CANDEE
Leaving Skid Marks
Yeah he is running 6a12 mivec heads on his car. Think he is going to run higher than stock compression. Think he was talking about running FJ20 rods & 4gaze or b18c pistons.
High Compression Ratio + Turbo
- Thread starter sideburns
- Start date
yeah Im doing the 2.5L mivec. will continue to update my build thread as progress is made so others can attempt it also.
pretty sure that the 6a12 mivec is 10:1
the combustion chamber volume of a 6a12 mivec head is 33cc compared to the 43cc of a 6a13tt head which means swapping the heads gives you a 10.4:1 CR ..
unfortunately it also would leave you with smaller valves, work to do on the intake arrangement, timing belt mods and possibly changing some oil feed points just to name a few of the issues that arise from swapping the heads...
spot on
6a13tt valves are 33mm Intake and 28 Exhaust
where the 6A12 MiVEC valves are 31mm Intake and 27 Exhaust
and yep the mivec 6A12 is stock 10:1 CR
there is no oil feed issues but neither the 6a13 nor the 6a12 intake manifold fits so one has to be made from scratch or the 6a13tt one has to be modified to fit. The exhaust manifolds however are identical.
the 6a12 nor the 6a13tt thermostat housing will bolt to a 6a13 with 6a12 heads.
The front bank of cams is also out of phase with crank (1/2 a tooth) when at TDC so you will need a set of adjustable cam gears to correct this phasing issue.
And of course the issue of the cam belt being too short. I have got around this by spacing the center idler bearing bracket up off the block ~25mm which made the stock 6A13TT belt perfect length, however this throws out the rear bank cam timing, Again easy fix just dial it back that half a tooth with adjustable cam gears.
As for the compression difference. Rockit have you actually CC'ed the 6A12 Mivec heads? or is that 43cc and 33cc difference just numbers from a website?
6a13tt valves are 33mm Intake and 28 Exhaust
where the 6A12 MiVEC valves are 31mm Intake and 27 Exhaust
and yep the mivec 6A12 is stock 10:1 CR
there is no oil feed issues but neither the 6a13 nor the 6a12 intake manifold fits so one has to be made from scratch or the 6a13tt one has to be modified to fit. The exhaust manifolds however are identical.
the 6a12 nor the 6a13tt thermostat housing will bolt to a 6a13 with 6a12 heads.
The front bank of cams is also out of phase with crank (1/2 a tooth) when at TDC so you will need a set of adjustable cam gears to correct this phasing issue.
And of course the issue of the cam belt being too short. I have got around this by spacing the center idler bearing bracket up off the block ~25mm which made the stock 6A13TT belt perfect length, however this throws out the rear bank cam timing, Again easy fix just dial it back that half a tooth with adjustable cam gears.
As for the compression difference. Rockit have you actually CC'ed the 6A12 Mivec heads? or is that 43cc and 33cc difference just numbers from a website?
the 43 and 33 are actual measurements of the heads(to the nearest CC as trying to get more accuracy was futile. three repeats all got the same figures though so must be close).
but the 33&43 are very close to the mathematically calculated chamber volumes if you take gasket thickness and piston dish into account (although this relies on data from websites for the compressed gasket thickness & piston dish as I haven't been bothered to measure these yet).
just realised the CR of swapping heads would actually be 10.15:1, not the 10.4 I said earlier, failed to take into account the extra volume of the gasket+piston dish in the calculation.
but the 33&43 are very close to the mathematically calculated chamber volumes if you take gasket thickness and piston dish into account (although this relies on data from websites for the compressed gasket thickness & piston dish as I haven't been bothered to measure these yet).
just realised the CR of swapping heads would actually be 10.15:1, not the 10.4 I said earlier, failed to take into account the extra volume of the gasket+piston dish in the calculation.
Ill have to CC the heads I have as I have a burette at work for doing it accurately
the mivec pistons have a negative dome volume for higher compression (although I have not measured what it is) although this is kind of irrelevent due to no-one wanting to use the 2000cc block
If people want to build high comp motors. looking at honda B series pistons is the way to go as alot of them are 81mm same as the 6a13. just a matter of getting rods to match. but thats easy
the mivec pistons have a negative dome volume for higher compression (although I have not measured what it is) although this is kind of irrelevent due to no-one wanting to use the 2000cc block
If people want to build high comp motors. looking at honda B series pistons is the way to go as alot of them are 81mm same as the 6a13. just a matter of getting rods to match. but thats easy
- Thread starter
- #27
Hey Akys that's a good post Thanks for all the info
I've built a high comp Honda motor before it is actually getting tuned by Steve now after numerous issues with the past tuner if a Honda piston will fit you can get it off the shelf in an diameter you want my motor I bored out to 86mm and stroked to get it to a 2l and put 12.5:1 comp pistons with a decked head and block
I've got heaps of spares from my build sitting at home so if anyone had a block we could try it all in I would love to have a play and figure it all out so I can do it to my motor
as for the mivec heads I want really planning on swapping heads at all but what's the real benefits of it?
I've built a high comp Honda motor before it is actually getting tuned by Steve now after numerous issues with the past tuner if a Honda piston will fit you can get it off the shelf in an diameter you want my motor I bored out to 86mm and stroked to get it to a 2l and put 12.5:1 comp pistons with a decked head and block
I've got heaps of spares from my build sitting at home so if anyone had a block we could try it all in I would love to have a play and figure it all out so I can do it to my motor
as for the mivec heads I want really planning on swapping heads at all but what's the real benefits of it?
apart from the obvious benifit of the Mivec mechanism which gives you the ability for a boader torque curve with custom cams.
the only other advantage is the Mivec heads are solid lifter and the rockers piviot off a shaft so they are much better for extreme RPM applications. this is why I chose the Mivec heads becase I plan on revving my engine far harder than the stock 7250 rev limit,
and as above the the valves are smaller, the intake ports are smaller and you basicly need a heavily modified intake manifold or custom manifold to get the mivec heads to work on the 2.5 block. so really its a case of 2 steps forward 3 steps back when going mivec.
Hopefully I will have acess to a flow bench soon so I can provide flow figures on both ported and standard 6a12-mivec heads and 6a13tt heads
the only other advantage is the Mivec heads are solid lifter and the rockers piviot off a shaft so they are much better for extreme RPM applications. this is why I chose the Mivec heads becase I plan on revving my engine far harder than the stock 7250 rev limit,
and as above the the valves are smaller, the intake ports are smaller and you basicly need a heavily modified intake manifold or custom manifold to get the mivec heads to work on the 2.5 block. so really its a case of 2 steps forward 3 steps back when going mivec.
Hopefully I will have acess to a flow bench soon so I can provide flow figures on both ported and standard 6a12-mivec heads and 6a13tt heads
- Thread starter
- #29
ok so the intake and exhaust manifolds off the 6a13tt don't fit on the 6a12 mivec heads yeah?
im not looking at having to fabricate and modify everything because the car is my daily so im going to look into cams pistons and strengthening the motor over all
im going to start looking for alternate pistons that will fit and go from there
im not looking at having to fabricate and modify everything because the car is my daily so im going to look into cams pistons and strengthening the motor over all
im going to start looking for alternate pistons that will fit and go from there
the exhaust manifolds are the same flange between 6a13 and 6a12, its just the intake that is different.
but remember that you need to mess about with the cambelt idlers and what not to make the 6a13 cambelt work with the taller mivec heads.
all depends on what your power goal is for the car. Like I said above the only reason I went for mivec is because I want to pull some disgustingly high RPM with my motor, just 'cos I can.
and I don't even know if ill keep the mivec mechanism, I may delete the high lobe off the cam and pull the secondary rocker off the mivec to reduce valvetrain mass.
but remember that you need to mess about with the cambelt idlers and what not to make the 6a13 cambelt work with the taller mivec heads.
all depends on what your power goal is for the car. Like I said above the only reason I went for mivec is because I want to pull some disgustingly high RPM with my motor, just 'cos I can.
and I don't even know if ill keep the mivec mechanism, I may delete the high lobe off the cam and pull the secondary rocker off the mivec to reduce valvetrain mass.
- Thread starter
- #31
Ok a bit of an update got a motor ive now stripped and am prepping for a rebuild and I'm measuring up Honda pistons to see the difference and if I can fit them to up the comp
I'm also looking into cams for the 6a13tt heads and coil over plug set up with e85 flex fuel Ile post up picks as I go and let everyone know what actually fits and works
I'm also looking into cams for the 6a13tt heads and coil over plug set up with e85 flex fuel Ile post up picks as I go and let everyone know what actually fits and works
- Thread starter
- #32
Got the motor all stripped and painted
the heads have been stripped and cleaned and im going to start porting and polishing tonight just waiting to hear back from Meek about a seal kit abs bearing etc
the heads have been stripped and cleaned and im going to start porting and polishing tonight just waiting to hear back from Meek about a seal kit abs bearing etc
Actually these guys were using race fuel in the US, higher octane than E85- it made ZERO
So again, it really does depend on a number of factors, just upping the comp ratio is incredibly simplistic in the big scheme of things. Don't expect any gains- Thread starter
- #36
Yeah you won't huge power figures but that's not what I'm looking for.
The higher CR will give alot more bottom end response and torque and should with the right gearing should be a pretty quick and fun Streeter with no drama's of big turbos conversions etc
The higher CR will give alot more bottom end response and torque and should with the right gearing should be a pretty quick and fun Streeter with no drama's of big turbos conversions etc
- Thread starter
- #37
Ohh and if e85 is set up right you will have a big difference between that and pump on high comp if you had ac drive of some of the high comp Hondas I've been in you would see the difference
- Thread starter
- #38
So I've done a bit of work the last few nights I've got the intake runners and heads nearly finished with porting and polishing the block is clean and ready to get machined now I've just got to find pistons and rods with the right cr
I had a talk with the guy from cp pistons and he wad about as useful as tits on a bull but if I can still source cp pistons I would be happier than going a different brand so if any one had any other ideas on where to check next let me know
I had a talk with the guy from cp pistons and he wad about as useful as tits on a bull but if I can still source cp pistons I would be happier than going a different brand so if any one had any other ideas on where to check next let me know
- Thread starter
- #39
Ok so in going with prism forged rods which I've picked up pretty cheap abs I found 9.5:1 pistons that in ordering from the states and I'll deck the block to get the cr in looking for but over all is 1700 for piston rod combo now im playing the waiting game till I get my parts
Keep in mind if you are going to skim the heads/block to gain the compresion you want you may run into some piston to valve clearance issues. especially with larger camshafts
Would pay to check this as you probably will need to machine the pistons to give larger valve reliefs.
What is your goal for static C/R? and what cam spec do you plan on using?
Would pay to check this as you probably will need to machine the pistons to give larger valve reliefs.
What is your goal for static C/R? and what cam spec do you plan on using?
