2.5" exhaust - Effect on high spec build

[EVOgone]

Interesting result on the rollers today in the Rallycar.

Origional build mapping showed 228bhp 182lb/ft with 2" diam exhaust, Peak about 7k, went on 3 different occasion and was withing 1/2bhp..

Full 2.5" exhaust fitted and full top end rebuild (everything the same inc. timing) the map needed a few tweeks as a result of the exhaust. Its now eliminated some of the bumps in the torque curve but in essance the exhaust just moved the torque curve 500rpm higher but after 6k held the torque curve longer before dropping off. Now the power is still climing after 7.5k where previously is was dropping off. Results where 243bhp @ 7800rpm torque 180ft/lb. We even backed the timing back a little and were conserative due to using pump fuel.

Cliokongen had a very similar effect on his ITB engine which is similar spec (slightly lower cr and milder cams) with the exhaust. He also modified his manifold to have the mating down pipe of 2.5" dia to match the exhaust. This is the next step for me the back to the rollers but conclusion is if you have a high spec build then 2.5" exhaust is a must...

Before everyone says figures are rubbish, these are all on the same rollers so the step change is the important aspect rather than the absolute figures BUT these rollers are hub rollers and read very consistantly and are very accurate based on lots of feedback. Although did have one scare when the fuel pressure dropped on one run but was due to the exhaust boiling the fuel up due to not air getting to it.!! covered the exhaust and was fine after that.

Car goes very well, at the last event a guy came across whos was running a Millington engined Mk2 freshly built by Melvin Evans and said F@*% thought i would catch you up on the straight but couldnt, was well impress with the car.

Hopefully cars all OK for the next round of the championship for the Promenade stages 7/8th September on New Brighton sea front, great event with night time super special stages on the Friday.​

 

[Chip-mk1]

Must say I am surprised at it being as much as 15bhp extra, but that said only 2" to begin with it was certainly on the small side for that power so gains were always on the cards really.

Did you find at very low rpm you had some losses, or is it a case of being a competition car you didnt pay too much attention to the real low rpm stuff?

When I went from 2" to 2.5" on my ITB 2.0 vauxhall engine I had to put a fair bit more fuel into the map at high rpm, and no significant loss anywhere (which surprised me TBH), it clearly made quite a difference having the extra flow at high rpm, although mine included a manifold upgrade too so its hard to attribute how much was due to each part.

 

[EVOgone]

The 2" exhaust did have a few scapes on it so this wouldnt have helped. The torque curve 3k- 6kis near identical but staggered 500rpm later but hold on after 6k where it would start to really tail off. Sub 3k didnt really touch as it drives fine and idles great (when warm, as no cold start map) spends all its life between 5-8k.

Another similar spec engine has showed very similar results.

 

[Chip-mk1]

Just goes to show how mismatched the original exhaust was then I guess for such a big spec engine.

Would be interesting to have seen it on a 2.25" as well, as that might have given you the same gains (after all a 2.5" is 56% bigger than a 2" which sounds overkill, and 2.25" is 26% bigger which is probably plenty) for less weight.

I normally work to the rule of thumb that 2" is good for being unrestrictive upto 200bhp, 2.25" is good for 250bhp and 2.5" is good for 300bhp

 

[EVOgone]

Getting "Mike the Pipe" ex Simpson to make a proper manifold when i get time.......and some money


Should be interesting as the joins and the collectors arnt great with whats on currently.

 

[Chip-mk1]

Not personally familiar with Mike, but simpsons is a great background.

What are you on at the moment? A supersprint?

 

[EVOgone]

Not personally familiar with Mike, but simpsons is a great background.

What are you on at the moment? A supersprint?

K-Tec supersprint replica.........

 

[Chip-mk1]

K-Tec supersprint replica.........

Is it much different to a genuine one? I must say the collectors on my supersprint dont look terrible TBH, altough they could certainly be improved upon still so at your level I can really see the advantage in getting something custom made.

 

[EVOgone]

Is it much different to a genuine one? I must say the collectors on my supersprint dont look terrible TBH, altough they could certainly be improved upon still so at your level I can really see the advantage in getting something custom made.

For 250 quid new you get what you pay for, its no supersprint (very good and twice the price in SS).

The pipe into collectors overlap so interally not very smooth etc...

 

[Chip-mk1]

Will you be going for a 182 style manifold in terms of no joins? Or keeping it like the supersprint with the join in the middle so its easier to fit and remove? (182 style means either drop the subframe or move the engine right forward to fit)

 

[EVOgone]

Will you be going for a 182 style manifold in terms of no joins? Or keeping it like the supersprint with the join in the middle so its easier to fit and remove? (182 style means either drop the subframe or move the engine right forward to fit)

Will wait and see what the experts say

 

[Chip-mk1]

Will wait and see what the experts say

Im sure they will say no join is best performance wise.

Mind you with it being a rally car, I guess there are impacts to think about as well of course, and the fact you might need to replace half of it during an event in a hurry.

So not as simple as question as it first looks I suppose.

 

[EVOgone]

2 piece most likely as it easier to drop the suframe when needed etc.

 

[alexapps]

2 piece most likely as it easier to drop the suframe when needed etc.

Seeing as you haven't replied to any of my 5 pm's I have sent you, or the three messages I have left you in the for sale thread since I paid for the pads last sunday. Perhaps you will reply in here?

Did you get them sent out as promised last monday? As I asked when I purchased the items I needed the pads by the following saturday (today) and despite you promising you would get them to me in time, there is still no sign of them.

I would very much appreciate a reply, even if you have not sent them I would like to know where I stand.

Alex

 

[EVOgone]

PM'd there on there way, been on holiday.

Any issues drop me a not etc.

 

[alexapps]

Thanks,

Just got your pm.

Alex