Engine Builders: Stumped looking for some input

[GPW]

I am working with a pre-cut off the shelf set of pistons on an all motor build with rods. I had to fly-cut the exhaust recess areas to get better piston to valve clearance which is presently at .074 -.070 would have been better but less reliable...the current setting works just great.

Piston to head clearance is at .042. there is a window of .038-.042 but the latter is more reliable.

My delema is the intake valve to piston clearance is real loose .084 ish due to the deep precut recess's.

The intake target is .050 for this particular application but I cannot add material to this piston. Angle cutting the head is a distant thought. Use of a larger intake cam with slightly higher lift is an option, retarding the intake cam which is presently at 106 is a poor option as power would suffer.

My real question is to the guys more experienced than I, what kind of power if any would I gain by reeling that large intake gap in assuming the piston to head number stays the same.

I shoot for perfection with everything I do, this however has me slightly stumped. Are my hands tied here??

The clay impressions below visually show the above problem.

Please if you have not built a Hayabusa motor with your own measurements leave this thread alone. Thank you in advance.

Intake UPPER section...

 

[GoldenChild]

Can I ask who made the pistons?

 

[GPW]

They were purchased by the customer they are JE. Not a manufacturer issue not complaining about the pistons. They just appear to not work best with the cam profiles.

 

[bbailey]

I'm sure there are people on here with ALOT more experince than me,but why are you trying to close up the gap in the valve relief?As long as you have the proper squish I would be more concerned with not having enough clearance than with having too much.changing the valve lift or cam timing just to use up the valve relief is what I don't get.Maybe I can learn something from this myself.
brian

 

[GPW]

Brian I agree with you on this but I want to be certain I am thinking correctly. Someone keeps pushing me to tighten things up further and I can't, further more I don't think we will see any additional power. Just being sure. I may be wrong but I am not too proud to ask. Thanks for the input. I know a tighter squish would get a little more at the risk of a full race season of reliability.

 

[BIG GUY]

Good luck on getting these issues all sorted out. How much power is the customer looking at getting out of this setup?

 

[GIXERHP]

Ive emailed this post, to a friend, im sure he will chime in soon!

 

[GoldenChild]

They were purchased by the customer they are JE. Not a manufacturer issue not complaining about the pistons. They just appear to not work best with the cam profiles.

ok was asking as they may have been custom with taller deck.Sorry to bother.

 

[GPW]

ok was asking as they may have been custom with taller deck.Sorry to bother.

They are 13.5:1 pistons, I was just clearing up that there was no specific problem with the piston its self.

 

[GPW]

Ive emailed this post, to a friend, im sure he will chime in soon!

As you know the intake valve follows the piston which is why the clearance can be much tighter on the intake to valve side. I have read from some the top builders that they push that clearance as tight as (Dave "o")

.036 in some cases. I just want to be certain since we can't reach these tighter piston/intake valve tolorences we are not leaving anything on the table.

A different piston to start with would eliminate the problem but this set-up "is what it is". Thanks for the input.

 

[Draco1340]

Greg,
If the pockets were .75 mm more shallow, the volume of the two pockets would be about 1.4 - 1.5 cc less. Making some assumptions on bore and stroke, I'll assume you have about 351.7 cc per cylinder (83 x 65 bore /stroke) and you realistically have about 29.3 cc of total compressed cylinder volume giving you somewere about 13:1 CR. If you subtract 1.4 cc for the valve pocket, it bumps your compression to about 13.6 : 1.

In answer to your question, its 6 tenths of a compression point your leaving on the table. Thats worth more than a couple of HP. If you provide specific engine dimensions I can make a more accurate calculation.

 

[GPW]

Greg,
If the pockets were .75 mm more shallow, the volume of the two pockets would be about 1.4 - 1.5 cc less. Making some assumptions on bore and stroke, I'll assume you have about 351.7 cc per cylinder (83 x 65 bore /stroke) and you realistically have about 29.3 cc of total compressed cylinder volume giving you somewere about 13:1 CR. If you subtract 1.4 cc for the valve pocket, it bumps your compression to about 13.6 : 1.

In answer to your question, its 6 tenths of a compression point your leaving on the table. Thats worth more than a couple of HP. If you provide specific engine dimensions I can make a more accurate calculation.

Thanks Draco, using a virtual engine builder program it had the CC's per cylinder at 361 I think. Your bore and stroke numbers are accurate. I did not CC thead so I don't have those numbers for you.

With the piston to head clearance at .042 now the tightest I would go safely would be .038, thats on the tight side and would put the exhaust valve to piston clearance at around .070 which "might" be ok but if we extend the rev limiter any there could be problems even with rods.

Gosh wish you were closer, I am sure we could build a few monsters together. You have the engineering/math side I am missing.

This is the info I was looking for thank you!

 

[Draco1340]

Greg,
To get back that lost 1.4 cc in the valve pockets, you would need to remove .25mm (.010") of head gasket or piston to head clearance. No can do. Sorry. Put bigger cams in instead perhaps?

 

[GPW]

Greg,
To get back that lost 1.4 cc in the valve pockets, you would need to remove .25mm (.010") of head gasket or piston to head clearance. No can do. Sorry. Put bigger cams in instead perhaps?

Thats what I was thinking an intake cam with higher lift might do the trick. I have been beating my head against the wall with this for over a week. Trying to do it all myself without any help. This a perfect example of "everyone" needs help sometimes. I will chat with the customer to see what he wants to do. I owe you big time! ....he may go for the dual cam upgrade.

 

[Professor]

Loose the base gasket (if there currently is one). Then cut the top of the piston to get your P-H clearance closer. I'm running .0365 p-h on mine with stock rods. I wouldn't spray mine with the clearance I have however. You can also get a .027" head gasket if you want to tighten that up. Thermal barrier coatings on the top of the piston can also be worth .002-.004"

I think what rods you are using and the intended use/abuse of the engine will be factors in how you can get closer to your desired numbers.

On the other hand, there is also the option of another set of pistons that closer meets your targets.

 

[GPW]

Loose the base gasket (if there currently is one). Then cut the top of the piston to get your P-H clearance closer. I'm running .0365 p-h on mine with stock rods. I wouldn't spray mine with the clearance I have however. You can also get a .027" head gasket if you want to tighten that up. Thermal barrier coatings on the top of the piston can also be worth .002-.004"

I think what rods you are using and the intended use/abuse of the engine will be factors in how you can get closer to your desired numbers.

On the other hand, there is also the option of another set of pistons that closer meets your targets.

What size cams are you using and what is your exhaust valve to piston clearance? Great info and thanks for chiming in. I agree on a fresh set of pistons. CP 13.25's seem to be the best fit with these cams they also require some trimming but the valve pockets will be much better.

 

[Professor]

I'd have to dig out some notes on the P-V. The Cams are the small webs. 378/330. This is in a Gen I engine with a Gen II crank. Stock bore still. Cam timing can make a pretty large change in clearance. My exhaust cam is pretty small compared to what most run regardless. It is pretty close to the stock 08 exhaust. Of course lift on the nose is not even close to where the clearance issue comes into play. My cams are not very agressive regardless.

 

[GPW]

I'd have to dig out some notes on the P-V. The Cams are the small webs. 378/330. This is in a Gen I engine with a Gen II crank. Stock bore still. Cam timing can make a pretty large change in clearance. My exhaust cam is pretty small compared to what most run regardless. It is pretty close to the stock 08 exhaust. Of course lift on the nose is not even close to where the clearance issue comes into play. My cams are not very agressive regardless.

Thats our big difference these cams are pretty aggressive. I like how they pull out the door and through the mid-range with zero bog or power drop off so adjusting the piston is the best route, great racing cams.

I am going to finalize all my measurements and have a few sets of custom pistons done for my personal kits to work with, so we can see max power vs. trying to make various off the shelf pistons work with this head/cam combo. Trying to decide on the brand. Thanks for the input.

 

[GoldenChild]

Be very careful with the cp pistons you order as we had a 12.5 to 1 stock spec'd motor with only a piston upgrade,cc out 14.5 to 1 as they made a 3mm bigger piston but with no dish and called it 12.5 to 1 pstons.we confronted them and was basically told oh well.

I would talk with John at Woessner and see what they might can do for you.

 

[bbailey]

I can see how making the valve relief smaller would effect the C.R.But what I dont see is how using all of the valve relief will affect power.I know a larger cam or cam timing or less valve relief cc will change something,but just using all of the space available should not change anything.