Author Topic: Flat top- dished pistons Engine rebuild problem (Read 581 times)

Steve · « **on:** May 03, 2005, 09:24:48 PM »

Advise welcome and needed please!

I instructed guy building my bottom end to get dished pistons. When I went to pick up my block it had flat top pistons(by probe). This makes the compression w/ the dart heads at 11.7 to 1.
According to my buddy who will be helping me put my motor together, he said that compression is too high to run pump gas; that the engine will ping like mad.

In order to reduce compression and get my motor back(the guy has had it WAY too long) my friend said we should go to a 72 cc combustion chamber pair of heads instead of the 64 cc heads we had originally ordered.

I trust this guy but I would welcome other opinions. Will the flat top pistons w/ the 72cc heads be comparable to what my initial setup was to be?

Any input welcome and appreciated. thank you!

stantond · « **Reply #1 on:** May 03, 2005, 10:04:40 PM »

Compression ratio is actually the volume (or volumetric) ratio. By increasing the clyinder head volume when using a flat piston you should get a similar volume ratio with the dished piston and standard heads. I say similar because I don't know all the details regarding your 'dished' pistons and original heads.

Yes, an 11.7:1 compression ratio is too high for 93 octane pump fuel. However, you might get by with an octane booster and retarding (delaying) the ignition timing if you want more horsepower. The solution your buddy suggests sounds pretty reasonable if you don't want a high compression engine.

Regards,

Malta

Geary420 · « **Reply #2 on:** May 03, 2005, 10:35:03 PM »

Well Steve, that 200 shot is deffinately out of the question now, might as well just box it up and send it my way

. Are you able to just send back your new Darts for 72cc set? Otherwise it would seem more reasonable to just have the guy swap out the slugs... If you ordered it right and he built it wrong its his problem not yours, I understand the frustration and wanting to get it done though.

~~Geary~~

john9001 · « **Reply #3 on:** May 03, 2005, 10:38:15 PM »

many questions.

are the heads on your block?

if he built the botton end why did he put the heads on?

are you really sure they are flat top?

how much trouble would it be to pull a head and see what pistons are in there?

did he give you a parts receipt, (check part numbers)

Steve · « **Reply #4 on:** May 03, 2005, 11:40:52 PM »

Geary, why is the 200 shot out? This is major.I am meeting him tomorrow at 9:30 AM>

The heads are not on the motor, but my friend has seen it and it is definitely flat top.

He bought the heads bare and assembled them, but they are not yet on the block. He says the assembled heads sometimes have valveguides that are too tight, causing the valve to hang up, then slapping the piston.
All the receipt says is "optional probe forged pistons"

MrCoffee · « **Reply #5 on:** May 04, 2005, 12:07:35 AM »

what you need is one of those how to build my engine books at the autoshop.

Geary420 · « **Reply #6 on:** May 04, 2005, 01:08:36 AM »

Comment on the 200 shot was with regards to the 11.7:1 compression ratio. High comp. + N2o = BOOM! If you get it back down to a reasonable level im sure it will be fine, but as previously stated i think 200 shot pushing it with that block. A lot of guys run a lot of spray through L98's and all is good, but most of them are 100 or 150 shots, and probably not on top of a setup thats already makin 450-75. Btw, what tranny are u running? Thanx for the tip on the pre-assembled heads, had not heard that before.

Steve · « **Reply #7 on:** May 04, 2005, 02:17:29 AM »

Gocha Geary. Ya, we are going to go to a bigger combustion chamber , 72 instead of 64. to pull the compression down to 9.5-10.

The 200 shot is a dual stage so I'm hoping it will be ok. If I get info that 200 is a bit much I'll get smaller jets. The tranny is the stock AT for now, until I blow it up I suppose. I'd like to get a nice 4 or 5 gear auto w/ an overdrive... I'm told I'm dreaming.

Same thing w/ the rear end.. dana 35. Will wreck it and get the 44. I don't really plan on racing it more than a handful of times just to see what it will do. I put less than 2000 miles on it in the year prior to tearing it down so I'm hoping tranny and rear end will hold up.

lazs2 · « **Reply #8 on:** May 04, 2005, 09:14:02 AM »

steve... that sounds wrong. It is hard to get allmost 12 to 1 compression with flat tops and 64 cc chambers. if you had pistons that did that then they would have to have huge valve relief's in em and.. I can't belive that going to 72 cc chambers would lower from 12to1 down to 9.5 to one. You might get on compression point for the difference in heads... say, from 10.5 down to 9.5.

lazs

Steve · « **Reply #9 on:** May 04, 2005, 11:24:04 AM »

Thanks lazs, I'm leaving to go to the shop and get this straightened out. I want this car to run on pump gas, so I have to make sure he gets this comp thing right.

Skydancer · « **Reply #10 on:** May 04, 2005, 12:03:40 PM »

My Triumph runs 12.0 to 1!

So would I be better running 97 oct super or standard (over here ) 95 oct?

Just wondering.

nuchpatrick · « **Reply #11 on:** May 04, 2005, 12:18:42 PM »

As long as it's tuned properly it can run 11.0 : 1 on pump gas.

I'm looking to build a 1.8L built to 2.0L (bored to 1995cc) with carrillo H beam rods, wiseco pistons and a few other goodies.. And it will be a 12.0:1 compression motor.

mora · « **Reply #12 on:** May 04, 2005, 12:49:43 PM »

Quote

Originally posted by Skydancer
My Triumph runs 12.0 to 1!

So would I be better running 97 oct super or standard (over here ) 95 oct?

Just wondering.

I'd use 97 if that's the highest you got. And make sure it's not knocking. That's pretty damn high comp ratio, is it a bike?

Skydancer · « **Reply #13 on:** May 04, 2005, 01:02:33 PM »

Yeah Its a 1200 daytona and thats how they build em probly why I'm getting 147 at the rear wheel.

Thanks for the tip been using 95 but will try 97

lazs2 · « **Reply #14 on:** May 04, 2005, 02:15:42 PM »

modern engine control systems allow for higher compression because the control not onlyu fuel mixture but advance curve. With a carb and mechanical/vacuum advance you need to run less lead.

A cam with a lot of overlap will negate some of the bad affects tho because it has lower compression at low RPM... aluminum heads cool better so can take a half to one point more than iron heads. sooooo... it all depends.

skyprancer... I believe your octane is rated differently than ours but I would go with the manufacturers recomendations... I also believe that the daytona is rated at 147 HP at the flywheel not the rear wheel.

lazs