Performance Boats Forum banner

1 - 20 of 43 Posts

·
senior member
Joined
·
1,814 Posts
Discussion Starter #1
not sure if i should stroke my 476 bbc or destroke it?(or leave it alone )i have a standard stroke motor now & was told that turbo motors like sm. ci motors. i can see if the turbo is not big enough to fill the new cyl. volume , you would get less boost. so on that theory should i destroke the motor for more boost & cram in the longest rods i can fit in there? right now i have rajay turbos with .65 turbine housing
 

·
Registered
Joined
·
2,234 Posts
If it was me, in the interest of curiosity, I would leave it alone and see what happens, I have heard also that camshaft selection is a larger player in the turbo "game" than CID, but don't know how much theory is built into that..boost is determined buy the speed at which the impeller spins and the size of the impellers and openings, which I guess could be translated into it has to come out before it goes in, meaning the pressure and volume of the exhaust gasses are what ultimately gives you the boost, I would translate that to meaning the exhaust duration should be much larger than the intake, but that's just my opinion, never tried it but should theoretically work...My buddy always uses different rocker ratios from intake to exhaust to enhance this further, has had large successes with big inch turbo power in pull trucks, maybe he's just lucky...

Would love to see some progress on this project;) There is nothing like turbos..."The quiet sleeper" :)devil

GT :)hand
 

·
senior member
Joined
·
1,814 Posts
Discussion Starter #3
do you know what rocker arm ratios he's running & what h.p. range were talking about & who does his engines?
 

·
Registered
Joined
·
2,234 Posts
GT Jets - Turbo cam grinds have short duration on the exhaust side. This produces velocity. Here is a Comp Cams baseline grind.

http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=426&sb=2[/url]
I thought I had it straight in my head, apparently not.... that actually makes perfect sense, keeping the pressure in the cylinder and rapidly releasing it would be more benificial, I had it in the memory banks backwards, thank you for the clarification.
That and I just had a tooth repaired yesterday afternoon :)grn, a little punchy...:)coffee

The fact remains that cam choice is of higher importance than bore/stroke right?

GT :)hand
 

·
Registered
Joined
·
2,234 Posts
do you know what rocker arm ratios he's running & what h.p. range were talking about & who does his engines?

He called his rocker arm ratio's a secret (I noticed because of two different brands of rocker and different style adjustable pushrods while in his shop). I do know he runs 510 CI and horsepower is always in the 1200-1500 range, he typically turns his motors at 6500 RPM + and they are very short legged (power curve comes in with boost) I don't think they really start working hard until 4500 RPM. I think I have a video around here somewhere, I'll see if I can convert it and put it on Photobucket (it's in VHSC).

GT:)hand
 

·
Registered
Joined
·
54 Posts
I thought I had it straight in my head, apparently not.... that actually makes perfect sense, keeping the pressure in the cylinder and rapidly releasing it would be more benificial, I had it in the memory banks backwards, thank you for the clarification.
That and I just had a tooth repaired yesterday afternoon :)grn, a little punchy...:)coffee

The fact remains that cam choice is of higher importance than bore/stroke right?

GT :)hand
The cam choice is very important. You got it straight.
 

·
mo balls than $cents$
Joined
·
11,734 Posts
not sure if i should stroke my 476 bbc or destroke it?(or leave it alone )i have a standard stroke motor now & was told that turbo motors like sm. ci motors. i can see if the turbo is not big enough to fill the new cyl. volume , you would get less boost. so on that theory should i destroke the motor for more boost & cram in the longest rods i can fit in there? right now i have rajay turbos with .65 turbine housing
leave it alone and get bigger turbos. hass828 is doin 120gps in his heavy stealth with a 468 and lil rayjay turbos:)bulb he's goin bigger turbos soon too.
 

·
mo balls than $cents$
Joined
·
11,734 Posts
just read you ar is .65, that's your issue, your turbos are spooling up to fast in the rpm and coming outta peak on the topend. you need bigger turbos with a 1.1 ar or something close, even a 1.32 ar would be alotttt better than your current deals.
 

·
senior member
Joined
·
1,814 Posts
Discussion Starter #10
i tried the 1.0 or 1.3, i can't remember, but i thought it went faster all the way around with the .65 . i better do some note reading on that day again. although i think i have been loosing the motor since laborday or prior. i know i hydro locked the motor & pulled all the plugs out on labor day & couldn't turn the motor over it was jammed . it finally, bearly moved( with the plugs out ) so maybey i should retest some things again when i get the motor back in .
 

·
mo balls than $cents$
Joined
·
11,734 Posts
i tried the 1.0 or 1.3, i can't remember, but i thought it went faster all the way around with the .65 . i better do some note reading on that day again. although i think i have been loosing the motor since laborday or prior. i know i hydro locked the motor & pulled all the plugs out on labor day & couldn't turn the motor over it was jammed . it finally, bearly moved( with the plugs out ) so maybey i should retest some things again when i get the motor back in .
think of it this way, you're peaking boost way low instead of leaning against the impellor with peak boost at efficiency. we're building a 496 twin turbo blow thru motor for my uncle and using 1.32a/r on it to lean up against a big impellor(aa-cut), the boost should be at peak efficiency on the 1.32 ar turbos from 6-7krpms, right where we need it. with a .65 ar , and the same motor, we'd peak boost from 3500-4500rpms and any rpms above that, is just excessive backpressure and heating up the compressors.sure boost may increase, but it isn't really boost you're seeing increase, its the heat expanding the air charge volume , and it turn, psi raises since the intake tract doesn't get any bigger.

been doin alot of research and reading(i even bought a 200 page book) to try and totally understand compressor maps /ar's/ compressor housing sizes etc.. and what i see would be the easiest thing to overlook and cause yourself hp loss is the wrong a/r. a small turbo with a bigger a/r will still do the job, but do it in a higher rpm range and not make alot of boost doin it, although its efficient boost, not heated boost with to much backpressure in the exhaust.

most guys who run nos will tell ya it doesn't help their mph much. well i've never had that issue and the reason why i the way i set up my pumps. i want my power leaning against the impellor instead of runnin past its peak or not turning it hard enough. same thing applies with boosted motors, lay that tq up against a big impellor so its usefull:)bulb

put a backpressure gauge on it, that'll tell ya alot!
 

·
steelcomp was here
Joined
·
26,512 Posts
Shoot for a rod ratio as long as you can, near 1.7 as possible. De stroking will allow longer rod...don't worry about losing displacement, you;re making that up with boost. The dynamic advantages of a longer rod will far outweigh losing a few inches.
JMO
 

·
Registered
Joined
·
103 Posts
My little blower deal runs a 1.782 r to str ratio. Always acts like a big guy. but is just a little fella. 6.700 oliver 3.760 crank 4.320 bore. Good or bad its been a lot of fun using an odd combo. It loves to spin. Next time I'm going to put one of the bigger inch fellas together. Jim
 

·
Premium Member
Joined
·
2,145 Posts
Shoot for a rod ratio as long as you can, near 1.7 as possible. De stroking will allow longer rod...don't worry about losing displacement, you;re making that up with boost. The dynamic advantages of a longer rod will far outweigh losing a few inches.
JMO
Agreed. Longer rod = more dwell time = more cylinder pressure exerting on the piston over time. IF you have to buy pistons and rods, I'd do as Steelcomp suggested. If the motor is already together, don't take it apart just for that.
 

·
Cantard
Joined
·
5,017 Posts
Shoot for a rod ratio as long as you can, near 1.7 as possible. De stroking will allow longer rod...don't worry about losing displacement, you;re making that up with boost. The dynamic advantages of a longer rod will far outweigh losing a few inches.
JMO
So something like a 4" stroke 6.385 rod and 4.125 bore would work ok?
 

·
Cantard
Joined
·
5,017 Posts
My little blower deal runs a 1.782 r to str ratio. Always acts like a big guy. but is just a little fella. 6.700 oliver 3.760 crank 4.320 bore. Good or bad its been a lot of fun using an odd combo. It loves to spin. Next time I'm going to put one of the bigger inch fellas together. Jim
Jez man.... I think you need some longer rods.:)hand
 

·
steelcomp was here
Joined
·
26,512 Posts
Agreed. Longer rod = more dwell time = more cylinder pressure exerting on the piston over time. IF you have to buy pistons and rods, I'd do as Steelcomp suggested. If the motor is already together, don't take it apart just for that.
That's all worth while stuff, but more important (IMO) is rod angle and piston side loading under boost. There's a lot of parasitic drag in short ratio dynamics. In a blown application, you really can't take advantage of the high piston acceleration rate of a short ratio so the added negatives are only that...negatives. Pushing the pin as high as possible in the piston also helps stabilize piston rock, which promotes better ring sealing.
 
1 - 20 of 43 Posts
Top