Big bore questions. Advice wanted"
11-26-2006, 12:03 AM
#16
Rennlist Member
Originally Posted by Tom M'Guinn
Impressive. How's the torque curve? Wild headwork and big cam?
The crazy power this car is making even with the problems is why I decided to go with a 2.8 stroker.
11-26-2006, 01:33 AM
#17
Rennlist Member
Originally Posted by RKD in OKC
LR has data from a cam provider that modeled the 944 valve train. This model was done to specifically determine the spring load needed to operate different cams. Their findings were that the factory stock cam, hydralic lifters, springs and valve sizes float at above 16psi of boost. This makes sense to me that Porsche would engineer the springs to be just enough to make the valve train as efficient as possible without floating. Operating a valve train uses some of the power from the motor. Stiffer springs use more power.
I'm not an expert and have no data or tests, but it just seems inconsistent with collective experience -- and I don't follow the logic -- where in the 4-stroke cycle will 17psi cause valves to float? At the end of the intake stroke, the cylinder pressure should be approaching boost pressure, no? I can see how higher revs could cause issues, but boost? Maybe I'm just not getting it?
11-26-2006, 03:24 AM
#18
Rennlist Member
Generally you need stiffer springs for higher RPM.
But, I guess, higher intake manifold pressures would have the effect of the spring having to close against the higher pressure on the backside of the intake valve.
But, I guess, higher intake manifold pressures would have the effect of the spring having to close against the higher pressure on the backside of the intake valve.
11-26-2006, 04:59 AM
#19
Rennlist Member
Thread Starter
I was really looking at the springs as an insurance as I've already broken an exhaust spring and taken a bite size chunk out of one of the exhaust valve. Not sure what happened first or what caused which, but better to be safe than sorry. We know that we were getting some spiking on the track which lead to much higher temps and therefore the valve 'melted'
Is there any negative aspect to the better/stronger springs? As for solid lifters I realise that they're more for out and out race engines which rev very high. I'm looking at mine to be revving only a little bit more than stock. Not quite sure what sort of limit. The idea being that it will work both for road and track but the emphasis will be for the track. Having said that my current stock setup K26/8 doesn't see 1 bar until about 3800 rpm so it's not like I'm dealing with a car that has masses of low end tq. anyway. I need to get stuck into the revs/boost to get it really moving.
It's interesting that the first questions people ask of you when you say you're going to mod the engine is "What are your power goals and what's your budget?" These are two very good questions and seemingly basic but it's not always so easy to answer them until you find out more about our cars. This is one of the purposes of this post. To ask some more questions, get some more answers so I can try and identify what my goals are. It's easy to say I want more power with a broad curve that's reliable but people will then start asking you more detailed questions and I can't always answer them. So in view of heads, intakes, exhausts/headers I am trying to understand what items do and what they cost and whether buying one is not worth it but buying three will make a noticeable difference and at what sort of costs. This impacts upon the budget so you have to decide whether to stick with mostly stock items or go all out and do it at one time. Also if these things can be easily added at a later date if you feel the need?
Anyway thanks for all your responses and keep 'em coming.
Is there any negative aspect to the better/stronger springs? As for solid lifters I realise that they're more for out and out race engines which rev very high. I'm looking at mine to be revving only a little bit more than stock. Not quite sure what sort of limit. The idea being that it will work both for road and track but the emphasis will be for the track. Having said that my current stock setup K26/8 doesn't see 1 bar until about 3800 rpm so it's not like I'm dealing with a car that has masses of low end tq. anyway. I need to get stuck into the revs/boost to get it really moving.
It's interesting that the first questions people ask of you when you say you're going to mod the engine is "What are your power goals and what's your budget?" These are two very good questions and seemingly basic but it's not always so easy to answer them until you find out more about our cars. This is one of the purposes of this post. To ask some more questions, get some more answers so I can try and identify what my goals are. It's easy to say I want more power with a broad curve that's reliable but people will then start asking you more detailed questions and I can't always answer them. So in view of heads, intakes, exhausts/headers I am trying to understand what items do and what they cost and whether buying one is not worth it but buying three will make a noticeable difference and at what sort of costs. This impacts upon the budget so you have to decide whether to stick with mostly stock items or go all out and do it at one time. Also if these things can be easily added at a later date if you feel the need?
Anyway thanks for all your responses and keep 'em coming.
Last edited by 333pg333; 11-26-2006 at 07:29 AM.
11-26-2006, 07:36 AM
#20
Rennlist Member
Thread Starter
Originally Posted by RolexNJ
- I think I know the guy you are talking about, it has nothing to do with headwork. Non-issue. He was having injector problem. Anyway, if you get the LR intake, you are going to shift the TQ to the right. That manifold is designed for more mid and high. And unless you've changed your goals, that isn't what you want Patrick?
The graduated 4 inch system from LR. Much debate here. For me, I felt a difference by the seat of my parts. Also, for what it's worth, they were used on Cup Cars, many people don't know that. Three, it is a bit tighter, but the installation is fine. I have 2 other friends of mine who did it too. The people who will "maximize" the most of a graduated system are poeple making some pretty big RWHP. But others can gain from it too. And it will not make the car sound different than a 3 inch alone. BS. I had both on. What makes the difference is the cat/or no cat, and which muffler you choice. Anyway, take a look here at the Lindsey Comparison Chart and see what they found out.
-Solid lifters? Again, for what you are doing Patrick, I would say definatlely no. Unless you are looking to reduce valve train weight, and rev a bit higher; the cost and ongoing maintenance isn't worth it. Plus, not sure if you care (I do), you will hear the vavle train noise. Will sound more mechanical coming from under the hood. Stiff springs? I would look into that if you plan to run higher boost. Lindsey makes some great Racing Springs that are better for long term with respect to running higher boost.
Hope that helps. In my email, I had addressed a lot of other issues that I'm not going to go back into here.
The graduated 4 inch system from LR. Much debate here. For me, I felt a difference by the seat of my parts. Also, for what it's worth, they were used on Cup Cars, many people don't know that. Three, it is a bit tighter, but the installation is fine. I have 2 other friends of mine who did it too. The people who will "maximize" the most of a graduated system are poeple making some pretty big RWHP. But others can gain from it too. And it will not make the car sound different than a 3 inch alone. BS. I had both on. What makes the difference is the cat/or no cat, and which muffler you choice. Anyway, take a look here at the Lindsey Comparison Chart and see what they found out.
-Solid lifters? Again, for what you are doing Patrick, I would say definatlely no. Unless you are looking to reduce valve train weight, and rev a bit higher; the cost and ongoing maintenance isn't worth it. Plus, not sure if you care (I do), you will hear the vavle train noise. Will sound more mechanical coming from under the hood. Stiff springs? I would look into that if you plan to run higher boost. Lindsey makes some great Racing Springs that are better for long term with respect to running higher boost.
Hope that helps. In my email, I had addressed a lot of other issues that I'm not going to go back into here.
I tend to agree with you that sometimes it is just the butt dyno that tells us something is better. As for the 4" exhaust well I knew that the cup cars had 3" into 4" and if you reckon that the noise isn't too droney that's good enough for me. I know that we have trouble fitting some mod's to our cars based on right hand drive, not sure whether there could be an issue with exhaust?
As for Hydraulics, well I'll leave that for another day as you say they're more for race engines and require maintenance.
Thanks
Patrick
11-26-2006, 09:00 AM
#21
Rennlist Member
Originally Posted by Tom M'Guinn
I'm not an expert and have no data or tests, but it just seems inconsistent with collective experience -- and I don't follow the logic -- where in the 4-stroke cycle will 17psi cause valves to float? At the end of the intake stroke, the cylinder pressure should be approaching boost pressure, no? I can see how higher revs could cause issues, but boost? Maybe I'm just not getting it?
11-26-2006, 11:46 AM
#22
Rennlist Member
Originally Posted by 333pg333
So in view of heads, intakes, exhausts/headers I am trying to understand what items do and what they cost and whether buying one is not worth it but buying three will make a noticeable difference and at what sort of costs. This impacts upon the budget so you have to decide whether to stick with mostly stock items or go all out and do it at one time. Also if these things can be easily added at a later date if you feel the need?
Anyway thanks for all your responses and keep 'em coming.
Anyway thanks for all your responses and keep 'em coming.
Head choice depends on which block you are using. If it's a 3L block, you're choices include a 2.7 head or a 2.5 head that has had its water passage modified. The 2.7 head offers bigger intake valves out of the box, and porting options on the exhaust. For the price of a 2.7 head, however, big valves, water passage modifications, and flow work can be done to a 2.5 -- so probably no clear winner there. If money is not an issue, probably more can be done with the 2.7 head.
If you are starting with a 2.5 block, then the 951 head is the obvious choice. And, other than the water passage, you can defer port/valve work on the 951 head without causing extra expense (assuming you do your own labor).
The 951 intake and headers have been used in plenty of 400+hp engines, so they are probably the kind of parts that cost a lot of $$ to squeeze out the last few HP on an already high output motor. With a solid bottom end, I'd concentrate on your choice of head, turbo, and engine management.
11-26-2006, 02:24 PM
#23
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Originally Posted by RKD in OKC
The crazy power this car is making even with the problems is why I decided to go with a 2.8 stroker.
Thanks,
Patrick: Email sent