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Discussion Starter #1
all right, so i took my block and crank into the machine shop today to be washed and miced out and all that good stuff. well when i told the machinist all the info on what i was going to do. which is im just taking a stock y7 block, rods, crank, head gasket and all and just throwing on some p29's ( .020 over) on the rods and thats it. Im not shaving the head down or anything. I just want a lil bit more compression from the pistions. but when i told him this, he asked so what kind of compression you going to be running and i said its gonna be around 12 to 1, according to http://www.zealautowerks.com/dseries.html. He was tellin me that im gonna have to be running race fuel all the time cause 93 octane wont cut it cause its gonna wanting to be pre-igniting. Is this true or can i run 93 octane without problems? All Info would be greatly appreciated!!! Thanks
 

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1993 Cobra
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your gonna wanna run 93 octane and a tune. I think he's talking about detonation/pinging. Tune + 93oct = happy you :TU:
 

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iirc, 12:1 is the limit for 91/93 oct.

btw under dynamic compression my boosted 9:1 y7 was actually a 14:1 engine. and it was running 91/93 oct. i had no issues with ignition. just get a tune and you should be good.
 

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Discussion Starter #4
I think i already know the answer to this question but cant tune a OBD2 ecu to at all to compenstate for the pre-ignition thats gonna be happening can you.
 

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you can pull timing from the distributor and change cam timing to help bleed off some of the pressure. it will not be the best way to do it, but i can be done. i would just swap over, but go expensive and get jumpers for everything you can. that way you can revert back if need be for smog.
 

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1993 Cobra
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^that plus you could get a safc hackjob to add more fuel.**

**Note: They mess up regularly, watch your afr's.
 

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Discussion Starter #7
so its probably just best to convert over to OBD1 with my p06 ecu and get a tune and see how it preforms from there. Thanks alot 99ejt and mightyjoecivic!!!
 

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allwheeldriveturboboxer
'08 Mazdaspeed3
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if i recall, p29's with an untouched y7 head will yield the same as a y8/p29 combo, which is over 12.5:1 CR. i thought it had a 32.8cc volume? anyway, that is extremely high for 93, even with a good tune. thankfully the combustion chamber is decent on the y7, but still...

i wouldn't do that using a stock headgasket. i'd look into a thicker HG like what i mentioned in the sticky that is floating in either this subforum or the all motor one.

what cam? that will make a difference too....

if the cam has little to no overlap, like a stock or reground cam, (overlap meaning time when both the intake and exhaust valves are open at the same time) then the compression will be higher when it's running (dynamic, as mentioned by 99EJ6T) than if you are running an aggressive cam profile. the aggressive profile will bleed off some compression during the compression stroke. i don't mean to insult your intelligence if you already know this.

now, 12:1 with bkr7e-11 NGK sparkplugs will be fine with 93 octane and a good tune. you can chip the p06, or p28, etc and see a good dyno tuner.

but 12.5-12.6:1...i'd think very hard about doing that.

whatever you do, trailer the car to the tuner. making a freshly built engine deal with overly rich conditions and retarded timing as a band-aid for the trip to the tuner can really hurt the engine's potential and/or kill it.

EDIT: looks like i was wrong about the CC of the y7 head, so, if that is true, you would indeed be back around 12:1 :)
 

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I run 92 octane with 12.5:1 at 28 degrees total timing.

preignition is a heat induced event

detonation is a pressure induced event

my motor does neither
 

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allwheeldriveturboboxer
'08 Mazdaspeed3
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I run 92 octane with 12.5:1 at 28 degrees total timing.
that isn't that much timing, plus you are cammed up : P

you can do different stuff to help an engine make it at higher compression levels, and alot of it is shit that should be done anyway while the engine is apart, and some is in the tuning (like running less timing, decent fuel pressures), but by philosophy is to leave some buffer room for a street car. i mean, if it's tuned close to the ragged edge, you are more likely to encounter issues, increased wear, popped headgaskets, etc, all for what- a few ftlbs.

it's a compromise- more timing, a little less compression might give you the same power, vs. retarded timing and a hair higher compression.

i put a heck of a lot of miles on my 12.X:1 compression y8 (like 55-65k, i forget). most of that was due to the Evans tune for sure, but the build wasn't bad either ; )

i have a similar notion for street turbo guys. if you bump up the CR, you don't have to worry about running AS big of turbo because you can get the cylinder pressures up so high with less boost, because of the compression, and at the same time you can help spool.
 

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allwheeldriveturboboxer
'08 Mazdaspeed3
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This guy is doing 13:1 with pump gas (92 oct). i can't see 12:1 is a limit..

http://speedfactory.auto.officelive.com/projects.aspx
They could have used a factory headgasket, with the associated lowered compression, both of which would heighten reliability, and still made the same power if they made a few changes in part selection, all of which would have cost no more than what they have. I'd like to see the graph close up so i can see the curve, because it looks like it's pretty jumpy.

not bad though, overall. glad to see anyone build a d at this point. i honestly don't think they care if they pop that thing, whereas someone like the OP would.

for a daily, stick with around 12:1 IMO.
 

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Brokedick Millionaire
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who let the dawg out??????

Someone kick the doghouse to rile up builthatch :)

I'm helping plan a 10.5:1 tiny turbo D15 for MILEAGE/PERFORMANCE, it shall be a blast.
 

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Discussion Starter #19
I'm helping plan a 10.5:1 tiny turbo D15 for MILEAGE/PERFORMANCE, it shall be a blast.
who you plan on building this build with and you ever gonna release the specs on it when you do get it going?
 

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Chances are also good that your machinist is used to working with V8's. Big bore engines have a much harder time with detonation and preignition at high compressions than small bore engines do.

A cam with some overlap should ease the pains for sure. :D
 
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