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Well the big engine makes a mountain of torque. It may now be time to move the discussion further on to engine breathing - improved air flow - for more power. Here we have cylinder head porting with larger valves, hotter cams, improved induction systems like ITB´s, long tube big pipe headers, dual 3" or 3.5" exhaust systems and more. What are your views guys?
Åke
I expect to change valves for 968, use headers (not planning on special big bore), Colin’s tri flow cams, twin 2 3/4” exhaust system with 3 free flow boxes and X section, individual throttle bodies.
The ITB will required significant effort so may end up being a phase 2 for the project as I would likely not
have enough time to deliver everything at the same time.
I already have the cams and headers so my plan would be building the bottom end, throwing in 968 valves and building a new exhaust system. Then comes the new intake manifold which could be ITB. I liked the design of inlet manifold that Greg produced as a test prototype some time back, that project is moving slowish but does have my attention. ITB would deliver lots of HP for the sacrifice of torque however that might seem incidental given the additional torque a 7.0 will produce.
I expect to change valves for 968, use headers (not planning on special big bore), Colin’s trip flow cams, twin 2 3/4” exhaust system with 3 free flow boxes and X section, individual throttle bodies.
The ITB will required significant effort so may end up being a phase 2 for the project as I would likely not have enough time to deliver everything at the same time.
That exhaust is small, 3” is minimum, I ran a 3” with my 5 litre and it’s only a two valver. You also need to be careful which throttle bodies you choose, what brand and size are you considering. You are staying with stock displacement right?
That exhaust is small, 3” is minimum, I ran a 3” with my 5 litre and it’s only a two valver. You also need to be careful which throttle bodies you choose, what brand and size are you considering. You are staying with stock displacement right?
Displacement would be 7.0, you really think dual 3” would be required?
I could imagine a single 3.5” which is really big but not a dual 3” seems overly large.
Not sure on the ITBs, I have seen BMW versions being used which would be easy to source in the UK but don’t know how much work is required to adapt these to fit.
Displacement would be 7.0, you really think dual 3” would be required?
I could imagine a single 3.5” which is really big but not a dual 3” seems overly large.
Not sure on the ITBs, I have seen BMW versions being used which would be easy to source in the UK but don’t know how much work is required to adapt these to fit.
Trust me everything needs to be custom made when the engine is 7.0 litres, I and Ake have throttle bodies for these engines, they are 55 mm. No BMW throttle will do. The exhaust for both our vehicles is duel 3.5”. Any larger gets very difficult to fabricate for the space. The cams need to be massive, mine will be made in four pieces, Ake has his, he is around 0.500” lift and lots of duration. I hope mine ends up at 0.550”. I’m not building this engine currently as I am concentrating on a simpler 2 valve build.
As a data point, the stock C6 Z06 NA LS7 7.0L at 505chp, has dual 2.5" id pipes into the mufflers, and 2.25" out of the mufflers.
Since I found this too, cam specs for LS7:
0.593/0.588 valve-lift, intake/exhaust
211/230 duration at 0.050, intake/exhaust
120/120 LSA/ICL
I don’t have any scientific approach to the size I suggested but I thought dual 3” would be overkill and 2 3/4” should be sufficient. That would be larger than the LS7. I would be continuing the size right through the silencers
That lift is massive, would require the head to be modified to get the lobe clearances.
The LS7 is a 2-valve engine having larger valves than a 4-valve engine hence more valve lift. For a high performance engine one should aim for a valve lift of 30% to 35% of the intake valve diameter. Attached a diagram for exhaust pipe diameter vs. horsepower.
Åke
Love the pic of your big valve upgrade, Ake. We use a 39mm valve on our intake when we go big, and get less shrouding. Your's look bigger, but they also look to have more shrouding. The flat face weighs a little more than the dished face, but helps us keep CR up and promotes a clean flame front.
another data point, the 6.2L, 700hp dodge HellCat uses dual 2.75" pipes, through EPA-CA cats and mufflers.
Originally Posted by SwayBar
As a data point, the stock C6 Z06 NA LS7 7.0L at 505chp, has dual 2.5" id pipes into the mufflers, and 2.25" out of the mufflers.
Since I found this too, cam specs for LS7:
0.593/0.588 valve-lift, intake/exhaust
211/230 duration at 0.050, intake/exhaust
120/120 LSA/ICL
Originally Posted by Marti
I don’t have any scientific approach to the size I suggested but I thought dual 3” would be overkill and 2 3/4” should be sufficient. That would be larger than the LS7. I would be continuing the size right through the silencers
That lift is massive, would require the head to be modified to get the lobe clearances.
Carl, so far I have only made some porting jobs on a test head to evaluate what will be the final design. I have tested intake ports with stock 37mm (358 CFM), oversize 39mm (399 CFM) and 42mm (438 CFM) valves. The flow data are at 14mm of valve lift. Start reading at post #218. https://rennlist.com/forums/928-foru...ughout-15.html
In spite of being the best flowing intake port I have ever tested, the 42mm intake port did not flow as much as expected. The problem seems to be the two intake valves sitting quite close together (C-C 44mm) shrouding each other. I have started (machine work is done) on an intake port where the 42mm valves are moved apart (C-C 47mm) to see what that will bring. The dished valves are some old valves I have taken from another engine and modified for testing purposes. The 7L stroker engine with small combustion chamber and large cylinder volume will actually have quite a high CR, dished pistons will be needed to get it down to an acceptable level.
Åke
Last edited by Strosek Ultra; 02-16-2018 at 01:59 PM.
Displacement would be 7.0, you really think dual 3” would be required?
I could imagine a single 3.5” which is really big but not a dual 3” seems overly large.
This picture might help. We used dual 3" exhaust for our 640 HP 5.0L motor with 15 pounds of boost, and it was right-sized. However, when we built our current 6.54L motor with 20 psi of boost, our goal was 900+ HP for Bonneville. We hoped we could save some work, so we fitted the previous years' 3” dual exhaust to the car for the break-in at the chassis dyno. At the dyno we saw the pressure in the intake plenum go up two PSI at full throttle because of this. This meant the previous dual 3” exhaust was too small for this new motor, and pressure was backing up all the way into the intake manifold as a result. Another tour through engine math proved it, and told us that we could not push the CFM we needed thru two 3” pipes. A dual 3.5” exhaust was necessary to support our HP goal. That's what we have on the race car now.
Carl, so far I have only made some porting jobs on a test head to evaluate what will be the final design. I have tested intake ports with stock 37mm (358 CFM), oversize 39mm (399 CFM) and 42mm (438 CFM) valves. The flow data are at 14mm of valve lift. Start reading at post #218. https://rennlist.com/forums/928-foru...ughout-15.html
In spite of being the best flowing intake port I have ever tested, the 42mm intake port did not flow as much as expected. The problem seems to be the two intake valves sitting quite close together (C-C 44mm) shrouding each other. I have started (machine work is done) on an intake port where the 42mm valves are moved apart (C-C 47mm) to see what that will bring. The dished valves are some old valves I have taken from another engine and modified for testing purposes. The 7L stroker engine with small combustion chamber and large cylinder volume will actually have quite a high CR, dished pistons will be needed to get it down to an acceptable level.
Åke
That will be interesting, Ake. I look forward to hearing how the engine reacts to that. I recently have gotten more CFM out of the heads by improved porting and narrower runners to increase velocity. We picked up quite a bit of flow, still with 39mm valves. My engine with these mods is still on the engine dyno. We found it under-cammed, so I'm making up another set of cams for it now. I'll post results when I have them.
In my opinion, there's no discrete cutoff in the exhaust size such that this size is too small and causes power loss and then at +1mm doesn't. Exhaust sizing after the cross-over is a continuum of tradeoffs, between back pressure, noise, weight, etc. With my turbo car, dual 3" exhaust without a cross-over and with two straight-thru mufflers caused about 2psi of back pressure when the power level was slightly over 700 rwhp (or about 800 hp at the crank if you make a wild a$$ guess of 15% driveline loss). Is 2psi a lot? It's not a lot at all for a factory stock car, but it's too much for what we're trying to do. Hence the move to dual 3.5" exhaust with a cross-over. Larger exhaust is usually noisier so now we have five straight-thru mufflers in there (or three + two cats if that's what you're into). Fitting anything bigger in there than dual 3.5" with mufflers is going to be a real challenge:
Does anyone know the true, measured lobe separation angle of the GTS cams? I'm not talking about the factory service manual info, but actual measurement if someone is willing to share.
02-16-2018, 07:14 AM
