I recently had a 350 late model roller motor for my 91 tpi and for some reason im only gettin 263 rwhp and 333 rwt and i have alot into this thing, lt4 hotcam, ported 083 heads with 2.02 1.60 valves, headers, throttle body, ported plenum, and the guy who tunned it said he did what he could without hurrtin the motor, and he acctualy asked me if it was a 5.0 block because it was not needing that much fuel and the horsepower and tourqe numbers reminded him of built 5.0 tpi`s and that i should be gettin alot better numbers. Any ideas on whats goin on here guys My only guess is that the guy who built my motor didnt put the right compression of piston in it because i asked hime to make it 9.7 to 1 but i think he made it like 8.5 to 1.<----- would this make my setup act that sluggish, and if so how could i check to make sure its 9.7 to 1 without taking the motor appart?

 

To measure the compression ratio, you would have to know exactly what pistons are in it and what head gaskets were used.

Just go rent a compression tester from auto zone. If you know what cam you have, and you find out your cranking compression numbers, you can roughly find out what your compression ratio is.

I'm not extremely familar with the TPI numbers, but those dont sound that bad. Thats probably a little over 300 horse and 400 foot pounds at the motor.

 

ok cool ill do that asap and post the numbers to see if there where they are suppose to be

 

Do a compression test to see what the cranking compression numbers are. That will give you an idea where your at and what performance to expect. Check all the cylinders to get an idea how each one is performing.

How ported are the TPI components? Did you port the base too or is it aftermarket? Stock TPI?

Those numbers arent too bad if stock TPI. Really arent all that bad for ported stock TPI. Run it at the track and see what it traps, dont necessarily go by dyno numbers. Now if the heads are ported fairly well and the intake stuff is aftermarket, then I'd expect a bit higher hp numbers and torque.

 

the base is stock the plenum ifs ported all the way for the 58mm throttle body, gonna do the compression test this evening hopefully the guy built it right and the dyno is just way off. Where should the compression range be for the 9.7 to 1 ratio?

 

Not 100% sure but most limit pump gas at 200 psi cranking compression....your build is not pushing the limits there, so i'd expect in the 150-170 range. 9.7 to 1 on an iron head is fine, usually over 10 to 1 starts to get alittle picky and needs a slightly larger cam. hotcam isnt huge but isnt really small either, so in that range above makes sense to me.

I think i have a calculator back home i'll try with those cam specs and see what it estimates.

 

cool i realy appreciate the help

 

150-170 range?

I just did my compression check and got inbetween 120-135 on all 8 cylinders

350, HSR, iron vortec heads, 268 cam.
My car also only made 246rwhp and 279rwt on a dyno tune.

 

If you can get access to a bore scope (a little camera on a 'snake' that goes through the spark plug hole) you can see the piston. A good machine shop may have one. That, along with knowing what heads you have by casting number, should give you an idea of what the CR is. Its the most accurate method short of removing the heads.

 

If this is a stock TPI motor with:

1. fairly mild Cam
2. Mild head porting
3. Mild manifold porting

I don't see how those numbers are so out of line.

The throttle body is useless and didn't gain you anything.

Those numbers don't sound that far off...

 

whats your static compression?

I just did a quick calc and I got 156 psi for the OP's hotcam motor above at 9.7 to 1 compression, at 150 rpm starter crank speed.

 

Static compression, when the engine is being turned by the starter, is right around 120-135psi on each cylinder.

 

I mean whats your static compression ratio? 9 to 1, 9.5 to 1? etc...

120-135 would usually mean less than optimal ring seal but these are just estimations/calculations.

 

Did you have the throttle open while doing the compression check? I've heard it can make a difference.

 

engine should be warm too

 

Engine not warm, throttle not open. I'll recheck a couple tonight with the TB open, but how would i get the thing warm? I already have the headers off. I do own a big kerosene heater, could blow that into the engine bay for an hour or so.

 

I assumed you had it running and could run it for a few minutes at idle, shut down and pop out the plugs to check the compression that way heater may work but even cold should show accurate enough information.


And FYI, i just did some searching on here and it seems that most TPI 305-350's are in the 170-180 range when stock with their cranking compression. So 120-135 is abit low, but you'd see abit less than stock due to larger cam/later intake closing, just dont think it would be that much of a difference.

 

Thanks Orr.

I'm not putting my hands around those headers after the cars been running for a minute or two. You can gladly come over and do that part for me

 

been there done that.. no thanks you can borrow my welding gloves, they should help