Compression ratio, DCR, cam duration, cast pistons, etc
Discussion
I'm posting this seperately from Puma Racings cranking pressure thread as I did not want to dilute the thread.
I am a hobbiest, have built a few engines, some more than once as I learn more and correct mistakes.
I read a lot and have a surface understanding of the relationship between compression ratio, cam duration, DCR & cranking pressure
Where I fall down is when static compression ratio is quoted as a maximum ( usually 10.5 to 1 for cast Pistons) and yet compression is down due to cam duration.
I do understand that when an engine comes on cam the pressures increase
But I still can't get my head around static compression ratio and cranking compression pressure.
Would all my problems disapear if I just fitted forged Pistons?
All my work so far has been with Pinto's however I'm soon to start on Duratec
I am a hobbiest, have built a few engines, some more than once as I learn more and correct mistakes.
I read a lot and have a surface understanding of the relationship between compression ratio, cam duration, DCR & cranking pressure
Where I fall down is when static compression ratio is quoted as a maximum ( usually 10.5 to 1 for cast Pistons) and yet compression is down due to cam duration.
I do understand that when an engine comes on cam the pressures increase
But I still can't get my head around static compression ratio and cranking compression pressure.
Would all my problems disapear if I just fitted forged Pistons?
All my work so far has been with Pinto's however I'm soon to start on Duratec
I have been told Ford cast Pistons are ok to 10.5 to 1, I have seen 10.7 to 1 work ok and understand that Det can break the ring lands
What I don't get is the relationship between a static compression ratio and measured cranking pressure as a limit
Yes I get that cam duration means the inlet closes later and exhaust opens earlier and so cranking pressure is reduced, I also understand as the engine comes on cam the dynamic flow effectivly increase the charge volume therefore pressure is higher
Do I build the engine for 10.5 to 1 or 190psi cranking
What I don't get is the relationship between a static compression ratio and measured cranking pressure as a limit
Yes I get that cam duration means the inlet closes later and exhaust opens earlier and so cranking pressure is reduced, I also understand as the engine comes on cam the dynamic flow effectivly increase the charge volume therefore pressure is higher
Do I build the engine for 10.5 to 1 or 190psi cranking
Thanks for that Dave
To recap
I should follow advice on static compression ratio for the Pistons fitted
I should be aware of the expected cranking pressure with the cams I have as an indication of how well the engine has been built
I can within a small margine affect the results by seining the cam
It's a Pinto, well 3 Pinto's 1 standard non injection, 1 x 205 block, injection head, +2.25 Pistons, 1mm gasket and FR32 cam
The other is similar except for a ported big inlet valve head, 42cc chambers, 1.3mm gasket to keep the compression down for the 93mm 2.8 V6 Pistons on Cosworth rods and a RL31 cam
The cam needs more compression than the Pistons can take so I have compromised wish I'd bought so forged Pistons from the start
To recap
I should follow advice on static compression ratio for the Pistons fitted
I should be aware of the expected cranking pressure with the cams I have as an indication of how well the engine has been built
I can within a small margine affect the results by seining the cam
It's a Pinto, well 3 Pinto's 1 standard non injection, 1 x 205 block, injection head, +2.25 Pistons, 1mm gasket and FR32 cam
The other is similar except for a ported big inlet valve head, 42cc chambers, 1.3mm gasket to keep the compression down for the 93mm 2.8 V6 Pistons on Cosworth rods and a RL31 cam
The cam needs more compression than the Pistons can take so I have compromised wish I'd bought so forged Pistons from the start
Don't prod me to hard Dave, I'm just a keen amature
I learn by absorbing as much info from the likes of you and others as I can.
If you say cast Pistons can take much more static and I should be looking for a certain cranking pressure with a certain cam, I get it
I have 1 Pinto built and 1 ready to rebuild, I can incorporate this new learning. I have several heads and gaskets 1 with plug Adaptors for smaller plugs which means the static compression can be over 11 to 1
I take it spark advance is critical with high compression engines and I have Megajolt fitted that I can build a conservative map with until the rolling road.
I learn by absorbing as much info from the likes of you and others as I can.
If you say cast Pistons can take much more static and I should be looking for a certain cranking pressure with a certain cam, I get it
I have 1 Pinto built and 1 ready to rebuild, I can incorporate this new learning. I have several heads and gaskets 1 with plug Adaptors for smaller plugs which means the static compression can be over 11 to 1
I take it spark advance is critical with high compression engines and I have Megajolt fitted that I can build a conservative map with until the rolling road.
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff