SBC TPI 383 ways to reduce torque

[nilak]

Hello,
I'm upgrading my 1991 Corvette to a 383SBC but the torque output is much higher than stock, especially with the stock manifold-long runners, 480Nm vs 650Nm. That would put stress on the transmission, diff and C beam, so I'm searching for ways to reduce max torque to about 500Nm. Power can still remain the same for the upper range.4500-5600. I will use stock manifold and large intake tubes AZS because I need the stock appearance so I need to do it by ECM tuning. One I know is by reducing the spark advance but this I understand would use the same amount of fuel and producing less torque/power. I thought of trying remapping the EGR to run at high throttle and mid rpm so that the amount of oxygen is reduced. Is this a bad idea?
Thank you.

[Fast355]

Hello,
I'm upgrading my 1991 Corvette to a 383SBC but the torque output is much higher than stock, especially with the stock manifold-long runners, 480Nm vs 650Nm. That would put stress on the transmission, diff and C beam, so I'm searching for ways to reduce max torque to about 500Nm. Power can still remain the same for the upper range.4500-5600. I will use stock manifold and large intake tubes AZS because I need the stock appearance so I need to do it by ECM tuning. One I know is by reducing the spark advance but this I understand would use the same amount of fuel and producing less torque/power. I thought of trying remapping the EGR to run at high throttle and mid rpm so that the amount of oxygen is reduced. Is this a bad idea?
Thank you.

EGR is a bad idea all the way around it will cause massive amounts of carbon buildup in the plenum, runners, etc as well as heat the plenum. Best to just cut torque by retarding timing like the OEMs do. Likely you only need to pull around where the TPI runners resonate the highest. Be careful though too much retard will create high exhaust gas temperatures. Another way to cut torque is to prevent power enrichment and run in stoich. Alot of GM and Dodge trucks run in that manner. They will stay in closed loop with less timing at 14.7:1 even at WOT.

[nilak]

Thank you fast355
How safe is to block the acceleration enrichment since less fuel means less cooling effect through evaporation of the fuel?

[sanderson231]

If torque is a concern why a stroker motor? A 350 can make 425 HP

[steveo]

let me ask the question in a way that makes more sense

"how do i reduce the volumetric efficiency of an engine within a certain powerband"

assuming the bore/stroke is already selected the following things affect volumetric efficiency in specific ranges (without affecting it across the board like a restrictor plate)

- camshaft profile (huge effect)
- ignition timing (medium effect)
- scavenging from exhaust configuration (small effect)
- intake runner length (small effect)

so yeah by that logic you probably want a different cam to achieve this goal that i don't agree with

How safe is to block the acceleration enrichment since less fuel means less cooling effect through evaporation of the fuel?

the cooling effect is small and needs to be proportioned against ignition timing to avoid knock. it's really hard to blow up an n/a engine even under heavy load unless you go really lean.

[nilak]

Thank you, The cam profile with a carburettor setup has 468lbft @4000rpm, and 400lbft@5500 420hp495”/.502” Lift, 220°/224° Duration @ .050”, 112 LSA, a different option they tested is 450hp[/B]: .510”/.521” Lift, 231°/236° Duration @ .050”, 111 LSA with 488lbft @4000. So the first cam seems good. I don’t know if it’s worth looking for a different cam with flatter curve, if that’s even possible. The TPI will probably make it worse. I can use large runners, these will raise the torque by like 40lbft everywhere according to a test I’ve seen. Probably they won’t be able to produce as much power as the carburettor configuration which has a smoother flow. I’ve seen test with as much as 80lbft reduction when using 11 degrees of total advance but for that I plan to install and EGT. I already have 1 extra slot for AFR and 2 more at the front of the preconverters (these are 100CPSI metallic).

[el Jefe]

Hello,
I'm upgrading my 1991 Corvette to a 383SBC but the torque output is much higher than stock, especially with the stock manifold-long runners, 480Nm vs 650Nm. That would put stress on the transmission, diff and C beam, so I'm searching for ways to reduce max torque to about 500Nm. Power can still remain the same for the upper range.4500-5600. I will use stock manifold and large intake tubes AZS because I need the stock appearance so I need to do it by ECM tuning. One I know is by reducing the spark advance but this I understand would use the same amount of fuel and producing less torque/power. I thought of trying remapping the EGR to run at high throttle and mid rpm so that the amount of oxygen is reduced. Is this a bad idea?
Thank you.

I face a similar problem with my 75' Vette so, I know where you are coming from. One thing I think you are missing is all the torque in the world won't do a thing if the tires aren't gripping. You can use your tires as a "safety valve" so to speak. Select tires that have a higher wear rating or a narrower footprint and they will begin to slip, AKA burnout, once your torque passes a certain level.

Or build a stronger transmission? I believe a C4 has 2 options for the differential, late C3 did. One is stronger than the other.

You've already hunted the bear, might as well enjoy the fruit of your labors...

[nilak]

Yes, tires will limit the differential torque, I think my car has a dana 44, they can handle street legal tires or better. Output rated torque is 3460lbft. A braking coefficient of 1 for a street tire would mean about 2000lbft.
Now in a low gear you will break traction at below 350lbft engine torque (@low rmp) but in a higher gear tires will not help and a high engine torque will have to be managed by the transmission gears. the transmission is said to be able to withstand even 480lbft but I prefer it as low as possible (400 is plentiful, lower than that would mean low power because I canÂ’t go higher than 5600rpm).
the plan wii be: timing retard and egt monitoring.

[sanderson231]

I still don't get it. Is someone giving you a 383? Everybody I know that stroked a 350 to 383 with the 400 crank was looking to increase torque. Just install a 350 and spin it faster.

[nilak]

I still don't get it. Is someone giving you a 383? Everybody I know that stroked a 350 to 383 with the 400 crank was looking to increase torque. Just install a 350 and spin it faster.

My old engine is at the point where it might need some refreshing soon, 135k miles, it still runs well though, but I wanted a bit more power, stock 245hp is not bad considering the high torque also. I donÂ’t think upgrades for this engine are a good idea because it would require a lot of components to be changed and there is a change of something not working as planned so while I still can I wanted to take the opportunity to buy a new engine. I can also try preserve the old one.
IÂ’ve been searching for a while for a new engine and the most interesting was this 383, a 350 wasnÂ’t available there because most people appreciate the 383. Anyway. At 4500-5000 rpm where I think is a safe limit to run it when I want to go fast the, 383 makes enough power. Would a 350 would make more power there or more likely above 5500 ?there it enters different territory upgrades. definitely not with a TPI manifold that I want to stick with, and less reliability

[sanderson231]

My old engine is at the point where it might need some refreshing soon, 135k miles, it still runs well though, but I wanted a bit more power, stock 245hp is not bad considering the high torque also. I donÂ’t think upgrades for this engine are a good idea because it would require a lot of components to be changed and there is a change of something not working as planned so while I still can I wanted to take the opportunity to buy a new engine. I can also try preserve the old one.
IÂ’ve been searching for a while for a new engine and the most interesting was this 383, a 350 wasnÂ’t available there because most people appreciate the 383. Anyway. At 4500-5000 rpm where I think is a safe limit to run it when I want to go fast the, 383 makes enough power. Would a 350 would make more power there or more likely above 5500 ?there it enters different territory upgrades. definitely not with a TPI manifold that I want to stick with, and less reliability

A TPI intake is like a restrictor plate on a 383. I'm not sure that peak torque will be a problem. I have DynoSim which has the TPI intake option built in. Tell me more about the engine and I will plug it into DynoSim and sees what it says about torque and HP.

[nilak]

That would be interesting
it is a restrictor but it also accelerates the flow at low rpm.
compression 9.8:1.
Rocker Arms: 1.5 Ratio, Full-Roller
Cylinder Heads: Aluminum, 180cc Intake Port, 2.02"/1.60" Valves, 65cc Chambers,
cam: 495”/.502” Lift, 220°/224° Duration @ .050”, 112 LSA
then stock exhaust manifold (then straight pipe track, cats for street) and intake manifold (probably will need match porting),
I’ll try TPIS throttle body but probably -minor differences

there is a guy on Yt, @richardholdener1727 and he did all sorts of tests with these engines, like tpi vs carb manifolds, you can get an idea of what kind of torque curves you can expect, even with different timing advance settings. I have the carburetor dyno chart.
the chart has max 468lbft. I’m expecting 500lbft and 380-400Hp. :)

[sanderson231]

I made some runs with DynoSIM. For ambient conditions I assumed 77 'F, 50% humidity and 1000 ft elevation

I first made a baseline run for a 350 with a TPI intake and the following:

stock port head head with 2.02" intake and 1.6" exhaust valves
9.5:1 compression ratio
48mm throttle body bores
Generic performance street cam
-0.552 valve lift intake and exhaust
-274/286 degrees duration @ 0" lift intake/exhaust
-230/236 degrees duration @ 0.050" lift intake/exhaust
-110 LSA
-60 degrees of overlap

This resulted in 262 HP @ 4500 rpm and 375 ft-lbs @ 3000 rpm. Stroking this engine to a 383 with 9.8:1 CR gives 270 HP @ 4000 rpm and 407 ft-lbs @ 3000 rpm

Using your cam specs for the 383, I get 262 HP @ 3500 rpm and 422 ft-lbs @ 2000 rpm. Increasing throttle body bores to 58mm gave 266 HP @ 4000 rpm and 423 ft-lbs @ 2500 rpm. Sorry but this is what DynoSIM predicts for a TPI intake.

If you want to stay with the 1990's look, you could consider picking up a used 1992-1997 LT1 and getting it rebuilt to your specs.

[nilak]

thank you, that’s a little disappointing :)
the guy that made those tests manage 330Hp on a mild cam 98 tpi, but with headers, no air filter, electric water pump, no alternator. I wonder why such differences
266 hp is ok but since this engine can do 420 I should search ways to make it faster, like large runners, with stock look but makes it harder to work on it.

[sanderson231]

The advertised specs for a 1991 TPI are 250 Hp and 375 ft-lbs of torque. DynoSIM was close to that so I think that it is a net HP (with accessories). I have also modeled a 2.3L w41 Quad 4 that is advertised for 190 HP and DyniSIM was also fairly close. I reran the L98 with large tube headers and an open exhaust and it made a huge difference - 330 HP @ 4500 rpm and 467 ft-lbs @ 2500 rpm

An LT-1 intake (requires LT-1 heads) from a 1992 vette has more upside for horsepower production with less torque. If you are locked into a max torque number a 350 will make more horsepower than a 383 at the same peak torque. DynoSIM included a model for a streetable 302 SBC (carbed). It made 433 HP @ 6500 rpm and 397 ft-lbs @ 4500 rpm. Note that HP is higher than torque. Fun engine with a standard transmission. I still dream of owning a 67-69 Z-28 Camaro some day.