Does anyone have data that correlates exhaust manifold pressure to the intake manifold absolute pressure (MAP) and rpm for a typical twin turbo setup on a VQ35DE?

If there is data for Greddy TD05-18G turbos with 3" downpipes and a Greddy Evo TT exhaust, that would be optimum, but I'll take what I can get.

I am looking to find out at what RPM the exhaust pressure starts to exceed the intake MAP (e.g. crossover point) for various boost levels.

 

wouldnt this vary greatly depending on exhaust back pressure? Ie. exhaust diameter + cats/no cats? Probably anytime you are boosting over 2-3psi your MAP will be higher than exhaust backpressure.. exhaust backpressure even on a stock setup is less than 3psi I believe.... so actually since you have greddy TT i bet you have very very minimal backpressure....

 

OK, this post is being significantly revised based on QuadCam's input and my previous misunderstanding of variable valve timing. Thank you QuadCam!!!!!

Based on what information I could find on the Internet, on factory turbochargers, the exhaust back pressure can exceed the boost pressure by as much as 50%. The information also seems to indicate that with a properly designed turbo system with a good exhaust system, this is not the case in the lower RPM range.

Nonetheless, that same source did indicate that, even with a properly designed turbo system, at some point in the upper RPM range there was a crossover point at which the exhaust back pressure starts to exceed the intake boost pressure. When this happens, it is better to have little or now over lap between the intake valve opening and the exhaust valve closing (I think that author suggested 10 degrees or less above the crossover point).

Now, based on what I learned from QuadCam, the intake valve opening is retarded as RPM increases, which will decrease the valve overlap as RPMs increase, which is what we want when we have boosted motors, especially as we reach the crossover point.

Now, does anybody know how much our cams retard and an approximate range of the crossover point on a Greddy TD05-18G kit (the exhaust system will have an effect, but I'm just looking for some general data)?

 

It certainly depends on the turbos, exhaust, cams, head flow characteristics etc...

This is a great way though to see exactly what is going on!! When your exhaust backpressure exceeds your manifold pressure your gains per psi of boost start to diminish. On our Z next year I will be monitoring EBP.


I have data on supras.... but that doenst help you. For example on a small 71 mm turbo on a supra EBP catches MP(manifold pressure) at about 20 psi. If you run 30 psi of MP then your EBP is in the 50!! When you hit 40 psi of MP your EBP is over 80!!

On my supra with the large 91 mm turbo my EBP did not exceed the MP until 42 psi.

 

What are you going to use to monitor EBP??? My engine is torn down right now. I'm leaning toward adding a sensor to monitor EBT.

On another note, has anyone played with the HKS Valcon to control the intake cam timing? Can you buy it without the HKS cams? Will it work with an F-CON V Pro V. 3.24, or only v. 3.3?

The F-CON I fried is v. 3.24, but I think (I have not confirmed this yet) that my new one is v. 3.3. I just realized that v. 3.3 will not work with my Navigator Pro. If the new one is v. 3.3, I'm thinking of repairing my v. 3.24 so that I can still use the Navigator Pro. However, if the Valcon is available without the HKS cams, but will not work with v. 3.24, I may sell the Navigator Pro and just use the F-CON v. 3.3.

The other option is Haltech, but I already have all of the HKS sensors and AFK Knock amp, so that is not very appealing to me.

 

[QUOTE=ttg35fort;6862367]What are you going to use to monitor EBP??? My engine is torn down right now. I'm leaning toward adding a sensor to monitor EBT.

QUOTE]



Yes, you need to install a sensor between the turbo and the collector. I use a compression fitting and about a foot of hard line, then go to the sensor so the heat stays away from the sensor.

 

this is getting deep

 

I could be wrong, but I think the VTC sprockets work to retard the cams. as they sit, the cams are fully advanced. not to be a smart ***, but remember that advancing/retarding of the cams works in the opposite way you may think. advancing the cam makes more low end power, while retarding them is for better top end.
so, as rpms rise, nissan retards the cams to extend the powerband.

I have a degree wheel but have yet to make an adapter to check the cam timing on these motors. I have curious about how nissan utilzes the VTC on these engines for quite some time.

 

there is no overlap between when the intake valve closes and the exhaust valve opens!!!!!! when the intake valve closes, you are already at the start of the compression stroke. YOu better hope that the exhaust valve is not opening!!!! if it is, then there are big problems. the exhaust valve shouldn't open up for another 300 (or so) degrees later of crank rotation.

I think you are speaking of the overlap of when the intake valve opens as compared to the closing of the exhaust valve. this occurs whent he crank is at then end of the exhaust stroke and continues through the beginning of the intake stroke.

 

this presure relationship has me curious. witht he greddy turbos, there is going to be a large amount of pressure built up between the heads and the turbines. Greddy has those little turbine housings and the castings/bolt holes never seem to line up right on the greddy casting either. port matching the greddy manifold to the greddy turbine housing will definately aid in reducing pressure in the exhaust manifold.

 

You are right. I must have been typing or trying to think to fast. Yes, at the end of the exhaust stroke, the intake valve opens slightly before the exhaust valve closes, in my case 26 degrees. Specifically, my intake valve opens 7.5 deg. before top dead center and my exhaust valve closes 18.5 deg. after top dead center.

I'll correct my post above.

 

Your absolutely NOT being a smart ***. It looks like my understanding of variable valve control timing was incorrect. I thought the intake cams advance with RPM, but if this is not the case, then I stand corrected and I need to put some more thought into this.

Nonetheless, retarding the intake cams as RPM increases now makes sense to me. As the back pressure increases, there will be a tendency to leave a positive pressure in the combusion chamber when the intake valve opens, thus inhibiting intake flow. By retarding the opening of the intake cam, it gives a little more time for the pressure within the combustion chamber to be reduced before the intake valve is opened. Ideally, the pressure will be reduced adequately below the intake pressure to help stimulate intake air flow.

Also, I see why a greater valve overlap is beneficial in the lower RPMs. At lower RPMs, the exhaust stroke is slower, thus allowing the exhaust gasses more time to exit through the exhaust valve, and thus resulting in less pressure in the combustion chamber. I'm assuming that at lower RPMs the pressure drops to a point that is beneficial for helping to draw in the intake charge sooner (with respect to cam angle), than at high RPMs.

I'll do some more edits to my post above. I hate to leave incorrect information posted.

Thank you QuadCam for your help and pointing me in the right direction!!!

You're not as bad as Larry said you were. Just kidding.

 

alot of retarding the intake cam (for higher rpm power) has to with the momentum of the incoming air at higher piston speeds. as the engine speed increaes, you can close the intake valve later and later due to the momentum of the incoming air charge. even though the the intake valve is still open as the piston is moving up in the compression stroke, the momentum of the air filling the cylinder can somewhat overcome the fact that the piston is now rising in the cylinder.
a "long" rod setup can take more a later intake valve closing better than a short rod setup, too......that's another topic alltogether, though.