Exhaust port timing ?'s

[N8tsNastyBlasty]

So I know this is a pretty closely guarded secret as far as all out drag/dune porting but that's what i like to do. I was fortunate enough to get some email feed back from flotek on some pointers and it totally helped tremendously on the path to a kickass diy port job. I think i might have gone to far but im not sure.I think i have the exhaust and tripple port timing at 193*-195*. the only way i figured out how to increase the duration was to raise the exhaust port by quite a bit and now it opens at like 77* atdc = or- a degree or 2. ive read that alot of ppl say 90* atdc is good. I put it together and it's way fast but the compression is only at 120lbs and that's with a vito's thin head gasket,thin base gasket,and .025 cut off the head. The one debilitating factor is that i got a good deal on 2 pistons thinking they would be over sized and they weren't. the bore was lightly honed and assembled with its third piston. I have a new piston to correct that issue. I'm getting ready to port my other jug and curious if anyone thinks my porting caused some of the low comp in the first build? thanx for any help.!

 

[OConnor Racing]

Is the head shaved or re-designed?
What is the squish?
What are the CC's of the head?

The timing seems OK. We all have our favorite numbers. One builder was running over 200 degrees on a cylinder I saw. You'll drop compression when you raise the exhaust port. You can make it up by changing the design of the head or the top of the cylinder or both. You need to watch out when you just mill the head. It can cause overheating problems.

 

[darederek]

you probably have a gasket leak. vitos gaskets are junk i would recommend only ever using yamaha OEM gaskets and many people on here will back me up on that. awk will tell you how easy he can collapse those cheap gaskets lol

 

[CHutzell]

if your Ex. port opens at approx 77 deg. ATDC your duration right now is approx 204 degrees of open time.Unless you have machined the cyl.head to match,your compression is going to be really low


by the way what cylinder are you using?

 

[N8tsNastyBlasty]

I'm using a stock cylinder with a thin base gasket the metal vito's kind.it's glued in with a thin layer of high temp copper rtv. I pressurized the jug with air and had no bubbles anywhere on the case. the head is a stock head that i had resurface down until the inside edge/lip is almost gone me and my bro measured it at about .025.I think he may have been rotating the engine the opposite direction when he said it opened at 77atdc i will double check it. I planned on checking the compression and adjusting the fuel octane.

 

[N8tsNastyBlasty]

do i measure from when the top of the piston begins to clear the port or the rings? what should i expect to see the exhaust open at atdc to achieve the desired 192* ? i set tdc by watchin the piston move and by pulling the connecting rod all the way out until it quit moving. btdc seems pretty spot on also.

 

[CHutzell]

Your best bet is to have someone modify your cylinder head,depending on what fuel you want to run,petrol or methanol a single stage or 2 stage hemi design is what you want.It really doesnt matter if you rotate the crank either way your wheel should give you the same reading.
How about your transfer port timing what is that?You said you modified your sub exhaust ports is this on the stock cylinder casting?

 

[CHutzell]

you want to measure from the edge of the piston crown yes.To get a duration of 192* open your exhuast port should open at approx 83.9* ATDC.

Are you using a dial indictator to locate TDC?

 

[N8tsNastyBlasty]

i have a digital caliper i could try to get bdc with the depht gauge on it. let me go double check it

 

[N8tsNastyBlasty]

talked to my bro and i'll have to get my numbers exact to get this right he said that 77* atdc wasnt accurate and that he just marked where it opened and closed then did the math. tdc wasnt spot on. thanx for the input for a newbie.

 

[N8tsNastyBlasty]

just reset my btdc with deph gauge on my unported jug/mock up engine and got 84* atdc for a duration of 192* just as Chutzell said i would. now im going to take the jug off my low comp engine and recheck my port timing.

 

[N8tsNastyBlasty]

double checked my timing with the degree wheel in place at bdc and found that the exhaust opens at 82.5* atdc and the transfers open at around 114*. good thing i took it apart cause i wasted the piston skirt.the head gasket was vito's thin one and had oil all the way to the outside edge. probably didn't seal well.I torqued it 1lb over spec when I assembled it. thanx for ll the help