Valve springs for back pressure ?

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parkland

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Apr 22, 2012
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Hi guys, I have a question that I can't seem to find the answer to.
And I hope this is a decently close enough sub forum to ask in.
I bought a bus for the dt360 mechanical diesel engine, and also bought a 2008 f250 truck to put it in.
I am planning to run about 500 hp of fuel from a p-7100 pump, and probably an he351ve turbo on the engine, and a atmosphere turbo, being something off a detroit diesel series 60 or so.
I don't plan on a high revving engine though, maybe 3200 RPM max. Just over stock.

My question is about valve springs. From what I've read, and who I've talked to, the stock springs should be fine at that speed.
But, what about the back pressure?
I've also read that these engines need stiffer springs to run an exhaust brake over 40 psi of back pressure.
However, with an exhaust brake, that back pressure exists with zero or little boost, where under power, the back pressure exists with a high boost intake.

Is it a fair argument to make, that the exhaust valve springs' closing ability will be the difference between boost and exhaust back pressure? Or is it going to be more absolute dependent on exhaust back pressure only?

What I'm saying, is if guys have trouble with valves holding 40 psi of back pressure with the engine brake, is that because they can't hold more than 40 psi, or is it because the intake charge is low?

I'm thinking that as long as the exhaust back pressure is less than 40 psi over the intake boost psi, it would be fine, but looking for opinions as to if you think that is correct, or totally wrong.
 
Also, and sorry to double the question up, but about back pressure killing head gaskets, how is that even possible?
Combustion pressure is way way higher than any back pressure, if the back pressure was high enough, it could make it hard to expel the exhaust, and let less fresh air in, which could be in essence like running EGR and lead to higher EGT's right?
But I still don't see how it could kill a head gasket.
If the pressure is not relieved into the exhaust enough, the fresh air just won't enter the cylinder on the intake stroke at the same rate or volume, right?

I don't see a scenario when high back pressure could cause higher combustion peak pressures, maybe I am missing something though.
 
Interesting questions. :pop:

First, you are trying to fix a float problem?

Things to consider IMO:
valve weight including retainer and keepers
rpm
Spring rating
Cam profile at all points
Spring condition (has it become weak)
Spring design

OR

Are you just striking some tech conversation up on said subject?

I would like to see with any spring used if it were to be proportional to the rating of the amount of back pressure needed to lift the valve off the seat compared to a pressure that keeps it open just from my own curiosity.
 
Fahlin Racing said:
Interesting questions. :pop:

First, you are trying to fix a float problem?

Things to consider IMO:
valve weight including retainer and keepers
rpm
Spring rating
Cam profile at all points
Spring condition (has it become weak)
Spring design

OR

Are you just striking some tech conversation up on said subject?

I would like to see with any spring used if it were to be proportional to the rating of the amount of back pressure needed to lift the valve off the seat compared to a pressure that keeps it open just from my own curiosity.
Click to expand...

The engine runs perfectly, but I am going to do an inframe rebuild on it before putting it into truck. Also, if it needs any upgraded parts, I would do that before putting it into the truck.

It runs perfect now, but I am trying to dig up info on valve springs if they are needed for not. I am going to add fuel and more boost, but it just a street truck, so no reason to increase RPM's. So that is the real question, everyone who seems to do valve springs it's because of wanting to run high RPM, so what do I need to do for higher boost and back pressure?
Some say upgrade them, some say it will be fine don't do a thing.

I don't want to crash valves around and blow motor, but I also don't want to install un needed spings that will wear out cam or something. Thus the delimna.

A lot of racing solutions are not great advice for an engine that is expected to last a long time.
I just want to try and find out if anyone has been down this road with this motor before, what boost and back pressure they managed on stock springs, etc.
 
What you are seeking takes test equipment to find float points which includes numerous points to be found. The weight of the valve is not a huge concern at normal engine speeds but if a spring has become weak you will float. Do what you need to do with tried and true combination of parts that don't break the bank. Become friends with a engine builder and ask for insight on rebuilds and ask if you can be around the shop to help understand. Not all will do it but there are quite a few generous guys out there in terms of helping out.

Find a reputable shop and be a patron of theirs.
 
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