Exhaust question for Duke

[Mike R.]

I am restoring a 69 L89 automatic car and I need an exhaust system. 1969 Corvettes all came with 2" systems. I would pain me greatly to chock my engine with a puny exhaust and I am curious how much power you think would be lost with the 2" system vs a 2.5" off road system from a 68?

I posted this instead of using a PM because I thought others would be interested in the answer.

Mike

[Duke W.]

Re: Exhaust question for Duke

All I can do is give you a rough estimate.

The 2.5" system with offroad mufflers is probably the lowest restriction factory installed system on any car of that era. On a "327 LT-1" configuration that is pushing close to 300 SAE corrected HP at the rear wheels it only generates about 3 psi backpressure, which is quite modest for a high performance road engine.

On a massaged SHP big block, which is pumping out close to 40 percent more exhaust volume, backpressue will about double since backpressure increases with the square of flow, and a properly massaged SHP big block should make at least 350 SAE corrected RWHP. On a lab dyno with headers the same engine would measure about 450 gross HP.

My estimate is that on a L-89, especially with massaged heads, the 2" system will cost 3-6 percent peak torque and 5-10 percent peak power.

Look at the JG and figure out the points loss. Most replacement systems will take a substantial deduction, and a 2.5" system will probably take a full deduction, so you have to trade off the performance potential versus the judging points loss, assuming you plan to have your car judged.

If it were my decision I would go with the 2.5" engine as I'm more interested in exploiting the maximum performance potential of whatever engine is installed without installing a bunch of aftermarket bubba parts in the engine.

Duke

[Gene M.]

Re: Exhaust question for Duke

The 2 1/2" system sounds like the way to go.

The idea of headers on an L89 sounds like a perfect match for running thru the gears if you are not concerned with NCRS points.

[Mike R.]

Re: Exhaust question for Duke

Thanks Duke, I knew I could count on you to talk me into what I wanted to do anyway!

BTW We talked last year about my 65 L84 engine. I found a lot of issues when I pulled the heads for porting and the pan to look at the rods. I am waiting for pistons and then it will go together. We blueprinted the block on a CNC after learning that the low point on the low deck was the stamp pad (talk about luck). I bought a standard crank and had it index ground .010/.010 and used the SCAT H beam rods. I chose the lighter ones that weigh about the same as stock. The valve throats were opened up with a radius cutter and the bowls blended. This will be about as strong a 327 as you can have and still be "stock"

Thanks again.

Mike

All I can do is give you a rough estimate.

The 2.5" system with offroad mufflers is probably the lowest restriction factory installed system on any car of that era. On a "327 LT-1" configuration that is pushing close to 300 SAE corrected HP at the rear wheels it only generates about 3 psi backpressure, which is quite modest for a high performance road engine.

On a massaged SHP big block, which is pumping out close to 40 percent more exhaust volume, backpressue will about double since backpressure increases with the square of flow, and a properly massaged SHP big block should make at least 350 SAE corrected RWHP. On a lab dyno with headers the same engine would measure about 450 gross HP.

My estimate is that on a L-89, especially with massaged heads, the 2" system will cost 3-6 percent peak torque and 5-10 percent peak power.

Look at the JG and figure out the points loss. Most replacement systems will take a substantial deduction, and a 2.5" system will probably take a full deduction, so you have to trade off the performance potential versus the judging points loss, assuming you plan to have your car judged.

If it were my decision I would go with the 2.5" engine as I'm more interested in exploiting the maximum performance potential of whatever engine is installed without installing a bunch of aftermarket bubba parts in the engine.

Duke

[Duke W.]

Re: Exhaust question for Duke

The idea of headers on an L89 sounds like a perfect match for running thru the gears if you are not concerned with NCRS points.

Actually, headers don't do much - even on a SHP engine with a road legal exhaust system. Properly designed headers, combined with a high overlap camshaft, do have a significant effect with open exhaust, but even with a relatively low restriction exhaust system, backpressure negates most of the wave dynamic effects. They do essentially nothing on an engine with a low overlap camshaft - a complete waste!

On a back to back lab dyno test of a "327 LT-1" dyno headers only made 2-3 percent more peak power than the OE 2.5" manifolds, but they did improve peak torque by about 7-8 percent. On the car the effect would be cut at least in half.

The peak power improvement was low because the 1 5/8" primary tubes were too small for this engine's output, which was in the 360 (honest) gross HP ballpark at 6500. Bigger primary tubes would have improved top end power, but reduced the peak torque improvement, and properly designed headers to maximize average power in the upper third of the rev range often create a big torque curve hole somewhere in the bottom half of the rev range, which is where we spend most of our timing driving.

Headers are really only useful for racing engines that run open exhaust with gearing that keeps revs in the upper third of the useable rev range.

For every psi of exhaust backpressure, the engine looses one psi BMEP, but that's not all. This backpressure also reduces volumetric efficiency because the cylinder can't start filling until cylinder pressure drops below exhaust backpressure, which doesn't happen until the exhaust valve closes, so a late closing exhaust valve is bad for a road engine.

The "327 LT-1" achieves, in the car with the OE exhaust system, about 90 percent VE. If you install a properly designed racing exhaust system - 1 7/8" OD primary pipes about 34" long with about 60" of 3 to 3.5" collector/tailpipe, typical of what you see on Corvette road racers, the peak VE improves to nearly 105 percent with a correspondingly large increase in torque and power.

Duke

[Gene M.]

Re: Exhaust question for Duke

Actually, headers don't do much - even on a SHP engine with a road legal exhaust system. Properly designed headers, combined with a high overlap camshaft, do have a significant effect with open exhaust, but even with a relatively low restriction exhaust system, back pressure negates most of the wave dynamic effects. They do essentially nothing on an engine with a low overlap camshaft - a complete waste!...........
Duke

I agree with your logic but there seems to still be more here to gain in clearing the cylinders out since there is a lot of cam timing on the L89.

BB's move a lot more air thru the cylinders than SB's. Are not the big block and small block like comparing apples and pears? The BB L89 has so much more overlap and cam timing over it's small block brother. Spec at E 32 ATDC, 94 BBDC and I 46 BTDC, 80 ABDC. It would appear that equal length tuned headers and a bigger free flowing exhaust would wake up the otherwise stock L89. Filling the cylinders using the exhaust due to the cam overlap is gotta help power. One would want the collectors to be longer as you mentioned to do the above mentioned. Off road mufflers with less baffling to cut out some back pressure (bad side is louder) to get closer to "open exhaust". But ya still want to be legal.

Just looking at the stock cast iron exhaust manifolds it can not allow any additional flow from cam overlap. There is no independence between cylinders. It's gotta be all turbulence.

The L89 would be a much better mannered street engine with about 20 degrees less cam timing. Then the whole issue with high overlap and no bottom end goes away.

I enjoy your posts and explainations, shows a lot of thought.

[Mike R.]

Re: Exhaust question for Duke

I think Duke's point is that headers are really only effective if there is negligible pressure in the collector and further downstream. With open exhaust you can have below atmospheric pressure at the port when the valve opens. You don't really get that on a street exhaust. If we accept that there is no accustical or inertial tuning in the collector with a streetable exhaust system then the BB manifolds start to look pretty good. Now if you had a 3 or 3.5" system with large mufflers, you might see more benefit to headers... besides I just spent a small fortune on an AIR system and would feel foolish putting it on headers...

I agree with your logic but there seems to still be more here to gain in clearing the cylinders out since there is a lot of cam timing on the L89.

BB's move a lot more air thru the cylinders than SB's. Are not the big block and small block like comparing apples and pears? The BB L89 has so much more overlap and cam timing over it's small block brother. Spec at E 32 ATDC, 94 BBDC and I 46 BTDC, 80 ABDC. It would appear that equal length tuned headers and a bigger free flowing exhaust would wake up the otherwise stock L89. Filling the cylinders using the exhaust due to the cam overlap is gotta help power. One would want the collectors to be longer as you mentioned to do the above mentioned. Off road mufflers with less baffling to cut out some back pressure (bad side is louder) to get closer to "open exhaust". But ya still want to be legal.

Just looking at the stock cast iron exhaust manifolds it can not allow any additional flow from cam overlap. There is no independence between cylinders. It's gotta be all turbulence.

The L89 would be a much better mannered street engine with about 20 degrees less cam timing. Then the whole issue with high overlap and no bottom end goes away.

I enjoy your posts and explainations, shows a lot of thought.

[Duke W.]

Re: Exhaust question for Duke

What Mike said... I'm trying to say that headers aren't worth the cost/hassle on a road engine... more heat radiated to the engine compartment, loosened bolts, burned wires, etc, and not much improvement in torque/power.

You are much better of putting the money into massaging the heads and doing whatever can be done to minimize exhaust system restriction.

The SHP big block mechanical lifter cam is actually pretty mild compared to SB mechanical lifter cams. It tells in the idle vacuum at 900 - about 14" for the BB cam, 12" for the Duntov and LT-1 cams, and only 10" for the 30-30 cam.

If it were me and I wanted max power, I would work the heads really well. I think the L-89 heads already have 1.84" exhaust valves.

The other thing I might do is increase the stroke and retard the OE cam 4-6 degrees. I worked with an owner of a L-71 on this issue It's a 4.25" stroker with massaged heads, the OE cam retarded 6 degrees, but no external changes.

It's got so much torque that first and second gear are practically unusable at WOT because the tires won't hold, but in normal part throttle driving the engine is quite docile. Top end power is restricted because it has OE side exhaust, which is more restrictive than the under-the-car exhaust. Last I heard he was planning on installing larger aftermarket pipes under the OE covers.

SAE corrected RWHP was in the 350-360 range at 5800 and torque was a whopping 425 lb-ft. That's about 500 lb-ft net flywheel torque, but only about 415 net HP at the crank due to the restrictive side pipes.

The biggest restriction to power on big blocks is the exhaust system. They need 3" pipes - like the modern LS7, but they probably won't fit under the car, so your best alternative is the current reproduction of the N-11 system.

If your car has OE side pipes, kiss top end power goodby. They're just to restrictive for the engine to achieve maximum power potential.

Duke

[Mike R.]

Re: Exhaust question for Duke

3" pipes will fit but you really need good worksmanship to keep the pipes close to the car to retain ground clearance. I had 3" pipes on my all aluminium 468 cu in 1969 coupe. I used huge mufflers and had to eliminate the spare tire and carrier. The system was fabricated with all mandrel bends. THere are a couple guys on the Corvette forum with nicely fabricated 3" and 3.5" systems.

Mike

What Mike said... I'm trying to say that headers aren't worth the cost/hassle on a road engine... more heat radiated to the engine compartment, loosened bolts, burned wires, etc, and not much improvement in torque/power.

You are much better of putting the money into massaging the heads and doing whatever can be done to minimize exhaust system restriction.

The SHP big block mechanical lifter cam is actually pretty mild compared to SB mechanical lifter cams. It tells in the idle vacuum at 900 - about 14" for the BB cam, 12" for the Duntov and LT-1 cams, and only 10" for the 30-30 cam.

If it were me and I wanted max power, I would work the heads really well. I think the L-89 heads already have 1.84" exhaust valves.

The other thing I might do is increase the stroke and retard the OE cam 4-6 degrees. I worked with an owner of a L-71 on this issue It's a 4.25" stroker with massaged heads, the OE cam retarded 6 degrees, but no external changes.

It's got so much torque that first and second gear are practically unusable at WOT because the tires won't hold, but in normal part throttle driving the engine is quite docile. Top end power is restricted because it has OE side exhaust, which is more restrictive than the under-the-car exhaust. Last I heard he was planning on installing larger aftermarket pipes under the OE covers.

SAE corrected RWHP was in the 350-360 range at 5800 and torque was a whopping 425 lb-ft. That's about 500 lb-ft net flywheel torque, but only about 415 net HP at the crank due to the restrictive side pipes.

The biggest restriction to power on big blocks is the exhaust system. They need 3" pipes - like the modern LS7, but they probably won't fit under the car, so your best alternative is the current reproduction of the N-11 system.

If your car has OE side pipes, kiss top end power goodby. They're just to restrictive for the engine to achieve maximum power potential.

Duke

[Duke W.]

Re: Exhaust question for Duke

Assuming one is willing to sacrifice orignality, a custom-made 3" mandrel bent system with the lowest restiction mufflers that you can stand (noise-wise) would be a much better investment than headers, and you would want as smooth a transition as possible from the 2.5" manifold outlets to the 3" head pipes.

The 3" piping and bypass mufflers are one of the secrets to the LS7's impressive output as are the hydroformed exhaust manifolds.

The vintage BB Corvette manifolds are pretty good. They just need to be matched up well to the head ports and headpipes.

The exhaust system is an afterthought to most guys. That's one reason why an engine that can make 500 SAE gross HP on a lab dyno can be challenged to break 350 SAE corrected at the rear wheels.

When system engineering an engine configuration, one has to consider everything from the induction air inlet to the tail pipe.

Duke