinning at the same RPM as the larger engine, and do so before the turbine side is out of its efficiency range.
There are a TON of other factors, but long story short, a given turbo is typically more efficient on a smaller engine (especially when it comes to V8s).
Want to make that 6.0 perform good on a 7875? Small cam, no overlap. You'll reduce the CFM demand of the engine with a small cam as compared to a larger one, and the negative overlap will prevent the backpressure from causing any issues building boost.
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Something not been asked, but why not go with a smaller engine? A built 5.3 will be stronger and will be much easier to hit your power goals with the 7875. It could also be cheaper to build than a 370 if you haven't already bought the parts.
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From: Ohio
The back pressure issues come up because you are trying to cram all the exhaust from a 370" motor through a 75mm turbine exducer and a T4 0.96 A/R housing. I'm sure that turbo will make 700hp on your motor but they are better choices if you are wanting to make more power. Cam will also come into play on the back pressure. You could get a cast wheel s480 for cheaper than a turbonetics unit and have room to grow.
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We are assuming this guy has the available room. I understand borgs get a lot of love round here, but Sheeezus those things take up a lot of room. I guess the question is, is this a street car......or is it a race car/no ac?
Thread Starter
Teching In
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Denmah made 700whp with the On3 7875 and a 6.0 through an unlocked 80e, so its definitely possible.
Turbos are complicated. Ever seen a compressor map?
They can flow a certain amount of CFM. They can also make a certain amount of pressure. Usually at the lower and upper ranges of their CFM capability, they aren't very good at making lots of boost efficiently. Put a turbo on a large engine with high CFM demand and it won't be able to make a lot of boost. Put it on a smaller engine that requires less CFM, and it will be able to make a ton of boost, and more efficiently.
Turbines are similar. Take a large engine and a small turbine and put them together. The turbine doesn't flow enough for the large engine to be efficient, so it spins the turbo at X RPM. Well at X RPM, the compressor isn't able to build much boost, or at least not efficiently. A smaller engine will be able to build boost with the wheel spinning at the same RPM as the larger engine, and do so before the turbine side is out of its efficiency range.
There are a TON of other factors, but long story short, a given turbo is typically more efficient on a smaller engine (especially when it comes to V8s).
Want to make that 6.0 perform good on a 7875? Small cam, no overlap. You'll reduce the CFM demand of the engine with a small cam as compared to a larger one, and the negative overlap will prevent the backpressure from causing any issues building boost.
Turbos are complicated. Ever seen a compressor map?
They can flow a certain amount of CFM. They can also make a certain amount of pressure. Usually at the lower and upper ranges of their CFM capability, they aren't very good at making lots of boost efficiently. Put a turbo on a large engine with high CFM demand and it won't be able to make a lot of boost. Put it on a smaller engine that requires less CFM, and it will be able to make a ton of boost, and more efficiently.
Turbines are similar. Take a large engine and a small turbine and put them together. The turbine doesn't flow enough for the large engine to be efficient, so it spins the turbo at X RPM. Well at X RPM, the compressor isn't able to build much boost, or at least not efficiently. A smaller engine will be able to build boost with the wheel spinning at the same RPM as the larger engine, and do so before the turbine side is out of its efficiency range.
There are a TON of other factors, but long story short, a given turbo is typically more efficient on a smaller engine (especially when it comes to V8s).
Want to make that 6.0 perform good on a 7875? Small cam, no overlap. You'll reduce the CFM demand of the engine with a small cam as compared to a larger one, and the negative overlap will prevent the backpressure from causing any issues building boost.
I guess at the end of the day its not the power really that im looking for. My car has NEVER been high HP but its always ran well. Then end goal is to get a 9 sec slip. 700hp comes from what i estimate it will take me to get there. The car has been 10.908 @ elevation of 4000ft and DA anywhere between 5500 and 6500 on 490HP...Its not a daily driver and hasnt been for years but i still cruise it on the weekends but gets trailered to the track, so is it a race car?...meh...lol..
I started into this with the wrong mentality i guess. I thought "build a big motor to leave room to grow in the future" when in turboland its the opposite. You big on the turbo..lol
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Joined: Mar 2014
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From: Ohio
Ask yourself that same question.
Sorry, I have been at SGMP No Mercy since Thursday of last week and I have tried to get back in touch with everyone who contacted me before I left and afterwards.
I'd be glad to assist with the proper camshaft!
I'd be glad to assist with the proper camshaft!
Remember that the compressor is connected to the turbine, and what the turbine does the compressor must do as well...or what the turbine can't do the compressor can neither do as well.
You can take a PTE billet 7675, put it on a 3.0 liter Supra and it will make 1000rwhp like it was a cake walk and some make 1100+. Take the same turbo and put it on a 6.0 liter LS motor and it might make 900rwhp.
It's all about airflow and where that airflow occurs in the RPM range that determines HP output. A smaller engine doesn't stretch the turbine until much higher RPM's where the smaller engine is starting to move and demand the most airflow. As we all should know, HP is a mathematical equation of work over time. The further you carry torque into the RPM range (even if it's a smaller amount) the more HP you make. A larger LS engine makes more torque and does it at a lower RPM than the 3.0 liter Supra thus maxing the turbine out at a much lower RPM. It cannot carry that torque far enough to match the HP output of the Supra. This is because the larger engine is maxing out the turbine at a lower RPM and thus achieves a lower HP number.
A 7875 will make around 700 without much headache through a TH400.
Last edited by Martin Smallwood; Oct 11, 2016 at 06:02 PM.